Sulfide is a potent phytotoxin and sulfide intrusion into seagrass tissues has been implicated in the declines of foundation seagrass species across the globe. To date, research has focused on understanding the physiological responses of individual plants and seagrass meadows to sulfide intrusion. The role of rhizosphere microorganisms in sulfide intrusion has not been thoroughly explored, despite the critical roles that microorganisms have in mediating marine sulfur cycling. We investigated if the composition of the sediment microbial community was linked to the vulnerability of seagrasses to sulfide intrusion by sampling sediments and seagrasses (Posidonia sinuosa) from six subtidal sites in Perth, Western Australia.
Seagrass δ34S signatures varied significantly across the study area, and sites with highest signatures of sulfide intrusion also had long-term declines in seagrass density. Microbial communities also differed between sites. Microbial communities at sites impacted by sulfide intrusion had a high relative abundance of Chromatiaceae and Ectothiorhodospiraceae; 2 families of purple sulfur bacteria that have a dominant role in sulfur oxidation processes and thrive in environments low in oxygen and high in sulfides. These sites also had a high abundance of functional genes related to sulfur metabolism. The relative abundance of sulfate reduction genes was negatively correlated with seagrass δ34S, suggesting a quantitative link between sediment microbial communities and seagrass sulfide intrusion. Fluorescent in-situ hybridization revealed differential abundances of sulfide oxidizing cable bacteria on P. sinuosa roots along a sulfide intrusion gradient, suggesting that this taxon plays on important role in sulfur cycling in seagrass sediments.
We finish by providing recommendations for future research priorities that will help establish a quantitative and mechanistic understanding of the relationship between seagrass health, sulfur cycling, and rhizosphere microbial communities that will be valuable to the management and restoration of seagrass ecosystems globally.