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Coral Structure and Function II
• Alga donates most of it’s fixed C to polyp– used for resp, growth, etc.
• Polyp respires– releases CO2 to alga
• Polyp excretes N waste - NH3
– used by alga
• Polyp also releases PO4-, SO4
-, NO3- to alga
– 1000x more conc. than in seawater– Algae grow faster - helps polyp
FOOD
CHOProtein
AAs Sugars Fatty acids
Alga
Polyp
NH3 CO2 O2
O2
CO2NH3
AAs
Protein
AAs Sugars
CHO
Lipid
ATPNADPH
Fatty acids
Growth & metabolism
Growth & metabolism
glycerol
H2O H2O
LIGHT
PO4- PO4
-
SO4- SO4
-
ATP
• Alga stores CHO – starch• Broken down at night
• Polyp stores lipid – fat bodies• Energy reserve
• Algal Photosynthesis• base of reef productivity• Energy source for reef building
• 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 & aragonite
Corals just aragonite
Some green algae just aragonite
Red algae magnesian calcite
Sponges 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 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.
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
• The visible light spectrum includes– the colors of light we can see– the wavelengths that drive photosynthesis
• Photosynthetic pigments absorb light
Light
ReflectedLight
Chloroplast
Absorbedlight
Granum
Transmittedlight
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
Light Reactions
• In the thylakoid membrane,
– chlorophyll molecules, other small molecules & proteins, are organized into photosystems
– photosystems composed of a reaction center surrounded by a number of light-harvesting complexes (LHC)
• LHC = pigment molecules bound to proteins
• 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 it’s 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
