Optimization of laccase production by Pleurotus ostreatus Florida and evaluation of metabolites generated during Kraft lignin biotransformation

White-rot fungi are efficient degraders of the lignin moiety due to the production of extracellular enzymes. Among these are laccases, which are capable of depolymerizing lignin by oxidative processes producing low-molecular weight phenolic and aromatic compounds. Among these, vanillic, protocatechuic, benzoic, ferulic and syringic acids, -hydroxylated phenol, guaiacol and vanillin have important industrial applications, as building blocks for chemical syntheses of high-valued compounds. Kraft lignin is an industrial lignin obtained from Kraft pulping of wood. Thus, the objective of this work was to optimize the culture conditions for laccase production by Pleurotus ostreatus Florida grown on Kraft lignin, and to evaluate lignin biodegradation products. Initially, the production of laccase by this fungal strain was evaluated in a Box-Behnken design with the independent variables: lignin concentration, yeast extract and time of cultivation. A second experimental design, using two variables at three levels, was performed for assaying laccase’s production exclusively using the substrate 2,6-dimethoxyphenol (2,6-DMP), and the degradation products were analyzed by LC–MS/MS. Concentration of lignin was the main variable that increased laccase production assayed against 2,6-DMP substrate. The optimal conditions for laccase production were variation of lignin and yeast extract concentrations and cultivation times: 0.87% (w/v) lignin, 4.5% (w/v) yeast extract and 10 days cultivation, respectively, resulting in a laccase titer of 5.68 U/mL, which corresponded to the value predicted by the experimental design. The lignin metabolites identified were 4-methoxycinnamaldehyde and cuminaldehyde. In addition, abscisic acid was produced; considered a stress-related hormone product of the fungus. These findings demonstrated a suitable source of fungal laccase for obtaining new metabolites 4-methoxycinnamaldehyde and cuminaldehyde from the biodegradation of Kraft lignin through fermentation. Graphical Abstract