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Microbial Interspecies Electron Transfer Via Electric Currents Through Conductive Minerals

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Microbial Interspecies Electron Transfer Via Electric Currents Through Conductive Minerals Microbial interspecies electron transfer via electric currents through conductive minerals Souichiro Katoa,1, Kazuhito Hashimotoa,b,c,2, and Kazuya Watanabea,b,d,2 aHashimoto Light Energy Conversion Project, Exploratory Research for Advanced Technology (ERATO), Japan Science and Technology Agency, Tokyo 113-8656, Japan; bResearch Center for Advanced Science and Technology, University of Tokyo, Tokyo 153-8904, Japan; cDepartment of Applied Chemistry, University of Tokyo 113-8656, Tokyo, Japan; and dSchool of Life Sciences, Tokyo University of Pharmacy and Life Sciences, Tokyo 192-0392, Japan Edited by James M. Tiedje, Michigan State University, East Lansing, MI, and approved May 03, 2012 (received for review October 27, 2011) In anaerobic biota, reducing equivalents (electrons) are transferred Concerning this idea, a recent study has suggested that spa- between different species of microbes [interspecies electron trans- tially separated biogeochemical redox processes in marine sedi- fer (IET)], establishing the basis of cooperative behaviors and ment (i.e., sulfide oxidation at depths of >10 mm and oxygen community functions. IET mechanisms described so far are based reduction at the surface) are connected by electric currents, but on diffusion of redox chemical species and/or direct contact in cell not by diffusion of redox molecules (12). Although that study did aggregates. Here, we show another possibility that IET also occurs not identify electron conduits and electrodes that could work in via electric currents through natural conductive minerals. Our marine sediments, possible mechanisms for electron transfer investigation revealed that electrically conductive magnetite have been discussed, including those via microbial nanowires, nanoparticles facilitated IET from Geobacter sulfurreducens to Thi- direct contact of outer-membrane cytochromes, and natural obacillus denitrificans, accomplishing acetate oxidation coupled to minerals, such as pyrite (12). Among them, we consider that nitrate reduction. This two-species cooperative catabolism also oc- natural minerals are likely candidates for natural electron con- curred, albeit one order of magnitude slower, in the presence of Fe duits for the following reasons. First, (semi)conductive minerals ions that worked as diffusive redox species. Semiconductive and [e.g., pyrite (semiconductive), magnetite (conductive), and he- insulating iron-oxide nanoparticles did not accelerate the cooper- matite (semiconductive)] are abundant in natural soil and sedi- ative catabolism. Our results suggest that microbes use conductive ments (13, 14); for instance, marine sediments in the Aarhus bay fi mineral particles as conduits of electrons, resulting in ef cient IET contain 30 μmol of pyrite per gram (12). Second, it has recently MICROBIOLOGY and cooperative catabolism. Furthermore, such natural mineral been shown that some bacteria use iron-oxide mineral nano- conduits are considered to provide ecological advantages for particles (magnetite and hematite) as conduits of electrons to- users, because their investments in IET can be reduced. Given that ward working electrodes in electrochemical chambers (15, 16). conductive minerals are ubiquitously and abundantly present in The present study was conducted to evaluate a possibility that nature, electric interactions between microbes and conductive conductive minerals can serve as electric conduits that connect minerals may contribute greatly to the coupling of biogeochem- redox reactions catalyzed by different species of microbes. In ical reactions. anaerobic biota, electrons are transferred between different species of microbes [interspecies electron transfer (IET)], bioenergy | biogeochemistry | electrode respiration | microbial ecology establishing a basis of cooperative behaviors and community functions (17–19). IET mechanisms described so far are based on iological energy conservation generally depends on the diffusion of redox chemical species (20) and/or direct contact in Bcoupling of oxidative and reductive chemical reactions. The cell aggregates (21). To address the involvement of conductive most prominent is organics oxidation coupled to oxygen re- minerals in IET, we constructed mixed cultures of two repre- G. sulfurreducens Thiobacillus duction; most of organisms, including animals and plants, exploit sentative soil bacteria, (2, 5) and denitrificans this type of catabolism for energy conservation (1). Recent (22), in the presence and absence of iron-oxide studies, however, have revealed that some bacteria perform minerals, and their IET-dependent cooperative catabolic reac- anodic and