Electromethanogenesis has emerged as a biological branch of Power-to-X technologies that implements methanogenic microorganisms, as an alternative to chemical Power-to-X, to convert electrical power from renewable sources, and CO2 into methane. Unlike biomethanation processes where CO2 is converted via exogenously added hydrogen, electromethanogenesis occurs in a bioelectrochemical set-up that combines electrodes and microorganisms. Thereby, mixed, or pure methanogenic cultures catalyze the reduction of CO2 to methane via reducing equivalents supplied by a cathode. Recent advances in electromethanogenesis have been driven by interdisciplinary research at the intersection of microbiology, electrochemistry, and engineering. Integrating the knowledge acquired from these areas is essential to address the specific challenges presented by this relatively young biotechnology, which include electron transfer limitations, low energy and product efficiencies, and reactor design to enable upscaling. This review approaches electromethanogenesis from a multidisciplinary perspective, putting emphasis on the extracellular electron uptake mechanisms that methanogens use to obtain energy from cathodes, since understanding these mechanisms is key to optimize the electrochemical conditions for the development of these systems. This work summarizes the direct and indirect extracellular electron uptake mechanisms that have been elucidated to date in methanogens, along with the ones that remain unsolved. As the study of microbial corrosion, a similar bioelectrochemical process with Fe0 as electron source, has contributed to elucidate different mechanisms on how methanogens use solid electron donors, insights from both fields, biocorrosion and electromethanogenesis, are combined. Based on the repertoire of mechanisms and their potential to convert CO2 to methane, we conclude that for future applications, electromethanogenesis should focus on the indirect mechanism with H2 as intermediary. By summarizing and linking the general aspects and challenges of this process, we hope that this review serves as a guide for researchers working on electromethanogenesis in different areas of expertise to overcome the current limitations and continue with the optimization of this promising interdisciplinary technology.

Measuring the distribution and dynamics of H2 in microbial electrochemical reactors is valuable to gain insights into the processes behind novel bioelectrochemical technologies, such as microbial electrosynthesis. Here, a microsensor method to measure and profile dissolved H2 concentrations in standard H-cell reactors is described. Graphite cathodes were oriented horizontally to enable the use of a motorized microprofiling system and a stereomicroscope was used to place the H2 microsensor precisely on the cathode surface. Profiling was performed towards the gas-liquid interface, while preserving the electric connections and flushing the headspace (to maintain anoxic conditions) and under strict temperature control (to overcome the temperature sensitivity of the microsensors). This method was tested by profiling six reactors, with and without inoculation of the acetogen Sporomusa ovata, at three different time points. H2 accumulated over time in the abiotic controls, while S. ovata maintained low H2 concentrations throughout the liquid phase (< 4 μM) during the whole experimental period. These results demonstrate that this setup generated insightful H2 profiles. However, various limitations of this microsensor method were identified, as headspace flushing lowered the dissolved H2 concentrations over time. Moreover, microsensors can likely not accurately measure H2 in the immediate vicinity of the solid cathode, because the solids cathode surface obstructs H2 diffusion into the microsensor. Finally, the reactors had to be discarded after microsensor profiling. Interested users should bear these considerations in mind when applying microsensors to characterize microbial electrochemical reactors.

Acetogenic bacteria play an important role in various biotechnological processes, because of their chemolithoautotrophic metabolism converting carbon dioxide with molecular hydrogen (H 2 ) as electron donor into acetate. As the main factor limiting acetogenesis is often H 2 , insights into the H 2 consumption kinetics of acetogens are required to assess their potential in biotechnological processes. In this study, initial H 2 consumption rates at a range of different initial H 2 concentrations were measured for three different acetogens. Interestingly, for all three strains, H 2 consumption was found to follow first-order kinetics, i.e. the H 2 consumption rate increased linearly with the dissolved H 2 concentration, up to almost saturated H 2 levels (600 µM). This is in contrast with Monod kinetics and low half-saturation concentrations, which have commonly been assumed for acetogens. The obtained biomass specific first-order rate coefficients ( k 1 X ) were further validated by comparison with values obtained by fitting first-order kinetics on previous time-course experimental results. The latter method was also used to determine the k 1 X value of five additional acetogens strains. Biomass specific first-order rate coefficients were found to vary up to six-fold, with the highest k 1 X for Acetobacterium wieringae and the lowest for Sporomusa sphaeroides . Overall, our results demonstrate the importance of the dissolved H 2 concentration to understand the rate of acetogenesis in biotechnological systems.

