Microbial community diversity and geochemistry inform bioremediation of molybdenum-contaminated groundwater

<i>In situ</i> remediation of groundwater at coal combustion product (CCP) sites can be challenging for elements such as molybdenum (Mo), which do not respond well to commonly used treatment. This research was initiated to improve the understanding of geochemistry and microbial diversity associated with a Mo plume at a CCP site toward the development of an <i>in situ</i> treatment scheme. Diffusive microbial samplers were designed and deployed at the study site for 9 weeks. Afterward, geochemical and community analyses were used as the basis to understand how microbial communities respond to elevated Mo concentrations within a plume. Our results show that the Mo and other constituents within the plume do not reduce the diversity of the community, in contrast to trends observed at other industrial sites with metals and metalloids in groundwater. Interestingly, bacteria of the order <i>Burkholderiales</i> were higher in abundance in wells where Mo >0.3 mg/L, and several sulfate-reducing bacteria were less abundant but not absent. Molybdenum sequestration experiments were also performed with sulfate-reducing bacteria enriched from groundwater samples collected at the site. The results show that <i>Desulfomicrobium escambiense</i> played a major role in Mo sequestration and activated a detoxification mechanism. This process involved the sequential activation of periplasmic heavy metal sensors, followed by the activation of <i>atpE</i> ATP synthase, which may function as an exporter of Mo to form Mo-S species in the periplasm of the cell. The results provide important considerations for bioremediation potential in groundwater settings impacted by Mo, especially those who seek to stimulate sulfate-reducing bacteria for Mo sequestration in biogenic sulfide solids.IMPORTANCEBioremediation of contaminated sites has become popular for chlorinated hydrocarbons, but it has not been widely applied to inorganic constituents outside of arsenic. Here, we show the potential for the development of geochemistry-informed bioremediation technologies of Mo-contaminated groundwater by leveraging Mo-tolerant communities despite the suppression of sulfate reduction by Mo.