Microbial Mineral Weathering for Nutrient Acquisition Releases Arsenic

Tens of millions of people in Southeast Asia drink groundwater contaminated with naturally occurring arsenic.  The process of arsenic release from the sediment to the groundwater remains poorly understood.  Experiments were performed to determine if microbial mineral weathering for nutrient acquisition can serve as a potential mechanism for arsenic mobilization.  We performed microcosm experiments with Burkholderia fungorum, phosphate free artificial groundwater, and natural apatite.  Controls included incubations with no cells and with killed cells.  Additionally, samples were treated with two spikes – an arsenic spike, to show that arsenic release is independent of the initial arsenic concentration, and a phosphate spike to determine whether release occurs at field relevant phosphate conditions.  We show in laboratory experiments that phosphate-limited cells of Burkholderia fungorum mobilize ancillary arsenic from apatite as a by-product of mineral weathering for nutrient acquisition.   The released arsenic does not undergo a redox transformation but appears to be solubilized from the apatite mineral lattice as arsenate during weathering.  Apatite has been shown to be commonly present in sediment samples from Bangladesh aquifers.  Analysis of apatite purified from the Ganges, Brahamputra, Meghna drainage basin shows 210 mg/kg of arsenic, which is higher than the average crustal level.  Finally, we demonstrate the presence of the microbial phenotype that releases arsenic from apatite in Bangladesh sediments. These results suggest that microbial weathering for nutrient acquisition could be an important mechanism for arsenic mobilization.