Combined toxicity of ferroferric oxide nanoparticles and arsenic to the ciliated protozoa Tetrahymena 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 3mg/L Fe₃O₄ NPs. After 24h 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.