Abstract
Quantitative and qualitative changes in bacterial communities from the Mediterranean Sea were compared in duplicate batch mesocosms with or without addition of inorganic nutrients. Methods including traditional microbial ecology techniques, molecular biology and flow cytometry were combined to determine abundances, production, cell size, activity, culturability and taxonomic diversity of bacterial cells. Addition of nutrients and confinement resulted in an increase of bacterial densities which were rapidly controlled by protozoan grazing. Changes in bacterial activity and morphology were observed during the growth phase of bacteria and under grazing pressure. The proportion of medium-size and culturable cells increased during the growth phase. These cells were preferentially consumed by grazers resulting in a strong limitation of bacterial production. As a consequence of the grazing pressure, large cells were produced and contributed to the remaining bacterial productivity after grazing. Grazing had an effect on the taxonomic composition of bacterial communities by preferentially eliminating γ-Proteobacteria, α-Proteobacteria were preserved. It seems that some species from the genera Ruegeria and Cytophaga may have developed defence strategies to escape predation.
Highlights
Knowledge on the role and control of bacterioplankton in biogeochemical cycles and energy £uxes is in permanent progress
The ¢rst growth phase was supported by dissolved organic carbon (DOC) present in the original seawater and from organic carbon released by protozoa during the grazing phase
Our results suggest that nutrients and grazing play a key role in shaping the morphologic, genotypic and phenotypic composition as well as the activity of bacterial communities by regulating abundances
Summary
Knowledge on the role and control of bacterioplankton in biogeochemical cycles and energy £uxes is in permanent progress. An important issue in aquatic microbial ecology is to elucidate the relative importance of resource limitation, grazing, and viral mortality on the functional and taxonomic diversity of bacterioplankton communities [5,6]. Nutrients may a¡ect the taxonomic structure of natural communities both temporally and spatially. It has been suggested that very small and large bacteria are partly or totally protected from nanoprotozoan grazing whereas active cells within the medium size-class are preferentially consumed [5,8]. It is unknown to what extent selection due to grazing pressure a¡ects the taxonomic structure and activity of natural communities. A key question is to identify and to understand the ecological role and behaviour of the bacterial cells which have developed adaptation strategies to escape grazing pressure in natural aquatic ecosystems
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