Microbial platforms for sustainable aviation fuel production: Metabolic pathways, engineering constraints, and...

Sustainable aviation fuels represent the most viable near-term option for reducing greenhouse gas emissions from the aviation sector, as they can be deployed within existing aircraft and fuel infrastructure. Despite significant advances in microbial metabolic engineering, the contribution of biological platforms to aviation fuel supply remains marginal, indicating persistent limitations that extend beyond laboratory-scale performance. This review provides a critical analysis of microbial systems proposed for sustainable aviation fuel production, examining their metabolic architectures, physiological boundaries, and compatibility with industrial fuel requirements. Major biological routes are evaluated, including fatty acid-based platforms, isoprenoid biosynthesis, direct biological hydrocarbon formation, and platform chemical intermediates. Instead of focusing solely on conventional metrics such as titer or yield, the analysis integrates biological constraints with downstream upgrading severity, hydrogen demand, regulatory blend limits, and biorefinery integration. Across diverse pathways, convergent bottlenecks emerge, including high redox and energy requirements for hydrocarbon biosynthesis, toxicity of fuel-range molecules to microbial hosts, kinetic limitations of terminal enzymes, and genetic instability under sustained production. In parallel, pathways that achieve robust fermentation performance frequently require energy- and hydrogen-intensive catalytic finishing to meet aviation fuel specifications, decoupling biological efficiency from overall process viability. These findings reveal a systemic misalignment between what microorganisms naturally produce and what aviation infrastructure can accept as certified fuel. The review concludes that transformative progress is unlikely to arise from incremental optimization of isolated metabolic pathways. Instead, it depends on integrated biorefinery concepts and hybrid bio-thermochemical strategies that explicitly co-design microbial metabolism with downstream processing requirements.