Fungi Employ Different Strategies to Degrade Extractives and Polysaccharides in Spruce Bark

Abstract

The bark represents the outer protective layer of trees and contains high concentrations of antimicrobial extractives. It also represents a side stream in forestry produced in millions of tons annually. In addition to cellulose, hemicelluloses, and lignin, spruce bark contains higher amounts of pectin and starch compared to spruce wood. Fungi are efficient lignocellulose degraders but their role in bark degradation is currently unclear. Cultivation of five fungi, Dichomitus squalens, Rhodonia placenta, Penicillium crustosum, Trichoderma sp. B1, and Trichoderma reesei on spruce bark over six months combined with chemical analyses of the bark revealed different degradation strategies. Toxic resin acids were degraded by Basidiomycetes but unmodified and tolerated by Ascomycetes. Substantial differences were also observed for glucuronoarabinoxylan and pectin degradation. The white-rot species D. squalens was further studied using proteomic analysis of its secreted proteins. Insight into fungal bark degradation strategies can inspire improved utilization of this abundant renewable resource.

Keywords: Spruce bark, fungi, Extractives, CAZymes, Proteomics