EVALUATION OF PHOTOSYNTHETIC CAPACITY IN PHYTOPLANKTON BY FLOW CYTOMETRIC ANALYSIS OF DCMU-ENHANCED CHLOROPHYLL FLUORESCENCE

A new method is presented to evaluate the photosynthetic capacity of individual phytoplankton cells using flow cytometry. The method is based on the well-known phenomenon in which chlorophyll a fluorescence is enhanced in the presence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU), an inhibitor of photosynthetic electron transport. Using a mercury-cadmium arc lamp-based flow cytometer, we found significant correlations between fluorescence enhancement and photosynthetic capacity. The correlation was dependent on species and growth irradiance. The regression coefficient ranged more than 5 times according to species, and cells under light-saturated growth were enhanced less than light-limited cells, although the former had higher photosynthetic capacity than the latter. The ability to measure DCMU-enhanced fluorescence in flow cytometry depends on a weak excitation beam and a long residence time in the sensing zone to ensure maximum variable fluorescence yield. We were unable to detect enhancement using laser-based flow cytometers. The variability in fluorescence enhancement is discussed in relation to its practical use in the field. The method was applied to natural assemblages in Bedford Basin (Canada): temporal variations of fluorescence enhancement were clearly evident in dominant diatoms and cryptophytes.