From Syngas Fermentation to Chain Elongation: the Role of Key Microorganisms and Multi-omics Analysis

Bioproduction of short chain carboxylic acids (SCCAs) and medium chain carboxylic acids (MCCAs) has emerged as an alternative strategy to upgrade low-value organic waste and reduce fossil fuels requirement. Using synthesis gas (syngas) to generate SCCAs and MCCAs by microbial communities would be an option to address part of the current energy challenge. Syngas fermentation offers a pathway for the sustainable synthesis of fuels and chemicals with advantages over catalytic syngas conversion. In the same way, chain elongation is an anaerobic microbial process driven by ethanol, carbohydrates, and SCCAs (e.g., acetate, lactate) to obtain high-value MCCAs (e.g., caproate, valerate). Because these technologies use organic wastes as feedstocks, mixed microbial communities are often considered biocatalysts. However, the management of microbial communities is the biggest bottleneck for efficient and simultaneous production of SCCAs and MCCAs. Understanding and steering these microbiomes is critical to optimize bioprocess performance. Therefore, this review discusses the metabolic pathways of both syngas fermentation and chain elongation. Also, to examine the overall performance of microbial communities involved in syngas fermentation and chain elongation, the influence of reactor parameters on the growth and metabolic activity of the key microorganisms is presented. The experimental strategies for simultaneous syngas fermentation and chain elongation processes are also presented and discussed. Finally, the use of multi-omics to better understand both syngas fermentation and chain elongation processes is discussed to steer these bioproduction processes towards full-scale applications.