Nostoc Colonization Driven by Sphagnum Host Genetic Variation

Sphagnum­ dominated peatlands sequester more carbon in temperate and boreal ecosystems than any other plant group. Sphagnum derive much of their nitrogen budget from an intimate symbiosis with nitrogen-fixing diazotroph bacteria, but little is known about the factors mediating this symbiotic relationship. Our research is guided by the overarching question: What are the genetic and physiological controls that shape Sphagnum-diazotroph symbiosis? Partnered with DOE JGI, we have (re)sequenced the genomes of 200 individuals from an S. fallax pedigree and genomes from a Sphagnum isolated diazotrophic cyanobacteria to create a QTL analysis to identify moss genes controlling the colonization. We hypothesize that there is variation in diazotroph colonization based on Sphagnum fallax genetic variation.Here I present results from two independent experiments. First, a pH gradient experiment was conducted that demonstrated diazotroph colonization was mediated by pH that drives the symbiosis to be either beneficial (mutualistic) or harmful (parasitic) for both organisms. Results show that while at low pH, both organisms benefit from this symbiosis and the symbiosis benefits cyanobacteria along the pH gradient. Using pH conditions that mediated mutualism, I explored the genetic controls on the Sphagnum-diazotroph initiation by conducting a Nostoc colonization experiment that demonstrated that there is variation in abundance of Nostoc colonization based on Sphagnum fallax genetic variation. I show a ranking of Sphagnum genotypes that had high Nostoc colonization to low Nostoc colonization. These extreme Sphagnum genotypes will be further explored to answer more research interests. Our research will provide a better understanding of plant-microbe interactions, ecological genomics, and peatland carbon and nitrogen cycling.