PHYTOPLANKTON PHOTOSYNTHETIC CHARACTERISTICS FROM FLUORESCENCE INDUCTION ASSAYS OF INDIVIDUAL CELLS

Saturating-flash fluorescence techniques, which can provide informatio n about the physiological state of phytoplankton, at present measure b ulk water samples and so provide ''averaged'' values for all the fluor escent particles present. In analyzing natural samples, however, more detailed information about the distribution of photosynthetic characte ristics among different cell types and(or) individual cells is desirab le. Therefore we developed two methods for applying a ''pump-during-pr obe'' technique on a cell-by-cell basis. We used either an epifluoresc ence microscope or a flow cytometer to make time-resolved measurements of the increase in chlorophyll fluorescence induced by a rectangular excitation pulse of 100-mu s duration. We used a biophysical model of fluorescence induction to obtain information about the quantum yield o f photochemistry in photosystem 2 (PS2) and the functional absorption cross-section for PS2. For several species (including the smallest phy toplankton, Prochlorococcus, which are 0.7 mu m in diameter), the maxi mum quantum yield of photochemistry in PS2 obtained by averaging data from many individual cells agreed well with estimates derived from bul k measurements of DCMU enhancement of Chl fluorescence.