Coral Structure and Function II

1. Coral Structure and Function II

2. Primary Production • Benthic • Cnidaria • Atoll analysis 1. Calcareousred algae 2. Calcareousgreen algae (Halimeda) 3. Foraminifera (20-40m protists, porous CaCO3 shell) 4. Corals • Coral polyp – predacious • Endosymbiont – zooxanthellae

3. Much of the productivity from corals • Cnidaria - from the Latin “nettle” – a plant • have often been mistaken for plants • attached to a substrate • do not wander about • same colour as many marine plants • same branched nature and growth habit

4. Much of the food needed by the polyp comes from the SYMBIONT • Many corals have different growth forms - can vary with local environment - light, depth etc. • Local environment affects distribution of the zooxanthellae

5. Zooxanthellae: • ZOO - animal • XANTHE - gold-coloured • single-celled alga, with 2 flagellae • a dinoflagellate • spherical, 8 - 12um dia • Most dinoflagellates are free-living • unusual group of algae • feeding modes ranging from photosynthetic autotrophy to heterotroph

6. mucus nematocysts

7. Zooxanthellae can live outside their host • essential in some species for finding a host • Dinomastigotesstage • motile free-living state, have two flagellae • Coccoid stage • living in animal cells, lack flagellae • In culture, zooxanthellae alternate between coccoid and dinomastigote stages

8. Almost all zooxanthellae are in the dinflagellate genus Symbiodinium (1959) • taxonomy of Symbiodinium in a state of flux • 1980 - Symbiodinium microadriaticumassumed to be the one species found in almost all corals

9. Recent work • great genetic diversity in zooxanthellae • clearly more than one species • now dozens of different algal taxa • zooxanthellae found in closely related coral species not necessarily closely related themselves • zooxanthellae found in distantly related coral species may, in fact, be closely related • may have multiple species in same coral

11. Usually a single Symbiodinium clone in a coral colony. Sometimes two. Adjacent colonies have same or similar Symbiodinium populations.

12. Symbiodinium Diversity Patterns

13. Acquisition of Zooxanthellae by Corals either 1. open (or indirect) transmission or acquisition • from the environment or 2. closed (or direct) transmission or acquisition - via gametes or - during asexual reproduction

14. Indirectacquisition • provides potential for host to establish a symbiosis with a different strain or species of zooxanthellae than was in symbiosis with the host’s parents • Coral bleaching • may also allow establishment of new symbiosis with different zooxanthellae strain, • has been proposed as a possible adaptive mechanism to environmental change • Shifting symbioses • controversial topic

15. Davy et al: Microbiol. Mol. Biol. Rev. June 2012 vol. 76 no. 2 229-261

16. In all hermatypic corals endosymbiotic algae provide an important source of nutrients • can demonstrate mutualistic relationship • feed 14CO2 to the coral • quickly taken up by alga and ends up in the polyp • feed zooplankton raised on 15N to coral • quickly taken up by polyp and ends up in the alga

17. clear they exchange a lot of material • benefit each other • reef-shading experiments • 3 months in the dark • algae expelled from the polyps • later the polyps died • Most coral polyps have absolute requirement for alga - but not vice-versa

18. MUTUALISM - benefits for algae? • shelter • protection from nematocysts, & other predation • receive waste products of polyp - CO2 & N • N is v.limiting in marine environment • the major limitation to plant growth • algal blooms occur in response to small changes in N • pressure exists to optimize N scavenging • favours such a mutualistic relationship • Disadvantage • algae restricted to shallow tropical waters

19. MUTUALISM - benefits for polyp? • food (CHO) • O2 • greatly increased ability to precipitate CaCO3 • without the alga, coral could not have such a high rate of metabolism • could not build such extensive reef structures

20. 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

21. 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”

22. 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

23. “host release factor”HRF • Various suggestions: • 10kDa protein • Free amino acid suite • Mycosporine-like amino acids • Taurine • Tension between HRFand PIF • photosynthesis-inhibiting factor Davy et al: Microbiol. Mol. Biol. Rev. June 2012 vol. 76 no. 2 229-261

24. 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

25. FOOD Polyp Protein CHO Lipid Growth & metabolism AAs Sugars Fatty acids ATP NH3 CO2 O2 NH3 CO2 O2 glycerol AAs AAs Sugars Fatty acids LIGHT ATP NADPH Protein CHO PO4- PO4- H2O H2O Growth & metabolism SO4- SO4- Alga

26. Mar Drugs. 2010; 8(10): 2546–2568.

27. Calcification - growth of the reef

28. 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 • Huge ATP demand

29. 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

30. CALCAREOUS ALGAE (greens & reds) are major contributors to reef calcification • the more flexible magnesian calcite • last 30 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)

31. 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

32. Calcification Calcite Aragonite Magnesian calcite (Mg carbonate)

33. Examples:

34. 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

35. Calcification