Corals just can't win. Global warming, sewage, over-fishing, and other human activities all damage reefs. Now, two of corals' fellow ocean dwellers--algae and bacteria--appear to be in cahoots to destroy coral populations further.
Algae are often found growing where corals once lived. Most researchers supposed that the algae simply moved in once corals were already dead or dying. But a recent study (Science, 24 February 2006) revealed that organic carbon--which often leaks out of some plants and algae--promotes microbial activity that kills coral.
To elucidate a possible connection between algae, microbes, and corals, marine ecologist Jennifer Smith of the University of California, Santa Barbara, and colleagues brought samples of coral and algae back to the lab from the Central Pacific. The team placed corals in tubs, half of which also contained algae. A filter separated the algae from the coral; it was fine enough to block bacteria and viruses, but large enough to allow passage of dissolved compounds.
Within 2 days, all the corals with algae neighbors turned white and died, while all the solo corals survived. Near the dying corals' surface, oxygen levels had plummeted, and the energy molecule ATP spiked--both signs of microbial activity. When the researchers performed the same experiments and added a broad-spectrum antibiotic to the tubs, none of the corals died. "We got the clearest results I've ever seen," says Smith. The organic carbon released by algae appears to be traveling through the filter and fostering bacteria, she says. Bacteria in turn suffocate the corals by using up the dissolved oxygen at their surface.
The researchers repeated their experiments on several different combinations of 10 coral genera and seven algae genera. Over 95% of corals suffered to some degree from being near algae, the team reports online 5 June in Ecology Letters. As more corals die, Smith says, there is more room for algae to settle and grow (algae can't grow effectively over living coral). "It can spin quickly into [a] positive feedback loop," says Smith.
The fact that the same results were observed with various genera of coral and algae "provide[s] an inkling that this could be fairly general" in many ocean habitats, says marine ecologist Drew Harvell of Cornell University in Ithaca, New York. But marine ecologist John Bruno of the University of North Carolina at Chapel Hill questions how often this really happens in coral reefs. "You always have algae next to corals, and corals aren't just dropping dead all the time," he notes.