Glowing Trio under the Sea: Nitrogen fixer joins algae inside coral

A Caribbean coral that fluoresces orange appears to be the first ever found to contain a symbiotic microbe that converts elemental nitrogen into a biologically usable form, as bacteria in the roots of bean plants do.

The glow of Montastraea cavernosa doesn’t come from the coral tissue itself but from cyanobacteria that take up residence there, say Michael P. Lesser of the University of New Hampshire in Durham and his colleagues.

Associations between corals and cyanobacteria have been under discussion for a while, says Lesser, and DNA analyses of coral samples have turned up signatures of those microbes. Lesser contends the new paper “is finally hard evidence that there are cyanobacteria living symbiotically in corals.”

The wavelength of the orange glow suggested to the researchers that the tissues contain a pigment called phycoerythrin, which many cyanobacteria produce to capture extra energy for their chlorophyll. The team then identified that pigment in the coral. When phycoerythrin absorbs the portions of sunlight that penetrate to the corals’ depth of at least 45 meters, it releases orange light.

Because many cyanobacteria capture elemental nitrogen dissolved in water, the researchers looked for evidence that the glowing microbes also do so. The team found that these bacteria make an enzyme, called nitrogenase, that’s critical to the nitrogen-fixing process.

The corals were already known to contain live-in algae that photosynthesize and contribute carbohydrates to their hosts, so the newly discovered nitrogen fixer makes the classic partnership a threesome, says Lesser’s team in the Aug. 13 Science.

Several imaging techniques reveal cyanobacteria inside the cells that form the outer layer of the coral polyp, say the researchers. The glowing corals seem to be thriving, so Lesser argues against considering the cyanobacteria a pathogen. The long-known algal partners of corals reside in the coral’s inner body layer.

The presence of a nitrogen fixer in the corals requires some rethinking about the coral-algal partnership, says Lesser. Scientists had proposed that algae don’t disrupt their coral’s host tissue because their growth is limited by a scarcity of nitrogen.

Lesser has a “very provocative finding, but it’s not definitive with regard to nitrogen fixation,” says Ed Carpenter of San Francisco State University. He adds, “You can have nitrogenase without nitrogen fixation.”

Both Carpenter and Douglas Capone of the University of Southern California in Los Angeles note that measurements of the rates of the nitrogenase reaction would have made the findings stronger. Capone ranks the new study’s molecular evidence of nitrogen fixation as “preliminary” but “relatively good.”

Forest Rohwer of San Diego State University says that “while previous studies have suggested that there might be this type of symbiosis, this work is much more complete.” He adds, “Understanding how the various components [of the coral community] work together is essential for understanding how corals live and die.”