Flow Cytometry Analysis of Marine Picoplankton

In the last decade, the use of flow cytometry (FCM) has become more and more popular among limnologists and marine biologists, both for laboratory studies and field research. FCM allows the analysis of phytoplanktonic cells that are too dim to be discriminated by epifluorescence microscopy. Its major advantages are to provide rapid and accurate measurements of individual particles and to allow the discrimination between- auto and heterotrophic populations as well as between cells and detritus or suspended sediments. FCM is particularly well suited for the study of the smallest size class of the plankton (below 2 μm), called picoplankton. Picoplankton is composed by 4 major groups: heterotrophic prokaryotes, prochlorophytes (Prochlorococcus),1 cyanobacteria (Synechococcus)2 and eukaryotes. These small organisms dominate the biomass in the open ocean, reaching respective concentration ranges of 106 – 105, 105 – 103, 105 – 103 and 104 – 102 cells per ml. The geographical distribution of these organisms, their biological characteristics (carbon and pigment content), and their dynamics in relation to the biotic factors are of major interest for the oceanographers. Initially used to descriminate and enumerate the different populations of phytoplankton, the application of flow cytometry has been extended to physiological analyses (e.g. DNA analysis) and more recently to phylogenetic analyses with the help of fluorescent molecular probes.