07 Oct 2013:
Nutrient Recycling by Sponges
Is Vital in Sustaining Reefs, Study Says
Sponges are the unsung heroes of coral reefs, helping the vibrant ecosystems thrive in the marine equivalent of a desert, a Dutch team working in the Caribbean has found
. Scientists had long questioned how reefs, some of the most productive communities on earth, were able to survive in low-nutrient tropical seas. Bacteria help recycle some nutrients, but the so-called "microbial loop" can't account for the high rates of carbon, nitrogen, and phosphorous recycling needed to maintain a coral reef, researchers say. Sponges fill that void by drawing in plankton and organic matter expelled by the corals and shedding cells that other reef organisms ingest as food, the researchers report in Science
. The "sponge loop," as the Dutch team calls the process, recycles 10 times more organic material than bacteria do and produces as many nutrients as all other primary producers in a coral reef combined, they say. The researchers measured nutrient cycling rates at reefs off the coast of the island of Curacao, where they found that organic matter sucked in by sponges reached snails, fish, and other reef organisms within two days. One sponge species in particular, Halisarca caerulea
, absorbs up to two-thirds of its weight in carbon daily, but barely grows in size because it sheds so many of its cells to the seafloor for other organisms to ingest.