Review of Literature - INFLIBNETshodhganga.inflibnet.ac.in/bitstream/10603/5345/9/09_chapter...

6

Review of Literature

2.1 Mangroves – an overview

The word "Mangrove" is considered to be a blend of the Portuguese word

"Mangue", meaning "tree", and the English word "grove", meaning a "grouping of

trees". Mangroves are a diverse group of unrelated trees, palms, shrubs, vines and ferns

that are having a common adaptation to live in waterlogged saline soils which are

dynamic in environmental conditions such as salinity, pH, temperature, redox potential,

soil texture and nutrients. There are about 80 species of mangroves found all over the

world between latitudes 32°N and 38°S (Saenger et al., 1983). As a common criteria,

the average water temperature should not fall below 24 °C and the average air

temperature should not be lower than 20 °C during the coldest month. Changes in air

temperature should not exceed 10 °C during the year. This limits the existence of

mangrove to certain extent only. They mostly present in tropical and subtropical coastal

areas subjected to tidal influences. An area influenced by tide can be a shoreline

inundated by the extremes of tides, or it can more widely refer to river-bank

communities where tides cause some fluctuation in water level (Tomlinson, 1986).

Mangroves can be found not only inhabiting tidal mud flats but also along freshwater

riverbanks. Repeatedly flooded, but well drained soil supports a luxurious growth of

mangrove plants. They normally grow poorly in stagnant waters and have rich growth

in the alluvial soil substrates with fine‐textured loose mud or silt, rich in humus and

sulphides (Kathiresan and Bingham, 2001).

Mangroves are possibly evolved just after the first angiosperms, around 114

million years ago (Duke, 1992). There are two categories of mangroves, true

mangroves and mangrove associates (Wang et al., 2010). The species which are

adapted to the mangrove environment and do not extend into other coastal plant

communities are called as true mangroves. True mangroves consist of a core group of

some 30-40 species such as Rhizophora apiculata, Kandelia candel, Ceriops tagal,

Bruguiera gymnorrhiza, Aegiceras corniculatum and Sonneratia caseolaris (Spalding,

2004). They are main component of the mangrove ecosystem both numerically and


7

structurally, and are found in almost all mangrove forests. Plants that exist in the

coastal environment and also found within mangroves are called as mangrove

associates such as Hibiscus tiliaceus and Barringtonia acutangula (Melana et al.,

2000). However, as for some fringe species straddle mainly on the landward

transitional zones, no discrete decisions among researchers could be arrived in favour

of this classification (Wang et al., 2010).

2.1.1 Adaptive features of mangrove plants

2.1.1.a . Strategies of mangrove plants to overcome high salinity

Mangroves are highly specialised plants that have developed extraordinary

adaptations to the unique ecological conditions. The strategy to combat high salinity is

differs from species to species. The first line of defence for many mangroves is to

prevent a large extent of the salt from entering by filtering it out at root level. Some

species can keep out more than 90 percent of salt eg. Rhizophora spp., Ceriops spp.,

Bruguiera spp, etc., It is essential to keep cytosolic salt concentration stable when

living in intertidal zones with high salinity (Liang et al., 2008). They possess various

mechanisms related with anatomic or physiological features to control salt absorption

and exclusion, such as ultrafiltration, salt-secretion and ion sequestration (Melana et al.,

2000; Mimura et al., 2003; Liang et al., 2008; Wang et al., 2008). Some species can

even accumulate saline ions as osmolytes to balance transmembrane osmotic potentials.

Different strategies of salt tolerance show that mangroves are having high salinity

adaptability in the anatomic and physiological levels. Accumulation of ions in the

vacuole is one of the most important strategies employed by plant cells against salt

stress (Melana et al., 2000; Mimura et al., 2003). Mangrove species like Kandelia

obovata, Avicennia marina, can accumulate inorganic ions and utilize them as

osmolytes to maintain osmotic and water potential. Increasing the concentration of Na+

in the vacuole and by removing potentially toxic Na+ from the cytoplasm is important

in maintaining cellular osmolarity. The higher ionic concentration in the vacuole

facilitates inflow of water and an increase in vacuolar volume during the cell growth

(Mimura et al., 2003; Liang et al., 2008). Thickening of leaves can reserve abundant

water by which absorbed salt was diluted and salt-induced damage was reduced to


8

some extent. For example, Lumnitzera racemosa can increase leaf thickness and water

content when stressed with high salinity (Liang et al., 2008). In some species of

mangroves, salt gland in the leaves is another typical structure in which excessive salt

can be secreted out. All species in genus Aegiceras, Avicennia, Acanthus and Aegialitis

have typical salt gland structures (Mimura et al., 2003; Liang et al., 2008).

2.1.1.b . Root adaptations for muddy environment

The root system of mangrove is diverse and quiet interesting adaptation to

survive in the muddy ecosystem (Fig.2). The major plant species of mangroves have

aerial roots, generally prop roots or even stilt roots eg. Rhizhophora spp. These root

systems are important in gas exchange, because the mangrove mud tends to be

anaerobic and can then absorb air through pores in their bark (Lenticels) (Naidoo and

Willert, 1999; Kathiresan and Bingham, 2001; Naidoo and Tuffers, 2002).

Rhizophora spp (Red mangroves) have prop roots downward from the trunk and

Figure 2. Various type of root system present in mangrove plants


9

branches, giving a steady support system. Some of the mangrove species, including

A. marina obtain stability with an extensive system of shallow, underground “cable

roots” that run out from the central trunk for a significant distance in all directions.

Pneumatophore which is also called as breathing roots extends from these cable roots.

These are filled with spongy tissue (arenchyma) and covered with lenticels that allow

oxygen to be transferred to the roots trapped below ground in the anoxygenic soils

2.1.1.c .Viviparous seedlings

The conditions like saline water, muddy saline soil with little or no oxygen is

not an advantageous environment for mangrove seeds to germinate and establish. To

triumph over this, mangrove species have unique way of reproduction, which is

generally known as vivipary. In viviparous method, seeds germinate and develop into

seedlings in the parent tree itself. These seedlings are called as propagules and they are

capable of doing photosynthesis. The parent tree supplies water and necessary

nutrients. They are buoyant and float in the water for sometime before rooting

themselves on suitable soil. The viviparous reproductive system allows seedlings to

develop salinity tolerance before being released from the parent tree (Kathiresan and

Bingham, 2001; Ye et al., 2005).

The mangrove forests were classified into six types as follows (Lugo and

Snedaker, 1974),

a. Fringe forests: This forest type occurs along fringes of protected shorelines and

islands with elevations higher than the mean high tide, inundated by most high

tides, rarely affected by strong wind, erosion and accretion.

b. Riverine forest: Forests along river and creek drainage areas, which are

inundated by most high tides and flooded during rainy seasons, and

therefore subjected to varying levels of salt concentrations.

c. Overwash forest: Small islands and peninsulas, which are completely

overwashed during all high tides.

d. Basin forest: This type exists in inland areas along drainage depressions.

Inundated by a few high tides during the dry seasons (also depending on the

distance to the coast) and more high tides during rainy seasons.


10

e. Dwarf forest: This forest type is limited to topographic flats above mean high

water levels. The trees are relatively small and may be stunted or dwarf-like;

additionally, the environment lacks external nutrient sources.

f. Hammock forest: This type is similar to the basin forest type, but the ground is

slightly elevated (5-10 cm) above the surrounding area.

Cintron and Novelli (1984) modified the above mentioned classification into

three types i.e., fringe forest, riverine forest and basin forest (Fig. 3). The classification

is mainly based on the topography and hydrology of the mangrove forest system

(Cintron and Novelli, 1984).

Figure 3. Classification of mangrove forests based on topography (Riverine forest type,

Fringe forest type, Basin forest type)


11

Figure 4. Schematic diagram depicts the factors involved in mangrove zonation (Lewis, 2005)

Mangroves often form noticeable zones or bands of species which are

sometimes generally parallel to the orientation of shore along with these marine plants

occur. The pattern of zonation may be attributed to a number of external factors such as

soil condition, nutrient and oxygen availability, and salinity (Cintron and Novelli, 1984).

Availability of freshwater and duration of daily tidal inundation which influences soil

conditions may limit the distribution of species to areas most suitable for their growth

(Bunt, 1996; Matthijs et al., 1999; Vilarrubia, 2000) (Fig. 4). The seedling dispersal,

size and growth rate of mangrove plants, destruction of seedlings by crabs and shade

tolerance also influences mangrove zonation. Zones are more complex and numerous

when a great number of species is present. However, the knowledge on mangrove

zonation is essential in restoration and afforestation of mangrove forests.


