Physiological responses of Chlorella pyrenoidosa to 1-hexyl-3-methyl chloride ionic liquids with different cations

Ionic liquids (ILs) are massively used in multiple fields of industry, and consequently, they have entered the environment and become potential threats to the respective ecosystems. In this paper, the toxicity of two different cationic types of ILs (1-hexyl-3-methyl pyridine chloride ([C₆Py]Cl) and 1-hexyl-3-methyl imidazole chloride ([C₆MIM]Cl)) to Chlorella pyrenoidosa (C. pyrenoidosa) was investigated. Growth inhibition increased with increasing ILs concentrations. C. pyrenoidosa showed a certain recovery at low ILs concentrations, the growth inhibition decreased from 6.13% to 1.57% of the control from 24 h to 96 h, respectively, in 0.5 mg/L [C₆MIM]Cl treatment. However, growth inhibition was negatively related with exposure time at high concentrations, and the strongest toxic effects were observed after 48 h. The IC₅₀ values (half inhibitory concentration) were 8.47, 6.65, 6.91 and 7.11 mg/L of [C₆MIM]Cl, respectively, in 24, 48, 72, and 96 h, and were 9.05, 6.83, 7.79 and 8.14 mg/L of [C₆Py]Cl, respectively. Chlorophyll content declined with higher concentrations of the ILs. The values of chlorophyll fluorescence parameters: the maximum effective quantum yield of photosystem II (PSII) (F_v/F_m), maximum quantum yield in PSII (F_v/F₀), and photosynthetic efficiency in PSII (Y(II)), decreased, whereas the minimal fluorescence (F₀) increased following the ILs treatment, indicating damage to the photosystem II. [C₆MIM]Cl and [C₆Py]Cl caused deformation of algae cells, plasmolysis, and damage of the cell membrane and cell wall, and affected organelle structure. Reactive oxygen species (ROS) concentrations increased with higher ILs concentrations from, and superoxide dismutase and catalase activity first increased and then decreased, indicating that the antioxidant defense system was activated to counteract ROS. ROS was the main stress in C. pyrenoidosa induced by ILs, and compared with [C₆Py]Cl, [C₆MIM]Cl were more toxic to C. pyrenoidosa.