Homoacetogenesis during hydrogen production by mixed cultures dark fermentation: Unresolved challenge

•Homoacetogenesis consumed 43% of the H2 yield in repeated batch fermentations.•Extent of the homoacetogenesis during continuous H2 fermentation is not well understood.•CO2 scavenging is a promising strategy towards controlling homoacetogenesis.

This paper is a comprehensive review of H2 consumption during anaerobic mixed culture H2 dark fermentation with a focus on homoacetogenesis. Homoacetogenesis consumed from 11% to 43% of the H2 yield in single and repeated batch fermentations, respectively. However, its quantification and extent during continuous fermentation are still not well understood. Models incorporating thermodynamic and kinetic controls are required to provide insight into the dynamic of homoacetogenesis during H2 dark fermentation. Currently, no adequate method exists to eliminate homoacetogenesis because it does not depend on the culture's source, pre-treatment, substrate, type of reactor, or operation conditions. Controlling CO2 concentrations during dark fermentation needs further investigation as a potential strategy towards controlling homoacetogenesis. Incorporating radioactive labeling technique in H2 fermentation research could provide information on simultaneous production and consumption of H2 during dark fermentation. Genetic studies investigating blocking H2 consuming pathways and enhancing H2 evolving hydrogenases are suggested towards controlling homoacetogenesis during dark fermentation.