How does rising atmospheric CO2 affect marine organisms?

Click to locate material archived on our website by topic

Coral Bleaching: Is It Physically or Biologically Driven?
Volume 8, Number 8: 23 February 2005

Coral bleaching or loss of endosymbiotic zooxanthellae is routinely ascribed to high water temperature, because it typically occurs during periods of maximum warmth in each hemisphere's summer season; and because of that fact, the climate-alarmist prescription for preventing coral bleaching is to vastly curtail the burning of fossil fuels, which process pumps CO2 into the air and, according to them, dramatically increases surface air temperature by enhancing the greenhouse effect of the atmosphere. It is possible, however, that a much more effective - not to say realistic - solution to the problem may eventually be devised, based upon a more complete understanding of the bleaching process.

The conceptual basis for obtaining such an understanding was recently developed by Rosenberg and Falkovitz (2004) in a review of what is known about the association between Oculina patagonica, a scleractinian coral of the Southwest Atlantic and Mediterranean Sea, and Vibrio shiloi, a gram-negative, motile, rod-shaped bacterium that has been demonstrated to be the causative agent of bleaching in this particular coral.

The two Tel Aviv University microbiologists begin unfolding their vision by noting that "several critical V. shiloi virulence factors are produced only at ... elevated summer water temperatures," which suggests, in their words, that "the primary effect of temperature is on the pathogen, not the host."

The first of these virulence factors is adhesion. At the low temperatures characteristic of winter, V. shiloi does not adhere to O. patagonica, irrespective of the temperature at which the coral has been maintained. When the bacteria have been grown at the high temperatures characteristic of summer, however, the authors report they have typically "adhered avidly to corals maintained at either low or high seawater temperatures."

The second factor is toxin. As described by Rosenberg and Falkovitz, "V. shiloi produces extracellular toxins that block photosynthesis, and bleach and lyse zooxanthellae." The particular toxin that blocks photosynthesis, called toxin P, is a proline-rich peptide that binds irreversibly to algal membranes; and, in their words, "the level of toxin P produced at 29C is 10-fold higher than that produced at 16C."

The third factor is the inability of V. shiloi to produce superoxide dismutase (SOD) at low temperatures. As Rosenberg and Falkovitz describe it, the "high concentration of oxygen and resulting oxygen radicals produced by the zooxanthellae during photosynthesis is highly toxic to bacteria and is one mechanism by which corals resist infection." At high temperatures, however, they report that "V. shiloi produces a potent SOD that helps it to survive in coral tissue." Consequently, lacking this potent SOD at low temperatures, the bacteria typically die during the winter and the coral rebounds the following spring.

So where does V. shiloi reside over winter, since it cannot survive in O. patagonica? The authors report that it takes up residence in the marine fireworm Hermodice carunculata, which also serves as a vector for transmitting the bleaching disease to the coral once the water has warmed in the spring and summer.

Progressing to a consideration of all corals, Rosenberg and Falkovitz present evidence that suggests there may be other bacteria-coral relationships that are analogous to that which exists between V. shiloi and O. patagonica. In addition, they note that "the random mosaic patterns of bleaching observed in coral colonies are difficult to attribute solely to environmental stress, since neighboring regions of the colony must be exposed to the same extrinsic conditions." Even more significant, in their opinion, is the fact that "the spreading nature of patchy coral bleaching is highly symptomatic of an infectious disease."

But, the world's climate alarmists may ask, does it really make any difference what is the true proximate cause of coral bleaching, as long as bleaching is facilitated by high water temperatures? Rosenberg and Falkovitz appear to think that it does, as they say that "in those cases where bleaching is the result of an infection, it should be possible to apply the vast experience that has been gained in studying infections diseases of other animals, plants, and humans."

We concur in this assessment, believing this course of action to be far more likely to produce positive results, and in a truly realistic time frame, than is the climate-alarmist proposal to try to change the climate of the planet via a plan that likely will have an imperceptible impact on mean global temperature, even a century into the future.

Sherwood, Keith and Craig Idso

Rosenberg, E. and Falkovitz, L. 2004. The Vibrio shiloi/Oculina patagonica model system of coral bleaching. Annual Review of Microbiology 58: 143-159.