Physiological and hydrological controls on mineral redox cycling by long-range electron transport by bacteria in anaerobic sediments

The cycling of iron and manganese oxides plays a critical role in the bioavailability of trace elements and macronutrients, the flux of carbon across terrestrial and atmospheric ecosystems, and the remediation of groundwater contaminated by toxic metals and radionuclides. Bacteria control one half of the redox cycle as the primary drivers of iron and manganese reduction in anaerobic soils and sediments. However, Fe(III) and Mn(IV) are almost exclusively present under anaerobic conditions as insoluble oxides, the reduction of which are facilitated by extracellular electron transport via conductive `nanowires', electron shuttling, and direct contact with outer membrane cytochromes. Our research focus is on the relative contribution of nanowires and electron shuttles under different physiological and hydrological conditions, which remains unexplored. We present a novel microfluidic platform that allows us to directly observe these phenomena under a controlled environment representative of groundwater conditions, monitor the metabolic activity and redox state of bacteria, and determine the presence of reduced products in-situ using Raman spectroscopy. Using Geobacter sulfurreducens and Shewanella oneidensis as model metal-reducing bacteria, and insoluble manganese dioxide (i.e. birnessite) as an electron acceptor, we show that 1) electron shuttling is more effective under static conditions 2) the presence of exogenous shuttles allows efficient electron transport under all flow regimes 3) redox potential of the bulk medium exerts significant control over reduction by both nanowires and electron shuttles 4) shuttling is amplified by orders of magnitude in nanopores.