Calcification

Calcification

• Calcite• Aragonite• Magnesian calcite

• DIC - dissolved inorganic carbon

– CO2 (aq)

– HCO3-

– CO3--

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

• Look at the chemistry

• 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.

• 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)

• Most calcareous algae in the Phyla: – RHODOPHYTA (REDS) & CHLOROPHYTA (greens)

– 1 genus in PHAEOPHYTA (brown - Padina)

• 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)

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

• 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

• 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

• 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

• 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.

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

• net productivity values (varies with lcation):

gC/m2/d

Calcareous reds 1 - 6

Halimeda 2 -3

Seagrass 1 - 7

N.S. kelp 5

• Overall productivity of the reef:

4.1 - 14.6 gC/m2/d

• includes – epilithic algae, on rock, sand etc., – few phytoplankton– seagrasses– coral etc.

• 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.

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