Meta-omics profiling of denitrifying bacterial communities with lignin as carbon source

Lignin is the most abundant renewable source of aromatic carbon and its microbial depolymerization and metabolism under aerobic conditions is well studied. However, lignin breakdown in the absence of oxygen remains poorly understood. In this study, we established long-term bacterial enrichment cultures supplied with diverse lignin preparations as the sole carbon source under denitrifying conditions. Denitrification dynamics were followed by monitoring nitrogenous gases. Metagenomics analysis of eight enrichments involving five lignins recovered 62 metagenome-assembled genomes (MAGs), several of which encoded enzymes for both denitrification and anaerobic metabolism of aromatic compounds. Quantitative metaproteomics confirmed expression of such enzymes and additionally showed that several MAGs expressed multiple oxidoreductases and uncharacterised proteins that are potential candidates for involvement in lignin modification. The detection of several oxygen-dependent oxidoreductases despite anaerobic conditions prompts intriguing discussion of potential mechanistic explanations. This systems-level study expands our understanding of bacterial processing of lignin-associated carbon in anaerobic environments and suggests enzymatic targets for further exploration of lignin depolymerization under oxygen-limited conditions.