Controlling electron transfer at the microbe-mineral interface.

Remarkably, many bacteria live and thrive in the earth’s subsurface by respiring extracellular insoluble minerals. Okamato et al. (1) in PNAS report how this process may be accelerated by the presence of flavins that bind as cofactors to electron transport proteins on the cell surface that are key to extracellular mineral respiration. Humans obtain the energy needed for life by respiring oxygen. This process involves using electrons extracted from food to reduce oxygen into water in our mitochondria. Free energy is released in this process, and we use this to make ATP, which is the universal energy currency of life. Our dependency on oxygen makes us obligate aerobes—take away the oxygen and we die. Thus, humans are confined to living on the surface of planet Earth where oxygen is freely available. However, the vast proportion of the earth’s habitable environments is not exploited by humans, but by a diversity of microorganisms, including bacteria, that can live in the absence of oxygen through a process called anaerobic respiration. Some of the best studied anaerobic respiratory electron acceptors are water soluble, such as nitrate or sulfate, and these anions can be transported relatively easily into bacterial cells where their respiratory reduction occurs. However, insoluble minerals, particularly minerals of iron and manganese, represent some of the most abundant respiratory substrates in the earth’s subsurface environments. In fact “mineral iron respiration” is among the most widespread respiratory processes in anoxic zones and so has wide environmental significance (2–4). For example, it directly impacts on the balance of several biogeochemical cycles such as the nitrogen, sulfur, and carbon cycles, and in turn influence the release of potent greenhouse gases, such as nitrous oxide. In some aspects, the way bacteria respire mineral iron (or manganese) is similar to the way in which our own mitochondria respire oxygen. Electrons generated from organic carbon metabolism are used to “reduce” the respiratory substrate. Thus, electrons generated by metabolism inside the bacterial cell are passed to the mineral iron, which is reduced from the ferric to ferrous state. However, because the ferric iron mineral is an insoluble particle, it cannot freely diffuse into bacterial cells. Consequently, if a bacterium is to be able to use a ferric mineral as a respiratory electron acceptor, it must have a molecular answer to the challenge of moving electrons generated by intracellular metabolism to minerals located outside of the cell (5). The mechanism by which electrons are transferred to, and across, the microbe– mineral interface is not fully resolved and represents a major question in the study of the biochemistry of an environmentally abundant group of bacteria. Answers will provide new insights into bacterial energetics. They are also likely to have important biotechnological impacts because there is potential for harnessing mineral-respiring bacteria in bioremediation processes for the clean-up of environments contaminated with toxic organic or inorganic pollutants. For example, reduction of soluble uranium (VI) to insoluble uranium (IV) can reduce the levels of uranium in the groundwater of contaminated sites. Mineral-respiring bacteria also have applications in microbial fuel cells and microbial electrosynthesis, both of which rely on electron exchange between microbes and solid extracellular substrates in the form of electrodes (6). It is such electron exchange, between bacteria and electrodes, that has been studied by Okamato et al. (1) using a technique called differential pulse voltammetry. Monolayer biofilms of the mineral ironreducing bacterium Shewanella oneidensis were grown on indium tin oxide (ITO) electrodes, and the flow of current due to interfacial electron exchange was measured in the absence and presence of added flavins. Bacteria from the genus Shewanella respire on extracellular substrates by transporting electrons to the cell surface via porin–cytochrome complexes in the outer membrane (7). These complexes facilitate electron transport by allowing periplasmic cytochromes to transfer electrons to cellsurface cytochromes through a transmembrane porin (Fig. 1). The best characterized of these porin–cytochrome complexes is the metal reduction (MtrCAB) complex from S. oneidensis, which binds a total of 20 electron-carrying heme cofactors that form an electron transfer conduit of some 20 nm. The Shewanella family of outer membrane multiheme cytochromes (OMMCs) form four major clades—outer membrane cytochrome A (OmcA), metal reducing cytochrome C (MtrC), undecyl cytochrome A (UndA), and metal reducing cytochrome F (MtrF). Recent work (8, 9), provided the first structures of OMMCs from Shewanella and revealed that ten hemes were bound as a “staggered cross” on the cell surface (Fig. 1). Of the Shewanella OMMCs, the S. oneidensis Fig. 1. A diagram of the transmembrane 20-heme cytochrome–porin MtrCAB complex. The staggered heme cross indicated in MtrC is expected to be a feature of both MtrC and OmcA studied by Okamoto et al. (1), who have proposed that flavin binds as a cofactor to these proteins and that this binding modulates the redox properties of the flavin, so enhancing the rate of electron transfer from MtrC and OmcA to insoluble extracellular substrates. The red spheres depict heme cofactors.