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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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slide2
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
slide3
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)
slide5
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
slide6
Calcification

Calcite

Aragonite

Magnesian calcite

(Mg carbonate)

slide8
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
slide10
CO2 and seawater
  • What forms of C are available to the coral ?
  • Organic and inorganic forms
  • DIC - dissolved inorganic carbon
    • CO2 (aq)
    • HCO3-
    • CO3--
slide11
DIC comes from:
    • Weathering
    • dissolution of oceanic rock
    • Run-off from land
    • Animal respiration
    • Atmosphere
    • etc.
slide12
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
slide13
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--
slide14
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

slide16
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

slide17
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
slide18
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
slide19
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
slide20
Environmental Effects of Calcification
  • When atmospheric [CO2] increases, what happens to calcification rate ?
    • goes down
    • more CO2 should help calcification ?
    • No
slide21
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--
slide22
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
slide23
So, as more CO2 dissolves,
  • more protons are released
  • acidifies the water
  • the carbonate combines with the protons
  • produces bicarbonate
  • decreases carbonate concentration
slide25
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
slide26
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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