Deciphering key stages, compounds, and microorganisms during microbial degradation of spruce bark

Conference poster, 2022

Bark acts as the skin and outer barrier of the tree, protecting it from microbial degradation and environmental stress [1]. The bark is a major forestry waste product, and while it is mainly composed of lignocellulose, it also contains a large amount of so-called extractive compounds, which typically have strong antimicrobial properties [2]. Despite the presence of these extractives, there are microorganisms capable of growing on bark [3]. However, it is currently not known which microorganisms are responsible for this biological degradation, or whether different species dominate different stages of bark degradation.

In this study, we have followed a microbial consortium growing on spruce bark for six months. We have analyzed the changes in the material over time using a combination of GC-MS, NMR, and HPAEC-PAD, both regarding extractive and carbohydrate content, and in parallel, we have mapped the microbial community using sequencing. These data enable correlation of the material changes with the variation of the bacterial and fungal population. We found that in the early degradation stages, the community is dominated by a few bacterial taxa, likely due to the antimicrobial properties of the bark. A significant increase in the microbial diversity was correlated with the degradation of resin acids, which thus likely are the main antimicrobial compounds. From one of the early samplings, we isolated one of the dominating organisms — a new Pseudomonas species able to metabolize different types of resin acids — which was phenotypically characterized and whole-genome-sequenced.

Our results suggests that the removal of resin acids from spruce bark is a critical step in the bark degradation process, and precedes degradation of carbohydrates and lignin, for which we did not observe major changes. This work forms a basis for understanding of which microorganisms and extractives are key players during microbial bark degradation.