ABSTRACT
Naturally occurring toxic arsenic (As) in alluvial groundwater systems represents one of the most serious abiotic contaminations with enormous public health concern. Globally, more than 100 million people are affected, while the Bengal Delta Plain (BDP), spread over the large areas of West Bengal (India) and Bangladesh, is the worst affected. Mobilization of As from alluvial sediments rich in Fe/Mn (Al) oxides/hydroxides is caused by complex interactions of hydrogeo-chemical and microbial processes. The evolutionary stratigraphy, geological settings, mineralogy of aquifer sediments, and biogeochemical factors are found to have extensive implications on the hydrogeochemistry of As in these aquifers. Bacterial community inhabiting As-rich aquifers play an important role in the geochemical transformation of As, mainly by facilitating redox transformation of As species. Bacterial groups inhabiting the western parts of BDP showed high metabolic diversity, particularly with respect to their abilities to utilize broad ranges of carbon sources, electron donors, and acceptors. Culture-independent molecular analysis of bacterial diversity revealed abundance of aerobic/facultative anaerobic, denitrifying, Fe2+/As3+ -oxidizing, and As-resistant Pseudomonas, Brevundimonas, Microbacterium, Acidovorax, Acinetobacter, and Hydrogenophaga, anaerobic Fe3+- and SO4 2−-reducing Geobacter and Geothrix, and methanogenic and methylotrophic populations. The revolutionary technology of sequencing, starting from shortgun to single-molecule long-read approach, its impact on our understanding of microbial evolution, and its function in ecophysiology and phylogenomics are discussed. In spite of high toxicity, inhabitant microbes are found to possess catabolic repertoire to transform As, to withstand its toxicity, or to use it as metabolic resource. Based on the current state of knowledge, an overall spectrum of microbiology of As-rich groundwater is presented, highlighting bacteria–As interaction and diversity of bacterial communities within contaminated groundwater of BDP.
