The electrochemical process of microbial electrosynthesis (MES) is used to drive the metabolism of electroactive microorganisms for the production of valuable chemicals and fuels. MES combines the advantages of electrochemistry, engineering, and microbiology and offers alternative production processes based on renewable raw materials and regenerative energies. In addition to the reactor concept and electrode design, the biocatalysts used have a significant influence on the performance of MES. Thus, pure and mixed cultures can be used as biocatalysts. By using mixed cultures, interactions between organisms, such as the direct interspecies electron transfer (DIET) or syntrophic interactions, influence the performance in terms of productivity and the product range of MES. This review focuses on the comparison of pure and mixed cultures in microbial electrosynthesis. The performance indicators, such as productivities and coulombic efficiencies (CEs), for both procedural methods are discussed. Typical products in MES are methane and acetate, therefore these processes are the focus of this review. In general, most studies used mixed cultures as biocatalyst, as more advanced performance of mixed cultures has been seen for both products. When comparing pure and mixed cultures in equivalent experimental setups a 3-fold higher methane and a nearly 2-fold higher acetate production rate can be achieved in mixed cultures. However, studies of pure culture MES for methane production have shown some improvement through reactor optimization and operational mode reaching similar performance indicators as mixed culture MES. Overall, the review gives an overview of the advantages and disadvantages of using pure or mixed cultures in MES. Key points • Undefined mixed cultures dominate as inoculums for the MES of methane and acetate, which comprise a high potential of improvement • Under similar conditions, mixed cultures outperform pure cultures in MES • Understanding the role of single species in mixed culture MES is essential for future industrial applications

Many homologous genes encoding β-oxidation enzymes were found in the genome of Cupriavidus necator H16 (synonym: Ralstonia eutropha H16). By proteome analysis, the degradation of adipic acid was investigated and showed differences to the degradation of hexanoic acid. During β-oxidation of adipic acid, activation with coenzyme A (CoA) is catalyzed by the two-subunit acyl-CoA ligase encoded by B0198 and B0199. The operon is completed by B0200 encoding a thiolase catalyzing the cleavage of acetyl-CoA at the end of the β-oxidation cycle. Strain C. necator ΔB0198-B0200 showed improved growth on adipic acid. Potential substitutes are B1239 for B0198-B0199 and A0170 as well as A1445 for B0200. A deletion mutant without all three thiolases showed diminished growth. The deletion of detected acyl-CoA dehydrogenase encoded by B2555 has an altered phenotype grown with sebacic acid but not adipic acid. With hexanoic acid, acyl-CoA dehydrogenase encoded by B0087 was detected on 2D gels. Both enzymes are active with adipoyl-CoA and hexanoyl-CoA as substrates, but specific activity indicates a higher activity of B2555 with adipoyl-CoA. 2D gels, growth experiments and enzyme assays suggest the specific expression of B2555 for the degradation of dicarboxylic acids. In C. necator H16 the degradation of carboxylic acids potentially changes with an increasing chain length. Two operons involved in growth with long-chain fatty acids seem to be replaced during growth on medium-chain carboxylic acids. Only two deletion mutants showed diminished growth. Replacement of deleted genes with one of the numerous homologous is likely. Importance The biotechnologically interesting bacterium Cupriavidus necator H16 was thoroughly investigated. Fifteen years ago, it was sequenced entirely and annotated (Pohlmann et al., 2006). Nevertheless, the degradation of monocarboxylic fatty acids and dicarboxylic acids has not been elucidated completely. C. necator is used to produce value-added products from affordable substrates. One of our investigations ' primary targets is the biotechnological production of organic acids with different and specific chain lengths. The versatile metabolism of carboxylic acids recommends C. necator H16 as a candidate for producing value-added organic products. Therefore, the metabolism of these compounds is of interest, and for different applications in industry, understanding such central metabolic pathways is crucial.

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