A phyllosphere bacterium Exiguobacterium aurantiacum S11 mitigates cadmium toxicity in seagrass (Zostera marina) via a synergistic "exclusion-redox-repair" network.

Heavy metal pollution, particularly cadmium (Cd), threatens coastal marine ecosystems and foundational species like the seagrass Zostera marina. Here, a multi-extremophilic phyllosphere bacterium, Exiguobacterium aurantiacum S11, was isolated to investigate its capacity and multi-dimensional molecular networks in mitigating acute Cd toxicity in Z. marina. In vitro assays confirmed high Cd tolerance in strain S11, driven by cell-surface biosorption and genomic active efflux systems (czcD, cadD, zupT, and znuA) regulated by cadC and resD. In an 18-day mesocosm challenge model utilizing 500 μM Cd, S11 inoculation reduced root Cd accumulation by 38.3% (p < 0.05) and attenuated systemic oxidative stress by suppressing malondialdehyde and stabilizing glutathione peroxidase activity. Furthermore, S11 specifically activated host folate biosynthesis, vitamin B6 metabolism, and DNA Base Excision Repair (BER) pathways, while promoting the accumulation of defensive diterpenes, flavonoids, and isoflavonoids. Overall, E. aurantiacum S11 establishes a synergistic "exclusion-redox-repair" network that enhances host resilience, offering a novel microbially assisted strategy for marine ecological restoration.