1996 Articles

Phytoplankton Photosynthetic Characteristics from Fluorescence Induction Assays of Individual Cells

Olson, Robert J.; Sosik, Heidi M.; Chekalyuk, Alexander M.

Saturating-flash fluorescence techniques, which can provide information about the physiological state of phytoplankton, at present measure bulk water samples and so provide "averaged" values for all the fluorescent particles present. In analyzing natural samples, however, more detailed information about the distribution of photosynthetic characteristics among different cell types and(or) individual cells is desirable. Therefore we developed two methods for applying a "pump-during-probe" technique on a cell-by-cell basis. We used either an epifluorescencemicroscope or a flow cytometer to make time-resolved measurements of the increase in chlorophyll fluorescence induced by a rectangular excitation pulse of 100-psduration. We used a biophysical model of fluorescence induction to obtain information about the quantum yield of photochemistry in photosystem 2 (PS2) and the functional absorption cross-section for PS2. For several species (including the smallest phytoplankton, Prochlorococcus, which are 0.7 pm in diameter), the maximum quantum yield of photochemistry in PS2 obtained by averaging data from many individual cells agreed well with estimates derived from bulk measurements of DCMU enhancement of Chl fluorescence.