By Gail Gallessich

Bacteria and algae are combining to kill coral—and human activities are compounding the problem.

Scientists have discovered an indirect microbial mechanism where algae help bacteria kill coral. Human activities are contributing to the growth of algae on coral reefs, setting the stage for the long-term decline of the coral.

Reporting in last month’s online version of the scientific journal Ecology Letters, scientists described laboratory experiments on coral and algae.

First author Jennifer Smith, a postdoctoral fellow at UC Santa Barbara’s National Center for Ecological Analysis and Synthesis (NCEAS), explained that scientists put algae and coral in chambers of seawater with filters between them. All of the corals with neighboring algae died, while coral without neighboring algae did not die. However, by adding an antibiotic, coral death was prevented even in the presence of algae, showing that bacteria fed by the algae are the agents of coral death. “We are the first to link these processes together,” said Smith.

“Our study shows that bacteria are the front line that kill corals,” she explained. “Algae release sugars, fueling bacterial growth on the corals. These bacteria suffocate the coral by cutting off the supply of oxygen. Once the corals die, this frees more space for more algae to grow. We think this process accelerates the rate of decline in already damaged reef ecosystems.”

“This tightly links the fields of microbiology with coral reef ecology to help guide reef conservation efforts,” said co-author Forest Rohwer, assistant professor of microbiology at San Diego State University.

The research resulted from a Scripps Institution of Oceanography-sponsored expedition to the northern Line Islands, a group of atolls and lightly populated islands about 1,000 miles south of the Hawaiian islands.

Co-author Enric Sala, leader of the Line Islands Expedition and a Scripps faculty member, said, “On certain coral heads I witnessed about half of the coral alive and half dead and covered by fleshy algae. In between the living half and the algae there was a band of sickness and death. What we found is that the algae are enhancing the coral disease.”

The report describes other conditions that put coral reefs at risk. Overfishing reduces the number of fish that graze on algae, thus increasing the amount of algae on the reef. Nutrients from sewage and agricultural run-off fertilize the algae. Warmer water and more intense hurricanes resulting from climate change are also blamed for coral death.

“Anyone who has been to the tropics and has had the experience of diving on a coral reef will not deny their spectacular beauty,” said Smith. “They support numerous species of animals and plants and many species that remain undiscovered to science. These ecosystems are particularly important to humans because they support abundant fisheries—commercial, subsistence, and recreational—and they generate a large tourism industry.”

She added that the reefs themselves protect coastal areas from erosion.