Relationships among bacterial cell size, productivity, and genetic diversity in aquatic environments using cell sorting and flow cytometry

The study of relationships between cell size and productivity is of key imp ortance in microbial ecology to understand which members of natural aquatic communities are responsible for the overall activity and/or productivity. Flow sorting of microorganisms from different environmental samples was use d to analyze the activity of bacterial cells depending on their biovolume. Bacterial cells from five different natural samples taken along the Mediter ranean coast including fresh- and seawaters were incubated with tritiated l eucine, then stained with SYTO 13 and sorted by flow cytometry according to their average side-angle-scattered (SSC) light. In all samples, a bell-sha ped relationship was found between cell biovolume and activity, whereas act ivity of a given cell-size class varied between samples. In contrast, an in verse relationship was found between biovolumes and abundances. These resul ts suggest that medium-sized cells with highest growth rates are probably s ubmitted to intense grazing. For one sample, bacteria within five different size classes were sorted and the genetic diversity of cells within each so rted size class and that of the whole community were analyzed by the denatu ring gradient gel electrophoresis (DGGE) method. The genetic diversity, as determined at the community level was highly represented into the pool of s mall cells, whereas only few species were present into larger cell subpopul ations. The results suggest that only a few genotypes may be dominant withi n the largest and most productive cells. Furthermore, cell size polymorphis m as well as heterogeneous cellular activities were found within some speci es.