Biodegradation characteristics of lignin in pulping wastewater by the thermophilic Serratia sp. AXJ-M: Performance, genetic background, metabolic pathway and toxicity assessment.

The key to the efficient removal of pulping wastewater lies in the effective degradation of lignin at high temperature. There is thus an urgent need to seek effective eco-environmental techniques to overcome this environmental limit for lignin degradation. The soil isolate thermophilic Serratia sp. AXJ-M efficiently metabolizes lignin. Nevertheless, the underlying comprehensive molecular mechanism of lignin degradation by thermophilic AXJ-M is poorly understood. Here, strain AXJ-M showed excellent degradation ability toward diverse lignin-related aromatic compounds. Functional genome analysis and RNA-Seq disclosed several traits which in joint consideration suggest a high efficiency of AXJ-M representative to the lignin degradation and environmental adaptation. Multiomics analyses combined with GC-MS revealed seven potential lignin biodegradation pathways. DyP was predicted to be involved in the breakdown of the β-O-4 ether bond, Cα-Cβ bond and Cα oxidation of lignin by prokaryotic expression and gene knockout and complementation. Molecular docking deepens the understanding of the interaction between DyP and lignin. Toxicity assessment experiments clearly indicated that AXJ-M significantly reduced the toxicity of the metabolites. This work expands the knowledge about the degradation mechanism of thermophilic lignin-degrading bacteria, most importantly, offers a new perspective on potential applications in utilizing this strain in pulping wastewater bioremediation.