We also analyze fluorescence on a single-celled basis. This is an extremely laborious procedure that involves putting a sample under a microscope, illuminating a single cell with pulses of light, and measuring the fluorescence of only that cell. Because the phytoplankton are composed of many different kinds of species, this technique can determine the different types of responses to various oceanographic conditions. We are using this to better understand why Phaeocystis antarctica or diatoms dominate in certain areas of the Ross Sea. We also conduct experiments by collecting water, adding various materials (such as iron or inhibitors of the uptake of silicon, an essential component of diatoms) to test the response. Because all phytoplankton are not alike in their responses, and because each has a unique role in the food web and the regional biogeochemical cycle, this approach allows us to understand how efficiently each taxon is operating in a given set of environmental conditions.
One of the aspects that interests us on CORSACS is called the "health index" of phytoplankton, which we call the quantum yield of photochemistry. We use a number of instruments to measure this, but one unusual one that VIMS has is the pulse amplitude modulated fluorometer. It measures fluorescence, the light that is absorbed by a chemical and re-emitted at a different wavelength (similar in many ways to fluorescent lights, in which a chemical such as neon is energized and gives off energy at a different wavelength; in neon's case, the energy is in the red region of the visible spectrum, so neon lights are always red). This fluorescence can be impacted by various environmental factors such as iron, light, and macronutrient concentrations. One of the advantages of measuring this fluorescence is that it allows us to ascertain the health of the phytoplankton assemblage in near real time. Nearly anything that stresses the algae will cause a decrease in fluorescence, but we handle the samples in a uniform manner to better understand the status of the phytoplankton, especially in assessing potential iron limitation. This is important to us since the phytoplankton bloom in the Ross Sea is believed to be seasonally limited by iron.