Productivity and the coral symbiosis iii
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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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Productivity and the Coral Symbiosis III


  • 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


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 CaC03

    • 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:


    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


    Calcification


    CO2 and seawater

    • What forms of C are available to the coral ?

    • Organic and inorganic forms

    • DIC - dissolved inorganic carbon

      • CO2 (aq)

      • HCO3-

      • CO3--


    • 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 = 1800-2300 mmol/Kg

    • Average seawater pH = 8.0 - 8.2

    • pH affects nature of DIC


    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--


    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


    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


    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


    • 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


    • 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


    Environmental Effects of Calcification

    • When atmospheric [CO2] increases, what happens to calcification rate ?

      • goes down

      • more CO2 should help calcification ?

      • No


    • 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--


    • 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


    • So, as more CO2 dissolves,

    • more protons are released

    • acidifies the water

    • the carbonate combines with the protons

    • produces bicarbonate

    • decreases carbonate concentration


    • 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


    • 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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