Filamentous fungi display different behavior during spruce bark degradation.

Conference poster, 2023

Microbial degradation of trees is hindered by their outermost tissue, the bark, which is also a material produced in vast amounts annually through debarking in the pulp and paper industry. While the bark is composed of the typical lignocellulose components cellulose, hemicellulose, and lignin, it also contains a large fraction of small molecules referred to as extractive compounds. The extractives are attributed to the strong anti-microbial properties of the bark, but despite their presence, filamentous fungi can frequently be found growing on the outside of fallen trees. How fungi deal with the presence of extractive compounds and polysaccharides while growing on bark is however not known today, and this precludes development of biological valorization methods.  
Here, we have followed fungi growing on spruce bark over six months, including white-rot (Dichomitus squalens), brown-rot (Postia placenta), as well as three Ascomycetes (Trichoderma reesei, Penicillium crustosum, Trichoderma sp. B1). The changes in the material were analyzed continuously using a combination of mass-loss determination, GC-MS, HPAEC-PAD, to monitor overall changes as well as detailed changes in extractives, carbohydrates, and lignin. These data enabled a comparison of the growth and substrate metabolism of five different fungi growing on the bark. The fungi exhibited clearly different approaches to the extractive compounds – from simply tolerating them, to detoxification and/or fully metabolizing them. Also carbohydrate analyses revealed significant differences among the fungi, with the brown-rot fungus P.  placenta displaying the typical hemicellulose first – cellulose second type of degradation pattern. From compositional analyses, D. squalens had the highest mass-loss (30%) and was quickest to reach stationary phase (12 weeks), and was able to significantly modify extractives and polysaccharides, in particular monosaccharides derived from pectin and xylan. Therefore, additional proteomic analyses was performed on D. squalens grown on bark, acetone-extracted bark (i.e. extractive-less), and galactomannan. The results revealed little difference in the proteome composition between acetone-extracted bark and bark, however, were identified. In particular, carbohydrate-active enzymes (CAZymes) related to pectin and xylan degradation were upregulated in the bark samples.  
Our results suggest that D. squalens, P. placenta, T. reesei, P. crustosum, Trichoderma sp. B1 have different substrate preferences, in particular, extractives are either degraded or tolerated. Significant changes could also be found in carbohydrate composition revealing pectin degradation. This work forms a basis for an understanding fungal degradation of bark.