Near reefs, microbial mix dictated by coral and algae

Coral and seaweed choose the bacteria that float in the waters above tropical reefs, a study of marine microbes suggests.

Microbial ecologist Linda Kelly of San Diego State University and colleagues examined microbes from the water just above 22 coral reefs. The organisms that claim the most real estate on the reef, coral or algae, determine which species of microbe live in the water, the researchers report June 30 in the Proceedings of the National Academy of Sciences.

Scientists have long known that fast-growing seaweed and algae can quickly smother slow-growing corals. But researchers didn’t know that the struggle directly affects bystanders such as ocean bacteria. “It’s not what was expected,” says Nancy Knowlton, a coral reef ecologist at the Smithsonian National Museum of Natural History in Washington, D.C. Most scientists suspected that water quality, especially nutrient concentrations, played the biggest role in determining which microbes live around a reef, she says.

More nutrients churn up from the deep ocean near the equator than elsewhere, making equatorial waters rich in phosphate and inorganic nitrogen. The team thought that reefs farther from the equator might have different microbes than equatorial reefs do.

The researchers tested that prediction in reefs throughout the Line Islands, a string of 11 atolls and coral islands south of Hawaii that straddle about 18 degrees of latitude across the equator. But latitude and nutrient composition played little role in determining the microbial makeup, the researchers found. Instead, coral-dominated reefs had different species of bacteria than did reefs ruled by seaweed, also called fleshy algae. The more coral in the reef, the more Cyanobacteria, Alphaproteobacteria and Firmicutes the team found in the water. Seaweed-covered reefs bathed in Gammaproteobacteria and Betaproteobacteria. Some of those bacteria are potential pathogens that might harm the seaweed-covered reefs’ remaining coral.

Although all of the coral-dominated reefs had roughly the same species of bacteria, the genes those bacteria carried differed based on the water’s nutrient content, the team discovered. Bacteria living in water chock full of nutrients carried a plethora of nutrient-processing genes, such as those involved in fixing nitrogen or in resisting the harmful effects of heavy metals such as cobalt, zinc and cadmium. Microbes in nutrient-poor waters tended to have more genes for photosynthesis. Kelly speculates that these microbes jettison extraneous genes for ones that allow the bacteria to make their own food.

People probably indirectly change reefs’ microbial mixes by fishing, the researchers say. Removing seaweed-munching fish from a reef may allow algae to take over and muck with the microbes. That can set up a vicious cycle in which the bacteria reinforce the change on the reef by making coral sick, Kelly says.