Short-term variations in specific biovolumes of different bacterial forms in aquatic ecosystems

Short-term and spatial fluctuations in specific biovolumes (volume x cell⁻¹) of different morphological categories of planktonic bacteria were estimated microscopically. Samples were taken from two lakes occurring in two different climatic systems: Lake Aydat (France) and Lake Cromwell (Canada). The study was done in summer, using 24-hour cycles of sampling. Due to their large size, the specific volume of filamentous bacteria constituted, on average, the major part (>70%) of the total specific volume of all bacterial forms considered. Greatest variations in specific biovolumes were recorded for filamentous bacteria (coefficients of variation ranged from 16 to 109%). These variations were more pronounced in the oxygenated and microaerophilic strata (DOC ≈1.5 mg liter⁻¹). Fluctuations in cell volume were high (coefficients of variation =12–80%) for coccal bacteria, whereas no marked fluctuations were found for the rod and vibrio bacteria (coefficients of variation =4–10%). Evidence of diel patterns of cell volume of filamentous bacteria is provided. These cells displayed their maximum size during the day until early night, indicating cell division was occurring at night. Homogeneous circadian patterns were not provided by specific volume variations of coccal, rod, and vibrio bacteria. Statistical relationships between bacterial specific biovolumes and the biotic and abiotic parameters considered are discussed.