Insights into the microbiomes for medium-chain carboxylic acids production from biowastes through chain elongation

Bioconversion of medium-chain carboxylic acids (MCCAs) from biowastes through anaerobic mixed-culture fermentation is undergoing a revolution in terms of mitigating the lower fossil fuels requirement and increasing biowaste treatment capacity. Benefiting from hydrophobicity and high energy density of MCCA, this high-valuable biofuel exhibits easier separation and wider application than traditional volatile fatty acid products. The biggest bottleneck for efficiently and simultaneously producing MCCAs by mixed-culture fermentation is complicated or even entangled microbial interaction. Therefore, this review aimed to supply guidelines to understand and steer these microbiomes toward the controllable ones. The metabolic pathway of chain elongation and associated cooperating and competing pathways were firstly discussed to understand the primary microbial interaction in mixed-culture fermentation. In an attempt to inspect the overall performance of mixed-culture CE reactor, the typical microbial community and its variation influenced by reactor parameters were also identified. The methods of in-line extraction of MCCAs for relieving toxicity inhibition on microbiome were also summarized regarding the difficulties lied in continuous MCCAs production. Finally, the future research directions of MCCAs production via mixed-culture fermentation would be proposed to help steer these novel bioproduction processes toward full-scale applications.