Influence of depth and time on diversity of free-living microbial community in the variably saturated zone of a granitic aquifer

We investigated the temporal and vertical changes in the microbial communities related to hydrological variations an aquifer (Brittany, France). Five water samplings were carried out, spanning three hydrological cycles in the variably and the permanently saturated zones. Seasonal variations in the major anion concentrations (NO3 -, SO4 2- and Cl(-) ) indicated that different physical processes occurred during the recharge process in the two zones. The variably saturated zone is mainly dominated by diffusion and advection processes from the soil, whereas the permanently saturated zone is controlled by moderate advective transfer from the variably saturated zone. Bacterial diversity was investigated by flow cytometry, 16S rRNA and narG genes analyses. Part of this diversity was new in that 6 of the 27 16S rRNA gene sequence phylotypes were unknown even at the class or phylum level. The narG gene analysis did not reveal any clear variation in time or depth within the nitrate reducers' community. In contrast, 16S rRNA gene analyses showed modifications of community composition that could be related to the hydrologic and chemical contrast between the two zones. It was concluded that the physical processes of water transfer could influence bacterial diversity at the soil-aquifer interface.