The first electron counts – how anaerobic microbes ‘breathe’ iron

Scientists at ETH Zurich and the Swiss Federal Institute of Aquatic Science and Technology have now reported that the speed of two-electron transport from microbes to extracellular minerals may be best described by just considering the ease with which the first electron hops over. Using insight from electrochemical experiments combined with UV-V is spectroscopy, the researchers reconciled literature data on the rate and energy balance of a reaction that is crucial to microorganisms in anaerobic environments. Most living species power various biological functions by passing electrons through a sequence of carriers of decreasing potential energy within a respiratory chain. Such a chain needs a constant source of high-energy electrons, usually from food (ingested or generated by photosynthesis) or another substrate, plus a sink to suck up low-energy electrons after they have done their useful work, for most ecosystems, oxygen acts as this terminal sink; but under low-oxygen conditions, cells need to find alternatives. For example, minerals in the soil that contain iron in an oxidized form can take up these electrons. Because these grains of rock are located outside the cell, some microbes use extracellular electron shuttles (extracellular electron shuttle) – small molecules that can transport one or two electrons – to ensure efficient delivery. Therefore, the final stage of respiration in such organisms involves the release of electrons stored in extracellular electron shuttles to the iron mineral, for any chemical reaction, two quantities are often of interest: how fast the reaction progresses – expressed as the reaction rate – and the free energy balance, which determines the direction of the reaction. Although these are not generally related parameters, "Marcus Theory" predicts that the transfer of an electron between two molecules will be faster if it is more energetically favorable (except in what is termed the “inverted” Marcus region). However, this correlation was not obvious in existing data describing the transport of two electrons from an extracellular electron shuttle to iron. This quandary motivated the Swiss team’s experiments. The energy change involved in a reaction can be found mathematically by subtracting reduction potentials of the participating molecules. “Our biggest challenge was to find reliable values for the standard reduction potentials of extracellular electron shuttle,” explains first author "Meret Aeppli" (now at Stanford University). These values were indispensable for the calculation of Gibbs free energies. Importantly, the researchers were able to determine this potential for each step of the two-electron process, not just an average value describing the overall reaction. Their results showed how the transfer of the first electron releases less energy than that of the second electron. When the researchers compared results from three different extracellular electron shuttles to the respective reaction speeds, they observed a consistent correlation only for the energy associated with the transfer of the first electron. “We show that rates of iron oxide reduction by reduced extracellular electron shuttle scale with the free energy of the less exergonic [less favorable] first of the two electron transfers,” says Aeppli. Moreover, the “free energy relationship unifies rate data from this and past work for different extracellular electron shuttle and different solution conditions”. While the research is completed for now as the final part of "Aeppli’s PhD" thesis, she sees potential in future investigations “extending the framework presented in the paper to other iron minerals and mixed mineral systems” as well as “conducting experiments with iron minerals and added microorganisms, extracellular electron shuttles and organic substrates”. Such studies could enable scientists to gain an even deeper understanding of the electron transport processes happening in and around anaerobic microbes. The research is described in the Proceedings of the National Academy of Sciences