Valorization of corn-silage anaerobic digestate through the cultivation of the white-rot fungus Pleurotus ostreatus.

The white-rot fungus Pleurotus ostreatus SMR 684 was cultivated on the solid digestate obtained from biogas production at commercial scale from corn silage (namely corn silage digestate, CSD), with the aim of i) producing edible mushrooms and ii) degrading lignin. The pure fungal species was grown in sterilized Petri dishes containing 3% malt extract and 1.5% agar. When the mycelium formed an abundant biomass, discs of 9 mm were cut and used for inoculation of the digestate. Each circle was placed in the center of a Petri dish (11 cm diameter) containing 15 g of wheat straw (WS, control) or CSD. Moisture was fixed at 70% in each substrate. The fungus was incubated for 42 days and samples were taken approximately twice per week. The mycelium in CSD grew as fast as in the control. Each sample was extracted with 0.1 M potassium phosphate buffer, filtered and centrifuged. The superrnatant was analyzed for the determination of proteins, ligninolytic activities (laccase, peroxidases) and cellulolytic and hemicellulolytic activities (endoglucanase, cellobiohydrolase, xylanase), while the solid residue was used for the determination of lignin. It is worth noting that the highest activities (total peroxidases: 4.06 IU/mg protein after 24 days; xylanase: 3.03 IU/mg protein after 17 days) were found in the mycelium grown on CSD. Lignin was reduced by 12% in CSD and 11% in WS after 42 days. This study indicated that CSD might represent a suitable feedstock for the production of fungal fruiting bodies. Moreover, the reduction of lignin percentage suggests that spent CSD after mushroom cultivation, enriched with fungal biomass, might be re-introduced into the anaerobic digester with the consequent increase of biogas yield, since fungal biomass is reported to represent a viable feedstock for biogas production.