Comparative growth inhibition of 6PPD and 6PPD-Q on microalgae Selenastrum capricornutum, with insights into 6PPD-induced phototoxicity and oxidative stress.

Widespread environmental detection of tire additive N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine (6PPD) and its toxic metabolite 6PPD-Q has raised great concerns for their potential impact on aquatic biota. This study investigated the effects of 6PPD and 6PPD-Q on the model green microalgae (Selenastrum capricornutum). Results showed that 6PPD at the concentrations of 1-5 mg·L^-1 stimulated S. capricornutum growth, while higher concentrations (10-50 mg·L^-1) inhibited growth with an IC_{50(96 h)} of 8.78 mg·L^-1. However, at concentrations up to 10 mg·L^-1, no toxicity was observed for S. capricornutum exposed to 6PPD-Q. Under the stress of 6PPD, S. capricornutum exhibited increased cellular membrane permeability and cell wall rupture, indicating structural damage to the algae cell. Microalgal oxidative stress was induced through the accumulation of reactive oxygen species (ROS), reaching levels of 1.65-5.29 times higher than the non-exposure cells, which altered enzymatic activities including superoxide dismutase (SOD) and catalase. Exposure to 6PPD at concentrations of 10-50 mg·L^-1 resulted in photosynthetic toxicity as evidenced by decreased Chlorophyll a (Chl a) content and adverse effects on chlorophyll fluorescence parameters, such as maximum photochemical quantum yield (Fv/Fm), PSII (photosystem II) effective quantum yield [Y(II)], and photosynthetic electron transfer rate (ETR). While the concentrations employed may be higher than those typically found in the environment, this study uncovers a significant finding that 6PPD may demonstrate even greater toxicity to microalgae than its derivative, 6PPD-Q. This underscores the need for further investigation into the ecological risks of 6PPD, particularly in the context of primary producers like microalgae.