Productivity and the Coral Symbiosis IV: Reef Photosynthesis

Productivity the production of organic compounds from inorganic atmospheric or aquatic carbon sources – mostly CO 2 principally through photosynthesis –chemosynthesis much less important. All life on earth is directly or indirectly dependant on primary production.

gC/m 2 /d TropicalCoral Reef Tropical open ocean Mangrove2.46 Tropical Rain Forest5.5 Oak Forest3.6

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/m 2 /d Calcareous reds1 - 6 Halimeda2 -3 Seagrass1 - 7 N.S. kelp5

Overall productivity of the reef: gC/m 2 /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 Reflected Light Chloroplast Absorbed light Granum Transmitted light

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 nm

Gamma rays X-raysUVInfrared Micro- waves Radio waves 10 –5 nm 10 –3 nm 1 nm 10 3 nm 10 6 nm 1 m 10 6 nm 10 3 m nm Visible light Shorter wavelength Higher energy Longer wavelength Lower energy

Gamma rays X-raysUVInfrared Micro- waves Radio waves 10 –5 nm 10 –3 nm 1 nm 10 3 nm 10 6 nm 1 m 10 6 nm 10 3 m 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 Outer membrane Intermembrane space Inner membrane Thylakoid space Thylakoid Granum Stroma 1 µm

CO 2 CALVIN CYCLE O2O2 [CH 2 O] (sugar) NADP  ADP + P i An overview of photosynthesis H2OH2O 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

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 election acceptor Photon Thylakoid Light-harvesting complexes Reaction center Photosystem STROMA Thylakoid membrane Transfer of energy Special chlorophyll a molecules Pigment molecules THYLAKOID SPACE (INTERIOR OF THYLAKOID) e–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)

Absorption of light by chloroplast pigments 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, c 1, c 2 & d a all green plants and algae b Chlorophyceae c 1 & c 2 Phaeophyceae dRhodophyceae

Chlorophyll a –Is the main photosynthetic pigment Chlorophyll b, c, d –Are accessory pigments C CH CH 2 C C C C C CN N C H3CH3C C C C C C C C C N C C C C N Mg H H3CH3C H C CH 2 CH 3 H C H H CH 2 H CH 3 C O O O O O CHO in chlorophyll a in chlorophyll b Porphyrin ring: Light-absorbing “head” of molecule note magnesium atom at center Hydrocarbon tail: interacts with hydrophobic regions of proteins inside thylakoid membranes of chloroplasts: H atoms not shown 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