Peatland ecosystems are estimated to store a third of terrestrial carbon as dead organic peat. The moss plant Sphagnum is a foundation genus in these ecosystems, with its biological function (e.g., photosynthetic CO2 gain, recalcitrant decomposition, acidification) and abiotic environment influencing ecosystem structure and function and potentially global C cycling. These nitrogen limiting peatland systems highlight an important association of Sphagnum plants with N2-fixing (diazotrophic) bacterial associates that may ultimately influence Nitrogen (N) and Carbon (C) cycling. Our research in this area is guided by multiple questions including, who are the diazotrophic members in the Sphagnum microbiome? How does community abundance change in relation to experimental warming? Does host carbon limitation influence diazotroph interaction? And what are the host - genetic and - physiological controls that shape microbiome structure and function? Here, we first explore how the Sphagnum associated microbiome changes in relation to the warming treatments at a large-scale peatland experimental manipulation (SPRUCE: mnspruce.ornl.gov) using 16S rRNA profiling. We then use this information to guide genotype and strain selection for simplified constructed communities in gnotobiotic microcosm experiments.
Results/Conclusions
With 16S rRNA profiling of experimentally warmed Sphagnum, we found all samples to be dominated by Alphaproteobacteria (45-51%) followed by Acidobacteria (11-16%) and Gammaproteobacteria (8-9%). We found functional member abundance to vary by warming treatment: diazotroph abundance decreased with increased temperature (6% in ambient control, 3% in ambient +6C) and methanotroph abundance increased with temperature (0.14% in ambient control, 1.3% in ambient +6C respectively). Guided by community profiling results, a diazotrophic bacterium (cyanobacterium) and fungal isolate (Trizodia spp.) were chosen and used to establish simplified constructed communities under gnotobiotic microcosm conditions. Initial results show that all three members can interact within the same space without observed negative consequences. Current experiments are exploring how increased temperature influences abundance, N2 – fixation and net photosynthesis. A current collaboration with the DOE Joint Genome Institute has now expanded the genomic resources for this project by providing two draft genomes for S. fallax and S. magellanicum and the resequencing of a 200 individual S. fallax pedigree. Together with isolated fungal and bacterial strains, this represents a tremendous resource to the biological community interested in plant – microbe interactions, evolutionary and ecological genomics, and peatland ecology.