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.