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.
Highlights
Aquifers are populated by a vast diversity of microorganisms, some of which can play a critical role in subsurface biogeochemical cycling and pollutant degradation (Yagi et al, 2010)
SB-O2 and SB-O1 wells are fed by this groundwater flow, though they were sampled at distinct depths (216 and 80 m), while the SB-O well is fed by more superficial loops resulting in a natural mixing of old and recent groundwater
Microorganisms detected in recent groundwater were highly associated with NO3− and O2 concentrations (Figures 3 and 5)
Summary
Aquifers are populated by a vast diversity of microorganisms, some of which can play a critical role in subsurface biogeochemical cycling and pollutant degradation (Yagi et al, 2010). Geochemical conditions, and in particular the availability of electron donors and acceptors, are a major driver of microbial community composition and diversity in groundwater and the geological substratum (Akob et al, 2007; Boyd et al, 2007; Flynn et al, 2012, 2013; Nyyssönen et al, 2014). To date very few studies have addressed the effect of hydrology on groundwater microbial communities, and those that have were restricted to a single aquifer (Roudnew et al, 2010; Bougon et al, 2012; Larned, 2012; Lin et al, 2012; Zhou et al, 2012). The comparison of several aquifers with well-defined hydrogeologic conditions allows to constrain the influence of groundwater flow in order to better understand biogeochemical functioning of groundwater ecosystems
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