Coral are nothing to sneeze at when it comes to generating mucus. One species common in Australia's Great Barrier Reef, for example, can exude up to 4.8 liters of mucus per square meter of reef a day. Scientists knew that the goo keeps the coral from drying out and protects them from infection, but now a research team proposes that the slime doesn't just help the coral. It helps the entire reef ecosystem save energy by recycling nutrients.
Coral reefs are diverse communities, but the tough skeleton that we think of as coral is created by calcareous coral polyps, critters closely related to sea anemones. Many corals form a partnership with photosynthetic algae called zooxanthellae; they live in the bony skeleton and pay rent by converting sunlight into carbon compounds that the coral can use for energy. But corals excrete as mucus up to half of the carbon made by their tenants. Scientists puzzled over why the coral would squander so much of this valuable resource.
To investigate, a team of researchers tracked the fate of mucus off Heron Island, part of the Great Barrier Reef. They found that mucus does much more than clear the coral's sinuses--it keeps nutrients and energy in the reef ecosystem. Some of the mucus dissolves near the reef and is recycled by floating microorganisms or by critters in the sediment. Large mats of undissolved slime--"mucus floats"--trap suspended particles and microorganisms before drifting into nearby lagoons, where they provide a rich source of nutrients for bacteria and organisms in the mucky bottom, the researchers report in the 4 March issue of Nature.
Previous work focused on pieces of the mucus story, says team member Christian Wild of the Max Planck Institute for Marine Microbiology in Bremen, Germany, but this study is the first to put them all together. Well, maybe not all, says Gisele Muller-Parker of Western Washington University in Bellingham, Washington. Although showing that mucus from the reef helps maintain life in the lagoons is "really novel and neat," she says, the recycling role of fish and similar-sized animals that eat mucus and contribute waste to the reef remains murky. "These guys kind of ignore the large critters," she adds.