Growth and physiological responses of a marine diatom (Phaeodactylum tricornutum) against two imidazolium-based ionic liquids ([C4mim]BF4 and [C8mim]BF4)

Ionic liquids (ILs) have been considered as “green” substitutes for traditional organic solvents in many existing biological and chemical areas. However, they have high solubility and poor biodegradability in water, suggesting that they could become persistent chemical pollutants in aquatic environment. The ability of two widely used imidazolium-based ILs to affect the growth and physiological characteristics of a marine diatom (Phaeodactylum tricornutum) was investigated in this study. The diatom was exposed to different concentrations of 1-butyl-3-methylimidazolium tetrafluoroborate ([C4mim]BF4) and 1-octyl-3-methylimidazolium tetrafluoroborate ([C8mim]BF4) for 96 h within a batch-culture system. Results showed that [C4mim]BF4 and [C8mim]BF4 were very stable in seawater during 96 h of exposure, and the compounds significantly inhibited the growth of P. tricornutum with 24, 48, 72 and 96 h EC50 values of 30.81, 28.53, 39.92, 45.88 mg L−1 and 30.17, 23.36, 28.62, 31.37 mg L−1, respectively. In addition, the photosynthetic activity and chlorophyll a synthesis of P. tricornutum were inhibited by [C4mim]BF4 and [C8mim]BF4, indicating that the structural integrity of chloroplasts of the diatom may be disrupted or damaged by the two ILs. Compared with that of the controls, reactive oxygen species (ROS) level was increased by 0.65, 1.17, 1.85, 3.13, 2.94 times and 0.55, 1.77, 2.42, 3.45, 3.47 times in 5, 10, 20, 40 and 60 mg L−1 [C4mim]BF4 and [C8mim]BF4 treatments, respectively. The excessive ROS may cause lipid peroxidation, shortage of metabolic energy and decline of photosynthetic efficiency, which may be the main reason for toxicity of the two ILs to marine diatoms. To withstand the damaging effects of excessive ROS, remarkable physiological and biochemical responses occurred in treatments with the two ILs to protect the cells of P. tricornutum. Parameters such as soluble protein content, soluble sugar content, and superoxide dismutase (SOD) and peroxidase (POD) activities of the diatom increased significantly with increasing concentrations of the two ILs at 96 h of exposure relative to the controls. These findings not only provide strong background for evaluating the ecological risks and toxicity of ILs in marine environment, but also help to unravel the toxic mechanism of the two ILs to marine diatoms.