One coral alga explodes with temperature increase

Unlike its neighbors, this rare species thrives in warmer waters

When Caribbean coral reefs are in hot water, one alga takes advantage of the situation — and possibly comes to the rescue.

SOME LIKE IT HOT The heat-tolerant alga Symbiodinium trenchi, a rare species in coral reefs in the Caribbean Sea, had a population boom when the water temperature rose in 2005. The alga inhabited both bleached (shown center) and unbleached (far right) corals. LaJeunesse et al./Proceedings of the Royal Society B 2009.

A rare type of alga proliferated in several species of coral in the Caribbean Sea while warming waters were killing other algal inhabitants, researchers report in the Sept. 8 Proceedings of the Royal Society B. The authors say that their two-year study raises questions about how opportunistic algae might affect coral colonies dealing with the physical stress of increasing ocean temperatures.

“I think it’s a pretty compelling paper,” says coral reef biologist Andrew Baker of the University of Miami.

Many coral species rely on particular algae for nourishment. These one-celled organisms squat in the coral polyps’ cells, making energy from sunlight and passing it on.

A temperature increase of just a few degrees can kill these beneficial algae, bleaching the coral and starving it in the process, says coauthor Todd LaJeunesse of Pennsylvania State University in University Park. But a few years ago scientists discovered that some species of algae thrive in warmer waters, taking over the dying coral and potentially protecting it from starvation (SN: 8/28/04, p. 142)

Now researchers have found one species that begins proliferating before the coral is bleached. During a period of unusually warm water near Barbados in 2005, LaJeunesse and his colleagues found that Symbiodinium trenchi, a rare alga species normally present in about one percent of the team’s samples, appeared in much higher concentrations than usual in some coral types. The authors say the alga’s early proliferation could mean the species is able to save some types of coral from bleaching and may be useful as an indicator of stress in a reef.

By monitoring the colonies, the scientists found that the heat-tolerant S. trenchi thrived while more heat-sensitive algae died back. When the team tested samples six months after the ocean had returned to its normal temperature, the rare alga had become the dominant algae in eight different coral species. The most bleached coral also had the highest concentrations of S. trenchi, suggesting that more stressed colonies were more accessible to the opportunistic algae.

But two years after the ocean’s hot flash, the S. trenchi population had shrunk again, and the more typical algae populations had returned.

Baker says the rare alga’s population boom likely helps keep the corals alive and offers hope that some corals may be able to survive warming waters, at least in the short term.

But LaJeunesse says it is not yet clear if and how the alga benefits these coral species. The scientists didn’t expect S. trenchi to take over before the corals were bleached and were surprised that typical populations returned when the water cooled back down. These findings raise questions about how heat-loving algae interact with corals, he says.

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