Seagrasses evolved from terrestrial plants into marine foundation species approximately 100 million years ago. However, their ecological success remains unclear. While symbiotic microorganisms are known to be critical for the development of seagrasses in nitrogen-limited and sulfide-accumulated (phytotoxic) marine sediments, our understanding of the symbiosis between seagrasses and bacteria is still limited. In this study, we found that the seagrass Halophila ovalis harbors various species of symbiotic bacteria, which exhibit spatial and temporal variability in the root system. All identified symbionts possess the capacity to fix nitrogen, suggesting they can provide ammonia and amino acids to their host in exchange for sugars. Additionally, some of these symbionts are sulfide-oxidizing bacteria, helping to reduce sulfide concentrations in the rhizosphere. These findings shed light on the long-term success of seagrasses in marine environments and present new opportunities for the conservation of seagrass ecosystems.

seagrass, nitrogen fixation, symbiosis, sulfide oxidation