title: Groundwater Isolation Governs Chemistry and Microbial Community Structure along Hydrologic Flowpaths.
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.
authors.affiliation: OSUR-UMR 6553 ECOBIO, Université de Rennes 1 and Centre National de la Recherche ScientifiqueRennes, France; OSUR-UMR 6118 Géosciences, Université de Rennes 1 and Centre National de la Recherche ScientifiqueRennes, France; BRGM, Laboratory DepartmentOrléans, France.
authors.forename: Sarah
authors.initials: S
authors.lastname: Ben Maamar
authors.affiliation: OSUR-UMR 6118 Géosciences, Université de Rennes 1 and Centre National de la Recherche Scientifique Rennes, France.
authors.forename: Luc
authors.initials: L
authors.lastname: Aquilina
authors.affiliation: OSUR-UMR 6553 ECOBIO, Université de Rennes 1 and Centre National de la Recherche Scientifique Rennes, France.
authors.forename: Achim
authors.initials: A
authors.lastname: Quaiser
authors.affiliation: BRGM, Laboratory Department Orléans, France.
authors.forename: Hélène
authors.initials: H
authors.lastname: Pauwels
authors.affiliation: OSUR-UMS 3343, Université de Rennes 1 and Centre National de la Recherche Scientifique Rennes, France.
authors.forename: Sophie
authors.initials: S
authors.lastname: Michon-Coudouel
authors.affiliation: OSUR-UMR 6118 Géosciences, Université de Rennes 1 and Centre National de la Recherche Scientifique Rennes, France.
authors.forename: Virginie
authors.initials: V
authors.lastname: Vergnaud-Ayraud
authors.affiliation: OSUR-UMR 6118 Géosciences, Université de Rennes 1 and Centre National de la Recherche Scientifique Rennes, France.
authors.forename: Thierry
authors.initials: T
authors.lastname: Labasque
authors.affiliation: OSUR-UMR 6118 Géosciences, Université de Rennes 1 and Centre National de la Recherche Scientifique Rennes, France.
authors.forename: Clément
authors.initials: C
authors.lastname: Roques
authors.affiliation: OSUR-UMS 3343, Université de Rennes 1 and Centre National de la Recherche Scientifique Rennes, France.
authors.forename: Benjamin W
authors.initials: BW
authors.lastname: Abbott
authors.affiliation: OSUR-UMR 6553 ECOBIO, Université de Rennes 1 and Centre National de la Recherche Scientifique Rennes, France.
authors.forename: Alexis
authors.initials: A
authors.lastname: Dufresne
keywords: 16S rRNA; aquifers; bacterial communities; groundwater; hydrology; iron oxidation; nitrate; pyrosequencing