Bio-electrochemical systems (BESs) have recently emerged as an exciting technology. In a BES, bacteria interact with electrodes using electrons, which are either removed or supplied through an electrical circuit. The most-described type of BES is microbial fuel cells (MFCs), in which useful power is generated from electron donors as, for example, present in wastewater. This form of charge transport, known as extracellular electron transfer, was previously extensively described with respect to metals such as iron and manganese. The importance of these interactions in global biogeochemical cycles is essentially undisputed. A wide variety of bacteria can participate in extracellular electron transfer, and this phenomenon is far more widespread than previously thought. The use of BESs in diverse research projects is helping elucidate the mechanism by which bacteria shuttle electrons externally. New forms of interactions between bacteria have been discovered demonstrating how multiple populations within microbial communities can co-operate to achieve energy generation. New environmental processes that were difficult to observe or study previously can now be simulated and improved via BESs. Whereas pure culture studies make up the majority of the studies performed thus far, even greater contributions of BESs are expected to occur in natural environments and with mixed microbial communities. Owing to their versatility, unmatched level of control and capacity to sustain novel processes, BESs might well serve as the foundation of a new environmental biotechnology. While highlighting some of the major breakthroughs and addressing only recently obtained data, this review points out that despite rapid progress, many questions remain unanswered.

Anaerobic digestion of organic waste into methane and carbon dioxide (biogas) is carried out by complex microbial communities. Here, we use full-length 16S rRNA gene sequencing of 285 full-scale anaerobic digesters (ADs) to expand our knowledge about diversity and function of the bacteria and archaea in ADs worldwide. The sequences are processed into full-length 16S rRNA amplicon sequence variants (FL-ASVs) and are used to expand the MiDAS 4 database for bacteria and archaea in wastewater treatment systems, creating MiDAS 5. The expansion of the MiDAS database increases the coverage for bacteria and archaea in ADs worldwide, leading to improved genus- and species-level classification. Using MiDAS 5, we carry out an amplicon-based, global-scale microbial community profiling of the sampled ADs using three common sets of primers targeting different regions of the 16S rRNA gene in bacteria and/or archaea. We reveal how environmental conditions and biogeography shape the AD microbiota. We also identify core and conditionally rare or abundant taxa, encompassing 692 genera and 1013 species. These represent 84–99% and 18–61% of the accumulated read abundance, respectively, across samples depending on the amplicon primers used. Finally, we examine the global diversity of functional groups with known importance for the anaerobic digestion process.

The on-going research towards sustainable fuel production entails the improvement of the microbial catalysts involved. The possible reversibility of specific anaerobic catabolic reactions opens up a range of possibilities for the development of novel reductive bioprocesses. These reductive biohydrogenation pathways enable production of high energy density chemicals of interest as biofuels such as alcohols and long chain fatty acids. Anaerobic bioprocesses take place under energy scarcity conditions due to the absence of strong electron acceptors such as oxygen, and provide metabolic pathways towards these energy dense (reduced) chemicals. Metabolic reactions take place very close to thermodynamic equilibrium with minimum energy dissipation and consequently, environmental changes in product and substrate concentrations can easily reverse the driving force of the chemical reaction catalysed. The objective of this work is to investigate the potential reversibility of specific anaerobic pathways of interest. The thermodynamics of the different steps in biochemical pathways are analysed and combined with assumptions concerning kinetic and physiological constraints to evaluate if pathways are potentially reversible by imposing changes in process conditions. The results suggest that (i) in homoacetogenesis they may operate in both reductive and oxidative directions depending on the hydrogen partial pressure in the system, (ii) acetate reduction to butyrate with hydrogen is not feasible, but no biochemical bottlenecks are apparent in butyrate production from acetate with ethanol or lactate as electron donors, (iii) the reduction of short chain to longer chain fatty acids with ethanol as the electron donor appears thermodynamically and kinetically feasible, and (iv) alcohol production from the corresponding fatty acids (e.g. ethanol from acetate) was found to require proton translocations at specific sites in the biochemical pathways in order to compensate for the ATP required for phosphatation of acetate and to enable energy harvesting. Overall, the methodology proposed here allows for analysing the potential reversibility of catabolic pathways and therewith contributes to the development of efficient and reliable anaerobic bioprocesses for the production of biofuels and chemicals.