Nitrate Aggravates While Ammonium Mitigates Thermal Bleaching in Corals through Divergent Lipid-Mediated Pathways and Stress Response

The increased input of inorganic nutrients, particularly dissolved inorganic nitrogen, has been associated with heightened susceptibility of reef-building corals to thermal bleaching; however, how nitrate and ammonium differentially modulate this response remains unclear. In the present study, we combined cellular physiology, transcriptomics, and quantitative lipidomics to test how nitrate or ammonium enrichment alters coral responses to ocean warming. The findings indicated that nitrate enrichment and an elevated temperature acted synergistically to aggravate oxidative stress and exacerbate bleaching. Under heat stress, nitrate reduced photosynthetic membrane lipids, ceramides, and coenzyme Q, lowering photosynthetic efficiency in Symbiodiniaceae, and induced upregulation of genes for pro-inflammatory lipid biosynthesis in the coral host. By contrast, ammonium enrichment and elevated temperature acted antagonistically to reduce susceptibility to bleaching. Under heat stress, ammonium increased photosynthetic membrane lipids and photosynthetic efficiency in Symbiodiniaceae, while in the coral host, it decreased pro-inflammatory lipids, increased ceramide levels, upregulated genes involved in heat shock protein 70 orthologues, and re-established symbiosis. These divergent effects indicate that nitrate and ammonium activate distinct lipid-mediated pathways and stress responses that differentially regulate coral susceptibility to heat stress, with important implications for managing nitrogen pollution under ocean warming.