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:

organism CaCO3

Molluscs calcite & aragoniteCorals just aragoniteSome green algae just aragoniteRed algae magnesian calciteSponges aragonite (with silica)Some bryozoans all 3

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 mol/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 thanin 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.