calcification. calcite aragonite magnesian calcite dic - dissolved inorganic carbon –co 2 (aq)...
Post on 22-Dec-2015
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Calcification
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Calcification
• Calcite• Aragonite• Magnesian calcite
• DIC - dissolved inorganic carbon
– CO2 (aq)
– HCO3-
– CO3--
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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
--
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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
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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
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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
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• 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
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• 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
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Environmental Effects of Calcification
• When atmospheric [CO2] increases, what happens to calcification rate ? – goes down
– more CO2 should help calcification ?
– No
• Look at the chemistry
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• 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
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• So, as more CO2 dissolves,
• more protons are released
• acidifies the water
• the carbonate combines with the protons
• produces bicarbonate
• decreases carbonate concentration
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• 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
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• 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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• a major source of calcium deposition on the reef
– the coral symbiosis
• However, CALCAREOUS ALGAE (greens & reds) also major contributors
– the more flexible magnesian calcite
• last 20 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)
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• Most calcareous algae in the Phyla: – RHODOPHYTA (REDS) & CHLOROPHYTA (greens)
– 1 genus in PHAEOPHYTA (brown - Padina)
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• Many not considered to be “plants” until 19th century– referred to as “corallines”
– calcareous horny sea organisms
• 3 genera particularly important in creating reef structure:
1. Halimeda (global)
2. Penicillus (Caribean)
3. Tydemania (Indo-pacific)
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Halimeda
• variety of substrates from sand to rock
• different species adapted to specific substrates
– lagoon - large holdfast (1-5cm) deep into the sand
– on rock - small (1cm) in crevices
– sprawl across coral debris - attached by threadlike filaments
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• variety allows Halimeda to colonize all zones of the reef– except very high energy areas like reef crest, (find
calcareous reds here)
• Halimeda particularly abundant in lagoon and the back- and fore-reef areas– so not much in Bonaire
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• Halimeda grows quickly
• produces a new segment overnight– a whitish mass– turns green in the morning– induction of chlorophyll synthesis by light– after greening, it lays down the magnesian
calcite and stiffens up
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• Estimates from Great Barrier Reef– Halimeda doubles its biomass every 15d.– equates to 7g dry wt. per day per sq m.
• Segments get broken off– settle on lagoon floor– in sand grooves– adding solid material
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• Halimeda grows down to 150m– light intensity is 0.05% of surface– grows slowly here, uses different pigments– this is about the limit for the Chlorophyta– algae growing deeper than this are in the
Rhodophyta
• Texts often say euphotic zone ends at 1% surface light– not the case, reds can be found as deep as 268m.
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Productivity
• no single major contributor to primary production
• due to a mixture of organisms - can be different at different locations
• Includes:– Fleshy and calcareous macroalgae– Sea grasses– Zooxanthellae
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• net productivity values (varies with lcation):
gC/m2/d
Calcareous reds 1 - 6
Halimeda 2 -3
Seagrass 1 - 7
N.S. kelp 5
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• Overall productivity of the reef:
4.1 - 14.6 gC/m2/d
• includes – epilithic algae, on rock, sand etc., – few phytoplankton– seagrasses– coral etc.
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• 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 greens and reds.
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gC/m2/d
TropicalCoral Reef 4.1 - 14.6
Tropical open ocean 0.06 - 0.27
Mangrove 2.46
Tropical Rain Forest 5.5
Oak Forest 3.6