Scientists have learned that coral’s symbiotic algae can scoop up available nitrogen, store the excess in crystal form and slowly feed it to the coral as needed.
Although it has been known for years that the algae serve nitrogen to their coral hosts, it was not known that they could store the nitrogen for later.
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.
The dinoflagellates respond to rich nitrogen water by rapidly storing the nitrogen as crystals of uric acid within its cells. But the dinoflagellates don’t hang onto the nitrogen for long. Starting at about six hours after exposure, the microbes begin translocating nitrogen-rich compounds to the coral host, where the nitrogen is used in specific cellular compartments all over the surface layers of the coral.
This storage and release process helps explain how these corals get through the ups and downs of nitrogen concentrations, says lead researcher Anders Meibom of the École Polytechnique Fédérale de Lausanne. “This gives the coral-algae symbiosis a very efficient way to deal with strong fluctuations in nitrogen availability,” he writes. “When the nitrogen availability suddenly becomes high, the algae can take-up large amounts of nitrogen on a timescale of a few hours, store it into crystals inside the algae cells and then release this stored nitrogen for metabolic processes and growth when the nitrogen levels become normal again.”
The research was published in mBio, the online open-access journal of the American Society for Microbiology.