Bioluminescent bays are among the rarest and most fragile of ecosystems. They form when large numbers of micro­organisms, often dinoflagellates such as Pyrodinium bahamense, congregate in a lagoon with an opening narrow enough to keep the organisms from escaping. The dinoflagellates feed on vitamin B12 produced by red mangrove trees and glow bluish-­green when disturbed by motion of any kind, although scientists have yet to fully understand the phenomenon. Because “bio bays” need very specific conditions to survive, there are only a handful worldwide, and most of the known ones are in the Caribbean.

In 2010 ecologists identified a new bio bay in Puerto Rico’s Humacao Natural Re­serve and are studying it for clues to how best to preserve these ecosystems. The Humacao bio bay formed after the U.S. Army Corps of Engineers built channels to protect near­by towns from flooding. The channels allowed saltwater from the Caribbean to flow into once brackish Humacao lagoons. “Along with the tide, in came a bioluminescent dinoflagellate,” says Ricardo Colón-Rivera, an ecology graduate student at Texas A&M University.

Colón-Rivera and his colleagues have found, to their surprise, that the dinoflagellate responsible for the light show may not be P. bahamense but another organism they have yet to identify. They are also hoping to understand the effects of salinity, rainfall and climate change on bioluminescence so they can help preserve bio bays like the one at Humacao. The Humacao bay came to life “because of a confluence of very unusual events,” says Rusty Feagin, a coastal ecologist at Texas A&M. “We need to understand and protect it—before its lights go out.”

This article was published in print as "Bright Microbes."