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Productivity and the Coral Symbiosis III

Productivity and the Coral Symbiosis III. Overall productivity of the reef: 4.1 - 14.6 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

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Productivity and the Coral Symbiosis III

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  1. Productivity and the Coral Symbiosis III

  2. Overall productivity of the reef: 4.1 - 14.6 gC/m2/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

  3. 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)

  4. Calcification - growth of the reef

  5. In ocean, mostly find 3 forms of CaC03 • Calcite • Mostly of mineral origin • Aragonite • Fibrous, crystalline form, mostly from corals • Magnesian calcite • Smaller crystals, mostly plant origin

  6. Calcification Calcite Aragonite Magnesian calcite (Mg carbonate)

  7. Examples:

  8. Corals • remove Ca++ & CO3-- from seawater • Combines them to CaCO3 • transports them to base of polyp • Calcicoblastic epidermis • minute crystals secreted from base of polyp • Energy expensive • Energy from metabolism of algal PS products

  9. Calcification

  10. CO2 and seawater • What forms of C are available to the coral ? • Organic and inorganic forms • DIC - dissolved inorganic carbon • CO2 (aq) • HCO3- • CO3--

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

  12. DIC in ocean constant over long periods • Can change suddenly on local scale • E.g. environmental change, pollution • Average seawater DIC = 1800-2300 mmol/Kg • Average seawater pH = 8.0 - 8.2 • pH affects nature of DIC

  13. Carbon and Seawater • normal seawater - more HCO3- than CO3-- • when atmospheric CO2 dissolves in water • only 1% stays as CO2 • rest dissociates to give HCO3- and CO3--

  14. H2O + CO2 (aq) H2CO3 HCO3- + H+ (1) HCO3-CO3-- + H+ (2) equilibrium will depend heavily on [H+] = pH relative amounts of different ions will depend on pH

  15. dissolved carbonate removed by corals to make aragonite Ca++ + CO3-- CaCO3 (3) pulls equilibrium (2) over, more HCO3- dissociates to CO3-- HCO3-CO3-- + H+ (2) removes HCO3-,pulls equilibrium in eq (1) to the right H2O + CO2 (aq) H2CO3 HCO3- + H+ (1) more CO2 reacts with water to replace HCO3-, thus more CO2 has to dissolve in the seawater

  16. Can re-write this carbon relationship: 2 HCO3-CO2 + CO3-- +H2O • used to be thought that • symbiotic zooxanthellae remove CO2 for PS • pulls equation to right • makes more CO3-- available for CaCO3 production by polyp • No

  17. demonstrated by experiments with DCMU • stops PS electron transport, not CO2 uptake • removed stimulatory effect of light on polyp CaCO3 deposition • therefore, CO2 removal was not playing a role • also, in deep water stony corals • if more food provided, more CaCO3 was deposited • more energy available for carbonate uptake & CaCO3 deposition

  18. Now clear that algae provide ATP (via CHO) to allow polyp to secrete the CaCO3 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

  19. Environmental Effects of Calcification • When atmospheric [CO2] increases, what happens to calcification rate ? • goes down • more CO2 should help calcification ? • No

  20. Add CO2 to water • quickly converted to carbonic acid • dissociates to bicarbonate: H2O + CO2 (aq) H2CO3 HCO3- + H+ (1) HCO3-CO3-- + H+ (2) • Looks useful - OK if polyp in control, removing CO3--

  21. Add CO2 to water • quickly converted to carbonic acid • dissociates to bicarbonate: H2O + CO2 (aq) H2CO3 HCO3- + H+(1) HCO3-CO3-- + H+(2) • Looks useful - OK if polyp in control, removing CO3-- • BUT, if CO2 increases, pushes eq (1) far to right • [H+] increases, carbonate converted to bicarbonate

  22. So, as more CO2 dissolves, • more protons are released • acidifies the water • the carbonate combines with the protons • produces bicarbonate • decreases carbonate concentration

  23. Also, increase in [CO2] • leads to a less stable reef structure • the dissolving of calcium carbonate H2O + CO2 + CaCO3 2HCO3- + Ca++ • addition of CO2 pushes equilibrium to right • increases the dissolution of CaCO3

  24. anything we do to increase atmospheric [CO2] leads to various deleterious effects on the reef: • Increases solubility of CaCO3 • Decreases [CO3--] decreasing calcification • Increases temperature, leads to increased bleaching • Increases UV - DNA, PS pigments etc.

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