A cytometric framework to assess trends in the morphological structure of bacterioplankton communities along freshwater environmental gradients.

Bacterioplankton communities are characterized by varying distributions of cell size, shape and internal complexity, and macromolecular composition, yet there have been few attempts to quantitatively describe this complex community structure and to assess how it varies among communities and habitats. Here we present a framework to assess this morphological structure, based on the analysis of dot clouds resulting from flow cytometric measurements of side and forward scatter and cell fluorescence of individual bacterioplankton cells. Each community has a characteristic cytometric dot cloud, which forms an ellipsoid that can be described by a combination of metrics that quantify its shape, elongation, volume, orientation, and internal complexity. We apply this framework to assess how the bacterioplankton morphological structure (BMS) varies in 637 lakes distributed across Canada, covering a wide range of limnological, watershed, and climatic features. We show that there is a BMS core, which is characterized by small, simple and oblate shapes, and low overall fluorescence i.e. present in all lakes but is prevalent in oligotrophic lakes with hydrologically less evaporated water and low retention time, likely reflecting mass effects and allochthonous bacterial inputs. We further show that along gradients of increasing network water residence time, system productivity and dissolved organic carbon enrichment, there is a clear succession wherein BMS becomes increasingly dispersed, complex, and prolate shapes, likely reflecting environmental selection of aquatic taxa.