Chain elongation into medium-chain carboxylates, such as n-caproate and n-caprylate, with ethanol as an electron donor and with open cultures of microbial consortia (i.e., reactor microbiomes) under anaerobic conditions is being developed as a biotechnological production platform. The goal is to use the high thermodynamic efficiency of anaerobic fermentation to convert organic biomass or organic wastes into valuable biochemicals that can be extracted. Several liter-scale studies have been completed and a first pilot-plant study is underway. However, the underlying microbial pathways are not always well understood. In addition, an interdisciplinary approach with knowledge from fields ranging from microbiology and chemical separations to biochemistry and environmental engineering is required. To bring together research from different fields, we reviewed the literature starting with the microbiology and ending with the bioprocess engineering studies that already have been performed. Because understanding the microbial pathways is so important to predict and steer performance, we delved into a stoichiometric and thermodynamic model that sheds light on the effect of substrate ratios and environmental conditions on product formation. Finally, we ended with an outlook.

SUMMARYThe world needs sustainable, efficient, and renewable energy production. We present the plant microbial fuel cell (plant-MFC), a concept that exploits a bioenergy source in situ. In the plant-MFC, plants and bacteria were present to convert solar energy into green electricity. The principal idea is that plants produce rhizodeposits, mostly in the form of carbohydrates, and the bacteria convert these rhizodeposits into electrical energy via the fuel cell. Here, we demonstrated the proof of principle using Reed mannagrass. We achieved a maximal electrical power production of 67 mW m À2 anode surface. This system was characterized by: (1) nondestructive, in situ harvesting of bioenergy; (2) potential implementation in wetlands and poor soils without competition to food or conventional bioenergy production, which makes it an additional bioenergy supply; (3) an estimated potential electricity production of 21 GJ ha À1 year À1 ð5800 kWh ha À1 year À1 Þ in Europe; and (4) carbon neutral and combustion emission-free operation.

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Bioelectrochemical systems (BESs) are emerging technologies which use microorganisms to catalyze the reactions at the anode and/or cathode. BES research is advancing rapidly, and a whole range of applications using different electron donors and acceptors has already been developed. In this mini review, we focus on technological aspects of the expanding application of BESs. We will analyze the anode and cathode half-reactions in terms of their standard and actual potential and report the overpotentials of these half-reactions by comparing the reported potentials with their theoretical potentials. When combining anodes with cathodes in a BES, new bottlenecks and opportunities arise. For application of BESs, it is crucial to lower the internal energy losses and increase productivity at the same time. Membranes are a crucial element to obtain high efficiencies and pure products but increase the internal resistance of BESs. The comparison between production of fuels and chemicals in BESs and in present production processes should gain more attention in future BES research. By making this comparison, it will become clear if the scope of BESs can and should be further developed into the field of biorefineries.