Physiological Response And Transcriptional Regulation Of Chlorococcum Sphacosum GD To Organic Matter Stress

The use of microalgae for wastewater treatment is economical,environmentally friendly and sustainable.By absorbing N,P and organic matter,microalgae can effectively achieve the dual benefits of wastewater treatment and biomass accumulation.However,alt-hough organic pollutants in wastewater can support the growth and development of micro-algae,high concentrations of organic matter can have an impact on the biological activity of microalgae and their efficiency in wastewater treatment.In order to reveal potential mechanisms,this study investigated the metabolic activity,physiological response and pol-lutant removal efficiency of Chlorococcum sphacosum GD at four levels（400,1500,4000and 12000 mg/L）of organic matter,and each concentration was set with three repeats.The results obtained from this study were as follows:（1）As the concentration of organic matter increased,the levels of hydrogen peroxide（H₂O₂）and reactive oxygen species（ROS）in cells of Chlorococcum sphacosum GD showed a trend of increasing first and then decreasing,and reached a maximum of 2.92 mmol/gprot and 143797.5（fluorescence intensity）at a concentration of 4000 mg/L,respectively.At the same time,there were corresponding changes in antioxidant enzymes,with peroxidase（POD）and catalase（CAT）levels increasing first and then decreasing,The cotent of both superoxide dismutase（SOD）and malondialdehyde（MDA）increased following the increase of the concentration of organic matter.In addition,the mortality rate of Chlorococcum sphacosum GD cells gradually increased with increasing organic matter concentration.The above results indicate that different concentrations of organic matter produced different de-grees of stress on Chlorococcum sphacosum GD,and the stress effect increased with in-creasing organic matter concentration.（2）The increase of organic matter concentration caused a significant decrease in pho-tosynthetic activity and photosynthetic pigment content of Chlorococcum sphacosum GD and an increase in biomass.A redistribution of cellular components occurred.The total pro-tein content decreased and the polysaccharide and total lipid content accumulated with in-creasing organic matter concentrations.In addition,the increase in organic matter concentration triggered an intracellular calcium signalling response.The level of both cal-cium-dependent protein kinase（CDPK）and calmodulin（CAM）both showed a trend of in-creasing first and then decreasing with increasing stress,and the peak occurred at 4000 mg/L organic matter,which were as high as 1469.81 and 801.78 ng/L,respectively.The level of calcium pump(Ca²⁺-ATPase)showed a trend of increasing.The above results indicated that the physiological activity and response of cells changed with the increase of organic matter stress.（3）Different organic matter concentrations resulted in differences in the degradation characteristics of the pollutants by Chlorococcum sphacosum GD.By the five day of the experiment,the COD removal efficiency was 99.45%,97.60%,92.00%and 42.9%in order of increasing organic matter concentration.In contrast,the organic matter concentration had less influence on the removal efficiency of ammonium and total phosphorus by Chlorococ-cum sphacosum GD.The removal efficiency of ammonium could reach about 90%in dif-ferent organic matter concentration groups,and the removal efficiency of total phosphorus reached more than 97%in all experimental groups except for 67.09%at 400 mg/L organic matter concentration.（4）A preliminary analysis of the molecular response of Chlorococcum sphacosum GD to different concentrations of organic matter was carried out based on transcriptomic tech-niques.Compared to the control（400 mg/L）,the number of DEGs（differentially expressed genes）gradually increased with increasing organic matter concentration,and were 1970,3261 and 4684,respectively.KEGG enrichment analysis showed that the shared DEGs were significantly enriched in ribosome biogenesis,cell cycle,RNA transport and nitrogen me-tabolism.GO functional annotation was mainly related to cellular components,molecular functions and biological processes.In addition,genes related to apoptosis,photosynthesis and antioxidant enzymes were analysed and compared separately,which to some extent re-vealed the consistency between molecular and physiological responses.