Background Salt marshes are dominated by the smooth cordgrass Spartina alterniflora on the US Atlantic and Gulf of Mexico coastlines. Although soil microorganisms are well known to mediate important biogeochemical cycles in salt marshes, little is known about the role of root microbiomes in supporting the health and productivity of marsh plant hosts. Leveraging in situ gradients in aboveground plant biomass as a natural laboratory, we investigated the relationships between S. alterniflora primary productivity, sediment redox potential, and the physiological ecology of bulk sediment, rhizosphere, and root microbial communities at two Georgia barrier islands over two growing seasons. Results A marked decrease in prokaryotic alpha diversity with high abundance and increased phylogenetic dispersion was found in the S. alterniflora root microbiome. Significantly higher rates of enzymatic organic matter decomposition, as well as the relative abundances of putative sulfur (S)-oxidizing, sulfate-reducing, and nitrifying prokaryotes correlated with plant productivity. Moreover, these functional guilds were overrepresented in the S. alterniflora rhizosphere and root core microbiomes. Core microbiome bacteria from the Candidatus Thiodiazotropha genus, with the metabolic potential to couple S oxidation with C and N fixation, were shown to be highly abundant in the root and rhizosphere of S. alterniflora. Conclusions The S. alterniflora root microbiome is dominated by highly active and competitive species taking advantage of available carbon substrates in the oxidized root zone. Two microbially mediated mechanisms are proposed to stimulate S. alterniflora primary productivity: (i) enhanced microbial activity replenishes nutrients and terminal electron acceptors in higher biomass stands, and (ii) coupling of chemolithotrophic S oxidation with carbon (C) and nitrogen (N) fixation by root- and rhizosphere-associated prokaryotes detoxifies sulfide in the root zone while potentially transferring fixed C and N to the host plant.

Northern peatlands store approximately one-third of terrestrial soil carbon. Climate warming is expected to stimulate the microbially-mediated degradation of peat soil organic matter (SOM), leading to increasing greenhouse gas (GHG; carbon dioxide, CO2; methane, CH4) production and emission. Porewater dissolved organic matter (DOM) plays a key role in SOM decomposition; however, the mechanisms controlling SOM decomposition and its response to warming remain unclear. The temperature dependence of GHG production and microbial community dynamics were investigated in anoxic peat from a Sphagnum-dominated peatland. In this study, peat decomposition, which was quantified by GHG production and carbon substrate utilization, is limited by terminal electron acceptors (TEA) and DOM, and these controls of microbially-mediated SOM degradation are temperature dependent. Elevated temperature led to a slight decrease in microbial diversity, and stimulated the growth of specific methanotrophic and syntrophic taxa. These results confirm that DOM is a major driver of decomposition in peatland soils, contains inhibitory compounds, but the inhibitory effect is alleviated by warming.

Symbiotic root microbiota are crucial for plant growth as they assist their hosts in nutrient acquisition. In the roots of coastal marine plants, heterotrophic activity in the rhizosphere by sulfate-reducing microorganisms has been linked to nitrogen fixation. In this study, we recovered 239 high-quality metagenome-assembled genomes (MAGs) from a salt marsh dominated by the foundation plant Spartina alterniflora, including diazotrophic sulfate-reducing and sulfur-oxidizing bacteria thriving in the root compartment. Here we show for the first time that highly-abundant sulfur-oxidizing bacteria in the roots of a coastal macrophyte encode and highly express genes for nitrogen fixation (nifHDK). Further, we leveraged a S. alterniflora biomass gradient to gain a mechanistic understanding on how root-microbe interactions respond to abiotic stress from anoxia and elevated sulfide concentration. We observed that the roots of the stressed S. alterniflora phenotype exhibited the highest rates of nitrogen fixation and expression levels of both the oxidative and reductive forms of the dissimilatory sulfite reductase gene (dsrAB). Approximately 25% and 15% of all sulfur-oxidizing dsrA and nitrogen-fixing nifK transcripts, respectively, were associated with novel MAGs of the Candidatus Thiodiazotropha genus in the roots of the stressed S. alterniflora phenotype. We conclude that the rapid cycling of sulfur in the dynamic S. alterniflora root zone is coupled to nitrogen fixation during both reductive and oxidative sulfur reactions, and that the S. alterniflora - Ca. Thiodiazotropha symbiosis is an adaptive response to anoxic and sulfidic sediment conditions, whereby the plants benefit from reduced sulfide toxicity and potential nitrogen acquisition.