Seagrass Survives Coastal Pollution by Splitting Microbial Labor Between Root Zones

Seagrass meadows are among the most valuable ecosystems on the planet, buffering coastlines, storing carbon at extraordinary rates, and serving as nurseries for commercially important fish. Yet these underwater flowering plants are under siege from a double threat that is intensifying as human populations crowd the world’s coasts: nutrient pollution from agriculture and sewage, and the toxic hydrogen sulfide that accumulates in the oxygen-starved sediments beneath them. A new study published in the journal Microbiome reveals that seagrasses do not face this combined assault alone. Instead, the plants appear to orchestrate a remarkable division of labor among the microscopic organisms living on and inside their roots, recruiting different microbial specialists to different root compartments depending on which stress dominates the environment.

The research, led by Wenqian Qi, Zhijian Jiang, Xiaoping Huang and colleagues at the South China Sea Institute of Oceanology of the Chinese Academy of Sciences, took an unusually comprehensive approach. Rather than running a single laboratory experiment, the team conducted a large-scale field investigation along a natural tropical gradient where nutrient loading and sulfide exposure vary from site to site. By sampling across this gradient, the researchers could observe how seagrass holobionts, the plant plus its full complement of associated microorganisms, respond to realistic combinations of pressures rather than to isolated stressors applied in isolation in a tank.

The methodological toolkit deployed in the study was correspondingly broad. The team integrated measurements of plant physiology with stable isotope analysis, metabolomics performed by ultra-high-performance liquid chromatography coupled to tandem mass spectrometry, 16S rRNA amplicon sequencing to profile bacterial communities, and shotgun metagenomics to reconstruct the functional genetic potential of those communities. This combination allowed the researchers to connect, at least correlatively, the chemistry of the plant, the chemistry of its root secretions, the identity of its microbial partners, and the genes those partners carry.

The first major finding concerns the plant itself. Across the gradient, the seagrass host displayed physiological and metabolic patterns consistent with what ecologists call a growth-defense trade-off. Where nutrients were abundant, the plant’s metabolite profile skewed toward growth-related compounds, reflecting the fact that nitrogen and phosphorus, usually limiting resources, were plentiful. Where sulfide exposure was intense, the profile shifted instead toward metabolites associated with antioxidant defense, a response that makes chemical sense because sulfide interferes with cellular respiration and generates oxidative stress. When both pressures coincided, the plant had to balance these competing metabolic demands simultaneously.

Crucially, this metabolic reprogramming did not show up primarily as a large shift in the bulk composition of the plant’s internal chemistry. Instead, the most consequential changes appeared in the relative abundance of specific root exudates, the chemical compounds that seagrasses release from their roots into the surrounding sediment. Two classes of compounds stood out: flavonoids and phenylpropanoids. These are well-known secondary metabolites in terrestrial plants, where they often serve as signaling molecules that attract or repel particular microorganisms. The study suggests that in seagrasses they may play a similar role, acting as a chemical language through which the plant communicates with its microbiome and reshapes which microbes gather at its roots.

The compartment-specific nature of the microbial response is the study’s most striking discovery. The researchers distinguished between two microbial habitats: the root endosphere, the community of microorganisms living inside the root tissue itself, and the rhizosphere, the zone of sediment immediately surrounding the roots that is influenced by root exudates. The differential exudates showed distinct relationships with microbes in each compartment. Under high nutrient conditions, taxa involved in nitrogen cycling were preferentially associated with the root endosphere, placing the nitrogen specialists in intimate contact with the plant’s own tissues. Under sulfide exposure, sulfur-cycling taxa were preferentially enriched in the rhizosphere, the outer zone where sulfide originating from anoxic sediments must first be intercepted.

The shotgun metagenomic data added a functional dimension to this spatial pattern. The rhizosphere microbiome showed stressor-tailored genomic potential, meaning that the genes present in the community shifted in ways that matched the dominant local stress. Under sulfide exposure, genes involved in sulfur oxidation and sulfur transfer, such as fccBA and dsrEF, became more prominent. These genes encode components of pathways that convert reduced sulfur compounds, including the sulfide that poisons plant roots, into less toxic oxidized forms. In effect, the rhizosphere community under sulfide stress carried the genetic equipment to act as a chemical detoxification shield for the plant.

Under nutrient enrichment, a different genomic signature emerged. The rhizosphere communities displayed redundant ammonia assimilation potential, exemplified by the gene gltD, which participates in the glutamate synthase pathway that incorporates ammonia into organic molecules. Redundancy in this context means that multiple microbial taxa carried the capacity, so the function is robust even if some members of the community decline. The researchers suggest this redundant assimilation potential may be relevant to nutrient stress mitigation, helping to buffer the plant against the imbalances that excess nitrogen can create in plant metabolism.

Perhaps the most intriguing result concerns what happens when both stressors strike at once. Under the dual pressure of high nutrients and sulfide, the putative functional division of labor appeared to be reconfigured. The endosphere communities, which under nutrient pressure alone had been dominated by nitrogen-cycling specialists, showed greater representation of taxa putatively involved in sulfur cycling. This suggests a degree of flexibility in the holobiont’s response: when the plant faces simultaneous threats, the internal root community takes on some of the sulfur-handling duties, potentially compensating for the fact that the plant’s metabolic resources are stretched between growth and defense demands. The division of labor, in other words, is not a fixed arrangement but a dynamic one that adjusts to the stress landscape.

The authors frame their findings within the holobiont concept, the idea that a host organism and its microbiome should be understood as a single ecological unit whose combined capabilities determine resilience. For seagrasses, whose sediments are naturally sulfide-rich and whose coastal habitats are increasingly eutrophic, this framework has direct practical implications. The researchers propose that the functional division of root microbiota provides a useful ecological framework for developing microbiome-informed seagrass restoration and conservation strategies. Restoration projects currently focus largely on transplanting healthy shoots into suitable sediments; a microbiome-informed approach might additionally consider inoculating transplants with beneficial sulfur-oxidizing or nitrogen-assimilating microbes, or selecting donor meadows whose root communities are pre-adapted to the stress conditions of the restoration site.

The study also carries a broader message about how coastal ecosystems cope with the cumulative effects of human activity. Eutrophication does not merely fertilize coastal waters; it triggers cascades that deplete oxygen, alter sediment chemistry, and generate sulfide, meaning that nutrient and sulfide stresses are physically coupled in the real world. By examining a natural gradient where these stresses co-occur, the study captures a more realistic picture of environmental change than single-stressor experiments can. The results suggest that the hidden microbial partners of seagrass roots constitute part of the plant’s stress-response machinery, and that protecting or restoring these partnerships may be as important as protecting the plants themselves. As seagrass meadows continue to decline globally, understanding the microscopic division of labor beneath the sediment surface may prove essential to keeping these underwater forests, and the carbon stores and fisheries they support, alive.

Subject of Research: Functional compartmentalization of seagrass root microbiomes under nutrient and sulfide stress

Article Title: Functional division between root endosphere and rhizosphere microbiomes supports seagrass holobiont resilience under high nutrient and sulfide exposure

Article References: Functional division between root endosphere and rhizosphere microbiomes supports seagrass holobiont resilience under high nutrient and sulfide exposure. (n.d.). https://doi.org/10.1186/s40168-026-02553-z

Image Credits: AI Generated

DOI: 10.1186/s40168-026-02553-z

Keywords: seagrass, holobiont, microbiome, rhizosphere, endosphere, eutrophication, sulfide, metabolomics, metagenomics, root exudates, sulfur oxidation, nitrogen cycling