How does rising atmospheric CO2 affect marine organisms?

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Short-Term Heating Helps Corals Survive Greater Long-Term Warming
Oliver, T.A. and Palumbi, S.R. 2011. Do fluctuating temperature environments elevate coral thermal tolerance? Coral Reefs 30: 429-440.

The authors write that "the vast majority of studies that investigate the effects of fluctuating thermal regimes on thermal tolerance have examined daily thermal cycles in which the hottest temperatures lasted on the timescale of hours," and they report that in eight of ten such studies -- performed on taxa as diverse as corals, crustaceans, fish, and amphibians -- "organisms that were acclimated to a daily fluctuating thermal regime showed thermal tolerance or tolerance-relevant gene expression (e.g., heat shock proteins) that was equal to, or greater than, that of organisms acclimated to a thermal regime held constant at the maximum temperature of the fluctuating regime," citing the studies of Hutchison and Ferrance (1970), Otto (1974), Sastry (1979), Thorp and Wineriter (1981), Threader and Houston (1983), Podrabsky and Somero (2004), Schaefer and Ryan (2006) and Putnam et al. (2010).

What was done
Taking advantage of back-reef pools in American Samoa that differ in diurnal thermal variation, Oliver and Palumbi experimentally heat-stressed Acropora hyacinthus corals from a thermally moderate lagoon pool and a more thermally-variable pool that naturally experienced 2- to 3-hour high temperature events during summer low tides, after which they compared coral mortality and photosystem II photochemical efficiency of colony fragments they collected from each of these lagoons that they exposed to either ambient (28.0°C) or elevated (31.5°C) water temperatures.

What was learned
The two researchers report that in the heated treatment, "moderate pool corals showed nearly 50% mortality whether they hosted heat-sensitive or heat-resistant symbionts," while "variable pool corals, all of which hosted heat-resistant symbionts, survived well, showing low mortalities statistically indistinguishable from controls held at ambient temperatures." Also in the heated treatment, they say that "moderate pool corals hosting heat-sensitive algae showed rapid rates of decline in algal photosystem II photochemical efficiency," while those "hosting heat-resistant algae showed intermediate levels of decline." And, as by now might have been expected, they state that "variable pool corals hosting heat-resistant algae showed the least decline."

What it means
Oliver and Palumbi say their results suggest that "previous exposure to an environmentally variable microhabitat adds substantially to coral-algal thermal tolerance, beyond that provided by heat-resistant symbionts alone," indicative of the latent ability of earth's corals to potentially adapt to warmer temperatures than have been believed to be possible in the past, as they -- or if they -- gradually begin to experience recurring daily episodes of greater warmth in a gradually warming world.

Hutchison, V.H. and Ferrance, M.R. 1970. Thermal tolerances of Rana pipiens acclimated to daily temperature cycles. Herpetologica 26: 1-8.

Otto, R. 1974. The effects of acclimation to cyclic thermal regimes on heat tolerance of the western mosquitofish. Transactions of the American Fisheries Society 103: 331-335.

Podrabsky, J. and Somero, G. 2004. Changes in gene expression associated with acclimation to constant temperatures and fluctuating daily temperatures in an annual killifish Austrofundulus limnaeus. Journal of Experimental Biology 207: 2237-2254.

Putnam, H., Edmunds, P. and Fan, T. 2010. Effect of a fluctuating thermal regime on adult and larval reef corals. Invertebrate Biology 129: 199-209.

Sastry, A. 1979. Metabolic adaptation of Cancer irroratus developmental stages to cyclic temperatures. Marine Biology 51: 243-250.

Schaefer, J. and Ryan, A. 2006. Developmental plasticity in the thermal tolerance of zebrafish Danio rerio. Journal of Fish Biology 69: 722-734.

Thorp, J.W. and Wineriter, S.A. 1981. Stress and growth response of juvenile crayfish to rhythmic and arrhythmic temperature fluctuations. Archives of Environmental Contamination and Toxicology 10: 69-77.

Threader, R. and Houston, A.H. 1983. Heat tolerance and resistance in juvenile rainbow trout acclimated to diurnally cycling temperatures. Comparative Biochemistry and Physiology Part A: Physiology 75: 153-155.

Reviewed 20 July 2011