Sulfur speciation in Sphagnum peat moss modified by mutualistic interactions with cyanobacteria

Abstract

Summary Peat moss (Sphagnum spp.) develops mutualistic interactions with cyanobacteria by providing carbohydrates and S compounds in exchange for N‐rich compounds, potentially facilitating N inputs into peatlands. Here, we evaluate how colonization of Sphagnum angustifolium hyaline cells by Nostoc muscorum modifies S abundance and speciation at the scales of individual cells and across whole leaves. For the first time, S K‐edge X‐ray Absorption Spectroscopy was used to identify bulk and micron‐scale S speciation across isolated cyanobacteria colonies, and in colonized and uncolonized leaves. Uncolonized leaves contained primarily reduced organic S and oxidized sulfonate‐ and sulfate‐containing compounds. Increasing Nostoc colonization resulted in an enrichment of S and changes in speciation, with increases in sulfate relative to reduced S and sulfonate. At the scale of individual hyaline cells, colonized cells exhibited localized enrichment of reduced S surrounded by diffuse sulfonate, similar to observations of cyanobacteria colonies cultured in the absence of leaves. We infer that colonization stimulates plant S uptake and the production of sulfate‐containing metabolites that are concentrated in stem tissues. Sulfate compounds that are produced in response to colonization become depleted in colonized cells where they may be converted into reduced S metabolites by cyanobacteria.