The two scientists from the University of Kansas (Lawrence, Kansas, USA) begin their review by stating that "the adaptive bleaching hypothesis (ABH) may be concisely defined as the proposal that the loss of photosymbionts has the potential to allow some representatives of the host species to re-establish a symbiosis with a different dominant alga, resulting in a new holobiont (also referred to as ecospecies or host-symbiont unit) that is better suited to the altered environmental circumstances," further noting that "such a re-shuffling has the potential to enhance long-term survival of the hosts, helping to reconcile the long-recognized paradox that coral reefs seem sensitive to environmental perturbation in the short term, but robust over geological time."

Fautin and Buddemeier next proceed to enunciate five fundamental assumptions that underlie the ABH hypothesis, and they review the scientific literature pertaining to each of them.  At the conclusion of this analysis, they find, in their words, that "virtually all evidence adduced thus far is consistent with, much supportive of, and none fatal to, the ABH."  Indeed, they say "there seems to be a virtually continual opportunity for creation of new holobionts upon which selection can act and thus provide prompt, dynamic response of the symbioses to changing environmental conditions," noting that "it now appears that adaptive change in the dominant symbiont can occur over the entire range from normal background fluctuations in algal populations to acute bleaching events."

With respect to the latter phenomenon of severe bleaching -- which can indeed be induced by rising temperatures, especially within the context of reduced reef vitality owing to the many local assaults of humanity upon the watery environment in which corals live (see our Editorials of 1 Jan 1999, 19 Sep 2001, 23 Jan 2002, 6 Mar 2002 and 26 Mar 2003) -- Fautin and Buddemeier report that bleaching may "(1) accelerate the process of change, and/or (2) change the set of possible trajectories for how these communities might recover by dramatically changing the starting abundance of each symbiont type and thereby shifting the system out of a region of local stability (where symbiont community structure tends to return to the same equilibrium after minor disturbance) into a region of relative instability (where many more recovery outcomes are possible)."

In closing, the marine biologists state that much additional research is needed to clarify the detailed mechanisms of the adaptive bleaching hypothesis, citing the conclusion of Baker (2002): "the real question is not whether coral-algal associations can adapt by recombining, but rather how, and over what timescales, they do so."  This is the area in which research dollars need to be invested, alongside funds earmarked for programs to reduce local threats to coral health throughout the world.  The channeling of hard-to-come-by funds into various fairy-tale schemes that are claimed to be capable of changing the temperature trajectory of the entire planet (but that will only squander unimaginable amounts of money and not achieve their goal in any event) is to insure that we will indeed lose these incomparable ecosystems, not to CO2-induced global warming, but to everyday run-of-the-mill human activities that adversely impact nearby coral reefs.

Sherwood, Keith and Craig Idso

References
Baker, A.C.  2002.  Is coral bleaching really adaptive?  Nature 415: 602.

Fautin, D.G. and Buddemeier, R.W.  2004.  Adaptive bleaching: a general phenomenon.  Hydrobiologia 530/531: 459-467.