The physical limits of metal reduction by long-range extracellular electron transfer, and the role of cytochrome-bound flavins

Abstract:

Microbial reduction of metals and radionuclides in the subsurface plays an essential role in the biogeochemical cycling of micronutrients and the remediation of contaminated groundwater. While recent advances in the field have improved our ability to understand and predict bioreduction in these environments, the contribution of long-range extracellular electron transfer (EET) by electron shuttling or reduction along conductive pili remains elusive. Long-range EET is implicated in the reduction of radionuclides like uranium that are reversibly sorbed in clay nanopores and exist as persistant sources of contamination. In regions of low hydraulic conductivity, electron shuttles and conductive pili may increase physical mixing beyond what is possible by advection and diffusion, resulting in reduction over a larger area than predicted by current models. We present a novel microfluidic platform that allows us to study long-range EET to the exclusion of other mechanisms, 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 Geobacter sulfurreducens as a model metal-reducing bacteria, insoluble manganese dioxide as an electron acceptor, and Escherichia coli K-12 as a reductant and redox buffer, we demonstate that 1) long-range EET by conductive pili requires the presence of flavins 2) Reduction by direct contact only requires the presence of a lowered electric potential 3) The limit of reduction by conductive pili is on the order of 15-20 microns. We are actively exploring the influence of hydrological conditions on the expression of different mechanisms of long-range EET, and the importance of extracellular cytochromes and pili conductivity on metal reduction.