12

2.2 . Global status of Mangroves

Globally, mangroves occur in 118 countries and territories (Spalding, 2004; Giri

et al., 2010). The total mangrove area cover in the year 2000 was 137,760 km2

in

countries and territories in the tropical and subtropical regions of the world (Fig. 6).

The total land area of mangrove forest cover is less than 1 percent of the entire

area of tropical forests and less than 0.4% of global forest cover (Spalding et al., 2010).

Around 75% of world’s mangroves are found in just 15 countries, and only 6.9% are

protected under the existing protected areas network. The most extensive mangrove

area is found in Asia, followed by Africa and North and Central America.

Table 1. The mangroves cover of countries and their percentage in global total (Giri et al.,

2010).

S.No Country Area (m

2) % of global total

1 Indonesia 3,112,989 22.6

2 Australia 977,975 7.1

3 Brazil 962,683 7

4 Mexico 741,917 5.4

5 Nigeria 653,669 4.7

6 Malaysia 505,386 3.7

7 Myanmar (Burma) 494,584 3.6

8 Papua New Guinea 480,121 3.5

9 Bangladesh 436,570 3.2

10 Cuba 421,538 3.1

11 India 368,276 2.7

12 Guinea Bissau 338,652 2.5

13 Mozambique 318,851 2.3

14 Madagascar 278,078 2

15 Philippines 263,137 1.9

Figure 5. Latitudinal distribution of mangrove forest of the world (Giri et al., 2010)


13

Figure 7. The reduction of mangrove area from the year 1980 to 2005

Figure 6. Global distribution of Mangroves (FAO 2007)


14

Five countries (Indonesia, Australia, Brazil, Nigeria and Mexico) together

account for 48 percent of the total global area (Table 1) (Giri et al., 2010). The global

patterns of biodiversity in mangroves are occurs in an interesting manner. In the

latitudinal pattern of mangrove flora, the highest species richness occurs around the

Equator and declines at higher latitudes both north and south (Fig. 5) (Duke et al.,

1998; Gopal and Chauhan, 2006; Giri et al., 2010). Although the literature on

mangrove forests is wide and abundant case studies describe their extent and losses

over time, global, comprehensive information on the status and trends in the extent of

mangroves has been lacking. However, the available mangrove estimates are not giving

a real picture and it shows ranges from 12 to 20 million hectares (FAO, 2007; Giri et

al., 2010). In the earlier studies, many of the countries with least cover of mangroves

were neglected and it may be due to lack of information (FAO, 2003; FAO, 2007).

FAO denoted the reduction of about 5 million hectares in global area of

mangrove forest. According to the survey, the most comprehensive data on the state of

the world’s mangrove forests revealed that the mangrove area worldwide had fallen

below 15 million hectares at the end of 2005 down from an estimated 19.8 million

hectares in 1980 (FAO, 2007; Giri et al., 2010) (Fig. 7). Every year 1 to 2% of

mangroves are disappearing worldwide, a rate greater than or equal to declines in

adjoining coral reefs or tropical rainforests. The loss occurs in almost every country

that has mangroves, and rates continue to rise more rapidly in developing countries,

where more than 90% of the world’s mangroves are found (Duke et al., 2007). By the

degradation mangrove areas are becoming smaller or fragmented which makes their

long-term survival at great risk, and essential ecosystem services may be lost.

The predictions on mangrove loss alarming that 30–40% of coastal wetlands

and 100% of mangrove forests (Duke et al., 2007) could be lost in the next 100 years if

the present rate of loss continues. At the country level, Indonesia, Mexico, Pakistan,

Papua New Guinea and Panama recorded the largest losses of mangroves during the

1980s. A total of some 1 million hectares were lost in these five countries. The

deforestation is because of many reasons especially anthropogenic effects rather than

natural calamity (Kairo et al., 2001). This human infringement of habitat destruction

includes exploitation of land for urbanization, agriculture, aquaculture and mining, and

clearing forest for fire wood and timber (Ellison and Farnsworth, 1997; Kairo et al.,


15

2001; Adeel and Pomeroy, 2002). Globally, initiatives have been taken to conserve and

protect mangroves, which resulted in decreased rate of mangrove loss (FAO, 2007).

Most countries have now banned the clearing of mangrove areas for aquaculture and

require environmental impact assessments prior to large-scale conversion of these areas

to other uses.

2.3 Mangroves of India

Mangroves in India occupy

about 5% of the global mangrove

vegetation and are spread over an

area of about 4,500 km2 along the

coastal States of the country. In the

past, total area of the mangroves in

India was around 6,740 km2 which

accounted about 7% of the world

mangroves and 8% of the Indian

coastline (Untawale, 1987). The

decline of mangrove in India is

about 33%, which is very alarming

and continuance of mangrove loss

at this rate may fade away the

ecosystem totally. In India, deltaic

and backwater-estuarine type of

mangroves occurring in the west coast (Arabian) which is characterized by typical

funnel-shaped estuaries of major rivers or backwaters, creeks, and neritic inlets. Insular

mangroves are present in Andaman and Nicobar islands which are formed by many

tidal estuaries, small rivers, neritic islets, and lagoons (Gopal and Krishnamurthy,

1993). The east coast of India harbours 70% of total mangrove vegetation and 12% on

the west coast (Fig. 8). The bay islands (Andaman and Nicobar) account for 18% of the

Country’s total mangrove area (Krishnamurthy et al., 1987; Kathiresan et al., 1995).

The east coast of India having immense mangrove cover due to the nutrient-rich

Figure 8. Mangrove sites in India (Kumar, 2000).


16

alluvial soil formed by the rivers Ganga, Brahmaputra, Mahanadhi, Godavari, Krishna

and Cauvery and a perennial supply of freshwater along the deltaic coast. A total of 82

mangrove species, distributed in 52 genera and 36 families, has been recorded by

various studies (Mandal and Naskar, 2008). Sundarban mangrove forest, West Bengal,

having the highest taxa diversity i.e, 69 species, 49 genera, 35 families, including two

species, viz. Scyphiphora hydrophyllacea and Atalentia corea reported for the first time

from Indian Sundarbans (Mandal and Naskar, 2008).

Table 2. Area distribution of mangroves in India (Kumar, 2000).

State/Union territory Government of India,

1987 (sq. km)

Government of India,

1997 (sq. km)

West Bengal (Sundarbans) 4,200 2,123

Andaman and Nicobar Islands 1,190 966

Maharashtra 330 124

Gujarat 260 991

Andhra Pradesh 200 383

Tamil Nadu 150 21

Orissa 150 211

Karnataka 60 3

Goa 200 5

Kerala Sparse Nil

Total 6,740 4,827

Mangroves in Tamil Nadu exist on the Cauvery deltaic areas. The major extent

of mangrove cover of Tamilnadu is present at Pichavaram and Muthupet. The present

work was carried out in both Pichavaram and Muthupet mangroves. However, when

compared to Pichavaram, Muthupet mangrove is least explored.

Pichavaram mangroves (Lat. 110 2’ N; Long. 79

0 47’ E) that extend between the

Vellar and Coleroon estuarine areas spread to an area of 21 km2 (Kannan, 1990).

Pichavaram has a well developed mangrove forest dominant with Rhizophora spp.,

Avicennia marina, Excoecaria agallocha, Bruguiera cylindrica, Lumnitzera racemosa,

Ceriops decandra and Aegiceras corniculatum (Kathiresan, 2000). In the point of

abundance, Avicennia marina is the most common species, followed by

Rhizophora apiculata, R. mucronata, Bruguiera cylindrica, Excoecaria agallocha,

Ceriops decandra, Avicennia officinalis, Aegiceras corniculatum,

Rhizophora annamalayana, Acanthus ilicifolius and Lumnitzera racemosa.

Xylocarpus granatum and Sonneratia apetala are rare. Kandelia candel is extinct from


17

the study area (Kathiresan, 2000). In this mangrove ecosystem, 13 species of mangrove

trees, with predominant Avicennia marina and Rhizophora species, as well 73 spp. of

other plants, 52 spp. of bacteria, 23 spp. of fungi, 82 spp. of phytoplankton, 22 spp. of

seaweeds, 3 spp. of seagrass, 95 spp. of zooplankton, 40 spp. of meiobenthos, 52 spp.

of macrobenthos, 177 spp. of fish and 200 spp. of birds were recorded (Kathiresan,

2000). The Pichavaram mangrove ecosystem has been broadly studied over past three

decades, but presently problem mangrove degradation aroused due to anthropogenic

pressures and coastal ecological changes.

