Coral reefs are among the richest and most diverse habitats on earth. Coral are members of the phylum cnidaria, which also comprises jellyfish and sea anemones, and they live in symbiosis with the alga Symbiodinium zooxanthellae. In return for housing, these algae supply chemical energy to meet the needs of the reefs, which are stationary herds of animals, some of which live for hundreds of years.
This stability can be lost, however, when YBD strikes. YBD, or yellow blotch disease, produces characteristic pale yellow lesions and eventually kills symbiotic algae by impairing mitosis and damaging the photosynthetic apparatus. This condition now affects coral reefs throughout the Caribbean along with a rising percentage of reefs in the Pacific, including near Southeast Asia, says Cervino, who has tracked coral pathology for two decades. In controlled experiments in aquaria, the consortium of vibrios causes YBD in the Caribbean coral genus Montastraea, which is a major coastal reefbuilder, he says. Single Vibrio species are far less virulent than are clusters of such species.
Although YBD infections start at ambient temperatures, they spread with increasing efficiency as temperatures rise from 29-33°C, according to Cervino. Despite the importance of YBD, however, thermal stress remains the number one killer of coral through damage inflicted when algae, in response to rising temperatures are expelled from corals, bleaching and killing them, he says. In addition to heat stress and pathogen-inflicted damage, coral is sensitive to, and YBD is exacerbated by, two- to five-fold increases in nitrogen and phosphate levels. Those nutrients also exacerbate Aspergillus infections of coral. "If global warming-induced thermal stress and YBD infections continue to proliferate in the tropics, this will contribute significantly to the collapse of the remaining reef habitats in the next decade," he and his collaborators warn.
Excesses of nonsymbiotic algae, a condition aggravated by overfishing, also can damage and kill coral, according to Eugene Rosenberg of Tel Aviv University in Israel. With the impact of overfishing in mind, Cervino plans to test whether microorganisms that grow in aquaculture pens also contribute to the progress of YBD when coral is exposed experimentally to Vibrio consortia.
Cervino's findings are "important in the field because there has been a great deal of debate between microbiologists, who believe that bacteria are largely causing bleaching and eventual death in corals, and marine biologists, who believe that abiotic factors such as temperature, UV, and pollution cause coral bleaching," says Rebecca Case of the Harvard University Center for the Environment, who was not involved in the research.