Symbiotic Algae Helps Coral Store Nitrogen
May 14, 2013

Symbiotic Algae Helps Coral Store Nitrogen

Brett Smith for — Your Universe Online

In a study that could have implications for ocean conservation and marine-based economic activity, researchers from the Ã‰cole Polytechnique Fédérale de Lausanne (EPFL) in Switzerland have found new details surrounding the symbiotic relationship between corals and algae.

"Coral reefs are the jungles of our oceans - hotspots of biodiversity that easily outcompete all other marine ecosystems," said the EPFL´s Christophe Kopp, first-author of the study that will appear in the peer-reviewed journal mBio later this week.

The research found the symbiotic algae are able to process and store valuable nutrients for themselves and their corals in the form of crystals.

"It was a great surprise to find the nitrogen-rich crystals inside the algae," said co-author Anders Meibom, a professor at the Swiss institute.

"It all makes perfect sense now,” Meibom said. “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."

To detect these tiny food banks, the EPFL team conducted a series of experiments with the Aquarium Tropicale Porte Dorée in Paris. At the aquarium, the researchers provided the corals nitrogen-rich compounds containing a radioactive nitrogen isotope that allowed them to track the path of the nutrients. Then the team periodically extracted bits of coral and analyzed them with an isotopic imaging device.

After tracking how the nitrogen was being processed and stored, the EPFL researchers created a timeline of the entire process using a series of images taken after each extraction. Both electron microscopy and mass spectrometry techniques allowed the team to study which cellular compartments processed the heavier nitrogen isotopes.

In their study, the researchers noted the corals depend strongly on the algae for sufficient nutrient extraction, particularly when exposed to nitrate, a compound they cannot process on their own.

Most notably, the researchers were able to show that algae temporarily store nitrogen as uric acid crystals. This enables the algae and coral to stockpile nutrients for future use when the supply suddenly drops.

"This gives the coral-algae symbiosis a very efficient way to deal with strong fluctuations in nitrogen availability," Meibom wrote. "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 marine biologist added that his team is currently investigating the same type of processing and storage processes for carbon-based nutrients.

While nitrogen is necessary to the development of all organisms, many corals inhabit regions where the element is not easily obtainable. For this aspect of coral homeostasis, algae play an important role in taking up ammonium and nitrate from seawater for itself and its coral host.

Experts say preserving coral reefs is important because they act as the foundation of both biodiversity and economic activity, as a tourist attraction and a facilitator of commercially caught fish.