Peatland ecosystems are estimated to store about a third of the Earth’s terrestrial carbon as dead organic peat. The moss plant Sphagnum is a keystone 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 of the Sphagnum microbiome? How does community abundance change in relation to experimental warming? Does the warming conditioned microbiome influence Sphagnum performance to heat? 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 perform microbiome transfer studies to determine if and how the warming altered microbiome benefits Sphagnum to elevated temperatures.
Results/Conclusions
The taxonomic diversity of overall microbial communities and diazotroph communities, as well as N2 fixation rates, decreased with warming (p < 0.05). Following warming, diazotrophs shifted from a mixed community of Nostocales (Cyanobacteria) and Rhizobiales (Alphaproteobacteria) to predominance of Nostocales. Microbiome community composition differed between years, with some diazotroph populations persisting while others declined in relative abundance in warmed plots in the second year.
To determine if the warming altered microbiome benefits the plant, we developed a microbiome transfer approach where field collected Sphagnum microbiomes, conditioned to three-years of elevated temperature (ambient + 9 ˚C) or ambient temperature, were isolated and applied to germ-free tissue culture Sphagnum and exposed to temperature manipulations. Consistent over two consecutive years, we found that Sphagnum grows better at elevated temperatures when inoculated with a warming-conditioned microbiome than when inoculated with an ambient microbiome or no microbes at all. Metatranscriptome data revealed that changes in microbiome nitrogen, carbon, sulfur metabolism and heat shock response differed as a result of microbiome origin (i.e., conditioned to warming or ambient field temperature). On the plant side, expression of stress related genes, including those encoding heat shock genes, were reduced at elevated temperatures when in symbiosis with a microbiome originating from warm field conditions. Future experiments will apply these communities across sequenced moss pedigrees for identification of plant genes mediating beneficial microbial interactions.