Flow cytometric determination of phytoplankton DNA in cultures and oceanic populations

A method to measure DNA and chlorophyll fluorescence simultaneously in single phytoplankton cells was developed and tested on cultured and natural populations. Samples were stored at -80°C following fixation with 1 % glutaraldehyde and freezing in Ilquid nitrogen, stained with DAPI (4',6-&amino-2-phenylindole), and analyzed by flow cytonletry. In cultures, cell DAPI-DNA fluorescence measured by flow cytometry was related to cell DNA content measured by fluorometry with DAPI, over almost 4 orders of magnitude. In natural populations, the method easily discriminated photosynthetic cells from other living and non-living particles and p e m t t e d computation of the fraction of particulate DNA contained in photosynthetic picoplankton. In the northwestern Mediterranean Sea in summer, this fraction was higher at neritic than at pelagic stations, and at the latter was maximum at mid-depth. In the future, this method should permit (1) better estimates of biomass partitioning among the different trophic compartments, and (2) studies of cell cychng of oceanic plankton populations.