Molecular mechanism of ethanol-H2 co-production fermentation in anaerobic acidogenesis: Challenges and perspectives

Ethanol-type fermentation (ETF) is one of three fermentation types during the acidogenesis of the anaerobic biological treatment. Ethanoligenens, a representative genus of ETF, displays acidophilic, autoaggregative, and ethanol-H2 co-producing characteristics and facilitates subsequent methanogenesis. Here, the latest advances in the molecular mechanisms of the metabolic regulation of ethanol-H2 co-producing bacteria based on multi-omics studies were comprehensively reviewed. Comparative genomics demonstrated a low genetic similarity between Ethanoligenens and other hydrogen-producing genera. FeFe‑hydrogenases (FeFe-H2ases) and pyruvate ferredoxin oxidoreductase (PFOR) played critical roles in the ethanol-H2 co-metabolic pathway of Ethanoligenens. Global transcriptome analysis revealed that highly expressed [FeFe]-H2ases and ferredoxins drove hydrogen production by Ethanoligenens at low pH conditions (4.0–4.5). Quantitative proteomic analysis also proved that this genus resists acetic acid-induced intracellular acidification through the up-regulated expression of pyrimidine metabolism related proteins. The autoaggregation of Ethanoligenen facilitated its granulation with acetate-oxidizing bacteria in co-culture systems and mitigated a fast pH drop, providing a new approach for solving a pH imbalance and improving hydrogen production. In-depth studies of the regulatory mechanism underlying ethanol-H2 co-production metabolism and the syntrophic interactions of ethanol-H2 co-producing Ethanoligenens with other microorganisms will provide insights into the improvement of bioenergy recovery in anaerobic biotechnology. The coupling of ETF with other biotechnologies, which based on the regulation of electron flow direction, syntrophic interaction, and metabolic flux, can be potential strategies to enhance the cascade recovery of energy and resources.