cathodic half reactions using electrodes (mostly tions were investigated. graphite carbons) as electron donors and acceptors, respectively. Results For example, dissimilatory iron-reducing bacteria (DMRB) Shewanella Geobacter Electrochemical Characteristics of Catabolic Reactions in Monocultures. (such as and ) oxidize organics and transfer We attempted to construct IET-dependent mixed cultures con- electrons to anodes, resulting in the current generation in sisting of two soil bacteria, G. sulfurreducens and T. denitrificans. G. G. metallireducens microbial fuel cells (2, 3). On the other hand, sulfurreducens is a widely studied DMRB (5) and has abilities to accepts electrons from graphite cathodes and use them for respire a metal-oxide electrode [a tin-doped In2O3 (ITO) glass nitrate reduction (4). electrode] coupled to acetate oxidation (Fig. 1A). The anodic Although microbe/electrode interactions promise to be ap- current [at +0.4 V vs. standard hydrogen electrode (SHE)] was − plied to bioelectrochemical processes, such as microbial fuel/ initially stable at ∼7 μAcm2, although it increased afterward, electrolysis cells (5–7) and bioremediation (8, 9), their ecological probably owing to the formation of conductive biofilms (23). In and evolutionary origins remain enigmatic. Although some sci- entists deduce that microbe/electrode interactions stem from the abilities of bacteria to reduce extracellular solid metals as ter- Author contributions: S.K., K.H., and K.W. designed research; S.K. performed research; minal electron acceptors (5), recent studies have disclosed that S.K. and K.W. analyzed data; and S.K. and K.W. wrote the paper. electron transfer to electrodes does not necessarily share iden- The authors declare no conflict of interest. tical cellular components to those for solid-metal reduction (10, This article is a PNAS Direct Submission. 1 11). It is, therefore, likely that electrodes (that do not accept Present address: Bioproduction Research Institute, National Institute of Advanced Indus- trial Science and Technology, Sapporo 062-8517, Japan. electrons by themselves but transfer electrons to electron 2To whom correspondence may be addressed. E-mail: [email protected] or acceptors) may also exist in the natural environment, allowing [email protected]. microbes to evolve abilities to interact with electrodes (either This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. natural or artificial). 1073/pnas.1117592109/-/DCSupplemental. www.pnas.org/cgi/doi/10.1073/pnas.1117592109 PNAS Early Edition | 1of5 Downloaded by guest on September 24, 2021 Fig. 1. Electrochemical characteristics of G. sulfurreducens (blue lines) and T. denitrificans (black lines) on ITO electrodes. Anodic currents were measured using an electrode poised at +0.4 V vs. SHE after supplemented with 20 mM acetate (A), whereas cathodic currents were measured at −0.4 V vs. SHE with − 30 mM nitrate (B). Linear sweep voltammograms (C) were measured at a scan rate of 1 mV s 1 30 h after initiating the incubation. contrast, T. denitrificans is a chemolithoautotrophic bacterium and Cooperative Catabolism in the Presence of Conductive Iron Minerals. known to use solid iron minerals (e.g., pyrite) as electron donors To examine IET between G. sulfurreducens and T. denitrificans, (22). We confirmed that the metal-oxide electrode (at −0.4 V vs. anaerobic culture media were inoculated with these bacteria and SHE) served as an electron donor for T. denitrificans in the pres- supplemented with acetate (20 mM) and nitrate (30 mM) as ence of nitrate (Fig. 1B), generating a constant cathodic current at potential electron donor and acceptor, respectively. In a single − 4–5 μAcm2. The stable current density could be attributed to culture of either of these bacteria, neither acetate oxidation nor limited electron transfer by bacterial cells that directly attached to nitrate reduction was observed (Fig. 2A), because G. sulfurre- the electrode. In the absence of nitrate, a current density was ducens is able to oxidize acetate but unable to reduce nitrate and − negligible; e.g., below 0.2 μAcm2 at hour 20. Electrochemical vice versa for T. denitrificans. Under this condition, IET between characteristics of G. sulfurreducens and T. denitrificans were further the two bacterial species is required for accomplishing the co- analyzed by linear sweep voltammetry (Fig. 1C). It is shown that G. operative catabolism (acetate oxidation coupled to nitrate re- sulfurreducens generated anodic current in a potential range of over duction). The reaction, however, did not occur even in their −0.3 V (vs. SHE), whereas T. denitrificans generated cathodic coculture (Fig. 2A). IETs mediated by H2 and formate do not current in a potential range of below 0 V (vs. SHE). These results occur in their coculture, because T. denitrificans cannot use these suggest that electric currents
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