Acetogenic bacteria play an important role in various environmental and biotechnological processes, because of their autotrophic metabolism converting carbon dioxide with molecular hydrogen (H2) as electron donor into acetate. The main factor limiting acetogenesis is often H2. Assessing the potential of acetogens in environmental and biotechnological processes thus requires insights into their H2 consumption kinetics. In this study, initial H2 consumption rates at a range of different initial H2 concentrations were measured for three different acetogens. Interesting, for all three strains, H2 consumption was found to follow first-order kinetics, i.e. the H2 consumption rate increased linearly with the dissolved H2 concentration up to almost saturated H2 levels. This contradicts Monod kinetics, which is commonly assumed for acetogens. The obtained first-order rate coefficients ( k 1) were further validated by fitting first-order kinetics on previous time-course experimental results. The latter method was also used to determine the k 1 value of five additional acetogens strains. Biomass specific first-order rate coefficients were found to vary up to six-fold, with the highest k 1 for Acetobacterium wieringae and the lowest for Sporomusa sphaeroides . Overall, our results demonstrate the importance of the dissolved H2 concentration to understand the rate of acetogenesis in environmental and biotechnological settings. Synopsis Acetogenic bacteria are diverse, although they catalyze the same overall reaction. This study provides insights into their different H2 consumption rates, which has implications for their role in the environment and in biotechnological applications. ### Competing Interest Statement The authors have declared no competing interest.

Shewanella species are frequently selected as model strains to investigate microbially influenced steel corrosion. This selection is due to their relevance for corrosion, but also because of their easy cultivation in aerobic media. Unfortunately, these cultivation advantages do not lead to a straight-forward interpretation of their corrosion inducing or inhibiting mechanisms. The metabolic versatility of Shewanellae indeed enables a wide variety of corrosion mechanisms. This work reviews the metabolic capacities and the extracellular electron transfer mechanisms of Shewanellae and explains how these abilities lead to the various mechanisms by which Shewanellae induce and inhibit corrosion. It should be emphasized that the medium composition (presence of electron donor, acceptor, carbon source) strongly affects which mechanism is in play. Overall, this work concludes that Shewanellae model strains offer great opportunities to study corrosion, thanks in part due to genetic engineering options, but the full complexity of their corrosion processes should always be kept mind.

Acetogens share the capacity to convert H2 and CO2 into acetate for energy conservation (ATP synthesis). This reaction is attractive for applications, such as gas fermentation and microbial electrosynthesis. Different H2 partial pressures prevail in these distinctive applications (low concentrations during microbial electrosynthesis [<40 Pa] vs. high concentrations with gas fermentation [>9%]). Strain selection thus requires understanding of how different acetogens perform under different H2 partial pressures. Here, we determined the H2 threshold (H2 partial pressure at which acetogenesis halts) for eight different acetogenic strains under comparable conditions. We found a three orders of magnitude difference between the lowest and highest H2 threshold (6 ± 2 Pa for Sporomusa ovata vs. 1990 ± 67 Pa for Clostridium autoethanogenum), while Acetobacterium strains had intermediate H2 thresholds. We used these H2 thresholds to estimate ATP gains, which ranged from 0.16 to 1.01 mol ATP per mol acetate (S. ovata vs. C. autoethanogenum). The experimental H2 thresholds thus suggest strong differences in the bioenergetics of acetogenic strains and possibly also in their growth yields and kinetics. We conclude that no acetogen is equal and that a good understanding of their differences is essential to select the most optimal strain for different biotechnological applications.

Some acetogenic bacteria are capable of using solid electron donors, such as a cathode or metallic iron [Fe(0)]. Acetogens using a cathode as electron donor are of interest for novel applications such as microbial electrosynthesis, while microorganisms using Fe(0) as electron donor cause detrimental microbial induced corrosion. The capacity to use solid electron donors strongly differs between acetogenic strains, which likely relates to their extracellular electron transfer (EET) mechanism. Different EET mechanisms have been proposed for acetogenic bacteria, including a direct mechanism and a H2 dependent indirect mechanism combined with extracellular hydrogenases catalyzing the H2 evolution reaction on the cathode or Fe(0) surface. Interestingly, low H2 partial pressures often prevail during acetogenesis with solid electron donors. Hence, an additional mechanism is here proposed: the maintenance of low H2 partial pressures by microbial H2 consumption, which thermodynamically favors the H2 evolution reaction on the cathode or Fe(0) surface. This work elaborates how the H2 partial pressure affects the H2 evolution onset potential and the H2 evolution rate on a cathode, as well as the free energy change of the anoxic corrosion reaction. In addition, the H2 consumption characteristics, i.e., H2 threshold (thermodynamic limit for H2 consumption) and H2 consumption kinetic parameters, of acetogenic bacteria are reviewed and evidence is discussed for strongly different H2 consumption characteristics. Different acetogenic strains are thus expected to maintain different H2 partial pressures on a cathode or Fe(0) surface, while those that maintain lower H2 partial pressures (lower H2 threshold, higher H2 affinity) more strongly increase the H2 evolution reaction. Consequently, I hypothesize that the different capacities of acetogenic bacteria to use solid electron donors are related to differences in their H2 consumption characteristics. The focus of this work is on acetogenic bacteria, but similar considerations are likely also relevant for other hydrogenotrophic microorganisms.

