Small scale variability of bacterial communities

Laboratory experiments suggest that bacterial colonization of suspended particles in lake water is governed by stochastic processes. Thus, the particle-attached subcommunities should exhibit high small-scale (cm to m) spatial beta diversity in situ. We explored this hypothesis by studying the horizontal variability of bacterial metacommunities in the mixed upper layer of a lake at three seasons with contrasting phytoplankton regimes (May, July, October). Ten sets of 10 samples were collected at 2 cm distances in 10 ml syringes. Bacterial community composition was analysed in 19-24 samples per campaign, selected according to contrasting concentration of dissolved free amino acids (DFAA). Bacterial taxa were assigned to either the 'free-living' (FL) or the 'particle-attached' (PA) subcommunity, depending on their presence or absence in large (600 ml) 5-µm prefiltered reference samples from the same depth layer. Only the FL subcommunity exhibited seasonal differences in composition and assembly processes. Several abundant FL taxa had significant spatial variability or were correlated with DFAA concentrations in May and July, respectively. Spatial heterogeneity in the October FL subcommunity was due to higher alpha and beta diversity of rare taxa with a presumably dual lifestyle (PA or FL). The most pronounced small-scale spatial beta diversity was found in the PA subcommunity at all seasons: on average, only 10 % of their total seasonal richness was present in any single syringe sample. Our study thus provides first field evidence for high compositional heterogeneity of the PA subcommunities at spatial scales of cm to m, possibly related to the diversity of particulate organic matter and stochastic assembly processes.