OOS 30-5 - Unraveling the environmental drivers of the feather moss bacteriome and its nitrogen fixation in the Canadian boreal forest

Background/Question/Methods
Feather mosses have essential biogeochemical and ecological functions in the boreal forest floor, where they importantly contribute to carbon and nitrogen cycles. Bacterial communities inhabiting mosses can perform nitrogen (N₂-) fixation and, therefore, constitute a critical source of nitrogen for the boreal forest ecosystem. Yet, these bacteria play many other roles (e.g., decomposition, nutrient acquisition) that remain poorly characterized. Global change is greatly affecting the boreal forest at an unprecedented speed. Thus, studying the moss bacteriome and its environmental drivers is crucial (i) to improve our understanding of microbial processes and nutrient cycles in the boreal forest, and (ii) to assess how they will respond to global change. In this study, we used cyanobacterial pigment extraction combined with cDNA amplicon sequencing of the 16S rRNA and nifH genes to explore the active global and diazotrophic bacteriomes of feather mosses. We investigated the environmental drivers of the feather moss bacteriome and its N₂-fixation (i) at the ecosystem scale, by collecting mosses on a 500-km natural gradient of climate and nutrient deposition in the Canadian boreal forest, and (ii) at the plant scale, along the senescence gradient of the moss shoot.

Results/Conclusions
We discovered that the global feather moss bacteriome comprised 290 genera but that the diazotrophic bacteriome was restricted to only 12 genera. We found that cyanobacteria were major actors of the feather moss bacteriome, accounting for 33% of global bacterial communities and 65% of diazotrophic communities, and that several cyanobacterial and methanotrophic genera were actively contributing to N₂-fixation. Moreover, we showed that bacteria were occupying ecological niches along the moss shoot, with phototrophs being dominant in the apical part and methanotrophs being dominant in the basal part. Finally, climate (temperature, precipitation), environmental variables (moss species, month, tree density) and nutrients (nitrogen, phosphorus, molybdenum, vanadium, iron) strongly shaped global and diazotrophic bacteriomes and influenced cyanobacterial biomass and N₂-fixation. In summary, this work presents evidence that the feather moss bacteriome plays crucial roles in supporting moss growth, health, and decomposition, as well as in the nutrient cycling of the boreal forest. Taken collectively, our results shed light on the diversity of diazotrophs in boreal ecosystems, as well as the environmental drivers of N₂-fixation, improving our ability to predict the impacts of global change on the nitrogen cycle in high-latitude habitats.