AUTHOR=Ben Maamar Sarah , Aquilina Luc , Quaiser Achim , Pauwels Hélène , Michon-Coudouel Sophie , Vergnaud-Ayraud Virginie , Labasque Thierry , Roques Clément , Abbott Benjamin W. , Dufresne Alexis TITLE=Groundwater Isolation Governs Chemistry and Microbial Community Structure along Hydrologic Flowpaths JOURNAL=Frontiers in Microbiology VOLUME=6 YEAR=2015 URL=https://www.frontiersin.org/journals/microbiology/articles/10.3389/fmicb.2015.01457 DOI=10.3389/fmicb.2015.01457 ISSN=1664-302X ABSTRACT=

This study deals with the effects of hydrodynamic functioning of hard-rock aquifers on microbial communities. In hard-rock aquifers, the heterogeneous hydrologic circulation strongly constrains groundwater residence time, hydrochemistry, and nutrient supply. Here, residence time and a wide range of environmental factors were used to test the influence of groundwater circulation on active microbial community composition, assessed by high throughput sequencing of 16S rRNA. Groundwater of different ages was sampled along hydrogeologic paths or loops, in three contrasting hard-rock aquifers in Brittany (France). Microbial community composition was driven by groundwater residence time and hydrogeologic loop position. In recent groundwater, in the upper section of the aquifers or in their recharge zone, surface water inputs caused high nitrate concentration and the predominance of putative denitrifiers. Although denitrification does not seem to fully decrease nitrate concentrations due to low dissolved organic carbon concentrations, nitrate input has a major effect on microbial communities. The occurrence of taxa possibly associated with the application of organic fertilizers was also noticed. In ancient isolated groundwater, an ecosystem based on Fe(II)/Fe(III) and S/SO4 redox cycling was observed down to several 100 of meters below the surface. In this depth section, microbial communities were dominated by iron oxidizing bacteria belonging to Gallionellaceae. The latter were associated to old groundwater with high Fe concentrations mixed to a small but not null percentage of recent groundwater inducing oxygen concentrations below 2.5 mg/L. These two types of microbial community were observed in the three sites, independently of site geology and aquifer geometry, indicating hydrogeologic circulation exercises a major control on microbial communities.