The Structure-activity Relationship Study Of Ionic Liquids To Algae Scenedesmus Obliquus

Ionic liquids(ILs)are considered as environmentally friendly solvents because of they are unlikely to act as air contaminants or inhalation toxins.As the production and practical use of ILs are increasing rapidly,the doubt about the "green solvent" has gradually emerged.Owing to their excellent water solubility and poor biodegradability,the release of ILs into aquatic environments may lead to water pollution and relative potential hazards.Hitherto,the tox:icity of ILs towards aquatic organisms at different trophic levels including marine bacteria,algae,invertebrates,and vertebrates had been reported.In general,the toxicity of ILs is correlated with the length of alkyl chain,cation family and anion moiety.This study discussed the effects of four ionic liquids with different cations and anions on the growth of algae Scenedesmus obliquus.The ILs examined in this study included 1-hexyl-3-methylimidazolium chloride([HMIM]Cl),1-hexyl-3-methylimidazolium nitrate([HMIM]NO3),N-hexyl-3-mety lpyrid inium chloride([HMPy]Cl),and N-hexyl-3-metylpyridinium bromide([HMPy]Br).The main research includes four parts as follows:The results of acute toxicity revealed that the inhibitory effect on the algal growth rate became stronger with increasing IL concentration,which showed a dose-response relationship.It can be observed that the S.obliquus culture at low concentrations was recovered to attain a final density close to the control group.The growth abilities of these algae cell cultures were restored under the low concentrations of ILs.In contrast,the cell growth was inhibited continuously throughout the experiment under the treatment of high concentrations of ILs,which indicated the effects of high concentrations of ILs on the S.obliquus were irreversible.The IC50 values of the four ILs indicated the toxicity increased with increasing incubation time from 24 to 48 or 72 h,and a trend of decreasing toxicity was observed afterwards.Besides,the IC50 obtained revealed that the toxic order was:[HMIM]NO3>[HMIM]C1>[HMPy]Br>[HMPy]Cl.According to Passino's classification,the four ILs could be classified as moderate toxic chemicals.The chlorophyll content and chlorophyll fluorescence intensity after 48,96,144 h were determined in this paper.The results showed that content of chlorophyll a,chlorophyll b and total chlorophyll decreased as the concentration of ILs increased,which showed a dose-response relationship.As the exposure time prolong,the negative effects of the four ILs were slightly alleviated.Additionally,the effects of imidazolium-based ILs on the chlorophyll content were more serious than that of the pyridinium-based ILs.The toxic order was:[HMIM]NO3>[HMIM]Cl>[HMPy]Br>[HMPy]Cl.Meanwhile,the fluorescence parameters(F0,Fv/Fm,Fv/F0)and rapid light response curves(Y(?)and NPQ)futher demonstrated the four ILs destroyed the photosynthetic systems of S.obliquus,and the negative effects were aggravated as the concentrations of ILs increased.But the photosynthetic systems could recover with the prolongation of exposure time from 48 h to 144h.Futhermore,the impact of imidazolium-based ILs treatment was greater than that of pyridinium-based ILs.The analysis of data signified that the toxic order was:[HMIM]NO3>[HMIM]Cl>[HMPy]Br>[HMPy]Cl.To further study the toxicity mechanism of S.obliquus when exposed to ILs,the cell membrane permeability was determined after 24,48,72 and 96 h exposure to different concentrations of the four ILs.In the present study,the cell membrane permeability of S.obliquus was all increased during the exposure period and showed an obvious concentration-dependent relationship.It could also be observed the effects of imidazolium-based ILs on the membrane permeability were more serious than that of the pyridinium-based ILs after 24 h exposure.The toxic order was:[HMIM]NO3>[HIVMIM]Cl>[HMPy]Br>[HMPy]Cl.The effect of the high concentration of ILs on cellular membrane permeability aggravated with the prolongation of the exposure time.In addition,the electron microscopic examination observed the disruption of cell wall,cell membrane,organelles,as well as the structural change of cells.The increased number and size of starch granules together with electron-dense deposits could also be observed in the ultrastructure of cells,which considered as a mechanism to resist BLs toxicity.The futher study showed that the four ILs all promoted the production of ROS in S.obliquus and that there was a positive correlation between the ILs concentration and ROS content.During the experimental period especially at 96 and 144 h,the accumulation of ROS in S.obliquus was more evident when treated with the imidazolium-based ILs than the pyridinium-based ILs.The toxic order was:[HMIM]NO3>[HMIM]Cl>[HMPy]Br>[HMPy]Cl.Otherwise,the laser confocal microscopy images showed the DCF fluorescence intensity was enhanced as the ILs concentration increased,which has a positive relationship with the ROS levels in cells.The slightly enhancement of soluble protein content at low concentration of ILs revealed the activation of defense mechanism in S.obliquus.Besides,the activities of SOD and CAT increased significantly when exposure to low ILs concentration,and then decreased when the ILs concentration increased afterwards,which reflected the oxidative stress response was activated.The inhibition of the SOD and CAT activities at the high concentration may be attributed to the excess of ROS that could not be overcome.The oxidative stress induced by the high concentration of ILs exceeded the protective capabilities of the algae that might lead to irreversible damage.In consequence,the study indicated the excess ROS production in S.obliquus triggered by the exposure to ILs may be one of the major contributors to the toxicity of ILs.The excess ROS in S.obliquus might account for the significant growth suppression,membrane damage and eventually cell death.