Another study site Muthupet is situated 400 km south of Chennai and lies close

to Point Calimere on the Southeast coast of Penisular India ( 100 25’ N ; 79

0 39’ E). It

is at the southern end of the Cauveri delta covering an area of approximately 6800 ha.

of which only 4% is occupied by well-grown mangroves. Various branch rivers of the

river Cauveri flow through Muthupet and adjacent villages. At the tail end, they form a

lagoon before meeting the Palk Strait. The northern and western sides of the lagoon are

occupied by a sand spit which is devoid of mangrove vegetation (Azariah et al., 1992).

The Muthupet mangrove ecosystem holds a heterogeneous blend of mangrove plants

and animals. Among the six principal mangrove species, Avicennia marina, is most

common and abundant followed by Exoecaria agallocha, Aegiceros corniculatum,

Acanthus ilicifolius, Suaeda maritima and Suaeda monoica in that order. Five species

of seaweeds viz. Chaetomorpha sp., Enteromorpha sp., Gracilaria sp., Hypnea sp.,

Ulva sp., and two species of seagrasses namely Halodule sp. and Halophila sp. are

present in the mangrove water channels. 76 species of phytoplankton, 90 species of

zooplankton, 113 species of insects, 3 species of amphibians 7 species of reptiles and

13 species of mammals have also been reported from this area (Ajith Kumar et al.,

2006).

The Muthupet mangrove can be divided into 4 zones based on the distribution

of flora, that are Prosopis zone, Suaeda zone, mangrove zone and mud flat zone

(Azariah et al., 1992). The multifarious uses caused reduction in the extent of

Muthupet mangroves over a period of time. The direct and indirect natural and

manmade pressures posed a threat on it. In addition, few brackish water aquaculture

farms and salt pans have been developed in and around this area. The degradation has

occurred mostly in sparse mangrove forests due to the expansion of saltpan and human


18

activities. The observations on the water quality of the Muthupet mangrove biotope

have revealed seasonal difference in the physical-chemical and biological

characteristics (Ajith Kumar et al., 2006). In India, National Mangrove Committee was

set up in 1976 by Ministry of Environment and Forests for the management and

conservation of mangroves.

2.4. Mangrove species

Mangroves form taxonomically diverse groups, the majority of which fit in to

four genera: Bruguiera, Sonneratia, and mainly Avicennia and Rhizophora. Tomlinson

(1986) described the major mangroves include 34 species in 9 general and 5 families.

The minor species contain 20 species in 11 genera and 11 families which give a total of

54 mangrove species in 20 genera and 16 families. Duke (1992) recorded 69 mangrove

species belonging to 26 genera in 20 families. Families containing exclusively

mangroves are the Aegialitidaceae, Avicenniaceae, Nypaceae and Pellicieraceae. Two

orders (Myrtales and Rhizophorales) contain 25% of all mangrove families (Kathiresan

and Bingham, 2001). By reconciling common features from Tomlinson (1986) and

Duke (1992), Kathiresan and Bingham (2001) tabulated 65 mangrove species in 22

genera and 16 families (Table 2). Southeast Asia is the centre of mangrove biodiversity

which is having up to 45 species of flora. In the Pacific Islands, many of which are

atolls, 31 species of mangroves and five hybrids have been reported (Ellison, 2008;

Gilman et al., 2008). Rhizophora stylosa, Sonneratia alba, Lumnitzera littorea,

Heritiera littoralis, Bruguiera gymnorhiza, Excoecaria agallocha and Xylocarpus

granatum are some of the common species found in these islands.

Table 3. List of mangrove species (Kathiresan and Bingham, 2001)

Family Mangrove Species

Avicenniaceae Avicennia alba Blume

Avicennia balanophora Stapf and Moldenke ex Molodenke

Avicennia bicolor Standley

Avicennia eucalyptifolia (Zipp. ex Miq.) Moldenke

Avicennia germinans (L.) Stearn

Avicennia lanata Ridley

Avicennia marina (Forsk.). Vierh. Avicennia officinalis L.

Avicennia schaueriana Stapf and Leechman ex Moldenke

Avicennia africana Palisot de Beauvois

Bignoniaceae Dolichandrone spathacea (L. f.) K. Schumann

Bombacaceae Camptostemon philippinensis (Vidal) Becc.


19

Camptostemon schultzii Masters

Caesalpiniaceae Cynometra iripa Kostel

Cynometra ramiflora L.

Combretaceae Conocarpus erectus L.

Laguncularia racemosa (L.) Gaertn. f. Lumnitzera littorea (Jack) Voigt.

Lumnitzera racemosa Willd.

Lumnitzera rosea (Gaud.) Presl. (hybrid of L. racemosa and L. littorea)

Euphorbiaceae Excoecaria agallocha L.

Excoecaria indica (Willd.) Muell. - Arg.

Excoecaria dallachyana (Baill.) Benth.

Lythraceae Pemphis acidula Forst.

Pemphis madagascariensis (Baker) Koehne

Meliaceae Aglaia cucullata (Pellegrin ) Roxb.

Xylocarpus granatum Koen.

Xylocarpus mekongensis Pierre

Xylocarpus moluccensis (Lamk.) Roem.

Myrsinaceae Aegiceras corniculatum (L.) Blanco

Aegiceras floridum Roemer and Schultes

Myrtaceae Osbornia octodonta F. Muell. loc. cit.

Pellicieraceae Pelliciera rhizophoreae Triana and Planchon

Plumbaginaceae Aegialitis annulata R. Brown

Aegialitis rotundifolia Roxburgh

Rhizophoraceae Bruguiera cylindrica (L.) Bl.

Bruguiera exaristata Ding Hou

Bruguiera gymnorrhiza (L.) Lamk.

Bruguiera hainesii C. G. Rogers

Bruguiera parviflora Wight and Arnold ex Griffith

Bruguiera sexangula (Lour.) Poir.

Ceriops decandra (Griff.) Ding Hou

Ceriops tagal (Perr.) C. B. Robinson

Kandelia candel (L.) Druce

Rhizophora apiculata Bl.

Rhizophora mangle L.

Rhizophora mucronata Poir.

Rhizophora racemosa Meyer

Rhizophora samoensis (Hochr.) Salvoza

Rhizophora stylosa Griff.

Rhizophora lamarckii Montr. (hybrid of R. apiculata and R. stylosa)

Rhizophora annamalayana Kathir. (hybrid of R. apiculata and R. mucronata ) Rhizophora selala (Salvoza) Tomlinson (hybrid of R. stylosa and R. samoensis)

Rhizophora harrisonii Leechman (hybrid of R.mangle and R. stylosa )

Rubiaceae Scyphiphora hydrophyllacea Gaetn. f.

Sonneratiaceae Sonneratia alba J. Smith

Sonneratia apetala Buch.-Ham.

Sonneratia caseolaris (L.) Engler

Sonneratia griffithii Kurz

Sonneratia lanceolata Blume

Sonneratia ovata Backer

Sonneratia gulngai Duke (hybrid of S. alba and S. caseolaris)

Sterculiaceae Heritiera fomes Buch-Ham. Heritiera globosa Kostermans

Heritiera littoralis Dryand. In Aiton


20

2.4.1 Endangered mangrove species

Increased habitat loss and localized threats are occurring in all tropical coastal

regions of the world declines the range of all mangrove species (FAO, 2007). However,

some regions show greater losses than others. The mangrove biodiversity is highest in

the Indo-Malay Philippine Archipelago (Fig. 9), with around 36 to 46 of the 70 known

mangrove species occurring in this region (Polidoro et al., 2010).

Globally, the major part of threatened mangrove species is found along the

Atlantic and Pacific coasts of Central America (Fig. 10). Around 40% of the mangrove

species present along the Pacific coasts of Costa Rica, Panama and Colombia are listed

in one of the three threatened group, and a fifth species Rhizophora samoensis is listed

as near threatened (Polidoro et al., 2010). Three of these species, Avicennia bicolor,

Mora oleifera and Tabebuia palustris all listed as vulnerable, are rare or uncommon

species only known from the Pacific coast of Central America. One of the rare species

Sonneratia griffithii is distributed in parts of India and Southeast Asia, where 80% loss

of all mangrove area has occurred over the past 60 years, primarily due to the clearing

of mangrove areas for rice farming, shrimp aquaculture, and coastal development. This

species is already reported to be locally extinct in a many areas within its range, and

less than 500 mature individuals are known from India (Jin-eong, 1995; Polidoro et al.,

2010). Bruguiera hainesii is also a rare species which exists only in few fragmented

locations of Indonesia, Malaysia, Thailand, Myanmar (Kress et al., 2003), Singapore

and Papua New Guinea. It has very low rates of propagation and low rates of

germination. It is estimated that there are less than 250 mature individuals remaining.

