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Coral reefs have come under siege in recent decades. A variety of threats from acidification and bleaching to pollution and overfishing contribute to a worsening outlook. Now, a new study reveals yet another overlooked threat: a surprisingly common bacterial parasite.

The bacterium is the first member of a newly identified genus, and was discovered during a study of the Caribbean staghorn coral microbiome by Oregon State University (OSU) molecular biologist Rebecca Vega Thurber and her colleagues.

Vega Thurber and her team were the first to sequence the entire genome of this newly characterized bacterium. They then compared the genomic data they obtained with data housed in several collaborative databases, the result of work by a global pool of researchers studying various aquatic microbiomes.

Using sophisticated computer programs and search algorithms to query the databases and map genetic associations, the researchers discovered that the newly named bacterium, Candidatus Aquarickettsia rohweri—as well as other closely related members of the newfound genus, Candidatus Aquarickettsia—are commonly found in the microbiomes of sponges, corals, and other aquatic organisms worldwide.

“The primary thing we want people to take away is that this bacterium is a parasite. We know that it causes the coral to not be able to grow, but only under specific conditions,” Vega Thurber says.

At low levels, Ca. A. rohweri seems to be harmless, but when it proliferates in a nutrient-rich environment—such as fertilizer-polluted water—corals suffer. The parasite steals energy from corals. Analysis shows that the bacterium carries a gene, Tlc1, which allows it to rob coral cells of adenosine triphosphate (ATP), their basic energy source.

Vega Thurber thinks the parasites kill corals in one of two ways: “They directly sap the coral of its nutrition and resources; or alternatively, make it susceptible to additional pathogens.” Whether the bacterium kills the coral on its own or compromises its immunity to other diseases, it’s clear that Ca. A. rohweri, when found in abundance, is a deadly threat.

Stuart Sandin, a coral reef ecologist at Scripps Institution of Oceanography at the University of California San Diego, remarked on the study: “It really highlights this parasitic microbe isn’t unique. It’s found everywhere, so it’s something we should be paying attention to.”

Sandin was also impressed with the approach used in this study. Grace Klinges, a graduate student at OSU and the study’s lead author, wants to apply these same methods to future efforts, including modeling the evolution of the bacterium throughout the Caribbean and studying similar bacteria in the Candidatus Aquarickettsia genus.

Klinges believes that analyzing genomic differences between bacteria is key to understanding how they affect the health of corals. She also thinks these new techniques could help unravel other microbial mysteries—for instance, the culprit behind an ongoing outbreak of coral disease in the Caribbean.

“They have no idea what the pathogen is yet, so the methods we’ve used to characterize this bacterium could be used to figure out what that bacterium is,” she explains.

The insights and techniques of this study may aid in the fight to save corals, though ultimately these findings underscore the susceptibility of corals to nutrient pollution.

“I think at the big picture level, this is about local water management,” Vega Thurber says. “But without efforts to control climate change, it’s not going to matter.”