Bacterial Communities Found Living Deep Inside Corals
July 9, 2013

Bacterial Communities Found Living Deep Inside Corals

April Flowers for - Your Universe Online

A new study by researchers at Woods Hole Oceanographic Institution (WHOI) and King Abdullah University of Science and Technology (KAUST) reveals corals may let certain bacteria get under their skin.

The study, soon to be published in Applied and Environmental Microbiology, offers the first direct evidence that Stylophora pistillata, a species of reef-building coral found throughout the Indian and west Pacific Oceans, harbors bacterial denizens deep within its tissues.

"We have evidence that other species of coral also host these bacteria, and that they may play an important role in keeping a coral healthy," says Amy Apprill, a WHOI assistant scientist.

For decades, researchers have known corals don't like to live alone. For example, reef building corals are known to have symbiotic, or mutually beneficial, relationships with single-celled algae. Recent studies have revealed evidence suggesting bacteria, fungi, and viruses are also part of the mix -- especially a group of bacteria called Endozoicomonas, which has been associated with a number of coral species around the world. Scientists have been unable, however, to pinpoint exactly where Endozoicomonas lives -- in the coral's tissues or on its surface layer -- or what it does there.

The research team was able to gain access to the pristine coral reef colonies of the Red Sea, where they used DNA-based techniques to uncover an abundance of Endozoicomonas genes associated with the coral Stylophora pistillata. A DNA "probe" -- a fragment of DNA designed to fit into the bacterium's genetic code like a puzzle piece -- was created to light up when it connected with Endozoicomonas genes. The researchers were guided by the probe's fluorescence to spot Endozoicomonas living deep within the coral's tissue.

"These weren't single cells--they were living together in a clump, like a bunch of grapes on a stem," says Apprill. "That was pretty exciting, because we had not thought about them living like this before."

Scientists had previously linked Endozoicomonas bacteria to coral colonies throughout the world's oceans, as well as to some species of sponges and sea slugs. This study, however, is the first to directly reveal the bacteria living within any marine animal. The real mystery, say the research team, is what it is doing there.

"When we look at healthy corals, we see these really well-established microbial relationships," says Apprill.

KAUST Assistant Professor Christian Voolstra adds, "Endozoicomonas make up a good portion of the bacterial biomass which further tells us that they must be doing something important."

The next task, according to the team, is to figure out what they're doing -- why the coral lets them in at all -- to understand how they benefit the coral.

The bacteria may enable the coral to recycle nutrients in order to stay healthy, Apprill suspects. The team is currently working on designing new experiments to determine how the coral's relationship with its bacterial companions works.

Apprill, who plans to draw from studies of the human bacterial microbiome to explore the role of the coral's bacterial communities, says this will be a difficult task. Searching the Endozoicomonas group's genetic code for bits of DNA that are associated with particular functions in other bacteria, as well as looking at other coral species to see if the bacteria lives inside them will be the focus of future studies.

The relationship of the bacteria with Stylophora pistillata and other reef-building species may prove to be a critical point for scientists to understand as corals face a growing number of threats to their health and survival.

"Corals are highly susceptible to the impacts of climate change, coastal development and overfishing," says Apprill. "In order for scientists to predict the future success of corals, we need to understand their basic biology, including how their microorganisms may aid in keeping them healthy."

In addition to shedding light on coral's symbiosis, the study is strengthening the multi-year partnership between WHOI and KAUST. The team from WHOI worked closely with a number of KAUST researchers to gather Stylophora pistillata samples, to extract and to analyze the genetic data it contained.

"This study wouldn't have been possible without this collaboration," says Apprill. "Working with people from different institutions who think differently from you leads you to think about things in new ways, and in this case to make new discoveries."