MICROBIAL IRON OXIDATION AND CONTRIBUTION TO FE OXIDE COATINGS IN AQUIFER SEDIMENT

Microbial Fe oxidation and biomineral formation is important in aquifers because the highly-reactive oxides can control the mobility of nutrients (e.g. phosphate, C) and metals (e.g. arsenic, uranium). Mineral formation also has the potential to affect groundwater flow, depending on the volume and distribution in pore spaces. Fe coatings are common on aquifer sands, but it has never been determined whether these coatings are a microbial product. In this study, we aimed to understand the role of microbial Fe-oxidizers in producing Fe sand coatings and how their biominerals affect, and are affected by groundwater flow. As part of work at the Rifle aquifer in Colorado, the presence of Fe-oxidizers was confirmed by the enrichment and isolation of two novel microaerophilic Fe-oxidizing Betaproteobacteria, Hydrogenophaga sp. P101 and Curvibacter sp. CD03. We used these two isolates, as well as the model Fe-oxidizing microorganism, Gallionellales Ferriphaselus sp. R-1, in our experimental aquifer systems. We developed a “micro-aquifer,” a sand-filled flow-through culture chamber that allows for imaging of sediment pore space with multiphoton confocal microscopy. Fe oxide biofilms formed on sand grains, demonstrating that FeOM produce Fe oxide sand coatings. While iron bacteria are well known to clog groundwater wells and aquifers, no clogging occurred in our experiments. Instead, Fe biofilm distribution was dynamic: they grew as coatings, then periodically sloughed off sand grains, with some flocs later caught in pore throats. This has implications for physical hydrology, including pore scale architecture, and element transport. The sloughing of coatings likely prevents the biominerals from clogging wells and aquifers, at least initially. Although attached biomineral coatings sequester Fe-associated elements (e.g. P, As, C, U), when biominerals detach, these elements are transported as particles through the aquifer. Currently, we are working to further examine the spatial association of the Fe bacteria and sand coatings, as well as the biotic acceleration of the coating formation. Our work shows that microbial mineralization impacts in aquifers are dynamic, and that the fate and transport of biomineral-associated elements depend not only on geochemical conditions, but also physical pore-scale processes.