Symbiotic algae that live within reef-forming corals scoop up available nitrogen, store the excess in the form or uric acid crystals, and slowly feed it to the coral as needed, according to a study in mBio this week. 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 with the ups and downs of nitrogen concentrations in the environment.
"It was a great surprise to find the nitrogen-rich crystals inside the algae," writes corresponding author Anders Meibom of the École Polytechnique Fédérale de Lausanne, Switzerland, in an email interview. But, he writes, "it all makes perfect sense now. The algae suck up the ammonium and nitrate like a sponge when the concentrations of these molecules increases, then stores this nitrogen as uric acid crystals for later use."
Hard coral: a nitrogen-based economy
Like all reef-forming corals, the species Kopp et al. studied, cauliflower coral (Pocillopora damicornis), is actually a symbiosis of two different organisms: the coral provides protection to a species of photosynthetic 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 - but probably detrimental - ways.
To better understand these exchanges of materials and to determine how an excess of nutrients might affect the balance, Kopp et al. exposed pieces of coral (called "nubbins" - isn't that cute?) to varying concentrations of isotopically-labeled ammonium, nitrate, and aspartic acid and applied nano-scale secondary ion mass-spectrometry (NanoSIMS) to follow the path of the nitrogen. NanoSIMS enabled them to visualize and quantify the uptake, movement, and accumulation of this labeled nitrogen within the coral.
Dinoflagellates store excess nitrogen as crystals
When supplied with any of the nitrogen-rich compounds, the dinoflagellates responded 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 epithelium of the coral.
This storage and release process helps explain how these corals get through the ups and downs of nitrogen concentrations, says Meibom. "This gives the coral-algae symbiosis a very efficient way to deal with strong fluctuations in nitrogen availability," writes Meibom. "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."
To follow up on this work, Meibom says he and his colleagues are now studying how carbon-based nutrients are taken up and distributed in the same coral-algae symbiosis.