The bacterial communities of Alaskan mosses and their contributions to N2-fixation

Abstract

Background: Mosses in high-latitude ecosystems harbor diverse bacterial taxa, including N₂-fixers which are key contributors to nitrogen dynamics in these systems. Yet the relative importance of moss host species, and environmental factors, in structuring these microbial communities and their N₂-fixing potential remains unclear. We studied 26 boreal and tundra moss species across 24 sites in Alaska, USA, from 61 to 69° N. We used cultivation-independent approaches to characterize the variation in moss-associated bacterial communities as a function of host species identity and site characteristics. We also measured N₂-fixation rates via ¹⁵N₂ isotopic enrichment and identified potential N₂-fixing bacteria using available literature and genomic information.

Results: Host species identity and host evolutionary history were both highly predictive of moss microbiome composition, highlighting strong phylogenetic coherence in these microbial communities. Although less important, light availability and temperature also influenced composition of the moss microbiome. Finally, we identified putative N₂-fixing bacteria specific to some moss hosts, including potential N₂-fixing bacteria outside well-studied cyanobacterial clades.

Conclusions: The strong effect of host identity on moss-associated bacterial communities demonstrates mosses' utility for understanding plant-microbe interactions in non-leguminous systems. Our work also highlights the likely importance of novel bacterial taxa to N₂-fixation in high-latitude ecosystems. Video Abstract.

Keywords: Bryophytes; Nitrogen fixation; Phyllosphere; Plant microbiome.

Fig. 1

A map summarizing the species and sampling locations of the 461 samples in this study. Images (left) from each transect location show the environmental heterogeneity across the sites. The table (top right) displays the list of species sampled, ordered by the total number of samples collected for each species (see Supplementary Tables S1 and S2). Dots indicate the sites where at least one sample of a particular species was collected. The blue boxes indicate the seven moss species identified as “abundant and ubiquitous” in our data set (see the “Methods” section). Images of these seven species are displayed at the bottom right corner of the figure. The sites with the most moss species that were used in the site-specific analyses are marked with an asterisk

Fig. 2

A heatmap showing the abundant bacterial families (x-axis) across different moss host species (y-axis) with a tree showing the phylogenetic relationships between the moss host species. Only 24 of the 26 moss species are included in this heatmap (see the “Methods” section). The outgroup host species, Ptilidium ciliare, is a liverwort. The colors indicate the relative abundance of each bacterial family with shades of yellow indicating higher relative abundance and shades of blue indicating lower relative abundance. Numbers in each cell of the heatmap show the percent relative abundance of a particular bacterial family

Fig. 3

N₂-fixation rates for the seven abundant and ubiquitous species in our data set as measured by isotope enrichment over a 1-day incubation period. Moss species are arranged from highest average fixation rate to lowest. All seven species hosted N₂-fixing microbes however rates were highly variable across host species. Each circle represents the measured fixation rate in one moss sample. Error bars represent 1 standard error above and below the mean (which is indicated by the height of the main bar). A more in-depth analysis of the differences in N₂-fixation across moss species can be found in Stuart et al. [31]

Fig. 4

Spearman correlations between the relative abundances of individual bacterial phylotypes and measured N₂-fixation rates. Bacterial phylotypes used in this analysis were limited to those that had an average relative abundance of > 0.1% across the more abundant and ubiquitous moss host species (156 phylotypes). The data is separated into panels based on the family-level taxonomy of the phylotypes. Colored points and lines represent bacterial phylotypes that were significantly positively correlated with N₂-fixation in a particular moss species. Although not indicated on the plot for clarity, correlation coefficients for each phylotype can be found in Supplementary Table S6. Lines and points are colored according to the moss species in question. Gray dots are measurements for 137 phylotypes that were not found to be significantly positively correlated with N₂-fixation rates. We note that, to improve clarity, the x and y scales vary between panels

Fig. 5

Correlation network showing phylotypes (represented by colored nodes) that had significant positive Spearman correlations with N₂-fixation rates (rho, 0.33–0.78; P < 0.05, see Supplementary Table S6 for details) within each of the abundant moss species. Colors of the nodes represent the family membership of each phylotype. Phylotypes that were confirmed as potential N₂-fixers are labeled with their unique identifier and asterisks (see Supplementary Table S5 for details)