Although Reverse Electrodialysis (RED) is most commonly known as a selective separation technology used for the production of sustainable energy, it can also serve as a valuable pre-desalination tool. By coupling RED to Reverse Osmosis (RO) for seawater desalination: (1) sustainable energy is produced in the RED process and (2) seawater is partially desalinated prior to RO thus, decreasing the energy demand. In this study, secondary-treated wastewater is proposed as the low salinity source in RED and suitable pre-treatment techniques for this effluent are investigated. Although it is generally accepted that RED is less prone to fouling than typical pressure driven membrane processes, results showed that pre-treatment is a key to ensure efficient operation of the wastewater-seawater RED. Both 100 mu m filtration and rapid sand filtration proved to be suitable, with an increase in pressure drop of only 0.09-0.18 bar and a permselectivity decrease of only approximately 20% during 40 days of continuous operation. Conversely, River bank filtration did not perform better than the non-pretreated sample. As such, 100 mu m filtration and rapid sand filtration are considered suitable, robust, and cost efficient pre-treatment options for wastewater fed RED, enabling the improvement of the hybrid process of RED-RO seawater desalination.

Sporomusa sphaeroides related strains are to date the only homoacetogens known to increase metallic iron corrosion. The goal of this work was to isolate additional homoacetogenic bacteria capable of using Fe(0) as electron donor and to explore their extracellular electron transfer mechanism. Enrichments were started from anoxic corrosion products and yielded Acetobacterium as main homoacetogenic genus. Isolations were performed with a new procedure using plates with a Fe(0) powder top layer. An Acetobacterium strain, closely related to A. malicum and A. wieringae, was isolated, in addition to a S. sphaeroides strain. The Acetobacterium isolate significantly increased Fe(0) corrosion ((1.44 ± 0.16)-fold) compared to abiotic controls. The increase of corrosion by type strains ranged from (1.28 ± 0.13)-fold for A. woodii to (2.03 ± 0.22)-fold for S. sphaeroides. Hydrogen mediated the electron uptake from Fe(0) by the acetogenic isolates and tested type strains. Exchange of the medium and SEM imaging suggested that cells were attached to Fe(0). The corrosion enhancement mechanism is for all tested strains likely related to free extracellular components catalyzing hydrogen formation on the Fe(0) surface, or to the maintenance of low hydrogen concentrations on the Fe(0) surface by attached cells thereby thermodynamically favoring hydrogen formation.

Shewanella spp. are frequently found on corroded metal structures. Their role in microbial influenced corrosion has been attributed mainly to their Fe(III)-reducing properties and, therefore, has been studied with the addition of an electron donor (lactate). Shewanella spp., however, can also use solid electron donors, such as cathodes and potentially Fe(0). In this work, we show that the electron acceptor fumarate supported the use of Fe(0) as the electron donor by Shewanella strain 4t3-1-2LB, which caused a (7.0 ± 0.6)-fold increase of the corrosion rate. The corrosion-enhancing mechanism likely involved cell surface-associated components in direct contact with the Fe(0) surface or maintenance of low hydrogen levels by attached cells, thereby favoring chemical hydrogen formation by Fe(0). This work sheds new light on the role of Shewanella spp. in biocorrosion, while the insights into the corrosion-enhancing mechanism contribute to the understanding of extracellular electron uptake processes.

The Back Cover picture illustrates the discharge of an electroactive biofilm recorded by double potential step chronoamperometry, allowing the fast determination of charge-transfer parameters across the biofilm (Artwork: A. Landi). More information can be found in the Article by A. Prévoteau and co-workers on page 1026 in Issue 5, 2017 (DOI: 10.1002/celc.201600853).

The mechanism of electron transport across electroactive biofilms (EABs) is of high interest and still a matter of debate. Quantitative assessments of their redox conduction take considerable time and require non-turnover conditions (absence of substrate), which can be detrimental to EABs. Here, we measure the charge-transport parameters of Geobacter spp. dominated EABs with double potential step chronoamperometry (DPSC) with Cottrell analysis. The DPSC measurement is simpler and much faster than usual techniques and allows the determination of the charge-transport parameters even under turnover conditions. The electrochemical responses were well-described by a model of redox conduction driven only by electron diffusion within the EAB. The apparent diffusion coefficient for the electron (Dapp) was measured as approximately 3.2 x 10(-7) cm(2) s(-1), a value similar to those recorded for pure Geobacter sulfurreducens EABs, or for some redox polymers with comparable redox center concentrations. This method will be valuable for assessing the impact of EAB characteristics and environmental factors on the charge-transport ability of the biofilm, and for determining the rate-limiting step(s) for current production.