Microbial conversion of carbon dioxide into premium medium-chain fatty acids: the progress, challenges, and prospects

The conversion of carbon dioxide to medium-chain fatty acids (CO2-to-MCFAs) through microbial processes represents a valuable technology for sequestering and exploiting CO2, generating superior bio-chemicals from the primary contributor to the greenhouse effect. However, a comprehensive overview and generalization of microbial CO2-to-MCFAs are presently deficient. Based on this, the present review systematically summarizes the research progress, explicates the process mechanisms, analyses the key challenges and possible solutions, and anticipates forthcoming research perspectives and priorities for the first time. We proposed two original strategies, namely the synchronous strategy and integrated strategy, from current research into microbial CO2-to-MCFAs. The synchronous strategy concurrently achieves hydrogen (H2) and CO2 assimilation, as well as MCFAs production, by employing a reactor that co-cultivates predominant H2/CO2-utilizing microorganisms and chain elongation microorganisms. The integrated approaches involve CO2-to-precursors (i.e., acetate and ethanol) and subsequent precursors-to-MCFAs, achieved through the use of two bioreactors for separately cultivating H2/CO2-utilizing microorganisms and chain elongation microorganisms. Mechanistic insights reveal that microbial CO2-to-MCFAs predominantly encompasses two processes: H2 and CO2 assimilation into precursor and subsequent precursors chain elongation into MCFAs, through a Wood-Ljungdahl pathway and a two-round elongation, respectively. The analyses of key challenges and possible solutions for microbial CO2-to-MCFAs underscore the imperative to enhance efficiency and economy and to shed light on metabolic mechanisms. Furthermore, in order to improve the strategy application potential of microbial CO2-to-MCFAs, future research perspectives and priorities, e.g. exploitation of functional pure bacteria, screening of functional pure bacteria, multi-omics analysis, genetic modification and enhancement, enhancement of bioreactor stability, specific MCFA production, development of coupled purification technology for MCFAs, and economic benefits and ecological environmental risks, are proposed and prospected. This work is expected to offer a thorough understanding of the microbial CO2-to-MCFAs, guide and inspire researchers to address critical challenges in-depth and propel the development of CO2-to-MCFAs.