Iron Cycling Potentials of Arsenic Contaminated Groundwater in Bangladesh as Revealed by Enrichment Cultivation

The activities of iron-oxidizing and reducing microorganisms impact the fate of arsenic in groundwater. Phylogenetic information cannot exclusively be used to infer the potential for iron oxidation or reduction in aquifers. Therefore, we complemented a previous cultivation-independent microbial community survey covering 22 arsenic contaminated drinking water wells in Bangladesh, with the characterization of enrichments of microaerophilic iron oxidizers and anaerobic iron reducers, conducted on the same water samples. All investigated samples revealed a potential for microbial iron oxidation and reduction. Microbial communities were phylogenetically diverse within and between enrichments as was also observed in the previous cultivation-independent analysis of the water samples from which these enrichments were derived. Enrichment uncovered a larger diversity in iron-cycling microorganisms than previously indicated. The iron-reducing enrichments revealed the presence of several 16S ribosomal RNA (16S rRNA) gene sequences most closely related to Acetobacterium, Clostridium, Bacillus, Rhizobiales, Desulfovibrio, Bacteroides, and Spirochaetes, in addition to well-known dissimilatory iron-reducing Geobacter and Geothrix species. Although a large diversity of Geobacteraceae was observed, they comprised only a small part of the iron-reducing consortia. Iron-oxidizing gradient tube enrichments were dominated by Comamonadaceae and Rhodocyclaceae instead of Gallionellaceae. Forty-five percent of these enrichments also revealed the presence of the gene encoding arsenite oxidase, which converts arsenite to less toxic and less mobile arsenate. Their potential for ferric (oxyhydr)oxides precipitation and arsenic immobilization makes these iron-oxidizing enrichments of interest for rational bioaugmentation of arsenite contaminated groundwater.