Seagrass-mediated rhizosphere redox gradients are linked with ammonium accumulation driven by diazotrophs
Authors: Brodersen Kasper Elgetti , Mosshammer Maria , Bittner Meriel J. , Hallstrøm Søren , Santner Jakob , Riemann Lasse , Kühl Michael 
Published: March 2024

Seagrasses can enhance nutrient mobilization in their rhizosphere via complex interactions with sediment redox conditions and microbial populations. Yet, limited knowledge exists on how seagrass-derived rhizosphere dynamics affect nitrogen cycling. Using optode and gel-sampler-based chemical imaging, we show that radial O
2
loss (ROL) from rhizomes and roots leads to the formation of redox gradients around below-ground tissues of seagrass (
Zostera marina
), which are co-localized with regions of high ammonium concentrations in the rhizosphere. Combining such chemical imaging with fine-scale sampling for microbial community and gene expression analyses indicated that multiple biogeochemical pathways and microbial players can lead to high ammonium concentration within the oxidized regions of the seagrass rhizosphere. Symbiotic N
2
-fixing bacteria (
Bradyrhizobium
) were particularly abundant and expressed the diazotroph functional marker gene
nifH
in
Z. marina
rhizosphere areas with high ammonium concentrations. Such an association between
Z. marina
and
Bradyrhizobium
can facilitate ammonium mobilization, the preferred nitrogen source for seagrasses, enhancing seagrass productivity within nitrogen-limited environments. ROL also caused strong gradients of sulfide at anoxic/oxic interfaces in rhizosphere areas, where we found enhanced
nifH
transcription by sulfate-reducing bacteria. Furthermore, we found a high abundance of methylotrophic and sulfide-oxidizing bacteria in rhizosphere areas, where O
2
was released from seagrass rhizomes and roots. These bacteria could play a beneficial role for the plants in terms of their methane and sulfide oxidation, as well as their formation of growth factors and phytohormones. ROL from below-ground tissues of seagrass, thus, seems crucial for ammonium production in the rhizosphere via stimulation of multiple diazotrophic associations.

IMPORTANCE

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
Rhizobia
, including the symbiotic N
2
-fixing bacteria
Bradyrhizobium
, can contribute to the bacterial ammonium production in the seagrass rhizosphere. The release of O
2
from rhizomes and roots also caused gradients of sulfide in rhizosphere areas with enhanced nifH transcription by sulfate-reducing bacteria. O
2
release from seagrass root systems thus seems crucial for ammonium production in the rhizosphere via stimulation of multiple diazotrophic associations.