In Indian mangrove systems, 100% of mangrove plant species, 92% of other flowering

plants, 60.8% of seaweeds, 23.8% of marine invertebrates and 21.2% of marine fish are

threatened. Among 35 plant species, 9 are critically endangered, 23 endangered and 3

vulnerable (Rao et al., 1998). There is an urgent protection is needed for remaining

individuals as well as research to conservation of those species.

2.5. Importance and uses of mangroves

Mangroves are highly dynamic, critically important ecosystems with a

huge biomass production and high levels of productivity. They provide a vast

quantity of products and ecosystem services; moreover, they support adjacent


21

ecosystems via exchanges of nutrients. These benefits can be separated into products

(socio-economic values) and services (ecological benefits). In fact, ecological services

also hold socio-economic value as humans are a part of the ecosystems, and

maintaining functions as well as cost reduction both carry economic implications.

Figure 10. Proportion of Threatened (Critically Endangered, Endangered, and Vulnerable)

Mangrove Species (Polidoro et al., 2010).

Figure 9. Mangrove Species Richness: Native distributions of mangrove species

(Polidoro et al., 2010).


22

2.5.1. Socioeconomic benefits

The bond between mangroves and man has been lasting for centuries; with first

recorded references dating back over 2000 years. The exploitation of mangroves has

given a wide range of valuable benefits to local communities. In many places, people

are dependent on products made or harvested from mangroves. Mangrove forests are

the economic foundations of many tropical coastal regions providing at least US$1.6

billion per year in ‘‘ecosystem services’’ worldwide. The products of mangroves can be

categorized into two main groups, timber and non-timber products (Bandaranayake,

1998; Bandaranayake, 2002; FAO, 2007). In general, timber products are wood from

mangrove trees of good quality. Nevertheless, mangroves are mainly hardwood and

have a high resistance not only to pests and fungi, but also to rotting agents and saline

water. Timber is used for poles and as construction wood.

The leaves of Nypa fruticans are used as thatch for roofing. In addition to that

timber products include fishing gear, wood chips, matchsticks and pulp for the paper

industry (Bandaranayake, 1998; Mcleod and Salm, 2006; Spalding et al., 2010).

Mangroves are generally used as fuelwood, and also used in large-scale production of

charcoal. The production of charcoal from mangrove trees is most common all over the

world. Rhizophora is predominantly used for charcoal production, as its billets have a

high caloric power, slow burning characteristics and creates very less smoke

(Bandaranayake, 1998).

The numbers of non-timber mangrove products are enormous. Fisheries

products (mainly fish, shrimps, clams and crabs) are highly valued in mangrove

systems; they give food and a good source of income. Mangrove ecosystems are also

used for aquaculture, both as open-water estuarine mariculture (e.g. oysters and

mussels) and as pond culture (mainly for shrimps) (Bandaranayake, 1998). In addition

to that non-timber products include tannins (used for leather preparation), food

(some fruits and leaves are comestible), fodder, vegetable oils, honey, wax,

spices, medicine, beverages, food additives (e.g. ash of Avicennia has a high mineral

content) and substitutes (e.g. tea, betel nut) (Bandaranayake, 1998; FAO, 2007). The

increasing popularity of ecotourism activities also represents a potentially valuable and

sustainable source of income for many local populations.


23

2.5.2. Ecological benefits

Mangroves sequester up to 25.5 million tons of carbon per year, and supply

more than 10% of necessary organic carbon to the global oceans (Polidoro et al., 2010).

The overall area of mangroves is relatively small compared to other tropical forests, but

they should not be overlooked in terms of CO2 sequestration as they have higher rates

of productivity (Spalding et al., 2010). Mangroves also provide a imperative habitat

for marine and coastal fisheries. The productivity and protection offered by

mangrove environment creates ideal nursery conditions, which fish and crustaceans

depend on for breeding and rearing of offspring. Mangroves are also important in

many food chains not only for fish, but also for birds, mammals, amphibians, reptiles

and insects (de Graaf and Xuan, 1998; Xuan et al., 2005; FAO, 2007). They also play

pivotal role in maintenance of natural ecological processes of the coastal zone,

production of biomass, and retention of nutrients (Pham, 2010).

The impact of mangroves in terms of coastal protection is one of the most

valuable functions. Biogeochemical and trophodynamic processes as well as forest

structure and growth are closely linked to water movement, due to tides and waves

within mangrove ecosystems. Thus, hydrodynamic and hydraulic factors play a major

role in the structure and function of mangroves (Massel et al., 1999). Mangroves

provide mechanical protection for the shorelines, flood protection structure (e.g. dykes

and water gates) and the neighbourhood. They create an effective buffer by supporting

soil consolidation and sedimentation processes. Mangroves not only decrease the

hydrodynamic impacts of waves and erosion processes under normal tide

conditions, but also under extreme situations such as hurricanes. This benefit

provided by mangroves will translate into a savings on dyke maintenance costs and also

protect human lives in extreme situations. In an extensive mangrove forest of sufficient

size, these benefits are provided at no cost. After the Indian Ocean Tsunami in 2004,

mangroves received increased attention in respect to coastal protection by the

international press and scientists (FAO, 2007).

2.6. Cyanobacteria – Diversity and taxonomy.

Cyanobacteria are oxygenic photosynthetic microorganisms which are

previously referred as blue green algae. The existence of cyanobacteria on earth is dates

back to 3.5 billion years ago which is evident from fossil records. The early evolution


24

of earth's oxygen-rich atmosphere is most likely due to cyanobacterial photosynthesis.

They are a large and morphologically and physiologically diverse group of

phototrophic prokaryotes, which occur in almost every habitat on earth. The available

taxonomy of cyanobacteria, unlike other bacteria, is still an enigma that challenging

cyanobacterialogists. However, it is estimated that around 50% of cyanobacterial

strains existing in culture collections have been identified incorrectly or have been

assigned to the wrong taxonomic group (Komárek and Anagnostidis, 1989). The

continuously changing classification system and a lack of a consensus phylogeny are

the impending evidences of the unresolved evolutionary relationships among

cyanobacteria. The use of culture-independent techniques has revolutionized the field

of microbial diversity (Hugenholtz et al., 1998; Muyzer and Smalla, 1998). In recent

years, diversity of cyanobacteria have been investigated in numerous natural habitats

mainly by using molecular techniques, but still the issue of identifying individual taxa

is problematic (Golubic and Seong-Joo, 1999; Komárek and Anagnostidis, 1999).

In general, two different approaches have been applied to cyanobacterial

taxonomy; the traditional botanical approach and the considerably more recent

bacteriological approach. Because of their photosynthetic properties and pigment

composition, cyanobacteria were at first classified together with eukaryotic algae. The

botanical approach to cyanobacteria taxonomy dates back to the 19th century and is

based on phenotypic descriptions, including cell and sheath morphology, colony

formation, pigmentation, mode of reproduction. There are over 2000 species of

cyanobacteria have been validly published under the botanical code of nomenclature.

However, the discovery of the prokaryotic nature of cyanobacteria and limitations of

the botanical approach, particularly for those cyanobacteria with simple morphologies,

compelled to follow the bacteriological approach (Stanier et al., 1971; Rippka et al.,

1979). The bacteriological approach to cyanobacterial taxonomy was first proposed by

Stanier et al., (1971). This approach is based on physiological and genotypic characters

such as pigment composition, fatty acid analysis, heterotrophic growth, nitrogenase

activity, DNA base composition and genome length. The taxonomy according to this

approach was published by Rippka et al., in 1979.


25

In the beginning, molecular tools for taxonomy are mainly based on the

chemotaxonomic markers like lipid composition, polyamines and carotenoids.

However, methods based on nucleic acids and proteins are now much more commonly

used. The 16S rRNA gene has recognized to be a useful marker for investigating

phylogenetic relationships and is the most widely used marker for distinguishing

identities between prokaryotic organisms at the genus level. The internal transcribed

spacer (ITS) between the 16S and 23S rRNA genes has also been used as marker

which is complement to 16S rRNA analysis for diversity studies (Taton et al., 2003).

The ITS marker has been used to distinguish between cultured strains (Scheldeman et

al., 1999; Iteman et al., 2000; Boyer et al., 2002). Also the nitrogenase encoding gene

nifH has been used extensively as a genetic maker among nitrogen-fixing

cyanobacteria. The most updated system based on the bacteriological approach is given

in Bergey’s manual of bacteriology (Garrity et al., 2007) where cyanobacteria is placed

in the group “Oxygenic photosynthetic bacteria”.

The term polyphasic taxonomy first introduced by Colwell (1970), a new

approach was attempted to reach a consensus in bacterial systematics by integrating

genotypic, phenotypic and phylogenetic information which is proved to be a valuable

tool for cyanobacteriologists (Vandamme et al., 1996; Lehtimaki et al., 2000; Abed et

al., 2002; Abed et al., 2003). Moreover, with new information, such as whole-genome

sequences accumulating, the taxonomic system will continue to transform and develop.

