From ¹³C-lignin to ¹³C-mycelium: Agaricus bisporus uses polymeric lignin as a carbon source

Plant biomass conversion by saprotrophic fungi plays a pivotal role in terrestrial carbon (C) cycling. The general consensus is that fungi metabolize carbohydrates, while lignin is only degraded and mineralized to CO₂. Recent research, however, demonstrated fungal conversion of ¹³C-monoaromatic compounds into proteinogenic amino acids. To unambiguously prove that polymeric lignin is not merely degraded, but also metabolized, carefully isolated ¹³C-labeled lignin served as substrate for Agaricus bisporus, the world's most consumed mushroom. The fungus formed a dense mycelial network, secreted lignin-active enzymes, depolymerized, and removed lignin. With a lignin carbon use efficiency of 0.14 (g/g) and fungal biomass enrichment in ¹³C, we demonstrate that A. bisporus assimilated and further metabolized lignin when offered as C-source. Amino acids were high in ¹³C-enrichment, while fungal-derived carbohydrates, fatty acids, and ergosterol showed traces of ¹³C. These results hint at lignin conversion via aromatic ring-cleaved intermediates to central metabolites, underlining lignin's metabolic value for fungi.