Microscopic algae that live within reef-forming corals scoop up available nitrogen, store the excess in crystal form, and slowly feed it to the coral as needed, according to a study published in mBio, the online open-access journal of the American Society for Microbiology. Scientists have known for years that these symbiotic microorganisms serve up nitrogen to their coral hosts, but this new study sheds light on the dynamics of the process and reveals that the algae have the ability to store excess nitrogen, a capability that could help corals cope in their chronically low-nitrogen environment.
"It was a great surprise to find the nitrogen-rich crystals inside the algae," says corresponding author Anders Meibom of the cole Polytechnique Fdrale de Lausanne, Switzerland. "It all makes perfect sense now. The algae suck up the ammonium and nitrate like a sponge when the concentration of these molecules increases, then store this nitrogen as uric acid crystals for later use."
Like all reef-forming corals, the species they studied, Pocillopora damicornis, is actually a symbiosis of two different organisms: the coral provides protection to a species of photosynthetic algae called dinoflagellates, which, in turn, provide sugars and nitrogen to the coral host. The symbiosis allows the coral to thrive in clear, tropical waters that are naturally nutrient-poor. In many places, however, coral reefs are suffering from an excess of nutrients - pollution from sewage and fertilizers that impacts the symbiotic relationship and the health of coral in unknown ways.
To better understand these exchanges of materials and to determine how an excess of nutrients might affect the balance, the researchers exposed pieces of coral to varying concentrations of isotopically-labeled nitrogen-rich compounds. Using the facilities at the Aquarium Tropicale Porte Dore in Paris, France, the scientists applied a relatively new analytic technique called
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American Society for Microbiology