Ectomycorrhizal decomposers – enzymes, ecology, and evolution

Ectomycorrhizal fungi are central to nutrient cycling and carbon dynamics in forest soils. Some ectomycorrhizal species, here called ectomycorrhizal decomposers, have maintained genes from their saprotrophic ancestors encoding for potent oxidative enzymes known as manganese peroxidases. This thesis aimed to deepen the understanding of ectomycorrhizal decomposers and their role in soil organic matter dynamics, with a focus on Cortinarius species. Specifically, I aimed to elucidate the mechanisms and implications of ectomycorrhizal manganese peroxidases using a combination of molecular methods (including ITS metabarcoding, in silico genome analysis, and metatranscriptomics), enzyme assays, and heterologous expression of recombinant enzymes.Recombinant manganese peroxidases from Cortinarius were found to oxidise organic compounds. Further, expression of manganese peroxidase genes in situ was correlated with expression of genes related to acquisition of organic nitrogen and phosphorus. These results are key steps forward in linking the genomic capacity of ectomycorrhizal decomposers to their suggested role in mining nutrients from recalcitrant soil organic matter. In addition, gene expression data suggested that oxidative decomposition was not associated with a high energy demand, at least relative to other mycelial activities. On the ecosystem scale, it appeared that certain ectomycorrhizal species were particularly important decomposers, with considerable trait variation at the genus-level. The species assigned here exhibited niche differentiation with respect to forest age and soil fertility.In nitrogen-limited boreal forests, the duality of mycorrhizal ecology and potent decomposer capacity is an especially important trait. As anthropogenic climate change progresses, understanding how ectomycorrhizal decomposers influence soil organic matter in different environments may be fundamental for predicting the capacity for boreal forest soils to sequester carbon and sustain tree productivity.