The most recent advancements in microbial diversity include the community genomic

or ‘metagenomic’ approach which is resulted in an enormous increase in genomic

information related to microorganisms during the last decade.

2.6.1. Cyanobacteria and Mangroves

The presence of cyanobacteria in several mangrove ecosystems was explored in

many countries (Santra et al., 1988; Kannan and Vasantha, 1992; Mani, 1992;

Hoffmann, 1999; Nogueira and Ferreira-Correia, 2001; Kyaruzi et al., 2003).

Mangroves are occupied by diverse cyanobacterial communities, which reside on leaf,

root litter, live roots, and often form extensive mats on the surrounding sediments;

many of these communities are capable of fixing atmospheric nitrogen (Hoffmann,

1999). The genera Oscillatoria, Lyngbya, Phormidium, and Microcoleus are


26

widespread in these habitats, as heterocystous genera, like Scytonema, in some areas

(Hoffmann, 1999). The taxonomic survey of the Cyanobacteria in a red mangrove

forest of the Brazil estuaries revealed total of 15 taxa in 8 families, as follows:

Synechococcaceae (2 taxa), Chroococcaceae (1 taxa), Hyellaceae (1 taxa),

Xenococcaceae (1 taxa), Oscillatoriaceae (1 taxa), Scytonemataceae (2 taxa),

Phormidiaceae (5 taxa), and Pseudanabaenaceae (2 taxa) (Nogueira and Ferreira-

Correia, 2001). A total of ten genera of cyanobacteria were recorded in mangrove

ecosystem adjacent to Zanzibar, these consist of: (i) the heterocystous cyanobacteria

genera viz., Anabaena and Rivularia and (ii) non-heterocystous genera viz.,

Aphanocapsa, Merismopedia, Lyngbya, Microcoleus, Oscillatoria, Phormidium,

Schizothrix, and Spirulina (Kyaruzi et al., 2003).

The colonization of different species prefers various parts of the tree. In many

cases, the aerial roots, especially pneumatophores, give refuge to specific

cyanobacterial populations that may show sharp vertical zonation (Kathiresan and

Bingham, 2001). Non-heterocystous, particularly filamentous cyanobacteria resembling

like Lyngbya sp. and Oscillatoria sp. colonized the bottom part near to the sediment,

the filamentous cyanobacteria resembling Microcoleus sp. colonized the central zone,

and by coccoidal cyanobacteria resembling Aphanothece sp. mixed with undefined

filamentous cyanobacteria colonized the upper part (Toledo et al., 1995a). Lyngbya sp.,

Polysiphonia sp., and Oscillatoria sp. are common epiphytic cyanobacteria of

submerged root system of Rhizophora sp., (Krishnamurthy and Jayaseelan, 1983). The

pneumatophores of mangroves in West Bengal, India, are colonized by a number of

cyanobacteria viz., species of Calothrix, Anabaena, Lyngbya, Hydrocoleum, and

Schizothrix (Santra et al., 1988).

The planktonic cyanobacteria found in West Bengal, India, were belong to the

species of Trichodesmium, Synechococcus, Aphanothece, Gloeocapsa, Gloeothece,

Merismopedia, Oscillatoria, Johannesbaptistia, Microcystis, and Stigonema (Table 2)

(Santra et al., 1988). Three species of planktonic cyanobacteria have been recorded in

Pichavaram mangroves, Anabaena sp., Oscillatoria sp., and Trichodesmium sp.,

(Kannan and Vasantha, 1992; Mani, 1992). Planktonic cyanobacterial species were


27

Table 4. Cyanobacteria associated with Mangroves (Sundararaman et al., 2007)

Site/category Cyanobacterial species Location Reference

Pneumatophore

Calothrix sp

Anabaena sp

Lyngbya sp

Hydrocoleum sp

Schizothrix sp

Microcoleus sp

Lyngbya sp

Plectonema sp

Anabaena sp

WestBengal, India

Mexico

(Santra et al., 1988)

(Toledo et al., 1995a)

Epiphytic Dermocarpa sp

Xenococcus sp Chamaesiphon sp

Stichosiphon sp

WestBengal, India (Santra et al., 1988)

Planktonic Trichodesmium sp

Synechococcus sp

Aphanothece sp

Gloeocapsa sp

Gloeothece sp

Merismopedia sp

Oscillatoria sp

Johannesbaptistia sp

Microcystis sp Stigonema sp

WestBengal, India (Santra et al., 1988)

Pseudanabaena sp

Anabaena sp

Coelosphaerium sp

Lyngbya sp

Merismopedia sp

Microcystis sp

Oscillatoria sp

Spirulina sp

WestBengal, India (Banerjee and Santra,

2001)

Trichodesmium sp

Anabaena sp

Oscillatoria sp

Pitchavaram,

Tamilnadu.

(Kannan and Vasantha,

1992; Mani, 1992)

isolated from Sundarban mangrove estuary in India (Mani, 1992). The population

density of phytoplankton is decreased in monsoon and increased in pre-monsoon

periods, but the percentage of cyanobacterial population has increased during monsoon

and decreased in premonsoon periods (Mani, 1992; Banerjee and Santra, 2001). The

change in population density of cyanobacteria is due to the variations in environmental

factors such as temperature, salinity, light, etc., (Banerjee and Santra, 2001). A study on

mangrove-associated cyanobacteria in Muthupet estuary region in India has recorded

the presence of 17 cyanobacterial species (Selvakumar and Sundararaman, 2001). The

said study was aimed at to reveal the unique associates of cyanobacteria on mangrove

trees. Cyanobacterial species viz., Aphanocapsa koordersi, Johannesbaptistia

pellucida, Oscillatoria vizagapatensis, and O. tenuis, colonized on Avicennia marina;


28

cyanobacterial species viz., Porphyrosiphon natarsii, Phormidium sp., Oscillatoria

calcuttensis, and Schizothrix telephorides were found only on Aegiceras corniculatum;

cyanobacterial species viz., Aphanocapsa bullosa and A. littoralis were specific to

Excoecaria agallocha and the cyanobacterial species Oscillatoria claricentrosa was

found only on Sueada martima (Selvakumar and Sundararaman, 2001). The species

specificity may be attributed to the root exudates and also the environment concern.

The root exudates may play a key role in forming a community structure by selectively

stimulate and enrich certain groups of bacteria (Burgmann et al., 2005). Hence, the

knowledge of species specificity of cyanobacteria is most important while applying it

as ‘biofertilizer’ for the restoration of mangrove ecosystem even at the nursery stage.

2.7. Plant growth promoting properties of microbes

2.7.1. IAA productionIAA productionIAA productionIAA production

Auxin is important group of hormones which regulates the plant growth. In 1885

Salkowski discovered indole-3-acetic acid (IAA) in fermentation media (Salkowski,

1885). The auxin produced in apical bud and diffused downwards that inhibits the

lateral bud development. It induces root and regulates the ethylene biosynthesis. It

regulates the gene transcription and expression of the AUX/IAA protein (Dharmasiri et

al., 2003). The discovery of IAA as a plant growth regulator coincided with the first

indication of the molecular mechanisms involved in tumorigenesis induced by

Agrobacterium. Agrobacterium-induced tumours were shown to be sources of IAA

which are capable of growing in plant tissue culture medium in the absence of plant

growth regulators.

Diverse bacterial species have the ability to produce IAA. Different

biosynthesis pathways have been recognized and redundancy for IAA biosynthesis is

widespread among plant-associated bacteria. As some plant-interacting bacteria release

high levels of IAA, they may locally change the endogenous phytohormone balance of

the host and trigger responses, the nature of which depends on the phytohormone

concentration generated, type of host and its sensitivity to hormones (Sergeeva et al.,

2002). Different bacterial pathways have been identified to synthesize IAA. A high

degree of similarity between IAA biosynthesis pathways in plants and bacteria was


29

observed. The overview of bacterial IAA biosynthesis pathways is depicted in Figure

11.

Tryptophan is generally considered as a precursor of IAA production, because

addition of this amino acid to cultures of bacteria triggers the synthesis of IAA results

in higher IAA production in some of microbes (Theunis et al., 2004). Erwinia

herbicola a non-photosynthetic bacterium produces the large amount of IAA which has

the ipdC gene encoding for the indolepyruvate decarboxylase (Brandl and Lindow,

1996).

Cyanobacteria also have the ipdC homologues gene for the synthesis of IAA.

The indole-3-acetamide (IAM) pathway is the well characterized pathway in bacteria.

