Seagrass-mediated rhizosphere redox gradients are linked with ammonium accumulation driven by diazotrophs

Abstract

Seagrasses are important marine habitats providing several ecosystem services in coastal waters worldwide, such as enhancing marine biodiversity and mitigating climate change through efficient carbon sequestration. Notably, the fitness of seagrasses is affected by plant-microbe interactions. However, these microscale interactions are challenging to study and large knowledge gaps prevail. Our study shows that redox microgradients in the rhizosphere of seagrass select for a unique microbial community that can enhance the ammonium availability for seagrass. We provide first experimental evidence that <i>Rhizobia</i>, including the symbiotic N₂-fixing bacteria <i>Bradyrhizobium</i>, can contribute to the bacterial ammonium production in the seagrass rhizosphere. The release of O₂ from rhizomes and roots also caused gradients of sulfide in rhizosphere areas with enhanced nifH transcription by sulfate-reducing bacteria. O₂ release from seagrass root systems thus seems crucial for ammonium production in the rhizosphere via stimulation of multiple diazotrophic associations.