Study On The Physiological Response And Antioxidant Defense Mechanism Of Microalgae Under Cadmium Stress

Microalgae has received extensive attention due to its effective removal effect and tolerance to heavy metals.Therefore,it is very important to explore the influence of heavy metal stress on the physiological response and antioxidant defense mechanism of microalgae.This article first studied the effect of cadmium stress on the physiological activity of microalgae,and selected algae species with strong cadmium tolerance and good removal effect through comprehensive analysis.Then under the cadmium stress,the antioxidant defense mechanism of this algae was studied.In addition,the introduction of K3 carrier further improves the removal effect and tolerance of microalgae to cadmium.Finally,the microalgae cells before and after cadmium stress were characterized and analyzed to clarify the mechanism of microalgae’s removal of cadmium and its detoxification mechanism.The main conclusions of this article are as follows:(1)Cadmium stress has an adverse result on the physiological activity of microalgae.The inhibitory effect of cadmium on the biomass of Chlorella pyrenoidosa and Scenedesmus obliquus(S.obliquus)was relatively small,and the biomass of the experimental group was only reduced by 2.40% and 1.56%compared with the control group.Cadmium stress has the least adverse result on the chlorophyll a and nitrate reductase(NR)of S.obliquus,at the end of the experiment,the average content of chlorophyll a of S.obliquus in the experimental group was 13.54% higher than the initial value,and the NR content of S.obliquus in the 15 mg/L experimental group was 27.06% lower than that in the control group during the same period.Furthermore,the average removal rate of cadmium(II)by S.obliquus reached 85.20%,which was 31.68%,13.77% and 21.39% higher than that of Chlorella pyrenoidosa,Microcystis aeruginosa and Cyclotella hebeiana respectively.Comprehensive analysis results show that S.obliquus is more suitable for the treatment of cadmium-containing wastewater.(2)Cadmium stress increases the antioxidant level of S.obliquus.The average activity or content of total antioxidant capacity(T-AOC),superoxide dismutase(SOD),catalase(CAT),peroxidase(POD)and glutathione(GSH)of the cadmium-containing experimental group increased by 17.71%,27.37%,4.05%,16.17% and 34.27% respectively compared with the control group,this is an important manifestation of the role of the antioxidant defense mechanism of microalgae.(3)The removal effect and tolerance of S.obliquus biofilm to cadmium are better than those of suspended S.obliquus.The average removal rate of cadmium by S.obliquus biofilm is 3.61% higher than that of suspended S.obliquus,and the average content of extracellular polymeric substances(EPS)is also 40.45% higher.After 72 hours of stress at a concentration of 20 mg/L cadmium,the contents of suspended S.obliquus chlorophyll a and chlorophyll b were only 42.79% and 58.54% of those in the control group,while the S.obliquus biofilms were 52.77% and 83.09%.Compared with the suspended S.obliquus,the S.obliquus biofilm SOD average activity of the 10 mg/L experimental group increased by 18.30%,and the GSH average content also increased by 35.73%.(4)Characterization analysis clarified the removal mechanism and detoxification mechanism of S.obliquus to cadmium.Scanning electron microscopy(SEM)images show that cadmium poisoning can cause changes in the surface morphology and structure of S.obliquus.Fourier transform infrared spectroscopy(FT-IR)results indicate that C-H,C=O,-OH or N-H amide I and other functional groups on the surface of S.obliquus were involved in the adsorption of cadmium(II)on the surface of S.obliquus.X-ray photoelectron spectroscopy(XPS)showed that C-C,C-H,C-O,C-N,C=O,O-C-O,O-H and other chemical bonds on the surface of S.obliquus were complexed with cadmium(II).X-ray diffraction(XRD)data analysis showed that the crystallinity of S.obliquus treated with cadmium changed significantly.