Fungal Microbiome-Metabolome Relationships in Sphagnum and Two Co-Occurring Alaskan Mosses

In boreal and tundra ecosystems, mosses are abundant and ecologically important members of the vegetation due to their ability to insulate permafrost and maintain soil moisture. Mosses also harbour diverse bacterial and fungal symbionts that can provide nutrients and protection against environmental stressors. Sphagnum mosses are particularly important due to their significant role in carbon sequestration, which has been attributed in part to the production of antimicrobial metabolites that slow decomposition. Although Sphagnum leachate has been shown to inhibit bacteria, how Sphagnum chemical traits impact fungal communities remains understudied. Here, we used culture-free and culture-based methods to examine the relationship between moss fungal communities and metabolomes in living and senescing tissues of Sphagnum and two co-occurring moss genera across four Alaskan boreal/tundra sites. Although their richness was similar among moss genera, fungal and metabolite composition differed significantly among moss genera, regardless of tissue age. Importantly, mosses with more similar metabolome composition harboured more similar fungal communities, particularly in living tissues. Numerous OTU-metabolite correlations suggest direct interactions whereby fungi may consume, degrade, and/or be inhibited by metabolites; however, in vitro growth of moss-associated fungi showed inhibition in only 25% of replicates with two phenolic metabolites. Overall, our data suggest that metabolites may be a key factor structuring fungal communities in Sphagnum and other mosses, although not solely via inhibitory effects. Given the significance of mosses to ecosystem function and carbon sequestration in northern regions, it is critical to better understand factors that shape fungal communities potentially involved in stress adaptation and decomposition.