A new study led by a researcher at Scripps Institution of Oceanography at UC San Diego is offering new ideas about ocean pollution and the resilience of the rich microbial communities living with reef-building corals.
An international research team tested the response of coral bacteria to pollution produced by nearby fish farms in the Philippines and published the findings in the October 6 issue of the journal PLoS ONE. The researchers documented dramatic shifts in the normal bacteria communities associated with the corals shortly after the study commenced, but, to their surprise, they also discovered that the communities rebounded to their initial healthy state by the conclusion of the 22-day study period.
While the researchers were excited by the results, they are unclear whether such resilience could be sustained over longer periods.
"We hope that (corals) might be able to deal with a short pulse of pollution if they had to, but it's a pretty big battle for them to sustain over the long term," said Melissa Garren, a Scripps graduate student and lead author of the study.
Knowing that coastal pollution is a major threat to coral reefs-and also that intensive fish farming is a rapidly growing source of such pollution-the authors established an experiment in May 2008 at Bolinao, a town along the northwest coast of Luzon, the Republic of Philippines. During the study they grew Porites cylindrica, a common reef-building coral, in five sites: two areas subjected to high fish farm effluent exposure, two with low exposure, and a control area safely buffered away from effluent.
Within five days, the high-exposure corals experienced dynamic and dramatic shifts at the hands of potentially disease-causing bacteria. Both human and coral pathogens began dominating the coral communities during this period. But by the conclusion of the study, the scientists documented a dramatic reversal as the communities somehow shifted back toward their original state.
"This study reveals fish farms as a likely source of pathogens with the potential to proliferate on corals and an unexpected short-term resilience of coral-associated bacterial communities..." the authors note in their report.
Garren said further investigations are needed to determine whether disease-causing bacteria are weakening the coral's defenses, or perhaps the pollution is opening the door to outside invaders and allowing them to take hold. Similarly, follow-up studies will help determine how the corals were able to weather and rebound from the initial appearance of pathogens.
"The big question is whether they can keep it up over the long term," Garren said. "It could take a toll in resources to continue the battle. Had there been any additional stressors, it may have been more difficult or not possible for the communities to rebound."
The study is the latest advancement in the young and growing field of coral microbiology, which is helping researchers discover more about threats to corals at the basic level. "Discovering limits of resilience to anthropogenic stresses is a critical and urgent goal in coral conservation," said Farooq Azam, a coauthor of the study and a distinguished professor in the Marine Biology Research Division at Scripps. "This study underscores that environmental stress could actually manifest as highly dynamic shifts in the microbial communities that live intimately with the corals. Future studies should integrate both short and long time scale responses of corals to anthropogenic stresses and explicitly including the dynamic roles of microbes."
Garren said the information from the new study and other experiments will give the researchers a basis from which to inform coastal managers and work with fish farmers to minimize damage to corals.
She said the study provides a useful model for studying point-source pollution in other locations and determining how sea bottom communities respond to similar threats.
"If we understand why and how communities become unbalanced, then we can look for ways to help," said Garren.
In addition to Garren and Azam, the study's coauthors include Laurie Raymundo of the University of Guam, James Guest of the National University of Singapore, and C. Drew Harvell of Cornell University.
This research was supported by grants from the Global Environment Facility (GEF) Coral Targeted Research Program, the Gordon and Betty Moore Foundation, the European Commission International Cooperation with Developing Countries REEFRES project, the National Science Foundation (NSF), NSF's Integrative Graduate Education and Research Training (IGERT) program, and an NSF Graduate Research Fellowship.