Resin acids play key roles in shaping microbial communities during degradation of spruce bark

The bark is the outermost defense of trees against microbial attack, largely thanks to toxicity and prevalence of extractive compounds. Nevertheless, bark decomposes in nature, though by which species and mechanisms remains unknown. Here, we have followed the development of microbial enrichments growing on spruce bark over six months, by monitoring both chemical changes in the material and performing community and metagenomic analyses. Carbohydrate metabolism was unexpectedly limited, and instead a key activity was metabolism of extractives. Resin acid degradation was principally linked to community diversification with specific bacteria revealed to dominate the process. Metagenome-guided isolation facilitated the recovery of the dominant enrichment strain in pure culture, which represents a new species ( Pseudomonas abieticivorans sp. nov.), that can grow on resin acids as a sole carbon source. Our results illuminate key stages in degradation of an abundant renewable resource, and how defensive extractive compounds have major roles in shaping microbiomes. ### Competing Interest Statement The authors have declared no competing interest. * ANI : average nucleotide identity BLAST : basic local alignment search tool Cazyme : carbohydrate-active enzyme FID : flame ionization detector GBDP : Genome Blast Distance Phylogeny GC : gas chromatography GH : glycoside hydrolase; GHxx : glycoside hydrolase family xx GT : glycosyltransferase GTDB : Genome Taxonomy Database HPAEC-PAD : high-performance anion-exchange chromatography with pulsed amperometric detection LB : Lysogeny broth MAG : metagenome-assembled genome PIA : Pseudomonas isolation agar PUL : polysaccharide utilization locus TYGS : Type (Strain) Genome Server