Electron shuttle-mediated microbial Fe(III) reduction under alkaline conditions

Abstract

Purpose: Extracellular Fe(III) reduction plays an important role in a variety of biogeochemical processes. Several mechanisms for microbial Fe(III) reduction in pH-neutral environments have been proposed, but pathways of microbial Fe(III) reduction within alkaline conditions have not been clearly identified. Alkaline soils are vastly distributed; thus, a better understanding of microbial Fe(III) reduction under alkaline conditions is of significance. The purpose of this study is to explore the dominant mechanism of bacterial iron reduction in alkaline environments. Materials and methods: We used antraquinone-2,6-disulfonate (AQDS) as a representative of quinone moities of humic substances and elemental sulfur and sulfate as sulfur species to investigate the potential role of humic substances and sulfur species in mediating microbial Fe(III) reduction in alkaline environments. We carried out thermodynamic calculations to predict the ability of bacteria to reduce Fe(III) (oxyhydr)oxides under alkaline conditions and the ability of AQDS and sulfur species to serve as electron acceptors for microbial anaerobic respiration in an assumed alkaline soil environments. A series of incubation experiments with two model dissimilatory metal reducing bacteria, Shewanella oneidensis MR-1 and Geobacter sulfurreducens PCA as well as mixed bacteria enriched from a soil were performed to confirm the contribution of AQDS and sulfur species to Fe(III) reduction under alkaline conditions. Results and discussion: Based on thermodynamic calculations, we predicted that, under alkaline conditions, the enzymatic reduction of Fe(III) (oxyhydr)oxides would be thermodynamically feasible but very weak. In our incubation experiments, the reduction of ferrihydrite by anaerobic cultures of Shewanella oneidensis MR-1, Geobacter sulfurreducens PCA or microbes enriched from a soil was significantly increased in the presence of S0 or AQDS. Notably, AQDS contributed more to promoting Fe(III) reduction as a soluble electron shuttle than S0 did under the alkaline conditions probably because of different mechanisms of microbial utilization of AQDS and S0. Conclusions: These results suggest that microbial reduction of Fe(III) (oxyhydr)oxides under alkaline conditions may proceed via a pathway mediated by electron shuttles such as AQDS and S0. Considering the high ability of electron shuttling and vast distribution of humic substances, we suggest that humic substance-mediated Fe(III) reduction may potentially be the dominant mechanism for Fe(III) reduction in alkaline environments

Thermodynamic energy of anaerobic microbial redox reactions couples elemental biogeochemical cycles

“…At alkaline pH (pH =~9), sulfate and sulfur reduction are the favored processes compared with the reduction of ferrihydrite and goethite. Our calculation (data not shown) points out that among the common Fe(III) minerals, only ferrihydrite, the most unstable one, can be reduced at alkaline pH up to 9 which is consistent with previous researches (Wang et al 2017). Actually, microbial reduction of Fe(III) minerals and sulfur species is often observed to occur simultaneously (Boesen and Postma 1988).…”

Redox Structures of Humic Acids Derived From Different Sediments and Their Effects on Microbial Reduction Reactions

“…The latter process, also known as ammonification or fermentative reduction, has recently attracted increased attention due to its significance for nitrogen cycling in various ecosystems, achieving greater N retention than denitrification, since the produced is more available for plant and microbial uptake but less prone to losses via leaching or volatilization compared with N 2 produced by denitrification ( Rütting et al, 2011 ). Dissimilatory microbial reduction of Fe(III) to Fe(II) plays a key role in environmental iron cycling and influences a number of biochemical processes such as the exchange of nutrients and trace metals between aquatic systems and sediments ( Wang et al, 2017 ). Thus, both DNRA and FeOOH reduction significantly affect organic matter exchange and geochemical cycling in anoxic systems.…”

Effect of Amino Acid Addition in Marine Sediment on Electrochemical Performance in Microbial Fuel Cells

“…The polarization curves of the different anodes are shown in Figure , and their anti‐polarization ability is Met > Thr > Blank > Lys. Methionine is a potential precursor of smaller electrochemically active compound methylated sulfur compounds . The methylated sulfur compounds could be utilized by microbial communities to produce methyl mercaptan.…”

“…Methionine, threonine and lysine are essential for the growth of microbial. For example, L‐methionine (L‐Met) is a latent precursor of methylated sulfur compounds as electrochemically active compound in anaerobic sediments, and microbial communities can metabolize volatile methylated sulfur compounds to produce methyl mercaptan. Therefore, in this study, amino acids are added into the sediments to promote the growth and attachment of elecroactive bacteria on the electrode surface and to increase the output power density of the battery .…”