Productivity and the Coral Symbiosis IV:

Reef Photosynthesis

Productivity

• the production of organic compounds from inorganic atmospheric or aquatic carbon sources – mostly CO2

• principally through photosynthesis– chemosynthesis much less important.

• All life on earth is directly or indirectly dependant on primary production.

gC/m2/d

TropicalCoral Reef 4.1 - 14.6

Tropical open ocean 0.06 - 0.27

Mangrove 2.46

Tropical Rain Forest 5.5

Oak Forest 3.6

Productivity

• no single major contributor to primary production on the reef

• a mixture of photosynthetic organisms– can be different at different locations

• net productivity values (varies with location):

gC/m2/d

Calcareous reds 1 - 6

Halimeda 2 -3

Seagrass 1 - 7

N.S. kelp 5

• Overall productivity of the reef:

4.1 - 14.6 gC/m2/d

• from– epilithic algae, on rock, sand etc., – few phytoplankton– seagrasses– Zooxanthellae (in coral etc.)– Fleshy and calcareous macroalgae

• One obvious differences between different algae is their colour

• Different colours due to the presence of different photosynthetic pigments

• The visible light spectrum includes– the colors of light we can see– the wavelengths that drive photosynthesis

• Photosynthetic pigments absorb light

Light

ReflectedLight

Chloroplast

Absorbedlight

Granum

Transmittedlight

Light and Photosynthesis

• Air & water both absorb light– a plant at sea level receives 20% less light than

a plant on a mountain at 4,000m

– this reduction occurs faster in seawater – depends a lot on location

• get 20% light reduction in 2m of tropical seawater

• get 20% light reduction in 20cm of Maritime seawater

Photosynthesis uses a very specific part of the EM spectrum

• PAR

• Photosynthetically Active Radiation

• 350-700 nm

Gammarays X-rays UV Infrared

Micro-waves

Radiowaves

10–5 nm 10–3 nm 1 nm 103 nm 106 nm1 m

106 nm 103 m

380 450 500 550 600 650 700 750 nm

Visible light

Shorter wavelength

Higher energy

Longer wavelength

Lower energy

Gammarays X-rays UV Infrared

Micro-waves

Radiowaves

10–5 nm 10–3 nm 1 nm 103 nm 106 nm1 m

106 nm 103 m

380 450 500 550 600 650 700 750 nm

Visible light

Shorter wavelength

Higher energy

Longer wavelength

Lower energy

• Measure it as IRRADIANCE– moles of photons per unit area per unit time– mol.m-2.s-1

– E = Einstein = 1 mole of photons

E.m-2.s-1

• As light passes through seawater it gets ABSORBED & SCATTERED – = ATTENUATION (a reduction in irradiance)

• pure water– attenuation lowest at 465nm

– increases towards UV and IR ends of spectrum

• TRANSMITTANCE is highest at 465nm

• not dealing with pure water– seawater has all kinds of dissolved salts, minerals,

suspended material etc.:

• Attenuation is different in different locations - different light transmittance spectra:

To fully exploit a particular location, marine plants have a wide variety of PS pigments they can use.

Chloroplast

Mesophyll

5 µm

Outermembrane

Intermembranespace

Innermembrane

Thylakoidspace

Thylakoid

GranumStroma

1 µm

CO2

CALVINCYCLE

O2

[CH2O](sugar)

NADP

ADP+ P i

An overview of photosynthesis

H2O

Light

LIGHT REACTIONS

Chloroplast

ATP

NADPH

Light Reactions

• In the thylakoid membrane,

– chlorophyll molecules, other small molecules & proteins, are organized into photosystems

– photosystems composed of a reaction center surrounded by a number of light-harvesting complexes (LHC)

• LHC = pigment molecules bound to proteins

• LHC = pigment molecules bound to proteins

• funnel energy of photons to the reaction center

• reaction-center chlorophyll absorbs energy– One of its electrons gets bumped up to a primary

electron acceptor– electron transport– ATP & NADPH production

Photosystems

Primary electionacceptor

Photon

Thylakoid

Light-harvestingcomplexes

Reactioncenter

Photosystem

STROMAT

hyla

koid

mem

bran

e

Transferof energy

Specialchlorophyll amolecules

Pigmentmolecules

THYLAKOID SPACE(INTERIOR OF THYLAKOID)

e–

• different pigments have different absorption spectra

• combine in different amounts in different species to give each a unique absorption spectrum

• tells us which wavelengths of light are being absorbed (and thus it’s colour)

Ab

sorp

tion

of

ligh

t b

ych

loro

pla

st p

igm

en

ts

Chlorophyll a

Wavelength of light (nm)

Chlorophyll b

Carotenoids

Absorption spectra of pigments

• doesn’t tell us what the alga is doing with the light

• For this you need to look at the ACTION SPECTRUM– measures photosynthesis at different

wavelengths

• The action spectrum of a pigment

– show relative effectiveness of different wavelengths of radiation in driving photosynthesis

• Plots rate of photosynthesis versus wavelength

Marine PS pigments

• 3 major groups of PS pigments in marine organisms

– Chlorophylls– Phycobiliproteins– Carotenoids

• Chlorophyll a is essential

– find it in all plants and algae

• the other pigments are accessory pigments

– in the antennae complexes – funnel electrons to chlorophyll a in the reaction

centres

• 5 types of chlorophyll commonly found in marine organisms

• all are tetrapyrrole rings with Mg++ in the middle

• chlorophyll a, b, c1, c2 & d

• a all green plants and algae• b Chlorophyceae• c1 & c2 Phaeophyceae• d Rhodophyceae

• Chlorophyll a– Is the main photosynthetic pigment

• Chlorophyll b, c, d– Are accessory pigments

C

CH

CH2

CC

CC

C

CNNC

H3C

C

CC

C C

C

C

C

N

CC

C

C N

MgH

H3C

H

C CH2CH3

H

CH3C

HHCH2

CH2

CH2

H CH3

C O

O

O

O

O

CH3

CH3

CHO

in chlorophyll a

in chlorophyll b

Porphyrin ring:Light-absorbing“head” of moleculenote magnesiumatom at center

Hydrocarbon tail:interacts with hydrophobicregions of proteins insidethylakoid membranes ofchloroplasts: H atoms notshown

Accessory pigments absorb different wavelengths of light and pass the energy to chlorophyll a

• Also a wide range of carotenoids– C40 TETRATERPENES– very hydrophobic– sit in membranes

• 2 types of carotenoids

– CAROTENES (hydrocarbons)– XANTHOPHYLLS (have 1 or 2 oxygens)

-CAROTENE is the most common carotenoid in marine organisms

• often see a mixture of -CAROTENE & FUCOXANTHIN in the Phaeophyceae– gives the brown colour

• PHYCOBILINS are linear tetrapyrroles attached to proteins– red pigments

– no ring, no chelation of a metal

• Only found in Rhodophyceae & Cyanophyceae– and a few species of Cryptophyceae

• Algae from different locations will often have different absorption and action spectra

– CHROMATIC ADAPTATION

• difference in pigment composition due to a difference in light quality

• most pronounced when comparing algae grown at different depths

• Allows for optimal PS with the different wavelengths of light seen at different depths

• occurs within and between species

• In general, less light means more pigment

• e.g. Sea Lettuce (Ulva spp)

• move from high to low light– 10x less: 300 to 30 E.m-2.s-1

• chl a,b & c go up 700%

• One pigment doesn’t respond in this way

• FUCOXANTHIN– yellowish pigment found in brown algae

– probably because it performs 2 functions

• light harvesting

• protection from high light levels