From Electron Transport to Electron Allocation in Medium-Chain Carboxylic Acid Biosynthesis from Waste Biomass: Critical Roles, Challenges, and Enhancement Strategies

Chain elongation (CE) biosynthesis represents a promising anaerobic fermentation technology that valorizes waste biomass into high-value medium-chain carboxylic acids (MCCAs). However, inefficient intracellular electron transport rate limits electron allocation efficiency and longer-chain products formation. Nonsterilized waste biomass substrates further exacerbates these electron transport limitations through community-level competitive metabolism, low substrate electron donor-to-electron acceptor (ED/EA) ratios, and undissociated MCCA inhibition. Therefore, the comprehensive understanding of the mechanisms of optimizing electron allocation efficiency by enhancing the intracellular electron transport system (CIET) is urgently needed. This Review aims to systematically explore CIET’s critical role in promoting MCCA biosynthesis with three focused discussions: (1) mechanisms of electron transport systems that drive carbon chain elongation through reducing power and electron delivery; (2) practical limitations from insufficient electron supply and transport efficiency; and (3) enhancement strategies including ED/EA ratio optimization, direct electron flow manipulation, chain-elongating bacteria (CEB) microbial community enrichment, and in situ MCCA product extraction that improve electron allocation efficiency by enhancing CIET efficiency. Finally, future perspectives on optimizing electron supply/transport and mitigating undissociated MCCA toxicity are outlined. By synthesizing current research findings, this Review clarifies the regulatory mechanisms governing electron transport and promoting electron allocation efficiency in MCCA biosynthesis from waste biomass.