Toward high carbon recovery: Novel strategies to hindering the occurrence of competitive reactions during chain elongation process

Productions of medium-chain fatty acids (MCFAs) from waste streams via chain elongation (CE) platforms are conducive to the development of a sustainable society. Open-culture anaerobic fermentation is a promising and cost-effective technique to reclaim MCFAs from waste. Nevertheless, due to the complex microbial structure and the intertwined metabolic interactions, unwanted competing reactions including methanogenesis, excessive ethanol oxidation (EEO) and acrylate pathways always persist in the fermentation systems. Successful production of MCFAs requires the suppression of these competing reactions that could consume the substrate or byproduct used for CE process. Aiming to enhance the production of such higher-value liquids, multiple strategies have been proposed to hinder the occurrence of these undesired reactions in open-culture systems. Compared to the MCFAs productions, the roles of competitive reactions in CE processes are less well elucidated and have never been specifically reviewed before. To this end, this review first gave a general overview of CE pathways at first, followed by the introduction of the common competitive reactions that existed in the fermentation systems. Both conventional and emerging strategies to prevent competing reactions from occurring were examined in detail from the aspects of mechanisms, practicability, and the potential for improvements. The outlook for strategies with better technical feasibility was then put forward to ensure more carbon is shunted to MCFAs productions rather than wasted via competitive reactions.