Are prokaryotic cell shape and size suitable to ecosystem characterization?

Microbial biomass estimation depends on cell shape and size determinations and thus there is a wide biovolume variability among morphotypes. Nevertheless, data on morphology and morphometry of prokaryotic cells under different trophic status are seldom published, due to methodological difficulties of cell measurements. The main question addressed in this paper concerns the suitability of prokaryotic size and shape for environmental characterization. Microbial biovolumes were compared among different ecosystems, located in temperate and tropical regions. Samples were taken from fresh, brackish, mixohaline and estuarine waters that were classified as oligo-, meso-, eu- and hypertrophic by comparing synoptically different trophic indices. Prokaryotic cell abundance and volume were quantified by and then used to calculate biomass and then they were correlated to environmental variables. Some samples were analysed by flow cytometry also and data from sub-populations with a different apparent DNA content were available. Prokaryotic abundances generally increased from oligo- to hypertrophic waters while cell volumes increased from oligotrophic to eutrophic waters. Although significant correlations between cell volumes and environmental variables were detected (positive with salinity and negative with Chlorophyll-a), different morphotypes dominated each studied regions. Our results sustain the hypothesis that prokaryotic cell size and shape could be useful to ecosystem characterization.