Interactive toxicity of 6PPD and Cd(II) to algae in microbial community stock culture

The toxicity of 6PPD (N-(1,3-Dimethylbutyl)-N'-phenyl-p-phenylenediamine) to algae raises concern due to its potential to disrupt primary productivity in aquatic ecosystems. This study investigates the interactive toxicity of 6PPD and Cd(II) to algae in the presence and absence of other microorganisms, evaluated 48 h after exposure. In pure culture, Cd(II) exhibited dominant toxicity, while the toxicity of 6PPD was minimal. In contrast, in microbial community stock culture, the toxicity of 6PPD significantly increased with concentration, resulting in decreased algae growth, reduced chlorophyll a content, and suppression of key photosynthesis parameters. Notably, the addition of 2 mg/L Cd(II) significantly attenuated the toxicity of 6PPD to algae, suggesting an antagonistic interaction. However, at 4 mg/L Cd(II), the toxicity of 6PPD to algae remained significant at higher concentrations. Gene expression analysis revealed that 6PPD upregulated oxidative stress genes and photosynthetic genes, indicating a stress response, while Cd(II) inhibited these responses. Additionally, Cd(II) downregulated the P450 enzyme, suggesting potential inhibition of microbial transform of 6PPD into more toxic metabolites, such as 6PPD-quinone. Fluorescence spectroscopy showed that 2 mg/L Cd(II) reduced humic acid-like and tryptophan-like dissolved organic matter (DOM) in the culture supernatants, potentially affecting the bioavailability of 6PPD. The findings suggest that the presence of other microorganisms amplifies the toxicity of 6PPD, with effects may be mitigated by Cd(II) via microbial transformation inhibition or DOM complexation. Overall, this study provides a comprehensive understanding of how the interaction of 6PPD and Cd(II) impacts the health and function of aquatic plants.