Researchers study coral impacts by pollution off Bolinao, Philippines.
Image: James Guest
Study Describes Surprising Coral Resilience
Scripps Institution of Oceanography/University of California, San Diego
Eye-opening finding about coral bacteria leads to new questions about the long-term effects of ocean
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
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
“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
— Mario Aguilera
October 8, 2009