25.1 Lichens Lichens –Leafy or encrusting microbial symbioses –Often found growing on bare...

25.1 Lichens

• Lichens– Leafy or encrusting microbial symbioses

– Often found growing on bare rocks, tree trunks, house roofs, and the surfaces of bare soils (Figure 25.1)

– A mutalistic relationship between a fungus and an alga (or cyanobacterium)

• Alga is photosynthetic and produces organic matter

• The fungus provides a structure within which the phototrophic partner can grow protected from erosion (Figure 25.2)

© 2012 Pearson Education, Inc.

Figure 25.1


Figure 25.2

Algal layer

Rootlikeconnectionto substrate

Fungalhyphae


25.2 “Chlorochromatium aggregatum”

• In freshwater there are microbial mutualisms called consortia

• Consist of green sulfur bacteria (called epibionts) and a flagellated rod-shaped bacterium (Figure 25.3 and 25.4)– Consortium given a “genus species” name

– Green sulfur bacteria are obligate anaerobic phototrophs

– Flagellated rod allows for movement


Figure 25.3


Figure 25.4


II. Plants as Microbial Habitats

• 25.3 The Legume–Root Nodule Symbiosis• 25.4 Agrobacterium and Crown Gall Disease• 25.5 Mycorrhizae


25.3 The Legume–Root Nodule Symbiosis

• The mutalistic relationship between leguminous plants and nitrogen-fixing bacteria is one of the most important symbioses known

• Examples of legumes include soybeans, clover, alfalfa, beans, and peas

• Rhizobia are the best-known nitrogen-fixing bacteria engaging in these symbioses


Animation: Root Nodule Bacteria and Symbioses with LegumesAnimation: Root Nodule Bacteria and Symbioses with Legumes


• Infection of legume roots by nitrogen-fixing bacteria leads to the formation of root nodules that fix nitrogen (Figure 25.7)– Leads to significant increases in combined

nitrogen in soil

• Nodulated legumes grow well in areas where other plants would not (Figure 25.8)


Figure 25.7


Figure 25.8



• Nitrogen-fixing bacteria need O2 to generate energy for N2 fixation, but nitrogenases are inactivated by O2

• In the nodule, O2 levels are controlled by the O2-binding protein leghemoglobin (Figure 25.9)


Figure 25.9



• Cross-inoculation group– Group of related legumes that can be infected

by a particular species of rhizobia



• Critical steps in root nodule formation (Figure 25.10):

– Step 1: Recognition and attachment of bacterium to root hairs (Figure 25.11)

– Step 2: Excretion of nod factors by the bacterium– Step 3: Bacterial invasion of the root hair– Step 4: Travel to the main root via the infection

thread– Step 5: Formation of bacteroid state within plant

cells– Step 6: Continued plant and bacterial division,

forming the mature root nodule


Figure 25.10Root hair

Rhizobial cell

Infection thread

Invaded plant cellsand those nearby arestimulated to divide

Nodules

Soil

Continued plant andbacterial cell divisionleads to nodules

Formation ofbacteroid state withinplant root cells

Bacteria in infectionthread grow towardroot cell

Invasion. Rhizobia penetrateroot hair and multiplywithin an “infection thread”

Excretion of nod factorsby bacterium causingroot hair curling

Recognition and attachment(rhicadhesin-mediated)



• Bacterial nod genes direct the steps in nodulation

• nodABC gene encodes proteins that produce oligosaccharides called nod factors (Figure 25.12)

• Nod factors– Induce root hair curling

– Trigger plant cell division



• The legume–bacteria symbiosis is characterized by several metabolic reactions and nutrient exchange (Figure 25.14)


Figure 25.14Plant cytoplasm Photosynthesis

Symbiosomemembrane

Bacteroidmembrane

Sugars

Organic acids

Bacteroid SuccinateMalate

Fumarate

Pyruvate

ee

Nitrogenase

Citricacidcycle

Protonmotiveforce

Electron transportchain

Lb Leghemoglobin

GlutamineAsparagine



• A few legume species form nodules on their stems (Figure 25.15)


Figure 25.15


25.5 Mycorrhizae

• Mycorrhizae– Mutualistic associations of plant roots and fungi

– Two classes:• Ectomycorrhizae• Endomycorrhizae


25.5 Mycorrhizae

• Ectomycorrhizae– Fungal cells form an extensive sheath around

the outside of the root with only a little penetration into the root tissue (Figure 25.21)

– Found primarily in forest trees, particularly boreal and temperate forests


Figure 25.21

Fungalfilament

Forkedroot


25.5 Mycorrhizae

• Endomycorrhizae– Fungal mycelium becomes deeply embedded

within the root tissue (Figure 25.22)