In this two-step pathway tryptophan is first converted to IAM by the enzyme

tryptophan- 2-monooxygenase (IaaM) is encoded by the iaaM gene. In the second step

IAM is converted to IAA by an IAM hydrolase (IaaH) which is encoded by iaaH

(Spaepen et al., 2007). Cyanobacteria including Anabaena, Anabaenopsis, Calothrix,

Chlorogloeopsis, Cylindrospermum, Gloeothece, Nostoc, Plectonema, and

Synechocystis have been reported to produce IAA (Sergeeva et al., 2002). It is also

found that IAA increases the level of the biosynthesis of certain enzymes and

stimulates nitrogen fixation in the cyanobacteria Anabaena doliolum and Nostoc

punctiforme (Tsavkelova et al., 2006).

Figure 11. IAA synthesis pathways in bacteria. The intermediate referring to the name of the

pathway or the pathway itself is underlined with a dashed line. IAAld, indole-3-acetaldehyde;

IAM, indole-3-acetamide; IPDC, indole-3-pyruvate decarboxylase; Trp, tryptophan. (Spaepen

et al., 2007)


30

The indole-3-pyruvate (IPyA) pathway is considered as a major pathway for

IAA biosynthesis in plants. However, the key genes/enzymes of this pathway in plant

are not identified yet. The first step in this pathway is the conversion of tryptophan to

IPyA by an aminotransferase (transamination). In the rate-limiting step, IPyA is

decarboxylated to indole-3-acetaldehyde (IAAld) by indole-3-pyruvate decarboxylase

(IPDC). In the last step IAAld is oxidized in IAA (Fig. 11). In bacteria, IAA production

via the IPyA pathway has been described in a broad range of bacteria, such as

Bradyrhizobium, Azospirillum, Rhizobium and Enterobacter cloacae, and

cyanobacteria.

The factors influencing IAA biosynthesis in diverse bacteria is may be related to

environmental stress, including acidic pH, osmotic stress, and carbon limitation. In

Azospirillum brasilense IAA production and expression of the key gene ipdC have been

shown to be increased under carbon limitation, during reduction in growth rate and

under acidic pH (Ona et al., 2003; Broek et al., 2005; Ona et al., 2005).

Figure 12. IAA in pathogenic and beneficial microorganism–plant interactions. The model

regenerated from Spaepen et al.,(2007). A model suggested for the role of bacterial IAA

production in microorganism–plant interactions. Signaling taking place in the plant is indicated

in gray boxes; signaling taking place in bacterial cells is indicated in white boxes. Full lines

indicate demonstrated links; dashed lines indicate hypothesized links. AHL, N-acyl homoserine

lactone; IAA, indole-3-acetic acid; MAMP, microbial-associated molecular pattern; PAMP,

pathogen-associated molecular pattern; QS, quorumsensing; TIR, transport inhibitor response.


31

IAA is considered to be reducing the integrity of plant cell walls by

upregulating the production of cellulases and hemicelluloses, resulting in the leakage of

some simple sugars and other nutrients that would benefit root-associated

microorganisms (Fry, 1989). As well, root growth would be beneficial to resident

bacteria due to the increased availability of root exudates and root surface for growth.

Microorganisms that produce IAA can influence the host plant and may function as

pathogens, symbionts, or a growth regulator which is depend on how the produced IAA

influences the concentration of the plant’s endogenous IAA pool and also on the

sensitivity of the plant to IAA (Gutierrez et al., 2009). Spaepen et al., (2007) put

forward a hypothesis that IAA production may contribute to evade the host defence

system by derepressing auxin signalling. By this way, IAA biosynthesis may play a

crucial role in bacterial resistance and colonization on the host plant (Fig. 12). Some of

the bacteria such as Erwinia chrysanthemi, Pseudomonas savastanoi, and

Agrobacterium tumefaciens are phytopathogens of many host plants. The bacteria

including Azospirillum brasilense and Pseudomonas putida GR12-2, have proven

beneficial to plants, and many IAA producers have been shown to stimulate increases

in root mass and/or length (Fry, 1989; Hugouvieux-Cotte-Pattat et al., 1996; Holguin et

al., 1999; Patten and Glick, 2002; Yang et al., 2007).

2.7.2. Nitrogen fixation

Nitrogen is one of the most essential nutrients for all living beings and

dinitrogen gas (N2) is the major component of the Earth’s atmosphere. However, the

inert gas is not biologically available to most organisms. Biological nitrogen fixation,

or diazotrophy, the fixation of atmospheric nitrogen gas (N2) into biologically available

ammonia (NH3), is important in making nitrogen available in many ecosystems

(Omoregie et al., 2004). Among cyanobacteria, many species combine the ability to

perform oxygenic photosynthesis and fix atmospheric dinitrogen (N2). The nitrogen

fixation is performed according to the following equation:

N2 + 8 e- + 8 H

+ → 2NH3 + H2

The process requires 16 ATP to reduce one molecule of dinitrogen to ammonia

(Gallon et al., 1988). The reaction is catalysed by the enzyme nitrogenase, comprise

two subunits: dinitrogenase reductase and dinitrogenase. The nitrogenase generally


32

present in cyanobacteria is often referred to as conventional or classic nitrogenase

which has two components. The minor component, dinitrogenase reductase (Fe

protein), is encoded by the nifH gene and the major dinitrogenase (MoFe protein) is

encoded by nifD and nifK. The genes nifE, nifN, nifB are involved in FeMo cofactor

biosynthesis. The function of dinitrogenase reductase is to supply electron for the

catalytic dinitrogenase. Dinitrogenase reductase has two identical subunits linked by a

single [4Fe-4S] cluster. Dinitrogenase is a tetramer, composed of two P clusters [8Fe-

7S] and two FeMo cofactors which are proposed binding sites for dinitrogen (Igarashi

and Seefeldt, 2003).

Nitrogenase is irreversibly inactivated when exposed to oxygen. The presence

of O2 not only does affect the protein structure but it can also inhibit synthesis of

nitrogenase in many diazotrophs (Peter Wolk, 1996; Bergman et al., 1997). For oxygen

evolving photosynthetic cyanobacteria it is necessary to protect the nitrogenase from

inactivation and for this purpose various strategies have evolved. The most evident

strategy is the differentiation of heterocysts; specialized cells which is devoid of

photosynthetic capability to facilitate nitrogen fixation. The heterocysts have thicker

cell walls to reduce O2 diffusion, no PSII activity and exhibit higher respiration rate

than vegetative cells. In a cyanobacterial filament about 5-10% of the cells

differentiates into heterocysts and evenly spaced (Adams and Duggan, 1999).

The non-heterocystous nitrogen-fixing cyanobacteria possess more variable and

complex strategies. The unicellular and other non-heterocystous cyanobacteria manage

the problem by fixing nitrogen during the night and doing oxygenic photosynthesis

during the day (Gallon et al., 1988; Reddy et al., 1993). Ultrastructure analysis of

nitrogen-fixing unicellular cyanobacteria revealed that during photosynthesis

carbohydrates are stored in polysaccharide granules as a strategy to later on supply cells

with energy during nocturnal nitrogen fixation (Falcon et al., 2010). In addition, a large

group of cyanobacteria can fix nitrogen only under micro-oxic or anoxic conditions

(Bergman et al., 1997).

The non-heterocystous genus Trichodesmium has a different strategy for

nitrogen fixation, combining spatial and temporal separation of nitrogen fixation in

which photosynthetic rates are decreasing at midday when nitrogenase activity is

peaked (Berman-frank et al., 2003). Immunological studies by showed that nitrogenase


33

is restricted to a novel type of specialized cell which is called as diazocyte (Bergman

and Carpenter, 1991). Diazocytes represent about 15% of total cells, organized into

subsets of cells (Janson et al., 1994; Lin et al., 1998; El-Shehawy et al., 2003).

Nitrogen-fixers are commonly referred to as diazotrophs and are widespread in

diverse habitats both free-living and in various symbiotic associations. In different

environments, nitrogen availability can limit the growth and ecosystem productivity

(Feller et al., 2003). Nitrogen fixation represents a new source of nitrogen to mangrove

forests, but the contribution of this process to the nitrogen budget of mangrove

wetlands remains poorly understood (Pelegri and Twilley, 1998). Rate of nitrogen

fixation in mangrove wetlands varies with species, community types of nitrogen-fixers,

concentration of organic carbon substrates, and ambient conditions such as inundation,

temperature and light (Gotto and Taylor, 1976; Zuberer and Silver, 1978; Potts, 1979;

Van der Valk and Attiwill, 1983).

