THE ROLE OF BACTERIAL COMMUNITIES ON IN-SITU ARSENIC AND IRON CYCLING IN RIVERBANKS ALONG THE MEGHNA RIVER, BANGLADESH

Along the Meghna River in Bangladesh, season and tidal river stage fluctuations cycles drive oxygen-rich river water into permeable banks, oxidizing dissolved iron (Fe(II)) and precipitating Fe(III)-oxyhydroxides (FeOOH). These Permeable Natural Reactive Barriers (PNRB) sorb arsenic (As) oxyanions arriving from groundwater discharge, but the role of bacteria in a PNRB has not previously been examined. We report a detailed analysis of microbial community structure, function, and biogeochemical transformations occurring within a previously characterized PNRB on the Meghna River during the early dry season (January), when gaining river conditions predominate. The PNRB is located up to 3 m depth along the river’s edge, located along the final 10 m of a well-studied 90 m groundwater flow path that is rich in dissolved Fe(II) and As. This zone undergoes robust mixing with the river and contains high solid-phase concentrations of Fe(III) and sorbed As within a zone of fine sand and silt at the low-tide river edge. The reducing aquifer up-gradient of the PNRB contains concentrations of As and Fe that are relatively constant year-round and even a microbial community populated by anaerobic bacteria that prefer strongly reducing conditions and employ the use of siderophores to dissolve Fe(III) in complex organo-mineral matrices, contributing to a steady release of As to groundwater. This contrast with the biogeochemical signals within the PNRB, wherein the community contains high relative abundances and increased diversity of bacteria with a range of chemoautotrophic and facultative, dissimilatory metabolisms capable of catalyzing the oxidation of Fe(II)/As(III), or reduction of Fe(III)/As(V) using labile dissolved organic carbon (DOC), respectively. At the time of sampling, we observed a simultaneous removal of Fe, As, and DOC within the PNRB. However, the metabolic flexibility of metabolisms, and range of aero-tolerances in these communities indicate they also possess the functional potential to switch between microbially-mediated oxidation and reduction, during high and low river stages, respectively. This study sheds new insights into how fine-scale aquifer structure interacts with local hydrodynamics and microorganisms to dynamically shift the sequestration and release of Fe and As to drinking water.