– Are more common than ectomycorrhizae

– Found in >80% of terrestrial plant species


Figure 25.22

Epidermis

Mycelium

S

S

HP

HP

A

A

Outercortex

Innercortex


25.5 Mycorrhizae

• Mycorrhizal fungi assist plants (Figure 25.23) – Improve nutrient absorption

• This is due to the greater surface area provided by the fungal mycelium

– Helping to promote plant diversity


Figure 25.23


III. Mammals as Microbial Habitats

• 25.6 The Mammalian Gut • 25.7 The Rumen and Ruminant Animals• 25.8 The Human Microbiome


25.6 The Mammalian Gut

• Herbivores – animals that consume plants • Carnivores – animals that consume meat• Omnivores – animals that consume both • Phylogenetics suggests that different lineages

evolved a herbivorous lifestyle (Figure 25.24)


Figure 25.24

HerbivoresCarnivoresOmnivores

Sheep and cow

Horse

Giant panda

Rabbit

Gorilla

Orangutan

Dog

Lion

Pig

Brown bear

Human

Baboon

Spider monkey

Lemur© 2012 Pearson Education, Inc.

25.6 The Mammalian Gut

• Microbial associations with certain animals led to ability to catabolize plant fibers

– Plant fibers composed of insoluble polysaccharides.

• Cellulose most abundant component

– Two digestive plans have evolved in herbivorous animals (Figure 25.25)

– Foregut fermentation – fermentation chamber precedes the small intestine

– Hindgut fermentation – uses cecum and/or large intestine


Figure 25.25

Foregutfermentationchamber

Hindgutfermentationchambers

Hindgut fermenters

Foregut fermenters

Examples: Cecalanimals (photos 3 and 4), primates,some rodents, some reptiles

Examples:Ruminants (photo 1), colobine monkeys,macropod marsupials, hoatzin (photo 2)

Acidicstomach

Smallintestine

Largeintestine(colon)

Cecum

1. 2.

3. 4.


25.7 The Rumen and Ruminant Animals

• Microbes form intimate symbiotic relationships with higher organisms

• Ruminants– Herbivorous mammals (e.g., cows, sheep, goats)

– Possess a special digestive organ (the rumen) • Cellulose and other plant polysaccharides are

digested with the help of microbes (Figure 25.26)

– Rumen well studied because of implanted sampling port


Figure 25.26 EsophagusFood

Cud

Reticulum

Smaller foodparticles

Omasum Abomasum

Small intestine

Rumen



• The rumen contains 1010–1011 microbes/g of rumen constituents

• Fermentation in the rumen is mediated by cellulolytic microbes that hydrolyze cellulose to free glucose that is then fermented, producing volatile fatty acids (e.g., acetic, propionic, butyric) and CH4 and CO2 (Figure 25.27)

• Fatty acids pass through rumen wall into bloodstream and are utilized by the animal as its main energy source


Figure 25.27 FEED, HAY, etc.

Cellulose, starch, sugars

Fermentation Fermentation

Cellulolysis, amylolysis

Formate

Pyruvate Succinate

Lactate Propionate CO2

Removed byeructation toatmosphere

Acetate

Ru

min

an

t b

loo

ds

tre

am

Ru

me

n w

all

VFAs

Acetate

Propionate

Butyrate

Overall stoichiometry of rumen fermentation:

SUGARS



• Rumen microbes also synthesize amino acids and vitamins for their animal host

• Rumen microbes themselves can serve as a source of protein to their host when they are directly digested

• Anaerobic bacteria dominate in the rumen (Figure 25.28)



• Abrupt changes in an animal’s diet can result in changes in the rumen flora

• Rumen acidification (acidosis) is one consequence of such a change

• Anaerobic protists and fungi are also abundant in the rumen

• Many perform metabolisms similar to those of their prokaryotic counterparts


25.10 Termites

• Termites decompose cellulose and hemicellulose • Termites classified as higher or lower based on

phylogeny • Termite gut consists of foregut, midgut, and

hindgut (Figure 25.34) – Posterior alimentary tract of higher termites

(Termitidae)• Diverse community of anaerobes including

cellulolytic anaerobes (Figure 25.35)– Lower termites

• Anaerobic bacteria and cellulolytic protists


Figure 25.34

Hindgut compartments

Paunch

2 mm

Foregut

Midgut

Hindgut

Anoxic

Microoxic

Glucose

0.5 mm

Cellulose

Acetate


25.1 Lichens Lichens –Leafy or encrusting microbial symbioses –Often found growing on bare...

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