While global warming is fatal to many reefs, some corals are able to fight the heat

However, she noted in presenting a paper at the annual meeting of the American Association for the Advancement of Science in San Francisco, Feb. 18, Caribbean gorgonian sea fan corals show surprising warm-weather tenacity — they not only are somewhat temperature resilient but can also boost their cellular and enzymatic defenses to fight lethal microorganisms as temperatures rise. These abilities may someday be harnessed to help protect other fragile coral reefs, Harvell said.

The head of the Coral Disease Working Group of the Global Environmental Facility Coral Reef Targeted Research Program, Harvell describes these latest findings in detail in the March issue of the journal Oceanography. The working group is an international collaboration with stations in the Caribbean, Philippines, Australia and East Africa that seeks to understand how rising ocean temperatures (which are monitored with high-resolution satellite detectors) are related to lethal coral reef epidemics.

The warm temperatures that have been occurring worldwide as a result of global warming appear to be facilitating fatal epidemics in coral reefs across the globe, according to Harvell and her co-researchers. For example, in 2005, the Caribbean experienced unusually warm weather and a spate of coral deaths caused directly by temperature stress and indirectly by opportunistic infections contracted during the warm period.

“Below the waves, the warm temperatures triggered a series of events lethal to coral reefs: massive loss of critical symbionts, compromised immunity and ability to fight pathogens, and increased susceptibility to lethal infectious microorganisms,” reported Harvell.

However, the precise connection between warmer waters and coral disease is unclear. “It is difficult to pinpoint yet how temperature stress decreases the immune response,” said Harvell. “Although corals are near the base of the evolutionary tree, they are complicated animals.”

The complexities arise from different levels of symbiosis within a coral, both with bacteria on the surface and algae in the interior. Moreover, the warmer waters cause increased virulence of some pathogens as well as a diminished ability of corals to fight them. But these complications can also be keys to coral reef survival.

“A first line of defense is provided by surface microorganisms … [that] produce antibiotics that can help corals fight pathogenic bacteria,” reported Harvell.

However, not all corals are able to produce such defenses under thermal stress. “Some corals show unusual resilience to the double threat [of pathogens and warming seas], and we should look to these hardy corals for clues that might enhance others,” said Harvell. “Other corals are more sensitive and will not survive the continued warming trend.”

Yet there seems to be future promise for boosting the immunity of coral reefs, Harvell reported. Collaborators Rotem Effrony and Eugene Rosenberg in Tel Aviv, Israel, have isolated viruses that can consume several bacteria that are deadly to the more sensitive corals.

Graduate student Anne Poduska is a writer intern at the Cornell Chronicle.