View
220
Download
0
Tags:
Embed Size (px)
Citation preview
Productivity and the Coral Symbiosis II
• Polyp can survive extended periods with no external food source
• Tight internal N-cycling and algal PS
• Polyp lays down extensive lipid reserves to be drawn on in times of starvation
• High light and high food availability– ejection of pellets containing viable algal cells
• Control of algal cell number ?
• Algae divide within host polyp
• Analyze algal cell
– C,H,O from PS– N,P,S, from host (normally limiting)
• Symbiosis controlled by host
• Polyp controls permeability of algal membrane
• “signal molecules”
• Freshly isolated zooxanthellae
• Incubate in light with 14CO2
• Release very little organic C into medium
• Add some polyp extract - releases lots of organic carbon into medium
• Other cnidarian extracts work
• 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 PS: 90% fixed C to coral host
• Used for metabolic functions• Growth, reproduction &• Calcium deposition
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
• 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--
• 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.