Calcification - growth of the reef

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 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

• 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++

• 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.

• Great Barrier Reef:• Calcification of Porites down 14% since 1990

– 328 Porites corals sampled at 69 locations

– compared several growth parameters over 400 years -1572 to 2005.

• Great Barrier Reef:• Calcification of Porites down 14% since 1990

– 328 Porites corals sampled at 69 locations

– compared several growth parameters over 400 years -1572 to 2005.

Productivity and the Coral Symbiosis:

Reef Photosynthesis

Productivity

• the production of organic compounds from inorganic atmospheric or aquatic carbon sources – mostly CO2

• principally through photosynthesis– chemosynthesis much less important.

• All life on earth is directly or indirectly dependant on primary production.

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

Productivity

• no single major contributor to primary production on the reef

• a mixture of photosynthetic organisms– can be different at different locations

• net productivity values (varies with location):

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

• from– epilithic algae, on rock, sand etc., – few phytoplankton– seagrasses– Zooxanthellae (in coral etc.)– Fleshy and calcareous macroalgae

• One obvious differences between different algae is their colour

• Different colours due to the presence of different photosynthetic pigments

Light and Photosynthesis

• Air & water both absorb light– a plant at sea level receives 20% less light than

a plant on a mountain at 4,000m

– this reduction occurs faster in seawater – depends a lot on location

• get 20% light reduction in 2m of tropical seawater

• get 20% light reduction in 20cm of Maritime seawater

Photosynthesis uses a very specific part of the EM spectrum

• PAR

• Photosynthetically Active Radiation

• 350-700 nm

Gammarays X-rays UV Infrared

Micro-waves

Radiowaves

10–5 nm 10–3 nm 1 nm 103 nm 106 nm1 m

106 nm 103 m

380 450 500 550 600 650 700 750 nm

Visible light

Shorter wavelength

Higher energy

Longer wavelength

Lower energy

• Measure it as IRRADIANCE– moles of photons per unit area per unit time– mol.m-2.s-1

– E = Einstein = 1 mole of photons

E.m-2.s-1

• As light passes through seawater it gets ABSORBED & SCATTERED – = ATTENUATION (a reduction in irradiance)

• pure water– attenuation lowest at 465nm

– increases towards UV and IR ends of spectrum

• TRANSMITTANCE is highest at 465nm

• not dealing with pure water– seawater has all kinds of dissolved salts, minerals,

suspended material etc.:

• Attenuation is different in different locations - different light transmittance spectra:

To fully exploit a particular location, marine plants have a wide variety of PS pigments they can use.

Chloroplast

Mesophyll

5 µm

Outermembrane

Intermembranespace

Innermembrane

Thylakoidspace

Thylakoid

GranumStroma

1 µm

CO2

CALVINCYCLE

O2

[CH2O](sugar)

NADP

ADP+ P i

An overview of photosynthesis

H2O

Light

LIGHT REACTIONS

Chloroplast

ATP

NADPH

• different pigments have different absorption spectra

• combine in different amounts in different species to give each a unique absorption spectrum

• tells us which wavelengths of light are being absorbed (and thus its colour)

Ab

sorp

tion

of

ligh

t b

ych

loro

pla

st p

igm

en

ts

Chlorophyll a

Wavelength of light (nm)

Chlorophyll b

Carotenoids

Absorption spectra of pigments

• doesn’t tell us what the alga is doing with the light

• For this you need to look at the ACTION SPECTRUM– measures photosynthesis at different

wavelengths

• The action spectrum of a pigment

– show relative effectiveness of different wavelengths of radiation in driving photosynthesis

• Plots rate of photosynthesis versus wavelength

Marine PS pigments

• 3 major groups of PS pigments in marine organisms

– Chlorophylls– Phycobiliproteins– Carotenoids

• Chlorophyll a is essential

– find it in all plants and algae

• the other pigments are accessory pigments

– in the antennae complexes – funnel electrons to chlorophyll a in the reaction

centres

• 5 types of chlorophyll commonly found in marine organisms

• all are tetrapyrrole rings with Mg++ in the middle

• chlorophyll a, b, c1, c2 & d

• a all green plants and algae• b Chlorophyceae• c1 & c2 Phaeophyceae• d Rhodophyceae