Fate and Toxicity of Carbon Black to Phytoplankton in Natural Lakes: Insight into the Role of Phototransformation

Concern over the contamination of freshwater ecosystems with carbon black (CB) is increasing. Here, the toxicity of CB to phytoplankton (<i>Chlorella pyrenoidosa</i>) was evaluated; upon exposure, the median effective concentration for 72 h was 23.4 mg/L. CB underwent significant photooxidation during 15 days of light irradiation, although phototransformation was generally completed by day 7. Algal growth inhibition induced by phototransformed CB (TCB) at 1 mg/L was 64.1% greater than that induced by parent CB. Mechanistically, 1) phototransformation triggered the release of highly toxic byproducts, which inhibited algal growth by 18.9%; 2) metabolomic results demonstrate that the suppression of carbon and nitrogen assimilation in algal cells induced by TCB was 13.2-53.7% greater than that induced by CB; 3) TCB exhibited reactive oxygen species production ability, which triggered more significant algal membrane damage. A full-factorial experiment (2⁶⁺¹ runs) showed that the combined effect of temperature and suspended mineral particles, as well as electrical conductivity, was the primary environmental factor that mediated CB and TCB toxicity, respectively. The predicted toxicity of CB and TCB in Taihu Lake exhibited significant regional distribution, and TCB posed a greater environmental risk in aquatic ecosystems than CB. These findings highlight the importance of particulate contaminant transformation and environmental factors when evaluating their environmental risk.