Ammonia is widely regarded as the primary chemical pollutant responsible for fish toxicity in aquaculture. Scatophagus argus is an economically important euryhaline species extensively cultured in marine aquaculture. To investigate its physiological responses and molecular mechanisms under ammonia exposure, we determined the 96 h median lethal concentration (LC₅₀-96 h) of total ammonia nitrogen (TAN) for S. argus juveniles. Histopathological analyses were conducted at TAN concentrations of 0 (control), 30, and 60 mg/L, with transcriptomic analysis performed at 0 and 60 mg/L. The results showed that the LC₅₀-96 h for S. argus was 59.43 mg/L. Histological analysis revealed lamellar epithelial detachment and hepatocyte vacuolization in S. argus exposed to 60 mg/L TAN, indicating substantial structural impairment under ammonia stress. Transcriptomic profiling identified 245 differentially expressed genes (DEGs), comprising 142 upregulated and 103 downregulated genes. KEGG enrichment analysis indicated that DEGs were primarily enriched in energy metabolism and immune-related pathways. Key genes involved in glucose metabolism, amino acid metabolism, and cellular regulation (e.g., PFKM, PGM1, MAT2A, DDIT4) were significantly upregulated in energy metabolism pathways. In immune-related pathways, immune regulatory genes such as GIMAP4 and ARRDC3 were upregulated, while NAMLAA, associated with inflammatory modulation, was downregulated. Collectively, these transcriptional changes suggest that S. argus responds to external ammonia stress through coordinated regulation of energy metabolism and immune function. This study provides novel insights into the physiological and molecular strategies employed by S. argus in response to ammonia toxicity, offering a reference for environmental risk assessment and aquaculture management.

Green and golden tides caused by Ulva prolifera and Sargassum horneri erupt annually along the coasts of China. During the late stages of algal blooms, massive macroalgal die-offs result in the release of dissolved organic matter (DOM), which can profoundly affect local marine environments and elemental cycles. However, studies addressing these impacts remain insufficient and urgently needed. This study investigates the two primary macroalgal contributors, Ulva prolifera and Sargassum horneri, by simulating their decay and DOM release in seawater using Bacteria-active (BA) and Bacteria-inhibited (BI) groups. The release characteristics of carbon, nitrogen, and phosphorus, as well as their effects on microbial communities, were analyzed. Results show that U. prolifera and S. horneri rapidly release DOM and nutrients during their decay. Significant differences in the quantity and rate of DOM and nutrient release were observed due to structural differences between the two macroalgae. Carbohydrates constitute the major component of the released organic matter and are rapidly utilized by microbes for growth and reproduction. Furthermore, the DOM and nutrients released by macroalgae reshape the composition of microbial communities in the marine microenvironment. This study provides new insights into the potential regional marine ecological impacts of green and golden tide outbreaks from a biogeochemical perspective.