Desulfurization of kraft lignin by white-rot fungus Obba rivulosa

The kraft process generates lignin with residual sulfur, which hinders its utilization in various applications. Here, the ability of O. rivulosa to desulfurize Lineo™ kraft lignin was studied. The sulfur content of Lineo™ decreased by 26 % in the fungal cultures during a one-week incubation compared to original sulfur content. After cultivation, the lignin structure was modified and condensed, with the average molecular weight of lignin increasing by 73 %. Cultivating O. rivulosa on Lineo™ yielded secretomes enriched in enzymes acting on plant cell wall. Glycosyl hydrolases targeting carbohydrates were abundant, followed by enzymes involved in lignin modification and H 2 O 2 production. This study demonstrates that fungal cultures, including secreted laccase 1, MnPs and H 2 O 2 -producing enzymes, may lead to cleavage of sulfur covalently attached to lignin α- and β‑carbons thereby decreasing sulfur content. Desulfurization and the previously shown condensation of kraft lignin by O. rivulosa make Lineo™ promising starting material for various applications. • Fungal treatment resulted in a polymerized kraft lignin with reduced sulfur content. • Structural changes in lignin are due to the activity of several detected enzymes. • Reduced sulfur in kraft lignin expands its use in advanced material creation. • Fungal treatment is applicable to broader context where desulfurization is required.