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Coral Bleaching: Probiotic Hypothesis to the Rescue!
Volume 10, Number 9: 28 February 2007

The authors of an intriguing paper recently published in Environmental Microbiology (Reshef et al., 2006) develop a case for what they call the Coral Probiotic Hypothesis. This concept, in their words, "posits that a dynamic relationship exists between symbiotic microorganisms and environmental conditions which brings about the selection of the most advantageous coral holobiont."

This concept is analogous to the adaptive bleaching hypothesis of Buddemeier and Fautin (1993), or what we and many others refer to as symbiont shuffling, wherein corals exposed to some type of stress - such as that induced by exposure to unusually high water temperatures - "first lose their dinoflagellate symbionts (bleach)," in the words of Reshef et al., "and then regain a new mixture of zooxanthellae that are better suited to the stress conditions." In fact, the two phenomena work in precisely the same way, in one case by the corals rearranging their zooxanthellae populations and in the other case by the corals rearranging their bacterial populations.

In seeking evidence for their Coral Probiotic Hypothesis, the team of Israeli researchers concentrated their efforts on looking for examples of corals developing resistance to emerging diseases. This approach makes sense, because corals lack an adaptive immune system, i.e., they possess no antibodies (Nair et al., 2005), and they therefore can protect themselves against specific diseases in no other way than to adjust the relative sizes of the diverse bacterial populations associated with their mucus and tissues so as to promote the growth of those types of bacteria that tend to mitigate most effectively against the specific disease that happens to be troubling them.

Reshef et al. begin by describing the discovery that bleaching of Oculina patagonica corals in the Mediterranean Sea was caused by the bacterium Vibrio shiloi, together with the finding that bleaching of Pocillopora damicornis corals in the Indian Ocean and Red Sea was the result of an infection with Vibrio coralliilyticus. But they then report that (1) "during the last two years O. patagonica has developed resistance to the infection by V. shiloi," that (2) "V. shiloi can no longer be found on the corals," and that (3) "V. shiloi that previously infected corals are unable to infect the existing corals." In fact, they say that "by some unknown mechanism, the coral is now able to lyse the intracellular V. shiloi and avoid the disease," and because corals lack the ability to produce antibodies and have no adaptive immune system, the only logical conclusion to be drawn from these observations is that the coral probiotic phenomenon, as described by Reshef et al., must be what produced the welcome results.

With respect to the future of earth's corals within the context of global warming, the Israeli scientists note that "Hoegh-Guldberg (1999, 2004) has predicted that coral reefs will have only remnant populations of reef-building corals by the middle of this century," based on "the assumption that corals can not adapt rapidly enough to the predicted temperatures in order to survive." However, they report that considerable evidence has been collected in support of the adaptive bleaching hypothesis; and they emphasize that the hundreds of different bacterial species associated with corals "give the coral holobiont an enormous [our italics] genetic potential to adapt rapidly [our italics] to changing environmental conditions." In fact, they say "it is not unreasonable to predict that under appropriate selection conditions, the change could take place in days or weeks, rather than decades required for classical Darwinian mutation and selection," and that "these rapid changes may allow the coral holobiont to use nutrients more efficiently, prevent colonization by specific pathogens and avoid death during bleaching by providing carbon and energy from photosynthetic prokaryotes," of which they say there is "a metabolically active, diverse pool" in most every coral.

Sherwood, Keith and Craig Idso

Buddemeier, R.W. and Fautin, D.G. 1993. Coral bleaching as an adaptive mechanism - a testable hypothesis. 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.

Hoegh-Guldberg, O. 2004. Coral reefs in a century of rapid environmental change. Symbiosis 37: 1-31.

Nair, S.V., Del Valle, H., Gross, P.S., Terwilliger, D.P. and Smith, L.C. 2005. Macroarray analysis of coelomocyte gene expression in response to LPS in the sea urchin. Identification of unexpected immune diversity in an invertebrate. Physiological Genomics 22: 33-47.

Reshef, L., Koren, O., Loya, Y., Zilber-Rosenberg, I. and Rosenberg, E. 2006. The coral probiotic hypothesis. Environmental Microbiology 8: 2068-2073.