Lignin valorization through microbial production of polyhydroxyalkanoates: Recent trends, challenges and opportunities

Abstract

Polyhydroxyalkanoates (PHA) are biopolymers produced by bacteria under nitrogen-limited and carbon-rich conditions and have been explored as a potential replacement for petroleum-based plastics. Despite their versatile applications and non-toxic and eco-friendly properties, PHAs currently hold a relatively low market share owing to their high production and downstream processing costs. However, lignin, a renewable aromatic source and byproduct of lignocellulose biorefineries, is considered a cheap substrate for the microbial production of PHA. Research over the past decade has demonstrated that microbes with diverse aromatic metabolic pathways can degrade lignin and effectively transform lignin-derived aromatic compounds into PHA by biofuneling them into their central carbon metabolism. Recent advances in lignin extraction, lignin depolymerisation, genetic, metabolic, and protein engineering, multiomics approaches, artificial intelligence, and development of efficient fermentation and downstream processing methods have paved the way for sustainable production of PHA from lignin. In light of these developments, this review comprehensively examines the metabolic pathways involved in the utilisation of lignin-derived aromatic compounds and recent developments in improving the microbial production of PHA. We also discuss the challenges and opportunities to improve several aspects of the bioconversion of lignin into PHA from the perspectives of both lignin and bacterial processes.