Heat and Nitrate Drive Metabolic and Immune Reprogramming Leading to the Collapse of Symbiosis in the Model Sea Anemone Aiptasia

The maintenance of endosymbiosis in cnidarians depends on the tight regulation of host immunity, cell cycle, and nutrient exchange, yet how these processes are impacted by interacting environmental stressors remains largely unknown. To address this, we employed physiological metrics, gene expression analysis, microbiome characterization, imaging (NF-{kappa}B localization, endoplasmic reticulum ultrastructure, EdU labeling), and stable isotope tracing in the model sea anemone Exaiptasia diaphana to examine the effects of heat and nitrate on these regulatory processes, individually and in combination. Heat treatment led to NF-{kappa}B activation, proteostatic stress, suppression of nutrient exchange, decreased cell-cycle progression, and microbiome restructuring, with all effects more pronounced in symbiotic than aposymbiotic anemones. In symbiotic anemones, nitrate partially offset these heat-induced responses through sustained carbon translocation, suggesting that the presence of symbionts, in conjunction with elevated nitrate, can temporarily buffer host thermal stress. However, prolonged combined exposure resulted in holobiont failure. These findings reveal that while nitrate enrichment can transiently delay the onset of bleaching, it does not preserve the regulatory networks required for symbiotic stability -- underscoring the vulnerability of cnidarian holobionts to the compounding effects of warming and nitrate pollution.