Electron Shuttling by Biochar

Electron transfer from Fe(III)-reducing bacteria to Fe(III) minerals depends on the accessibility of Fe(III) to the cells. It has been demonstrated that direct contact and electron transfer via outer membrane cytochromes or nanowires, electron shuttling via redox-active organic and inorganic molecules, or even Fe(III) solubilization via chelators is an efficient strategy for transferring electrons from cells to Fe(III). (47, 48) In our non-growth cell suspension experiments, during which the synthesis of cellularly derived electron shuttles, nanowires, and Fe(III) chelators should be limited if not completely prevented (although nutrients from lysed cells might facilitate low levels of biosynthesis), we observed that addition of redox-active biochar particles at concentrations of 5 and 10 g Lstimulated electron transfer. Abiotic controls with biochar and ferrihydrite showed that even in the absence of cells the biochar contained electrons that can be transferred to Fe(III) [∼10 and ∼18% Fe(II) formation for 5 and 10 g Lbiochar setups, respectively (Figure 1 A and Figure S2 of the Supporting Information )]. However, in the presence of cells and 5 and 10 g Lbiochar, microbial reduction rates and extents increased significantly beyond the additive values of biochar with ferrihydrite and cells with ferrihydrite, respectively. This suggests that biochar itself can stimulate electron transfer by functioning as an electron acceptor forMR-1 and by transferring electrons from microbially reduced biochar to the Fe(III) mineral ferrihydrite. A similar stimulation of microbial ferrihydrite reduction by redox-active particulate compounds has been described for solid-phase humic substances (33) and for redox-active/conductive iron minerals. (49) For biochar, it has also been demonstrated that it can be reduced electrochemically. Depending on the biochar feedstock and charring temperature, biochar can store several hundred micromoles of electrons per gram of biochar. (22) These values of electron uptake capacities are in a range similar to that observed for humic substances, (36, 50) further supporting the potential role of biochar as an electron shuttle between Fe(III)-reducing bacteria and Fe(III) minerals.