Cyanobacteria, common in mangroves may play a pivotal role in fixing

atmospheric nitrogen (Gotto and Taylor, 1976; Van der Valk and Attiwill, 1983).

Nitrogen fixation in a mangrove ecosystem in South Australia could supply about 40%

of the annual nitrogen requirement, estimated to be 13 g N m-2

year-1

for Avicennia

trees (Van der Valk and Attiwill, 1983). Mangrove in Florida, biological nitrogen

fixation could supply up to 60% of the nitrogen requirement (Zuberer and Silver, 1978)

and nitrogen-fixation rates associated with different locations of coastal environments

was elaborately given in Table 3 (Howarth et al., 1988; Sundararaman et al., 2007). In

mangrove ecosystems, high rates of nitrogen fixation have been associated with dead

and decomposing leaves (Gotto and Taylor, 1976; Zuberer and Silver, 1978; Zuberer

and Silver, 1979; Van der Valk and Attiwill, 1983; Hicks and Silvester, 1985; Mann

and Steinke, 1993) pneumatophores (Potts, 1979; Zuberer and Silver, 1979; Hicks and

Silvester, 1985; Toledo et al., 1995a; Holguin et al., 2003), the rhizosphere soil

(Zuberer and Silver, 1978), tree bark (Uchino et al., 1984), and cyanobacterial mats

covering the surface of the sediment (Toledo et al., 1995b). Most nitrogen fixation by

planktons is by cyanobacteria rather than by heterotrophic bacteria, and rates of fixation

are reasonably correlated with the biomass of nitrogen-fixing cyanobacteria (Howarth

et al., 1988). The compounds like polysaccharides secreted by plant roots may

influence the diazotrophs to fix nitrogen (Burgmann et al., 2005). A positive correlation

was found between acetylene-reduction rates and the availability of organic matter


34

(Holguin et al., 2001). It is also possible that heterotrophic bacteria are involved in

nitrogen fixation; fueled by phototrophs (Kellart and Paerl, 1980; Omoregie et al.,

2004). The rate of nitrogen fixation varies with different seasons. Nitrogen fixation

associated with Avicennia germinans aerial roots in a Mexican mangrove showed that

rates were up to ten times higher during the summer than during autumn and winter.

The light intensity and water temperature are main factors influencing nitrogen fixation

(Toledo et al., 1995a). Nitrogenase activity associated with pneumatophores was light

dependent and was probably attributable to one or more species of cyanobacteria

present as an epiphyte (Hicks and Silvester, 1985).

Table 5. Nitrogen fixation rates in different locations of coastal environment (Sundararaman et al.,

2007).

Nature of Location Location Nitrogen fixation Rate (ARA)

g N m-2

Yr-1

Planktonic Nitrogen

fixation in estuaries

S.W. Bothnian Sea (Baltic)

Aland Sea (Baltic) Asko area (Baltic)

Stockholm archipelago, Sweden

Harvey estuary, Australia

0.06

0.07 0.80

0.013-1.8

1.2

Sediments and well

developed

cyanobacterial mats in

Estuaries and coastal

seas.

Vostok Bay, Japan

Upper Cook Inlet, Alaska

Narragansett Bay, Rhode Island

Kamishak Bay, Alaska

Norton Sound, Alaska

Beaufort Sea

Elso Lagoon, Bcaufort Sea Shelikoff Strait, Alaska

Rhode River estuary, Maryland

Lune estuary, England

Waccasassa estuary, Florida

Bank End, England

Kancohe Bay, Hawaii

Flax Pond mud flat, New York

Barataria Basin, Louisiana

0.002

0.01

0.03

0.03

0.03

0.03

0.07 0.08

0.13

0.14

0.37

0.43

0.60

0.65

1.56

Cyanobacterial mats of

mangrove forests and

coastal seas

Great Barrier Reef, Australia

Sippewissett marsh,

Massachusetts Enewetak Atoll, Marshall Islands

Sippewissett marsh,

Massachusetts

Texas Gulf Coast

Colne Point marsh, England

Gulf of Elat

Hiddensec Island, Baltic

Aldabra Atoll, Indian Ocean

Flax Pond salt marsh, New York

Bank End, England

Enewetak Atoll, Marshall Islands Kaneohe Bay, Hawaii

1.32

1.42 2-40

2.28

4.00

5.99

7.59

7.60

12.20

13.44

20.06

65.70

76.00


35

The application of cyanobacteria, Microcoleus sp. was alone studied for the

growth of mangroves (Toledo et al., 1995b; Bashan et al., 1998). The filamentous

cyanobacterium Microcoleus sp. was isolated and inoculated on to young mangrove

seedlings. Cyanobacterial filaments colonized the roots by gradual production of

biofilm (Toledo et al., 1995b). In another study, filamentous cyanobacteria

Microcoleus chthonoplastes was inoculated on to mangrove seedlings and found that

the total nitrogen and 15

N incorporation levels increased (Bashan et al., 1998). Hence, it

is imperative to explore the potential usage of cyanobacteria for the restoration of

degraded mangrove forests.

2.7.3. Phosphate solubilisation

Phosphate (Pi) is one of the key substrates in energy metabolism and

biosynthesis of nucleic acids and membranes. It also plays a vital role in

photosynthesis, respiration, and regulation of a number of enzymes. Among the many

inorganic nutrients essential for plants, P is one of the most important elements that

affect the plant growth and metabolism significantly. Plants acquire P from soil as

phosphate anions. However, phosphate anions are extremely reactive and may be

immobilized through precipitation with cations such as Ca2+

, Mg2+

, Fe3+

and Al3+

,

depending on the particular properties of a soil. In these forms, P is highly insoluble

and unavailable to plants. As a result of this, the amount available to plant is usually a

small fraction of this total. In marine sediments, phosphates usually precipitate because

of the abundance of cations in the interstitial water, making phosphorus largely

unavailable to plants. Phosphate- solubilizing microorganisms are potential suppliers of

soluble forms of phosphorus, which is beneficiary to mangrove plant.

Microbial solubilization of mineral-phase phosphates has been reported in a

variety of microbes including Mycorrhizae, Aspergillus, Bacillus, and Pseudomonas.

Freitas de et al., (1997) stressed the importance of Bacillus sp. as being the most

abundant phosphate solubilising bacterium in soil. It has been constantly stressed that

phosphate solubilization is a key plant growth-promoting mechanism (de Freitas et al.,

1997; Bashan et al., 1998; Rodríguez and Fraga, 1999). A number of mechanisms for

increasing the availability of phosphorus to plants have been described (Rodríguez and

Fraga, 1999). These mechanisms include; reducing the pH in the rhizosphere,

producing organic acids/chelates, and excreting phosphatases to free phosphorous

bound in organic matter. The production of organic acids and acid phosphatases play a


36

major role in the mineralization of organic phosphorus in soil. Production of organic

acids results in acidification of the microbial cell and its surroundings. It is found that,

only very few research has been focused on this group of bacteria in the marine

environment, either in temperate or tropical regions (Ayyakkannu and Chandramohan,

1971; Devendran et al., 1974; Craven and Hayasaka, 1982; Goldstein, 1996).

Phosphorus can be released from organic compounds in soil by three groups of

enzymes: (1) Nonspecific phosphatases, which dephosphorylate phosphor-ester or

phospho-anhydride bonds in organic matter, (2) Phytases, which specifically release

Phosphourus from phytic acid, and (3) Phosphonatases and C–P Lyases, enzymes that

perform C–P cleavage in organophosphonates. Among these three groups,

phosphatases and phytases are playing major role in phosphate solubilisation and it may

because of the predominant presence of their substrates in soil (Rodriguez et al., 2006).

However, mineral phosphate solubilisation by most bacteria has done by the production

of organic acid (Rodríguez and Fraga, 1999; Rodríguez et al., 2006). Goldstein (1996)

proposed direct glucose oxidation to gluconic acid (GA) as a major mechanism for

mineral phosphate solubilisation.

In an arid mangrove ecosystem in Mexico, nine strains of phosphate-

solubilizing bacteria were isolated from the roots of black mangrove (A. germinans);

Bacillus amyloliquefaciens, B. atrophaeus, Paenibacillus macerans, Xanthobacter

agilis, Vibrio proteolyticus, Enterobacter aerogenes, E. taylorae, E. asburiae, and

Kluyvera cryocrescens, and three strains viz., B. licheniformis, Chryseomonas luteola,

and Pseudomonas stutzeri were isolated from white mangrove (Languncularia

racemosa) roots (Vazquez et al., 2000). This was the first report of phosphate-

solubilising microbes collected from mangrove roots. Vazquez et al., (2000) studied the

solubilising potential of fungi isolated from mangrove roots. The mangrove ecosystem

has a very low amount of bioavailable phosphate (18 µg/ml of orthophosphates

compared to 73 µg/ml in seawater) due to the high pH (8.2) of the environment. The

authors identified a number of organic acids produced by phosphate-solubilising fungi

including acetic acid, propionic acid, isobutyric acid, isovaleric acid, valeric acid, lactic

acid, fumaric acid, succinic acid, and other unidentified compounds. There are no

reports on phosphate solubilisation by cyanobacteria from marine environment.

