Toxicological assessment of tyre-derived compounds: effects of 6PPD-quinone, diphenylguanidine, and mercaptobenzothiazole on Phaeodactylum tricornutum.

Urban runoff and industrial discharges introduce toxic compounds into marine ecosystems, threatening aquatic life and ecosystem functions. This study aimed to assess the acute toxic effects of three widely occurring tyre-derived contaminants (mercaptobenzothiazole (MBT), diphenylguanidine (DPG), and 6PPD-quinone (6PPD-Q)) on the growth of the marine diatom Phaeodactylum tricornutum. Growth inhibition assays were performed following ISO 10,253:2016 guidelines. Among the three compounds tested, DPG was the most toxic (EC50 = 0.101 µg/L), followed closely by MBT (EC50 = 0.212 µg/L). In contrast, 6PPD-Q was comparatively less toxic (EC50 = 4.072 µg/L). The no-observed-effect concentration (NOEC) and lowest-observed-effect concentration (LOEC) for DPG were 0.02 and 0.062 µg/L, respectively, and for MBT were 0.0948 and 0.299 µg/L, indicating high toxicity at environmentally relevant levels. In comparison, 6PPD-Q showed only moderate toxicity (NOEC = 3 µg/L; LOEC = 9 µg/L). Statistical analysis (ANOVA with Tukey's HSD post-hoc) confirmed significant dose-dependent inhibition for each compound (p < 0.001). Notably, at the lowest 6PPD-Q concentrations some replicates exhibited slight growth stimulation (negative inhibition ∼4 %), suggesting a potential hormetic effect. These findings underscore the ecological risks of tyre contaminants, especially in urban coastal areas where environmental concentrations oftern approach or exceed the EC50 and LOEC values found here. To our knowledge, this study contributes novel results by evaluating the toxic effects of MBT, DPG, and 6PPD-Q on P. tricornutum. The results highlight the urgent need for regulatory measures to limit these contaminants. Future research should investigate long-term, multi-stressor exposures and pollutant mixtures to better understand combined effects on marine primary producers and ecosystem resilience.