Time-dependent combined toxicity of four quaternary ammonium compounds to Vibrio qinghaiensis sp.-Q67: a synergistic trend shifting to additive action.

The ecological risks posed by mixtures of quaternary ammonium compounds (QACs) to aquatic ecosystems, especially their time-dependent combined toxicity, remain insufficiently characterized. Therefore, four QACs, benzyl dodecyl dimethyl ammonium bromide (BEN), didodecyl dimethyl ammonium bromide (DID), domiphen bromide (DOM) and tetradecyl dimethyl benzyl ammonium chloride (TET), were selected as experimental target contaminants to explore their time-dependent combined toxicity (0.25, 2, 4, 8, and 12 h) by taking Vibrio qinghaiensis sp.-Q67 (Q67) as the indicator organism. Here, the mixtures of four QACs, including four ternary mixture systems (BEN-DID-DOM, BEN-DID-TET, BEN-DOM-TET and DID-DOM-TET) and one quaternary mixture system (BEN-DID-DOM-TET), were designed by the uniform design ray method. The synergism-antagonism heatmap based on the concentration addition model (denoted SAHmapCA) was developed to systematically evaluate the interactions in various mixtures. The results showed that under acute exposure, all four QACs exhibited relatively strong toxicity. With prolonged exposure time, the toxicity of DID increased markedly, whereas that of the other three QACs varied only slightly. The four QACs induced cell morphological changes and possibly altered DNA structure in Q67, whereas no obvious effect on protein structure was observed at the tested concentrations. All mixtures showed obvious toxicity to Q67, similar to their individual components. DID-containing mixture rays showed a synergistic trend at the early exposure times (0.25 and 2 h), but exhibited additive action at later exposure times, whereas DID-free mixture rays exhibited additive action at all exposure time points. These results reveal a time-dependent shift from a synergistic trend to additive action in DID-containing QAC mixtures.