However, phosphate solubilisation by terrestrial bacteria has been studied well and its

potent use in plant growth promotion in agriculture is evidenced.


37

2.7.4. Other growth promoting substances

In addition to above mentioned plant growth promoting activities, the microbes

influence the plant growth by other ways too. Iron, an element essential for microbial

growth, is mostly unavailable because it is mainly present in soil in a hard-to-solubilize

mineral form. To sequester iron from the environment, numerous soil microorganisms

secrete low-molecular-weight, ironbinding molecules, called siderophores, which have

a high capacity for binding Fe3+

. The now-soluble, bound iron is transported back to the

microbial cell and is available for growth. However, it is evidenced that a number of

plant species can absorb bacterial Fe3+-

siderophore complexes (Bar-Ness et al., 1991;

Wang et al., 1993). The siderophores are Fe chelating metabolites produced by

microorganisms which will help plants by assimilating iron. Many bacteria and

cyanobacteria are known to produce siderophores (Gross and Martin, 1996;

Umamaheswari et al., 1997; Ison et al., 1999; Hersman et al., 2000; Rajkumar et al.,

2006)

In addition many beneficial bacteria and cyanobacteria act as a biocontrol

agents and protect plant from pathogenic microbes by the way of producing large array

of microbial substances which are involved in the suppression of pathogenic growth.

These substances include antibiotics, siderophores, small molecules such as hydrogen

cyanide (HCN), and hydrolytic enzymes such as chitinase, laminarinase, glucanase,

protease, and lipase.

2.8. Conservation and Restoration of mangroves

Mangrove forests around the world have suffered from advanced degradation

and overexploitation through human interactions over the last decades. The

degradation of mangroves can be due to two main forces; human-induced

degradation and natural disturbance-related degradation. In addition to deforestation for

timber products, land conversion for agricultural use and aquaculture such as shrimp

farming and salt plantations made a big impact on the mangrove cover. The diverted

use of freshwater for irrigation and land recovery has minimized the freshwater input

that lead to rapid destruction of mangrove forests. Further, dam construction has also

affected the freshwater inflow (Kathiresan and Bingham, 2001). The major reasons for

mangrove degradation are generally categorized as given in Figure 13. There is a


38

critical need to ameliorate the above said problems for the betterment of mangrove

ecosystems.

The conservation and restoration of threatened mangrove ecosystem were

already insisted (Field, 1999; Milano, 1999; Kairo et al., 2001; Toledo et al., 2001).

Conservation of mangrove is an important commitment for all the countries that

possess mangrove ecosystem. Awareness spreading globally on the value of such

ecosystem is increasing. Hence, governments of these countries need to conserve by

legislating new laws. Adger et al., (2005) stated the enhancement of social–ecological

resilience as an immediate necessity for coastal disaster management. The public

awareness should be spread all over the world. This will promote the understanding of

the incredible mangrove ecosystem and make them to involve in the conservation

programs.

Restoration of an ecosystem is the act of bringing an ecosystem back to, as

nearly as possible, its original condition (Field, 1999). Milano (1999) has dealt with the

success of restoration and enhancement program (over 121.5 hectares) in Florida and

ultimately suggested such program for global consideration by adopting different

approaches. Conservation strategies for mangroves should consider not only the plant

but the ecosystem as a whole, including all the physical, chemical, and ecological

processes that maintain productive mangroves (Fig. 14). This is especially important in

mangroves that do not receive external terrestrial nutrients from rivers or other sources.

Figure 13. Reasons for mangrove degradation (Sundararaman et al., 2007)


39

It is vital that the health of the benthic microbial communities be maintained because

these organisms are responsible for conserving the scarce nutrients within the

ecosystem (Holguin et al., 2001).

Ecological engineering is a developing field of science which resolves problems

pertaining to any of the ecosystem. Mesocosm is a closed system that contains all the

biotic and abiotic components of interest, and mimics environmental conditions in the

laboratory. Mesocosm study of mangrove forest was carried out to understand ecology

and species composition (Adey et al., 1996). A long practice of restoration programmes

is found over the world as well and has led to an increase in the extent of mangrove

area (Das et al., 1997; Kairo et al., 2001; Lewis, 2001; Kairo et al., 2002; Lewis, 2005).

The recolonization and natural regeneration properties of these forests also accounts in

their increased extent. In Ecuador, for example, the abandoning of shrimp cultivation

and salt production ponds resulted in a recolonization of various sites (FAO, 2007).

It has been reported that mangrove forests around the world can self-repair or

successfully undergo secondary succession over periods of 15–30 years if: (1) the

normal tidal hydrology has not been disrupted and (2) the availability of waterborne

propagules from adjacent stands is not limited or blocked (Lewis, 2001; Lewis, 2005).

It was observed that physical alteration in hydrology of mangrove ecosystem a highly

Figure 14. Strategies for mangrove afforestation.


40

successful approach for such restoration (Lewis, 2005). Hence, for restoration program

ecological engineering should also be considered for its success.

2.8.1. Biotechnological approach for mangrove conservation

Biotechnological approach includes production of genetically improved trees,

application of plant growth promoting microorganisms, propagation of mangroves by

micropropagation or vegetative propagation or seed propagation (Toledo et al., 1995a;

Toledo et al., 1995b; Bashan et al., 1998; Vazquez et al., 2000; Holguin et al., 2001;

Toledo et al., 2001; Bashan and Holguin, 2002) . Many of these methods should be

taken for detailed research in the view toward future conservation and restoration of

mangrove forest and it requires standardization for every mangrove species. Further,

though during the last decade of the twentieth century, attempts have been initiated to

apply microorganisms for the growth of mangroves (Toledo et al., 1995b; Bashan et

al., 1998; Rojas et al., 2001) it is not fully established.

2.8.1.a. The use of Growth Supplement (GS) medium for mangrove growth promotion

In India, organic farming was a well developed and systematized agricultural

practice during the past and this ‘ancient wisdom’ acquired through Indian knowledge

systems such as ‘Vedas’ described the use of ‘panchagavya’ in agriculture for the

health of soil, plants and humans. In Sanskrit, panchagavya means the blend of five

products obtained from cow, namely cow dung, cow urine, cow milk, curd and ghee

(Sugha, 2005). However, the farmers in the southern parts of India have used these

formulations of panchagavya and found to enhance the biological efficiency of the crop

plants and the quality of fruits and vegetables (Natarajan, 2002). Thus the idea of using

modified panchakavya as a growth supplement medium for the mangrove growth

promotion was implemented in this study.

2.8.1.b. Nursery development for successful mangrove restoration

The establishment of nursery of coastal plants especially well-adopted

mangroves will help in such process of resilience to mitigate the effect of coastal

disasters. Nursery approach is one of the best traditional methods of conserving man-

groves. The establishment of nursery is essential in reforestation programs, provides

good quality of seedlings at the right quantity in time (Melana et al., 2000; Kairo et al.,

2001; Kairo et al., 2002). Since, the availability of seedlings is limited to seasons in


41

many mangrove species, it is important to maintain a nursery to make the seedlings

available throughout the year (Rajiv, 1999). Experiments and field trials were

performed on this approach and the feasibility of this approach was already recognized

(Field, 1999; Rajiv, 1999; Kairo et al., 2001; Toledo et al., 2001; Clarke and Johns,

2002; Clarke, 2006). There were many considerations to succeed in this approach with

proper nursery technique (Toledo et al., 2001; Clarke and Johns, 2002).

Modern biotechnological approach is considered a necessity for

development of nursery. Microbiology of mangrove forests has pivotal consideration in

conservation and restoration especially, epiphytic phototropic, nitrogen-fixing and other

cyanobacteria. Already cyanobacteria were exploited in agriculture as biofertilizer,

especially on rice fields. The ex situ conservation of mangroves by rearing nursery is a

new concept for future afforestation programs. Thus, the exploitation of epiphytic

cyanobacteria along with other beneficial bacteria as biofertilizer with apposite field

during the development of nursery would be more sensible.

Review of Literature - INFLIBNETshodhganga.inflibnet.ac.in/bitstream/10603/5345/9/09_chapter...

Documents

Transcript of Review of Literature - INFLIBNETshodhganga.inflibnet.ac.in/bitstream/10603/5345/9/09_chapter...