How does rising atmospheric CO2 affect marine organisms?

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Recovery from Bleaching in Corals
Grottoli, A.G., Rodrigues, L.J. and Palardy, J.E. 2006. Heterotrophic plasticity and resilience in bleached corals. Nature 440: 1186-1189.

As described by the authors, "at elevated seawater temperatures, scleractinian corals lose their endosymbiotic dinoflagellates (zooxanthellae), which renders the colony pale or white in color, giving it a bleached appearance and often resulting in death," from which "some corals are able to recover," although "the mechanisms underlying such resilience are poorly understood."

What was done
In an attempt to better elucidate the mechanisms of coral recovery from bleaching, Grottoli et al. induced bleaching in branches of healthy Porites compressa and Montipora capitata coral colonies from Kaneohe Bay, Hawaii, by exposing them out-of-doors in flow-through filtered-seawater tanks devoid of zooplankton and maintained at an elevated temperature of 30C, while an equal number of coral branches were maintained in identical conditions but at the lower ambient seawater temperature of 27C. Then, "after 30 days," as they describe it, "half of the treatment and control branches were collected for analyses and the remaining branches were returned to the reef to recover in situ at ambient seawater temperatures (27C) and zooplankton concentrations."

What was learned
The researchers' experiment revealed that in P. compressa, total energy reserves and total biomass decreased significantly during bleaching, and continued to decrease during six weeks of recovery, because the bleached and recovering P. compressa corals met their daily metabolic energy requirements by consuming existing energy reserves and were largely dependant on significant inputs of zooxanthellae-derived photosynthetic carbon to recover those reserves. In the case of M. capitata, total energy reserves and biomass also decreased significantly during bleaching; but they were fully replenished after six weeks of recovery, because the bleached and recovering M. capitata corals met more than 100% of their daily metabolic energy requirements by markedly increasing (1) their rates of feeding on zooplankton and (2) the percent contribution of heterotrophically-acquired carbon to their daily respiration (CHAR), whereas P. compressa did not.

What it means
Grottoli et al. say their findings suggest that "coral species with high-CHAR capability during bleaching and recovery ... [1] will be more resilient to bleaching events over the long term, [2] could become the dominant coral species on reefs, and [3] may help to safeguard affected reefs from potential local and global extinction."

Reviewed 9 August 2006