A number of studies have clearly delineated the role of bacterial infections in causing coral reef bleaching (Hayes and Goreau, 1998; Ritchie and Smith, 1998). In a study of the coral Oculina patagonica, for example, it was found that bleaching of colonies of this coral along the Mediterranean coast has its origin in bacterial infection by the bacterium Vibrio AK-1 (Kushmaro et al., 1997) and that warmer temperatures may lower the resistance of the coral to infection and/or increase the virulence of this bacterium. In subsequent studies of the same coral and bacterium, Toren et al. (1998) and Kushmaro et al. (1998) further demonstrated that this high temperature effect may operate by enhancing the ability of the bacterium to adhere to the coral.
In discussing their findings, Kushmaro et al. (1998) comment on the "speculation that increased seawater temperature, resulting from global warming or El Niño events, is the direct cause of coral bleaching." In contradiction of this presumption, they cite several studies of coral bleaching events that were not associated with any major sea surface temperature anomalies. Hence, they state "it is not yet possible to determine conclusively that bleaching episodes and the consequent damage to reefs is due to global climate change." Likewise, Toren et al. (1998) note that the extensive bleaching that occurred on the Great Barrier Reef during the summer of 1982 was also not associated with any major seawater temperature increase. Additionally, they state that several authors have reported on the patchy spatial distribution and spreading nature of coral bleaching, which they correctly note is inconsistent with the global-warming-induced coral bleaching hypothesis. Instead, the answer may rest in the dynamics of host-pathogen interactions that appear to be bringing marine life into contact with diseases to which they are not normally exposed.
Such has also been the thinking of scientists in explaining the general history of coral reef decline throughout the Caribbean over the past quarter century (Harvell et al., 1999). According to Shinn et al. (2000), coincidental with the decline of Caribbean coral reefs over the past 25 years, there has been a "sharp increase in the transport of African dust to the western Atlantic." Of particular note is their statement that "Caribbean-wide mortalities of acroporid corals and … coral bleaching beginning in 1987, correlate with the years of maximum dust flux into the Caribbean." Hence, the authors put forward the hypothesis that the influx of dust and its attendant bacteria, spores and viruses has been partially to blame for much of the declining health of Caribbean corals, including that manifest as coral bleaching, over the past 25 years.
In conclusion, there is considerable evidence to suggest that disease plays an important role in coral bleaching and that recent bleaching events often ascribed to global warming may actually have been caused, or accelerated, by opportunistic infections.
References
Harvell, C.D., Kim, K., Burkholder, J.M., Colwell, R.R., Epstein, P.R., Grimes, D.J., Hofmann, E.E., Lipp, E.K., Osterhaus, A.D.M.E., Overstreet, R.M., Porter, J.W., Smith, G.W. and Vasta, G.R. 1999. Emerging marine diseases - climate links and anthropogenic factors. Science 285: 1505-1510.
Hayes, R.L. and Goreau, N.I. 1998. The significance of emerging diseases in the tropical coral reef ecosystem. Revista de Biologia Tropical 46 Supl. 5: 173-185.
Kushmaro, A., Loya, Y., Fine, M. and Rosenberg, E. 1996. Bacterial infection and coral bleaching. Nature 380: 396.
Kushmaro, A., Rosenberg, E., Fine, M. and Loya, Y. 1997. Bleaching of the coral Oculina patagonica by Vibrio AK-1. Marine Ecology Progress Series 147: 159-165.
Kushmaro, A., Rosenberg, E., Fine, M., Ben Haim, Y. and Loya, Y. 1998. Effect of temperature on bleaching of the coral Oculina patagonica by Vibrio AK-1. Marine Ecology Progress Series 171: 131-137.
Ritchie, K.B. and Smith, G.W. 1998. Type II white-band disease. Revista de Biologia Tropical 46: 199-203.
Shinn, E.A., Smith, G.W., Prospero, J.M., Betzer, P., Hayes, M.L., Garrison, V. and Barber, R.T. 2000. African dust and the demise of Caribbean coral reefs. Geophysical Research Letters 27: 3029-3032.
Toren, A., Landau, L., Kushmaro, A., Loya, Y. and Rosenberg, E. 1998. Effect of temperature on adhesion of Vibrio Strain AK-1 to Oculina patagonica and on coral bleaching. Applied and Environmental Microbiology 64: 1379-1384.