DISSOLVED ORGANIC CARBON IMPACT MICROBIAL ARSENIC CYCLING IN AQUIFERS OF WEST BENGAL (INDIA)

Organic matter (OM) in the Bengal Delta Plain (BDP) aquifers is widely acknowledged to sustain microbes participating in arsenic (As) biogeochemical cycling. Our ongoing studies in As-rich aquifers in Nadia district indicated presence of arsenite oxidizing bacterial (based on aioA gene) communities in the shallow Holocene Grey Sand Aquifers (GSA), which are absent in the deep Pleistocene Brown Sand Aquifers (BSA). BSAs are labeled as ‘As-safe’ aquifers. Of these, very few aquifers had been characterized earlier for their lipid biomarkers indicated the presence of thermally mature petroleum derived hydrocarbons, and their role in inducing microbial mediated redox dissolution of As.

We studied the hydrological contrasts between GSA and BSA. Different fractions of total lipid extracts (TLE) were extracted from groundwater. Limited terrestrial input of OM is observed in the GSAs, and TLEs indicate dominant low molecular weight (LMW) hydrocarbons (n-alkane, n-alkanols and n-alkanoic acids). The TLEs indicate absence of thermally mature hydrocarbon complexes and their degraded products in dissolved organic carbon (DOC) of BSA. These compounds are however present in GSA groundwater. Experiments with mixed cultures of dominant arsenite oxidizing bacterial strains, isolated from the groundwater indicated preferential utilization of GSA derived TLE as C source in comparison to BSA derived TLE. The GSA groundwaters also had higher Fe and Mn concentrations than BSA. The study establishes the characteristic differences in DOC composition in two types of aquifers GSA and BSA groundwater, which critically influence selective microbial assemblages, and affect elemental cycling including that of As in the BDP aquifers.