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Bioelectrochemical power-to-gas presents a promising technology for long-term storage of excess renewable energy in the form of methane. The transition of the technology from laboratory to applied scale is currently challenged by low volumetric production rates, energy losses at the cathode, as well as the unknown physiology of the microbes in an electrochemical reactor. Here, we introduce a stable electromethanogenesis system based on efficient in situ hydrogen production by non-precious-metal catalysts and effective hydrogen uptake by the methanogenic microorganisms. Using NiMo-cathodes and pure cultures of hydrogenotrophic Methanococcus maripaludis, our system achieved an unprecedented volumetric methane production rate from CO2 of 1.4 L methane per L per day. The system performed stably for over 4 weeks with columbic efficiencies steadily above 90%. A physiological analysis of cells in the electromethanogenic reactor revealed robustly-growing cells with nearly identical protein expression patterns to gas-fed controls. Local pH fluctuations at the surface of cathode and cation exchange membrane resulted in a small but noticeable fraction of cell lysis. Our data collectively indicate that physiologically uncompromised cells of a pure methanogenic culture can perform methanogenesis robustly at high specific rate in a biocompatible electromethanogenic reactor using inexpensive, earth-abundant cathode materials.

A microbial electrosynthesis cell comprising two biological cathode chambers sharing the same anode compartment is used to promote the production of C2-C4 carboxylic acids and alcohols from carbon dioxide. Each cathode chamber provides ideal pH conditions to favor acetogenesis/carbon chain elongation (pH = 6.9), and solventogenesis (pH = 4.9), respectively, without the requirement of external acid/base dosing.

CO2 emissions can be transformed into high-added-value commodities through CO2 electrocatalysis; however, efficient low-cost electrocatalysts are needed for global scale-up. Inspired by other emerging technologies, the authors report the development of a gas diffusion electrode containing highly dispersed Ag sites in a low-cost Zn matrix. This catalyst shows unprecedented Ag mass activity for CO production: -614 mA cm-2 at 0.17 mg of Ag. Subsequent electrolyte engineering demonstrates that halide anions can further improve stability and activity of the Zn-Ag catalyst, outperforming pure Ag and Au. Membrane electrode assemblies are constructed and coupled to a microbial process that converts the CO to acetate and ethanol. Combined, these concepts present pathways to design catalysts and systems for CO2 conversion toward sought-after products.

The potent greenhouse gas methane presents a widely accessible resource, being the primary component in natural gas as well as in bio-gas from anaerobic digesters. Given its relatively low heating-value and several issues concerning its storage and transportation, methane upgrading to liquid fuels is of particular interest. Microbial methane conversion/utilization and upgrading is gaining increasing interest due to its high conversion efficiency. In this study we computationally compare aerobic and anaerobic microbial pathways for CH4 oxidation and discuss theoretically achievable biomass yields as well as the possibility for building synthetic biological production platforms for liquid fuels. Specifically, the presented in-silico work investigates the potential of microbial methane upgrading in a metabolic network analysis by means of elementary flux modes. Aerobic fixation of methane via conversion of methane to methanol by a methane monooxygenase (MMO) and different subsequent formaldehyde assimilation pathways (Serine-cycle, RuMP, XMP/DHA-pathway) is compared with anaerobic pathways for oxidation of methane (AOM) by means of reverse-methanogenesis or via a presumed glycyl-radical enzyme, which uses fumarate for activation of methane. The different pathways for aerobic and anaerobic methane oxidation are compared in different central carbon-metabolism envelopes in order to identify highest achievable carbon yields. The capability of efficient CO2 fixation, as well as energy preservation in form of reducing equivalents is identified as crucial to enable high yields, which ranged from 22% to 100%. The potential of the different microbes to grow on these gas streams is assessed by means of the maximum achievable biomass yield and the CO2/CH4 uptake ratio. CO2 co-utilisation, by transferring reducing power between the two co-substrates, is highest, when combining reverse-methanogenesis with the Wood-Ljungdahl pathway, effectively replacing the need for H2 with CH4. Further, the possibility to upgrade methane into liquid (drop-in) bio-fuels is investigated. Established routes to methanol, ethanol, C4-alcoholes and farnesene are evaluated in the most promising substrate-pathway / organism combinations. Stoichiometric, thermodynamic and kinetic limitations are assessed and recommendations regarding potential industrial feasibility are given. The results presented here should guide future research efforts in search for feasible ways of (co)utilizing novel carbon substrates for sustainable production of fuels and chemicals.

Amphiphilic polycations are an alternative to biocides but also toxic to mammalian cells. Antimicrobially active hydrophilic polycations based on 1,4-dibromo-2-butene and tetramethyl-1,3-propanediamine named PBI are not hemotoxic for porcine red blood cells with a hemocytotoxicity (HC50) of more than 40,000 μg · mL(-1). They are quickly killing bacterial cells at their MIC (minimal inhibitory concentration). The highest found selectivity HC50 /MIC is more than 20,000 for S. epidermidis. Investigations on sequentially prepared PBIs with defined molecular weight Mn and tailored end groups revealed that there is a dependence of antimicrobial activity and selectivity on Mn and nature of the end groups.

Microbial electrosynthesis is a highly promising application of microbial electrochemical technologies for the sustainable production of organic compounds. At the same time a multitude of questions need to be answered and challenges to be met. Central for its further development is using appropriate electroactive microorganisms and their efficient extracellular electron transfer (EET) as well as wiring of the metabolism to EET. Among others, Clostridia are believed to represent electroactive microbes being highly promising for microbial electrosynthesis. We investigated the potential steps and challenges for the bio-electrochemical fermentation (electro-fermentation) of mid-chain organic acids using Clostridium kluyveri. Starting from a metabolic model the potential limitations of the metabolism as well as beneficial scenarios for electrochemical stimulation were identified and experimentally investigated. C. kluyveri was shown to not be able to exchange electrons with an electrode directly. Therefore, exogenous mediators (2-hydroxy-1,4-naphthoquinone, potassium ferrocyanide, neutral red, methyl viologen, methylene blue, and the macrocyclic cobalt hexaamine [Co(trans-diammac)]3+) were tested for their toxicity and electro-fermentations were performed in 1L bioreactors covering 38 biotic and 8 abiotic runs. When using C. kluyveri and mediators, maximum absolute current densities higher than the abiotic controls were detected for all runs. At the same time, no significant impact on the cell metabolism (product formation, carbon recovery, growth rate) was found. From this observation, we deduce general potential limitations of electro-fermentations with C. kluyveri and discuss strategies to successfully overcome them.

The replacement of petrochemical aromatics with bio-based molecules is a key area of current biotechnology research. To date, a small number of aromatics have been produced by recombinant bacteria in laboratory scale while industrial production still requires further strain development. While each study includes some distinct analytical methodology to quantify certain aromatics, a method that can reliably quantify a great number of aromatic products and relevant pathway intermediates is needed to accelerate strain development. In this study, we developed a robust reverse phase high performance liquid chromatography method to quantify a wide range of aromatic metabolites present in host microorganisms using the shikimate pathway, which is the major metabolic pathway for biosynthesis of aromatics. Twenty-three metabolites can be quantified precisely with the optimized method using standard HPLC equipment and UV detection, with the mobile phase used for chromatography also compatible with mass spectrometry (MS). The limit of quantification/detection is as low as 10 to 10 mol, respectively, which makes this method feasible for quantification of intracellular metabolites. This method covers most metabolic routes for aromatics biosynthesis, it is inexpensive, robust, simple, precise and sensitive, and has been demonstrated on cell extracts from S. cerevisiae genetically engineered to overproduce aromatics.