Effects of Small-Scale Turbulence on Bacteria: A Matter of Size A. Malits 1 , F. Peters 1 , M. Bayer–Giraldi 1 , C. Marrase ´ 1 , A. Zoppini 2 , O. Guadayol 1 and M. Alcaraz 1 (1) Institut de Cie `ncies del Mar, CMIMA (CSIC), Passeig Marı´tim de la Barceloneta 37-49, 08003 Barcelona, Spain (2) Istituto di Ricerca Sulle Acque (CNR), Via Reno l, 00198 Rome, Italy Received: 24 June 2003 / Accepted: 26 September 2003 / Online publication: 24 August 2004 Abstract We examined the influence of small-scale turbulence and its associated shear on bacterioplankton abundance and cell size. We incubated natural microbial assemblages and bacteria-only fractions and subjected them to treatments with turbulence and additions of mineral nutrients and/ or organic carbon. Bacterial abundance was not affected directly by turbulence in bacteria-only incubations. In natural microbial assemblage incubations, bacterial con- centrations were higher under turbulence than in still- water controls when nutrients were added. In general, in the turbulence treatments bacteria increased significantly in size, mainly due to elongation of cells. The addition of inorganic nutrients had a negative effect on bacterial size, but a significantly positive effect on abundance inde- pendently of other factors such as turbulence and the presence of predators. Flagellate grazing did not trigger an increase in bacterial size as a grazing resistance re- sponse in unmixed containers. With the addition of or- ganic carbon, bacteria elongated and partly settled to the bottom of the containers, in both the turbulent and still treatment, but bacterial abundance did not further in- crease. Furthermore, bacteria aggregated in the turbu- lence treatments after the second day of incubation even in the absence of other components of the microbial community. We found that turbulence and the associated shear increase bacterial size and change bacterial mor- phology, at least under certain nutrient conditions. This might be due to a physiological response (enhanced growth rate and/or unbalanced growth) or due to the selection of opportunistic strains when organic carbon is in excess compared to mineral nutrients. We suggest that shear associated with turbulent flow enhances 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. Introduction The evidence for an effect of small-scale turbulence on bacterial growth is scarce and contradictory. Uptake rates of radiolabeled macromolecules in pure cultures of the bacterium Zoogloea ramigera increased in stirred bottles by a factor of 12.6 and 6.2 for bovine serum albumin and dextran, respectively [4]. In contrast, laminar fluid shear rates <2.1 s )1 (e = 0.04 cm 2 s )3 ) were found to have no discernible effect on bacterial uptake of low molecular weight in natural assemblages [20]. Moeseneder and Herndl [27] detected that bacterial production was af- fected negatively by turbulence in the presence of parti- cles other than bacteria. They explained their findings as due to homogenization of the nutrient field so that bacteria would not be able to find nutrient-rich micro- patches by chemotaxis. However, the turbulence levels they applied were three to four orders of magnitude higher than found under natural conditions. After introducing more natural turbulence levels to microbial assemblages, the laminar shear field derived from tur- bulence enhanced bacterial abundance, at least when other parts of the microbial community were present [31, 33]. This was attributed to a partial grazing pressure relief on bacteria when flagellates shift in part to bigger phy- toplankton because of higher encounter rates under turbulence. From theory we know that the flux of solutes toward a cell can be increased by the relative motion of water with respect to the cell. Turbulence and the shear asso- ciated to it can increase this relative motion. The Sher- wood number (Sh) describes the relative increase in nutrient flux arriving to the cell surface under a certain hydrodynamic condition with respect to purely diffu- Correspondence to: A. Malits at present address: LOV, CNRS UMR 7093, BP 28, Station Zoologique, 06234 Villefranche-sur-mer, France; E-mail: malits@obs-vlfr.fr DOI: 10.1007/s00248-004-0133-4 d Volume 48, 287–299 (2004) d Ó Springer Science+Business Media, Inc. 2004 287