How does rising atmospheric CO2 affect marine organisms?

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Coral Response to Thermal Stress: Symbiont Shuffling Plus
Fitt, W.K., Gates, R.D., Hoegh-Guldberg, O., Bythell, J.C., Jatkar, A., Grottoli, A.G., Gomez, M., Fisher, P., Lajuenesse, T.C., Pantos, O., Iglesias-Prieto, R., Franklin, D.J., Rodrigues, L.J., Torregiani, J.M., van Woesik, R. and Lesser, M.P. 2009. Response of two species of Indo-Pacific corals, Porites cylindrica and Stylophora pistillata, to short-term thermal stress: The host does matter in determining the tolerance of corals to bleaching. Journal of Experimental Marine Biology and Ecology 373: 102-110.

The authors write that the various Symbiodinium clades that comprise the algal symbiont found in their coral host have been thought for some time now "to exert a major influence on the ability of reef-building corals to survive high-temperature stress," which is indeed correct; but they add that if the host itself plays a role in this process, "the hypothesis that corals simply shuffle or swap their Symbiodinium for clades that are more thermally tolerant does not tell the whole story." In fact, it suggests that corals may be even more adept at successfully coping with rising temperatures than has previously been believed.

What was done
To explore this possibility, Fitt et al. studied a number of coral host and Symbiodinium properties and processes in two ubiquitous Indo-Pacific reef corals that are known to be either very susceptible (Stylophora pistillata) or resistant (Porites cylindrica) to heat stress, while exposing them to seawater temperatures of either 28C (normal ambient) or 32C (elevated) for five days before returning them to the normal ambient temperature.

What was learned
The sixteen scientists report finding "both physiological and biochemical differences of both symbiont and host origin in the response to high-temperature stress." Consequently, they say that "hypotheses that talk only in terms of the thermal characteristics of the symbiont may miss critical information concerning questions surrounding the thermal tolerance of corals in the coming century." In this regard, they note "there are dynamic photoprotective mechanisms in both the host and zooxanthellae that include ultraviolet radiation absorbing mycosporine-like amino acids (Shick and Dunlap, 2002; Lesser, 2004), excess excitation energy dissipation in photosystem II via the xanthophyll cycle (Brown et al. 1999; Gorbunov et al., 2001), the expression of heat-shock proteins and other stress markers (Black et al., 1995; Downs et al., 2000; Lesser and Farrell, 2004), the up-regulation of antioxidant enzymes (Lesser, 1996; Lesser and Farrell, 2004; Lesser, 2006), host energy reserve utilization (Porter et al., 1989; Grottoli et al., 2004, 2006), and heterotrophic plasticity (Grotolli et al., 2006)," all of which phenomena, in their words, "presumably have underlying influences on any response to thermal stress, and hence, contribute to the overall differences within and between species in regard to their bleaching sensitivity."

What it means
In light of this diverse group of phenomena that can help both the coral host and its algal symbionts adjust to rising temperatures, it would appear that earth's corals are well equipped to deal with whatever further warming may come their way. They've done it in the past, and they will do it in the future, but only if the more direct assaults of humanity upon them and their local environs are not better addressed.

Black, N.A., Voellmy, R. and Szmant, A.M. 1995. Heat shock protein induction in Montastraea faveolata and Aiptasia pallida to elevated temperatures. Biological Bulletin 188: 234-240.

Brown, B.E., Ambarsari, I., Warner, M.E., Fitt, W.K., Dunne, R.P. and Gibb, S.W. 1999. Cummings DG Diurnal changes in photochemical efficiency and xanthophyll concentrations in shallow water reef corals: evidence for photoinhibition and photoprotection. Coral Reefs 18: 99-105.

Downs, C.A., Mueller, E., Phillips, S., Fauth, J.E. and Woodley, C.M. 2000. A molecular biomarker system for assessing the health of coral (Montastraea faveolata) during heat stress. Marine Biotechnology 2: 533-544.

Grottoli, A.G., Rodrigues, I.J. and Juarez, C. 2004. Lipids and stable carbon isotopes in two species of Hawaiian corals, Porites compressa and Montipora verrucosa, following a bleaching event. Marine Biology 145: 621-631.

Grotolli, A.G., Rodriguez, I.J. and Palardy, J.E. 2006. Heterotrophic plasticity and resilience in bleached corals. Nature 440: 1186-1189.

Gorbunov, M.Y., Kolber, Z.S., Lesser, M.P. and Falkowski, P.G. 2001. Photosynthesis and photoprotection in symbiotic corals. Limnology and Oceanography 46: 75-85.

Lesser, M.P. 1996. Exposure of symbiotic dinoflagellates to elevated temperatures and ultraviolet radiation causes oxidative stress and inhibits photosynthesis. Limnology and Oceanography 41: 271-283.

Lesser, M.P. 2004. Experimental coral reef biology. Journal of Experimental Marine Biology and Ecology 300: 227-252.

Lesser, M.P. 2006. Oxidative stress in marine environments: biochemistry and physiological ecology. Annual Review of Physiology 68: 253-278.

Lesser, M.P. and Farrell, J.H. 2004. Solar radiation increases the damage to both host tissues and algal symbionts of corals exposed to thermal stress. Coral Reefs 23: 367-377.

Porter, J.W., Fitt, W.K., Spero, J.H., Rogers, C.S. and White, M.W. 1989. Bleaching in reef corals: physiological and stable isotopic responses. Proceedings of the National Academy of Sciences USA 86: 9342-9346.

Shick, J.M. and Dunlap, W.C. 2002. Mycosporine-like amino acids and related gadusols: biosynthesis, accumulation, and UV-protective functions in aquatic organisms. Annual Review of Physiology 64: 223-262.

Reviewed 16 September 2009