Corals therefore capture zooplankton to get nutrients required for growth, like nitrogen, phosphorus, amino acids, or vitamins and minerals.  If the coral lives deep in the water where there is little light, then it also requires food to supplement the limited energy it can get from the sun.

 As I began to study more about corals, I learned that nutrients from the zooplankton were made available to the symbiotic zooxanthella e. The coral trades the extra nutrients from capturing zooplankton with the zooxanthellae for their energy-rich compounds from such as sugars & fats (we call that the "junk food diet"). Also like any animal, the coral releases waste products that are good fertilizer for the zooxanthellae, just like we take manure and spread it on our plants on land. The zooxanthellae can produce small building blocks for proteins called amino acids that the animal cannot make (thats why this tight recycling of nutrients between the coral host and its symbiotic zooxanthellae is thought to be one reason why corals flourish in such nutrient-poor waters.

However, these "facts" that were in my textbooks were based mostly on indirect evidence and assumptions. No one had actually measured nitrogen from zooplankton appearing in zooxanthellae, it just seems reasonable! I decided to answer questions such as: Do zooxanthellae really get nitrogen from their animal host? If zooxanthellae use nitrogen from host feeding to make amino acids, is any of that nitrogen returned to the animal host, or are the amino acids incorporated into protein in the zooxanthellae? How does the manner in which the animal and its symbiotic zooxanthellae use nitrogen depend on food supply?

The only way to find out what is happening is to have a "tag" on the atoms in the food and then follow the tagged atoms as they move around in the coral. Many atoms can have several different forms, called isotopes, that have different weights. Since I'm working on nitrogen, the two important isotopes are the most common form of the nitrogen atom (called ¹⁴N but pronounced nitrogen 14) and the heavier isotope (¹⁵N). Since ¹⁵N is not very abundant in nature (only 0.37% of all of the atoms in the world are¹⁵N), if we add more ¹⁵N than naturally occurs to the food, we can "see" the different parts of the animal become heavier. The extra wieght is far too small to measure the change using a scale or balance so I use a special machine called an isotope ratio mass spectrometer (That's one in the picture of me at the top of the page) 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. 

 Since I'm working on nitrogen from host feeding, I need to get the ¹⁵N label into food to feed to anemones and corals. To do this, algae are grown in seawater that contains nitrogen to help them grow, but all the nitrogen is ¹⁵N. I can then feed the algae to brine shrimp (aka sea monkeys) for a week or two to put the label into something anemones and corals will eat. When the brine shrimp are fed to the anemones, it puts the label into the symbiosis, beginning with the animal host. By sampling at different times, I can look to see whether the zooxanthellae are using the nitrogen from the animal host, and I can look for the label in different biochemical pools in both the animal host and the zooxanthellae.

I also plan to follow the carbon atoms as they move from the zooxanthellae to the animal. Again I can use an isotope, 13C, as part of carbon dioxide (13CO₂), to tag photosynthesis. If the textbooks are correct, I should "see" the tagged carbon atoms from the zooxanthellae mix with the tagged nitrogen from the animal as they share nutrients and energy. The animal host has first access to the nitrogen label, while the zooxanthellae have first shot at the carbon label. The interesting thing is to look for any amino acids that have both labels, to suggest where the carbon and nitrogen cycles are linked within the symbiosis. This approach addresses several questions. Do zooxanthellae use nitrogen from host feeding to make amino acids? If so, are they returning any of these amino acids to the host? Are any of the amino acids the host gets from its zooxanthellae essential amino acids (i.e., amino acids the animal host can't make by itself)? Or, is the animal host using its nitrogen to make amino acids from carbon skeletons it gets from the zooxanthellae?