POTENTIAL OF FLOW-CYTOMETRY FOR PUMP AND PROBE FLUORESCENCE MEASUREMENTS OF PHYTOPLANKTON PHOTOSYNTHETIC CHARACTERISTICS

''Pump and probe'' fluorescence assays provide information about the photosynthetic characteristics of phytoplankton in situ, but at present they measure bulk water samples and thus provide only average values for the whole community (including detritus). We used dual beam flow cytometry to investigate the potential for measuring ''pump and probe'' fluorescence of single cells. For Lauderia borealis (a centric diatom similar to 30 mu m in diameter) in different stages of N-limited batch culture, the fluorescence yield enhancement due to the presence of a ''pump'' beam was correlated with DCMU-enhanced bulk fluorescence, as expected. However, at the appropriate excitation power, fluorescence signals from single cells are so small that for most types of phytoplankton in the oceans, population averaging of signals will be necessary.