Productivity and the Coral Symbiosis III

Overall productivity of the reef: gC/m 2 /d this is organic carbon production must also consider carbonate production (deposition of physical structure of the reef) –Get about half of this from the coral symbiosis –the rest from the calcareous green & reds algae

CALCAREOUS ALGAE (greens & reds) are major contributors to reef calcification –the more flexible magnesian calcite last 25 years - role of these algae receive more attention –play a much bigger role in calcium deposition than previously thought 10% of all algae CALCIFY (about 100 genera)

Calcification - growth of the reef

In ocean, mostly find 3 forms of CaC0 3 Calcite –Mostly of mineral origin Aragonite –Fibrous, crystalline form, mostly from corals Magnesian calcite –Smaller crystals, mostly plant origin

Calcification Calcite Aragonite Magnesian calcite (Mg carbonate)

Examples: organismCaCO 3 Molluscscalcite & aragonite Coralsjust aragonite Some green algaejust aragonite Red algaemagnesian calcite Spongesaragonite (with silica) Some bryozoansall 3

Corals remove Ca ++ & CO 3 -- from seawater Combines them to CaCO 3 transports them to base of polyp –Calcicoblastic epidermis minute crystals secreted from base of polyp Energy expensive –Energy from metabolism of algal PS products

Calcification

CO 2 and seawater What forms of C are available to the coral ? Organic and inorganic forms DIC - dissolved inorganic carbon –CO 2 (aq) –HCO 3 - –CO 3 --

DIC comes from: –Weathering –dissolution of oceanic rock –Run-off from land –Animal respiration –Atmosphere –etc.

DIC in ocean constant over long periods Can change suddenly on local scale –E.g. environmental change, pollution Average seawater DIC =  mol/Kg Average seawater pH = pH affects nature of DIC

Carbon and Seawater normal seawater - more HCO 3 - than CO 3 -- when atmospheric CO 2 dissolves in water –only 1% stays as CO 2 –rest dissociates to give HCO 3 - and CO 3 --

H 2 O + CO 2 (aq) H 2 CO 3 HCO H + (1) HCO 3 - CO H + (2) equilibrium will depend heavily on [H + ] = pH relative amounts of different ions will depend on pH

dissolved carbonate removed by corals to make aragonite Ca ++ + CO 3 -- CaCO 3 (3) pulls equilibrium (2) over, more HCO 3 - dissociates to CO 3 -- HCO 3 - CO H + (2) removes HCO 3 -, pulls equilibrium in eq (1) to the right H 2 O + CO 2 (aq) H 2 CO 3 HCO H + (1) more CO 2 reacts with water to replace HCO 3 -, thus more CO 2 has to dissolve in the seawater

Can re-write this carbon relationship: 2 HCO 3 - CO 2 + CO H 2 O used to be thought that –symbiotic zooxanthellae remove CO 2 for PS –pulls equation to right –makes more CO 3 -- available for CaCO 3 production by polyp No

demonstrated by experiments with DCMU –stops PS electron transport, not CO 2 uptake removed stimulatory effect of light on polyp CaCO 3 deposition therefore, CO 2 removal was not playing a role also, in deep water stony corals –if more food provided, more CaCO 3 was deposited –more energy available for carbonate uptake & CaCO 3 deposition

Now clear that algae provide ATP (via CHO) to allow polyp to secrete the CaCO 3 and its organic fibrous matrix Calcification occurs 14 times faster in open than in shaded corals Cloudy days: calcification rate is 50% of rate on sunny days There is a background, non-algal-dependent rate

Environmental Effects of Calcification When atmospheric [CO 2 ] increases, what happens to calcification rate ? –goes down –more CO 2 should help calcification ? –No

Add CO 2 to water –quickly converted to carbonic acid –dissociates to bicarbonate: H 2 O + CO 2 (aq) H 2 CO 3 HCO H + (1) HCO 3 - CO H + (2) Looks useful - OK if polyp in control, removing CO 3 --

Add CO 2 to water –quickly converted to carbonic acid –dissociates to bicarbonate: H 2 O + CO 2 (aq) H 2 CO 3 HCO H + (1) HCO 3 - CO H + (2) Looks useful - OK if polyp in control, removing CO 3 -- BUT, if CO 2 increases, pushes eq (1) far to right [H + ] increases, carbonate converted to bicarbonate

So, as more CO 2 dissolves, more protons are released acidifies the water the carbonate combines with the protons produces bicarbonate decreases carbonate concentration

Also, increase in [CO 2 ] –leads to a less stable reef structure –the dissolving of calcium carbonate H 2 O + CO 2 + CaCO 3 2HCO Ca ++ addition of CO 2 pushes equilibrium to right – increases the dissolution of CaCO 3

anything we do to increase atmospheric [CO 2 ] leads to various deleterious effects on the reef: Increases solubility of CaCO 3 Decreases [CO 3 -- ] decreasing calcification Increases temperature, leads to increased bleaching Increases UV - DNA, PS pigments etc.