Biological degradation of lignin: A critical review on progress and perspectives

As the second most abundant biopolymer on earth after cellulose, lignin is considered to be an alternative source of various polymers and biomaterials since it can be cleaved into a large number of phenolic compounds. Due to its complex structure and low reactivity, depolymerization of lignin is the premise and rate-limiting step of its valorization. In nature, lignin can be degraded by microorganisms and their secreted enzymes into building blocks that can potentially be converted into high-value chemicals. In this review, the research progress on lignin biodegradation is discussed, focusing on the key enzymes and reaction mechanisms of lignin degradation, which provides a molecular basis for lignin bio-depolymerization. In addition, the metabolic pathways of different lignin derivatives in microorganisms are summarized, and value-added compounds derived from lignin are reviewed. Finally, the challenges and prospects of lignin biorefinery concepts are discussed.