How does rising atmospheric CO2 affect marine organisms?

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The Potential for "Symbiont Shuffling" in Corals
Mieog, J.C., van Oppen, M.J.H., Cantin, N.E., Stam, W.T. and Olsen, J.L. 2007. Real-time PCR reveals a high incidence of Symbiodinium clade D at low levels in four scleractinian corals across the Great Barrier Reef: implications for symbiont shuffling. Coral Reefs 26: 449-457.

Reef corals form associations with an array of genetically and physiologically distinct algal endosymbionts from the genus Symbiodinium; and it has been proposed that in response to certain environmental stimuli, such as an increase in water temperature that induces bleaching, the relative abundances of these clades may change through a process called symbiont shuffling, which enables the coral-algal association to better withstand, in the case of the example given, still higher water temperatures. "However," as the authors report, "only a minority of coral species have been found to harbor more than one symbiont clade simultaneously, and the current view is that the potential for symbiont shuffling is limited."

What was done
Using a newly developed real-time polymerase chain reaction assay, which they say "is able to detect Symbiodinium clades C and D with >100-fold higher sensitivity compared to conventional techniques," Mieog et al. tested 82 colonies of four common scleractinian corals (Acropora millepora, Acropora tenuis, Stylophora pistillata and Turbinaria reniformis) from eleven different locations on Australia's Great Barrier Reef for evidence of the presence of background Symbiodinium clades.

What was learned
The six researchers determined that "ninety-three percent of the colonies tested were dominated by clade C and 76% of these had a D background," the latter of which symbionts, in their words, "are amongst the most thermo-tolerant types known to date," being found "on reefs that chronically experience unusually high temperatures or that have recently been impacted by bleaching events, suggesting that temperature stress can favor clade D."

What it means
Mieog et al. conclude that the clade D symbiont backgrounds detected in their study can potentially act as safety-parachutes, "allowing corals to become more thermo-tolerant through symbiont shuffling as seawater temperatures rise due to global warming." As a result, they suggest that symbiont shuffling is likely to play a role in the way "corals cope with global warming conditions," leading to new competitive hierarchies and, ultimately, "the coral community assemblages of the future."

Reviewed 16 January 2008