How does rising atmospheric CO2 affect marine organisms?

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The Functioning of Symbiodinium Clade A and B Algae in Giant Sea Anemones
Loram, J.E., Trapido-Rosenthal, G. and Douglas, A.E. 2007. Functional significance of genetically different symbiotic algae Symbiodinium in a coral reef symbiosis. Molecular Ecology 16: 4849-4857.

Dinoflagellate algae of the genus Symbiodinium are functionally diverse symbionts of earth's corals, displaying, in the words of the authors, "variation in thermal tolerance and photosynthetic response to irradiance," which they say "is ecologically significant because high temperature and irradiance are the primary triggers of coral bleaching," which "results in depressed coral growth and calcification rates and, if prolonged, mortality of the colony." These facts are particularly important, because, as the three researchers continue, "recovery from bleaching is, in some cases, accompanied by the replacement of a previously dominant bleaching-susceptible form [of symbiont] by the alternative form that was previously at low abundance in the symbiosis," a phenomenon often referred to as symbiont shuffling.

What was done
Loram et al. studied the giant sea anemone Condylactis gigantea that occurs on the Bermuda platform in symbiosis with algae of Symbiodinium clade A, clade B and clade A + B mixtures, conducting a set of experiments that were designed to determine the fate of photosynthetically-fixed carbon, as well as the proportions of that carbon translocated to the anemone host and the distinctive patterns of its incorporation into host and algal compounds.

What was learned
The three researchers report that the key result of their study was the finding that symbioses of C. gigantea with dinoflagellate algae of Symbiodinium clades A and B "are functionally different," specifically noting that "the incorporation of algal photosynthetic carbon into animal lipids and amino acid pools was significantly higher in symbioses with algae of clade A than of clade B," and they speculate that "the profile of photosynthetic products recovered from the host fraction of C. gigantea suggests that amino acids, lipids and lipid precursors may be more important components of the mobile compounds of clade A than clade B," and that "the mobile compounds from the algae of clade A may be of greater nutritional value than those from the clade B algae in C. gigantea." They also note that "at elevated temperature, the symbioses with algae of clade B fixed carbon at a lower rate," and that "the C. gigantea symbiosis with clade B is bleaching susceptible, responding to elevated temperature by massive algal expulsion."

What it means
Loram et al. conclude that "the recovery of some symbioses after bleaching can involve the replacement of the resident alga by bleaching-resistant Symbiodinium of a different clade," and that "this shift can be adaptive for the host through enhanced resistance to subsequent bleaching stress," so that symbioses comprised of mixed algal symbionts may "be at an advantage in times of rapid global climate change," the potential for which adaptive survival strategy is clearly indicated by the results of their detailed experiments.

Reviewed 16 January 2008