Microbial photoproduction of n-heptane

The photoenzyme fatty acid photodecarboxylase (FAP) has emerged as a promising catalyst for the redox-neutral biological production of hydrocarbons. Previous studies have shown that FAP can efficiently convert medium-chain fatty acids such as n-octanoic acid into hydrocarbons, outperforming its natural long-chain fatty acid substrates (C16-C18). Such observation expands the potential applications of FAP to include solvents and jet fuels. However, the limited availability of natural sources of n-octanoic acid poses a challenge to the industrial implementation of n-heptane bioproduction. This study investigates the hydrocarbon synthesis capacity of an E. coli strain that expresses FAP and produces n-octanoic acid, the precursor to n-heptane, via a specific octanoyl-ACP thioesterase. Several FAPs and thioesterases were tested. A blue light-inducible promoter ensured high expression of both enzymes, eliminating the need for chemical inducers. Fusion of FAP with thioredoxin increased n-heptane production 12-fold. Using a co-cultivation strategy, where one strain produces n-octanoic acid and another strain converts it to n-heptane, increased hydrocarbon production 14-fold compared to co-expressing FAP and thioesterase. Co-cultures operated in batch mode in 100-mL photobioreactors enabled the recovery of >90%-pure n-heptane, yielding 272 mg·L-1 over 56 h. This work lays the foundation for the development of an industrial bioproduction of n-heptane.