Carbon, Metals, and Grain Size Correlate with Bacterial Community Structure in Sediments of a High Arsenic Aquifer

Bacterial communities can exert significant influence on the biogeochemical cycling of arsenic (As). This has globally important implications since As in drinking water affects the health of over 100 million people worldwide, including in the Ganges-Brahmaputra Delta region of Bangladesh where geogenic arsenic in groundwater can reach concentrations of more than 10 times the World Health Organization's limit. Thus, the goal of this research was to investigate patterns in bacterial community composition across gradients in sediment texture and chemistry in an aquifer with elevated groundwater As concentrations in Araihazar, Bangladesh. We characterized the bacterial community by pyrosequencing 16S rRNA genes from aquifer sediment samples collected at three locations along a groundwater flow path at a range of depths between 1.5 and 15 m. We identified significant differences in bacterial community composition between locations in the aquifer. In addition, we found that bacterial community structure was significantly related to sediment grain size, and sediment carbon (C), manganese (Mn), and iron (Fe) concentrations. Deltaproteobacteria and Chloroflexi were found in higher proportions in silty sediments with higher concentrations of C, Fe, and Mn. By contrast, Alphaproteobacteria and Betaproteobacteria were in higher proportions in sandy sediments with lower concentrations of C and metals. Based on the phylogenetic affiliations of these taxa, these results may indicate a shift to more Fe-, Mn-, and humic substance-reducers in the high C and metal sediments. It is well-documented that C, Mn, and Fe may influence the mobility of groundwater arsenic, and it is intriguing that these constituents may also structure the bacterial community.