Do corals recycle?

You probably never look at pictures of colorful fish or dramatic coral reefs and wonder how they get their nitrogen.  But as a biogeochemist, I can't help but look at the abundance and diversity of life on a coral reef and wonder how all these organisms can exist in waters that are the ocean equivalent of a desert when it comes to nutrients like nitrogen.

All living things need the element nitrogen to build the proteins that make up their cells.  But too much nitrogen is a bad thing, and you and I have ways of getting rid of our extra nitrogen in our kidneys before it builds up to dangerous levels in our blood.  Corals, too, have nitrogen waste products. But what happens to this waste nitrogen is not well understood.  We think a lot of it may be recycled many many times by the organisms that make up the reef. 

One way we are investigating this is conducting incubation experiments that allow us to track the movement and transformation of nitrogen through the different parts of the coral, and between the coral and its microbial friends.  In this picture, I'm injecting a nitrogen-containing tracer called ammonium into a bottle containing a small piece of coral.  The ammonium tracer I use in my experiments is slightly different than the ammonium normally found on the reef and acts like an invisible dye to measure how much gets taken up by the coral, how much is transformed by bacteria living with the coral, and how much is transformed by bacteria living in the surrounding water.  Understanding how coral reefs are so efficient at recycling their nitrogen waste might help us understand how reefs can be productive ecosystems in such nutrient-poor waters.