Environmental Concentrations of Herbicide Prometryn Increase the Susceptibility of Reef-Building Corals to Nitrate Stress

Herbicide and nitrate pollution frequently co-occur in coastal waters, while their combined effect on corals has been a critical knowledge gap. Although corals can exhibit acclimation to nitrate enrichment, it is unclear whether this acclimation persists under coexposure to PSII herbicides. Here, we investigated the combined effects of nitrate and the PSII herbicide prometryn on the reef-building coralAcropora hyacinthusby integrating physiology, ultrastructure, and host-symbiont multiomics. The results demonstrate that prometryn acts as the primary driver of photodamage and photosynthetic toxicity in the symbiotic Symbiodiniaceae under costress. Transmission electron microscopy revealed that prometryn induced activated symbiophagy and chloroplast degradation with loosely stacked thylakoids in Symbiodiniaceae. Transcriptomics and proteomics analyses predicted that nitrate enrichment stimulated host glutathione-based antioxidant responses. However, prometryn-induced phototoxicity impaired photosynthate supply and disrupted this nitrate acclimation mechanism. Moreover, reduced photosynthate transfer further impaired the host energy balance and intensified oxidative stress. These findings show that PSII herbicides can override short-term acclimation to nitrate enrichment, underscoring the need for the joint regulation of herbicide and nutrient inputs in reef ecosystems.