Metabolomics revealed more deleterious toxicity induced by the combined exposure of ammonia and nitrite on Ruditapes philippinarum compared to single exposure

NH3–N and NO2–N always co-exist in the aquatic environment, but there is not a clear opinion on their joint toxicities to the molluscs. Presently, clams Ruditapes philippinarum were challenged by environmental concentrations of NH3–N and NO2–N, singly or in combination, and analyzed by metabolomics approaches, enzyme assays and transmission electron microscope (TEM) observation. Results showed that some same KEGG pathways with different enriched-metabolites were detected in the three exposed groups within one day, and completely different profiles of metabolites were found in the rest of the exposure period. The combined exposure induced heavier and more lasting toxicities to the clams compared with their single exposure. ACP activity and the number of secondary lysosomes were significantly increased after the combined exposure. The present study shed light on the joint-toxicity mechanism of NH3–N and NO2–N, and provided fundamental data for the toxicity research on inorganic nitrogen. •Untargeted metabolomics was a useful approach for joint toxicity study.•NO2–N, NH3–N and Mix exposures induced similar but different metabolite profiles.•Combination exposure would induce heavier toxicity earlier and more lasting.•Combination exposure significantly enhanced activities of ACP and AKP.•More secondary lysosomes were detected in the Mix group.