How does rising atmospheric CO2 affect marine organisms?

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Zooxanthellae Responses to Bleaching in Great Barrier Reef Corals
Van Oppen, M.J.H., Mahiny, A.J. and Done, T.J. 2005. Geographic distribution of zooxanthella types in three coral species on the Great Barrier Reef sampled after the 2002 bleaching event. Coral Reefs 24: 482-487.

What was done
The authors sampled zooxanthellae from three common species of scleractinian corals at 17 sites along a latitudinal and cross-shelf gradient in the central and southern sections of the Great Barrier Reef some four to five months after the major bleaching event of 2002, recording the health status of each colony at the time of its collection and identifying its zooxanthella genotypes, of which there are eight distinct clades (A-H) with clade D being the most heat-tolerant.

What was learned
Van Oppen et al. report "there were no simple correlations between symbiont types and either the level of bleaching of individual colonies or indicators of heat stress at individual sites." However, they say "there was a very high post-bleaching abundance of the heat tolerant symbiont type D in one coral population at the most heat-stressed site."

What it means
With respect to the post-bleaching abundance of clade D zooxanthellae at the high heat-stress site, the Australian researchers say they suspect it was due to "a proliferation in the absolute abundance of clade D within existing colonies that were previously dominated by clade C zooxanthellae," and that in the four to five months before sampling them, "mixed C-D colonies that had bleached but survived may have shifted (shuffling) from C-dominance to D-dominance, and/or C-dominated colonies may have suffered higher mortality during the 2002 bleaching event" and subsequently been repopulated by a predominance of clade D genotypes. In either event, it is clear that "symbiont shuffling," either in anticipation of bleaching or after the fact, represents a viable mechanism by which coral colonies may adapt to rising temperatures and maintain their existence in the face of global warming and the higher base level it provides for periodic El Niņo-driven temperature spikes. For more on this topic, see Coral Reefs (Bleaching - Responses: Symbiont Shuffling) in our Subject Index.

Reviewed 10 May 2006