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The Symbiont Shuffle
Volume 7, Number 33: 18 August 2004

How will earth's corals ever escape the mass extinction that climate alarmists have decreed will come upon them as the planet warms and rising water temperatures bleach the living daylights ? make that the symbiotic algae ? out of them?  One promising strategy would be to change algal partners.

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); and in light of the significant 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, i.e., coral bleaching, 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."

Critical of this concept, Hoegh-Guldberg (1999) has challenged the symbiont shuffling hypothesis on the basis that its key mechanism - the stress-induced replacement of a less-stress-tolerant variety of zooxanthellae by a more-stress-tolerant variety - "has never been observed."  With the recent publication of two important papers in Nature, however, that criticism no longer holds water.

In a paper that provides a firm foundation for the thesis of the previously unobserved process of symbiont shuffling, Rowan (2004) describes how he measured the photosynthetic responses of two zooxanthellae genotypes or clades -- Symbiodinium C and Symbiodinium D -- to increasing water temperature, finding that the photosynthetic prowess of the former decreased at higher temperatures while that of the latter increased.  He then notes that "adaptation to higher temperature in Symbiodinium D can explain why Pocillopora spp. hosting them [in the vicinity of Guam] resist warm-water bleaching whereas corals hosting Symbiodinium C do not," as per his personal experience in that part of the world, and that "it can also explain why Pocillopora spp. living in frequently warm habitats host only Symbiodinium D, and, perhaps, why those living in cooler habitats predominantly host Symbiodinium C," again per his own experience, concluding that these observations "indicate that symbiosis recombination may be one mechanism by which corals adapt, in part, to global warming."

Clinching the concept, is the study of Baker et al. (2004), who "undertook molecular surveys of Symbiodinium in shallow scleractinian corals from five locations in the Indo-Pacific that had been differently affected by the 1997-98 El Niņo-Southern Oscillation (ENSO) bleaching event."  Along the coasts of Panama, they surveyed ecologically dominant corals in the genus Pocillopora before, during and after ENSO bleaching, finding that "colonies containing Symbiodinium in clade D were already common (43%) in 1995 and were unaffected by bleaching in 1997, while colonies containing clade C bleached severely."  Even more importantly, they found that "by 2001, colonies containing clade D had become dominant (63%) on these reefs."

After describing similar observations in the Persian (Arabian) Gulf and the western Indian Ocean along the coast of Kenya, Baker et al. summarize their results by stating they indicate that "corals containing thermally tolerant Symbiodinium in clade D are more abundant on reefs after episodes of severe bleaching and mortality, and that surviving coral symbioses on these reefs more closely resemble those found in high-temperature environments," where clad D predominates.  Hence, they confidently conclude their landmark paper by proposing that the symbiont changes they observed "are a common feature of severe bleaching and mortality events," and by predicting that "these adaptive shifts will increase the resistance of these recovering reefs to future bleaching."

Wonderfully, these two Nature papers not only successfully explain the past, they similarly foretell the future.  And that future bears absolutely no resemblance to what the world's climate alarmists claim it will be, which, of course, should come as no surprise to anyone.

Sherwood, Keith and Craig Idso

References
Baker, A.C., Starger, C.J., McClanahan, T.R. and Glynn, P.W.  2004.  Corals' adaptive response to climate change.  Nature 430: 741.

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

Hoegh-Guldberg, O.  1999.  Climate change, coral bleaching and the future of the world's coral reefs.  Marine and Freshwater Research 50: 839-866.

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

Rowan, R.  2004.  Thermal adaptation in reef coral symbionts.  Nature 430: 742.

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.