Once the particles land on the corals, the corals have two choices; either get rid of the particles, since they are mostly little 'rocks' or to "eat" them first and then get rid of them. In contrast to most scientists, I think that many corals do the latter! What use could such particles be to the corals? The particles are usually covered with bacteria and other organic matter that has a high nutritional value because of the content.  Getting enough nitrogen is as important for corals as for any animals, (that's why Mom is always worried that you get enough protein!). In addition, to move the particles off  their surface, the corals wrap them in mucus which would be silly to throw away. They they digest the mucus and particles in their guts and therefore receive nutrients and energy while concentrating the particles before finally ejecting them.

So, what  I do is try and figure out if corals do get nutrition from the particles. I need some way to label or "tag" the nitrogen on the particles so that if the corals remove it and use it, the labeled nitrogen will appear in their bodies. Each element in the periodic table can have several different forms, or isotopes, which are named according to their atomic weight. Since I'm working on nitrogen, the two important isotopes are normal nitrogen (¹⁴N) and a heavier stable isotope (¹⁵N) which is not very abundant in nature (only 0.37% of all of the atoms in the world are ¹⁵N). So if we add more ¹⁵N than naturally occurs to the food, we can "see" the different parts of the animal become heavier. We don't actually measure the change in weight using a scale or balance but use a special machine called an isotope ratio mass spectrometer (That's one in the picture of me at the top of the page) can be used to measure how much of each isotope is in any given sample. By following the fate of the 15N label you can figure out what happens to normal nitrogen in the system you're working with.

I catch some particles using a sediment trap placed on the reef and then expose the particles to ¹⁵N as ammonium (NH₄⁺) so that the bacteria can incorporate it into their bodies. Then after washing the particles really well, I "feed" it to corals in special chambers like this one. The coral is removed from the reef with a hole saw to make uniform pieces (try clicking on them in the figure) and set in the chamber where the pump (the red thing at the bottom right) gently moves the water with particles and keeps the particles suspended so that they can settle onto the coral surface in a natural manner.