Microbial reduction of ferrihydrite-organic matter coprecipitates by Shewanella putrefaciens and Geobacter metallireducens in comparison to mediated electrochemical reduction

Abstract

Despite numerous studies seeking to elucidate the effect of various specific organic compounds on the reactivity and stability of Fe oxyhydroxides in soil, studies examining the effect of natural organic matter (NOM) on the microbial reduction of Fe-organic matter (OM) coprecipitates are still rare. In this study, pure ferrihydrite (Fh) and Fe-OM coprecipitates were synthesized using three different types of NOM (extracellular polymeric substances extracted from Bacillus subtilis, OM extracted from the Oi horizon of a Cambisol, and OM extracted from the Oa horizon of a Podzol). These phases were characterized by N2 gas adsorption, nuclear magnetic resonance spectroscopy (NMR), Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), NOM desorption experiments, and mediated electrochemical reduction (MER). Iron(III) reduction under anaerobic conditions was monitored for 16days using two different strains of dissimilatory Fe(III)-reducing bacteria (Shewanella putrefaciens, Geobacter metallireducens). Mineral transformation during reduction was determined by XRD and FTIR of the solid post-incubation phases. Fe(III) reduction by Shewanella putrefaciens was influenced by the amount of available electron shuttling molecules provided by the NOM, whereas the Fe(III) reduction by Geobacter metallireducens as well as abiotic Fe(III) reduction was influenced by particle size and NOM-induced aggregation. The specific surface area proved to be a poor predictor of Fe reduction of Fe-OM coprecipitates. This study emphasizes that certain physicochemical properties of natural Fe oxyhydroxides (composition of sorbed NOM and aggregation state) impact the Fe reduction by distinct microorganisms to differing degrees. Understanding environmental Fe and C cycling, therefore, requires experimental approaches extending beyond the use of pure Fe oxyhydroxides and model organisms.