Combined toxicity of ferroferric oxide nanoparticles and arsenic to the ciliated protozoa Tetrahymena Pyriformis

•Fe3O4 NPs alone were not only non-toxic, but also promoted the growth of T. pyriformis in experimental doses.•The inhibition of As(V) toxicity by Fe3O4 NPs was weaken over time.•The intracellular ROS levels of T. pyriformis were significantly increased in the coexistence of Fe3O4 NPs and As(V).•Coexistence of Fe3O4 NPs and As(V) induced the oxidative damage of T. pyriformis.

Fe3O4 nanoparticles (NPs) have a high affinity for arsenic. As a result of this association, Fe3O4 NPs loaded with high concentration of arsenic can enter into organisms and produce locally high concentrations of arsenic, which may lead to some unexpected toxicity to aquatic organisms. The objectives of this research were to investigate the toxic effect of Fe3O4 NPs in combination with As(V). Cultured Tetrahymena pyriformis was chosen as a research model organism to evaluate the toxic effects of the combined agents. The results showed that after 24 h of As(V) exposure, the median effective concentration of As(V) to T. pyriformis was 1.29 mg/L. Fe3O4 NPs alone were not only non-toxic, but actually promoted the growth of T. pyriformis at the experimental doses. After 24 h exposure, the cell number increased by 32.2% at an exposure level of 3 mg/L Fe3O4 NPs. After 24 h exposure to 1.0 mg/L As(V), the survival rate increased from 60.5% in the absence of Fe3O4 NPs to 73.8% and 83.8% in the presence of 13 mg/L and 19 mg/L Fe3O4 NPs, respectively. However, after 30 h, the combined toxic effect of As(V) and Fe3O4 NPs on T. pyriformis was significantly enhanced and the survival rates for co-exposure to 1.5 mg/L As(V) and 13 mg/L Fe3O4 NPs decreased from 92.3% after 18 h to 45.3% after 30 h. After 18 h of exposure to Fe3O4 NPs alone, the intracellular ROS levels were markedly increased and achieved steady state. Compared with the control group, the intracellular ROS levels were significantly increased (2.56-fold) by the combination of 19 mg/L Fe3O4 NPs and 1.0 mg/L As(V). Accumulation of As(V) in T. pyriformis led to an increase in trivalent arsenics as a result of the saturation of the cellular arsenic methylation capability or/and redox reactions. These exposures also resulted in an imbalance between oxidants and antioxidants, resulting in oxidative damage and cell death.