Effects of small-scale turbulence on bacteria: a matter of size

We examined natural assemblages of marine bacterioplankton from several microcosm experiments by image analysis to test whether small scale turbulence and its associated shear affect bacterial size-distribution and morphology. The natural microbial community and bacteria-only fractions were incubated with a turbulence treatment, with addition of mineral nutrients and/or carbon and under control conditions. Throughout the experiments bacterial numbers increased significantly faster in the turbulence treatments, at least when incubated with the natural microbial community. In general, in the turbulence treatments bacteria increased significantly in size, mainly due to elongation of cells towards filaments. The addition of inorganic nutrients had a systematically negative effect on bacterial size, but a significantly positive effect on abundance independently of other factors like turbulence and the presence of predators. Flagellate grazing did not trigger an increase in bacterial size as a grazing resistance response in unmixed containers. With the addition of organic carbon bacteria elongated (up to 300 ìm) and settled down, even in the absence of other components of microbial community, but bacterial numbers did not further increase. Furthermore, bacteria aggregated in the turbulence treatments after the second day of incubation. We found that turbulence and the associated shear certainly affect bacterial size and morphology. This might be due to en enhanced growth rate and/or unbalanced growth when organic carbon is in excess compared to mineral nutrients. Shear associated with turbulent flow might enhance the DOM flux to bacteria directly as well as indirectly through enhanced grazing activity and photosynthetic release. The formation of bacterial aggregates and filaments under turbulence might give selective advantage to bacteria in terms of nutrient uptake and grazing resistance as well as increase bacterial sedimentation.