Salinity Modulates Hormetic Responses of the Estuarine Diatom Chaetoceros muelleri to Short-Chain Chlorinated Paraffins: Insights from Physiology and Transcriptomics

Salinity fluctuations and chemical pollution co-occur frequently in estuarine environments, yet their combined effects on estuarine organisms and the underlying mechanisms remain largely unexplored. In this study, we investigated the physiological and transcriptomic responses of the estuarine diatom Chaetoceros muelleri to short-chain chlorinated paraffins (SCCPs) under three salinity conditions (5‰, 15‰, and 25‰). Reduced salinity (5‰) tended to decrease the diatom sensitivity to SCCPs, as evidenced by a lower EC 50 (150 ± 63 μg L –1 ) compared to 15‰ salinity (194 ± 74 μg L –1 ). Hormetic responses were observed at both 5‰ and 15‰, with maximum stimulation of 58.06% and 27.10%, respectively. In contrast, SCCPs at 25‰ salinity presented no hormetic effect but caused chlorophyll depletion and oxidative stress. Transcriptomic analysis indicated that a low concentration of SCCPs (nominal 10 μg L –1 ) at 5‰ salinity down-regulated light-harvesting complexes and reallocated energy toward cell growth, which contributed to the observed hormesis under this condition. At 25‰ salinity, SCCPs induced no detectable transcriptional changes (0 differentially expressed genes), likely due to the reduced bioavailability under hyperosmotic conditions. These results demonstrated that salinity critically modulated SCCPs toxicity and highlighted the necessity of incorporating dynamic environmental factors into estuarine ecological risk assessments.