Microbial electrosynthesis (MES) is engineered to use electric power and carbon dioxide (CO2) as the only energy and carbon sources in reductive bioelectrochemical processes for biosynthesis. This technology is conducted in bioelectrochemical systems (BES) and takes advantage of electroactive microorganisms. In MES, hydrogen (H2) has been highlighted as the key intermediate element involved in a whole range of microbial metabolisms for the reduction of CO2. Basic processes in MES rely on the transformation of electric power (electrons) into chemical energy in a process called extracellular electron transfer (EET) An electromethanogenic reactor was used to study putative genes taking part in EET. Microbial community composition analysis through both DNA and cDNA signatures revealed that electromethanogenesis was conducted by Methanobacterium sp. Short-time changes in electron flow (closed and open electric circuits) were used to determine the gene expression levels of [NiFe]-hydrogenases (Eha, Ehb, and Mvh), heterodisulfide reductase (Hdr), coenzyme F420-reducing [NiFe]-hydrogenase (Frh), and hydrogenase maturation protein (HypD). According to RT-PCR data, suspected mechanisms were not regulated at the transcriptional level. Some microorganisms could serve as potentially interesting sustainable H2 producers in biocathodes. We have studied the biological H2 production in biocathodes operated at -1.0 V vs. Ag/AgCl, using a highly comparable technology and using CO2 as the sole carbon feedstock. Ten different bacterial strains were chosen from genera Rhodobacter, Rhodopseudomonas, Rhodocyclus, Desulfovibrio, and Sporomusa, all described as hydrogen-producing candidates. Eight over ten bacterial strains showed electroactivity and H2 production rates increased significantly (2 to 8-fold) compared to abiotic conditions for two of them (Desulfovibrio paquesii DSM 16681 and Desulfovibrio desulfuricans DSM 642). The application of bacteria-coated cathodes for sustainable H2 production may not be efficient enough to maintain H2 biosynthetic requirements for highly efficient producing strains. Here, we applied genetic engineering tools intending to further increase the H2 production ability of D. paquesii. [Fe]-only hydrogenase and tetraheme cytochrome c3 were selected as genes of interest to be overexpressed in D. vulgaris DSM 644 and D. paquesii DSM 16681. Different conditions and described protocols were tested towards implementing the proper mechanisms to ensure overexpression of the selected genes. The presented approaches might have contributed to a better understanding of the key role of H2 during microbial electrosynthesis and derive some conclusions. First, enhancing the current knowledge of extracellular electron transfer may lead to better control of reductive BES. Second, the required H2 supply for sustainable electrochemical bioprocesses may be provided in a more efficient way using bio-H2 evolving microorganisms. Finally, the application of synthetic biology and defined consortia should be considered for new and promising contributions in the METs field.