Microbial diversity and impact on carbonate geochemistry across a changing geochemical gradient in a karst aquifer

Abstract

Although microbes are known to influence karst (carbonate) aquifer ecosystem-level processes, comparatively little information is available regarding the diversity of microbial activities that could influence water quality and geological modification. To assess microbial diversity in the context of aquifer geochemistry, we coupled 16S rRNA Sanger sequencing and 454 tag pyrosequencing to in situ microcosm experiments from wells that cross the transition from fresh to saline and sulfidic water in the Edwards Aquifer of central Texas, one of the largest karst aquifers in the United States. The distribution of microbial groups across the transition zone correlated with dissolved oxygen and sulfide concentration, and significant variations in community composition were explained by local carbonate geochemistry, specifically calcium concentration and alkalinity. The waters were supersaturated with respect to prevalent aquifer minerals, calcite and dolomite, but in situ microcosm experiments containing these minerals revealed significant mass loss from dissolution when colonized by microbes. Despite differences in cell density on the experimental surfaces, carbonate loss was greater from freshwater wells than saline, sulfidic wells. However, as cell density increased, which was correlated to and controlled by local geochemistry, dissolution rates decreased. Surface colonization by metabolically active cells promotes dissolution by creating local disequilibria between bulk aquifer fluids and mineral surfaces, but this also controls rates of karst aquifer modification. These results expand our understanding of microbial diversity in karst aquifers and emphasize the importance of evaluating active microbial processes that could affect carbonate weathering in the subsurface.

How Deep Can Surface Signals Be Traced in the Critical Zone? Merging Biodiversity with Biogeochemistry Research in a Central German Muschelkalk Landscape

“…Previous investigations of the microbial inventory of karst aquifers primarily focused on spring water or caves that allow easier access, pointing to the presence of autochthonous microbial endokarst communities but also to high temporal dynamics, indicated by the episodic occurrence of allochthonous soil bacteria within the spring water communities (Farnleitner et al, 2005;Pronk et al, 2008;Shabarova et al, 2013). However, less information is available for groundwater obtained from drilled boreholes within a karstic limestone setting (Gray and Engel, 2013). Studies of carbon cycling in kart environments have often focused on using isotopes to quantify the dissolution and precipitation of carbonates (models reviewed in Han and Plummer, 2013) or to understand the degree to which C derived in soils can be recorded in cave deposition (Noronha et al, 2014).…”

Large Fractions of CO 2 -Fixing Microorganisms in Pristine Limestone Aquifers Appear To Be Involved in the Oxidation of Reduced Sulfur and Nitrogen Compounds

“…Recent studies addressing the microbial diversity in aquifers or drinking water wells by 16S rRNA gene-targeted pyrosequencing allowed comprehensive insight into the composition of microbial communities (14,(27)(28)(29). However, those studies were DNA based and did not allow characterization of active aquifer microbial community members, providing only limited insight into the physiologic strategies that are dominant under the given environmental conditions.…”

Microbial communities in UK aquifers: current understanding and future research needs

“…Lin et al 2012;Pilloni et al 2012;Gray & Engel 2013;Wilkins et al 2013). Although there are many examples of molecular techniques applied to contaminated aquifers (e.g.…”