Phosphorus-potassium fertilizer exposure induces oxidative stress and riboflavin metabolism disruption in juvenile turbot (Scophthalmus maximus).

Intensive agriculture's excessive fertilizer use significantly contributes to coastal nutrient pollution. While phosphorus's ecological impacts are recognized, the toxicity of combined phosphorus nutrients on marine life remains unclear. This study examined phosphorus-potassium fertilizer's (P-K fertilizer) effects on juvenile turbot (Scophthalmus maximus) via a 96-hour acute exposure experiment. Exposure solutions were prepared at environmentally relevant concentrations, resulting in elevated seawater phosphorus and potassium levels by 117.25 mg/L and 152.94 mg/L, respectively. Compound fertilizer aggravated multi-tissue damage, such as gill epithelial cell proliferation. Untargeted metabolomics identified that P-K fertilizer disrupts riboflavin metabolism, leading to reduced riboflavin levels. This deficiency impaired the production of flavin mononucleotide and flavin adenine dinucleotide. Meanwhile, excess potassium ions triggered Na+/K+ ATPase pump and gene of the renal outer medullary K+ channel to maintain ionic balance. Following riboflavin metabolism disruption, juvenile turbot exhibited impaired antioxidant capacity, with significantly decreased activities of glutathione peroxidase (by 54.5 %) and superoxide dismutase (by 12.6 %), a significant increase in catalase activity, obvious accumulation of malondialdehyde, and a reduction in total antioxidant capacity. The changes in related gene expression measured by real-time qPCR were consistent with the observed alterations in enzyme activities. Overall, compared to exposure to PO43--P alone, the combined P-K fertilizer exposure resulted in more severe disruption of riboflavin metabolism and exacerbated oxidative damage in marine fish, indicating that K+ potentiated the adverse effects through synergistic interactions. This work provides critical insights for managing nutrient pollution in marine ecosystems and safeguarding coastal biodiversity and ecosystem services.