How does rising atmospheric CO2 affect marine organisms?

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Algal Symbiont Fluctuations in Response to Warming
Chen, C.A., Wang, J.-T., Fang, L.-S. and Yang, Y.W.  2005.  Fluctuating algal symbiont communities in Acropora palifera (Schleractinia: Acroporidae) from Taiwan.  Marine Ecology Progress Series 295: 113-121.

How will earth's corals ever escape the mass extinction climate alarmists have decreed will come upon them as the planet warms and rising water temperatures cause them to expel their symbiotic algae in the process of bleaching?  One promising strategy would be to change algal partners via a mechanism described as symbiont shuffling.  This concept is based on the fact that the zooxanthellae that reside within membrane-bound vacuoles in the cells of coral hosts are highly diverse, comprising perhaps hundreds of different species or subspecies, of which several are typically found in each type of coral (Rowan and Powers, 1991).  In light of the great diversity represented by these many species and subspecies, Rowan et al. (1997) have suggested that "coral communities may adjust to climate change by recombining their existing host and symbiont genetic diversities," thereby reducing the amount of damage that might otherwise be expected from subsequent occurrences of the same climate-induced stress.  In fact, Buddemeier and Fautin (1993) have suggested that coral bleaching is actually an adaptive strategy for "shuffling" symbiont genotypes to create associations best adapted to new environmental conditions that challenge the status quo of reef communities.  Kinzie (1999) has likewise suggested that coral bleaching "might not be simply a breakdown of a stable relationship that serves as a symptom of degenerating environmental conditions," but that it "may be part of a mutualistic relationship on a larger temporal scale, wherein the identity of algal symbionts changes in response to a changing environment."  Nevertheless, the question remains: Does the process occur in nature?

What was done
In an attempt to detect real-world symbiont shuffling, Chen et al. studied the seasonal dynamics of Symbiodinium algal phylotypes via bimonthly sampling over an 18-month period of Acropora palifera coral on a reef flat at Tantzel Bay, Kenting National Park, southern Taiwan.

What was learned
Periodic molecular phylotyping revealed two levels of synbiont shuffling in host corals: (1) between Symbiodinium phylotypes C and D, and (2) among different variants within each phylotype.  Furthermore, the most significant changes in symbiont composition occurred at times of significant increases in seawater temperature during late spring/early summer, perhaps as a consequence of enhanced stress experienced at that time.

What it means
Chen et al. note that prior to their study long-term changes in symbiont communities had never been detected, and that their work thus reveals "the first evidence that the symbiont community within coral colonies is dynamic ... involving changes in Symbiodinium phylotypes."  This finding lends ever more credence to the symbiont shuffling concept of dealing with thermal stress and provides hope that earth's corals will indeed be able to weather whatever "global warming storm" nature may throw at them in the years and decades to come.

Buddemeier, R.W. and Fautin, D.G.  1993.  Coral bleaching as an adaptive mechanism.  BioScience 43: 320-326.

Kinzie III, R.A.  1999.  Sex, symbiosis and coral reef communities.  American Zoologist 39: 80-91.

Rowan, R. and Powers, D.  1991.  Molecular genetic identification of symbiotic dinoflagellates (zooxanthellae).  Marine Ecology Progress Series 71: 65-73; 1991.

Rowan, R., Knowlton, N., Baker, A. and Jara, J.  1997.  Landscape ecology of algal symbionts creates variation in episodes of coral bleaching.  Nature 388: 265-269.

Reviewed 30 November 2005