Exoelectrogens like bacteria from the Geobacter and Shewanella species have the ability to transfer electrons extracellularly to minerals for redox balancing of fermentation or to respire on extracellular electron acceptors (Shi et al., 2016; Lovley and Holmes, 2022). Extracellular electron exchange is also used between microbes in multi-species biofilms in anaerobic digestion. Over the last two decades, this exquisite ability of nature to transfer electrons extracellularly has seen a spurge of research into potential new applications. Replacing extracellular electron acceptors with macroscopic electrodes creates microbial fuel cells to convert the oxidation of organics from, for instance, wastewater into electricity (Logan et al., 2019). In a reversed direction of electron transfer (ie ‘inwards’ rather than ‘outwards’), microbes gather energy from extracellular electron donors, such as Fe (II)-bearing minerals. This has important consequences to the economy as it enhances biocorrosion, but the same ability is currently being developed for applications in electrobiosynthesis. Biofilms on conductive materials can be ‘fed’by applying electric potentials to drive synthesis. For some bacteria such as Shewanella oneidensis, it has been discovered that their natural ‘outward’electron transfer direction can be reversed (Ross et al., 2011), significantly widening the number of microbes that can be engineered for electrobiosynthesis. More recently, exoelectrogens, as well as nonexoelectrogens, have been coupled to light-harvesting nanomaterials, mainly CdS quantum dots, creating semiartificial photosynthetic biohybrids (Sakimoto et al., 2016; Wang et al., 2019; Martins et al., 2021). Such biohybrids could in principle be engineered to use light energy to drive either outward and inward electron transfer (Piper et al., 2021). So far, the focus has mainly been on photobioelectrosynthesis, where light-driven electron transfer into microbes is utilized for the synthesis of organic materials, ammonia or hydrogen. Early on in these studies, an important model exoelectrogen, Shewanella oneindensis MR-1, was developed for application in bioremediation (Marshall et al., 2006; Shi et al., 2016). S. oneindensis MR-1 is a dissimilatory metal ion-reducing bacterium with extreme diverse respiratory capabilities. Pollutants such chromium and radioactive uranium are reduced by S. oneindensis MR-1 from the soluble Cr (VI) and U (VI), to the insoluble Cr (IV) and U (IV), thereby preventing leaching from the polluted grounds. Key proteins from the metal-reducing (Mtr) pathway were subsequently found to also be responsible for exoelectrogenic capabilities of S. oneidensis MR-1 (Marshall et al., 2006). Especially the protein complex MtrCAB was found to transfer electrons across the outer membrane from the periplasm to the extracellular environment. Following on from this early work in metal reduction, it was shown that S. oneidensis MR-1 could reductively bleach a range of azo dye pollutants and the Mtr pathway was often identified to contribute to the rate of decolouration (Watanabe et al., 2009; Liu et al., 2016).

In spite of the inspiring capability of S. oneidensis MR-1 for extracellular electron transfer, it should not be forgotten that many reduction processes take place in the periplasm of Gram-negative bacteria. Furthermore, the degradation of chemicals and pollutants in general is often due to chemical processes in the cytoplasm. In work by Zhu et al.(2022), reported in this issue of Environmental Microbiology, it is indeed observed that nitroaromatic compounds and other pollutants are not necessary reduced outside the bacteria. In particular, they show that the inner-membrane protein CymA reduces …