| Microalgal cells can effectively purify wastewater while accumulating microalgal biomass,which has great potential for application in wastewater treatment.Chlorella sorokiniana is a photosynthetic organism,and the biochemical and metabolic processes of microalgae can be regulated by changing the light quality.In this paper,the growth and physiological characteristics of Chlorella sorokiniana under red,blue,green and white light were analyzed,and elucidated the metabolic regulation mechanism of Chlorella sorokiniana in response to different light quality with combined transcriptome analysis.The differences of pollutant removal and biomass accumulation of Chlorella sorokiniana under red,blue,white and alternating red and blue light modes were compared.The effectiveness of dynamic operation of the photobioreactor in treating wastewater and the stability of the system were verified.The main research results are as follows:（1）Red and blue light were the effective light qualities for cultivating Chlorella sorokiniana.The specific growth rates of microalgal cells under red and blue light were significantly higher than those in the green and white light groups,and the algal cell densities of the red and blue light groups were 52.96%and 61.11%higher than those in the white light control group,respectively.The carbohydrate content and lipid content of Chlorella sorokiniana were the highest after 7 days of culture under red light,reaching 72.84μg/mg and 18.64%,respectively.The protein concentration of microalgae in the blue light group was higher than that in the red light group,reaching 20.73 mg/L on the 5th day.Long-term green light stress accelerated the decay phase of Chlorella sorokiniana,which required a large amount of carbohydrates to maintain the normal physiological activities of the cells.（2）Differential expression of genes existed in Chlorella sorokiniana cultured under different light qualities.Compared with the white light control group,7713,9157 and 10837 unigenes were differentially expressed in the three experimental groups（red light,blue light and green light group）,respectively.Most of the differentially expressed unigene in the red light group were involved in fatty acid synthesis and carbon fixation process,and the gene expression of carbonic anhydrase,acetyl coenzyme A carboxylase and fatty acid synthase were higher.The differential genes enriched in the photosynthetic system under blue light were all up-regulated and the expression of Rubis CO enzyme（ribulose-1,5-bisphosphate carboxylase）was the highest,and the photosynthetic and carbon fixation rates were fast.The expression of genes involved in ROS synthesis in microalgae under green light was extremely high,and microalgae cells may respond to the stressful environment of green light through endocytosis and antioxidant mechanism of peroxisome.（3）Alternating red and blue light illumination（red light for 6 h+blue light for 6 h,alternating cycles）was a suitable light mode for Chlorella sorokiniana for deep nitrogen and phosphorus removal from wastewater and accumulation of biomass.In this mode,the removal rates of TN,TP and COD were 97.73%,93.02%and 88.78%,respectively.The OD680 and dry weight of microalgae after wastewater treatment reached 1.802 and0.525 g/L,respectively.（4）Under the alternating red and blue light illumination mode,the microalgal biofilm in the photobioreactor was able to adhere to the carrier surface stably,and the dynamic operation of the reactor treated the wastewater well.The effluent concentrations of NH3-N,TN,TP and COD in the sedimentation tank were 3.87 mg/L,8.83 mg/L,0.23 mg/L and 30.44mg/L,and the removal rates were 90.68%,80.19%,93.59%and 83.65%,respectively.The p H of the effluent of the system was maintained below8.0,and the effluent quality of the sedimentation tank was stable and reached the Grade A standard.（5）The removal of pollutants lies in the assimilation of microalgae biofilms in the photobioreactors.The plate photobioreactors contributed53.91%,54.88%,63.38%and 61.67%to the removal of NH3-N,TN,TP and COD,respectively,and the tubular photobioreactors contributed to the removal of NH3-N,TN,TP and COD The rates were 36.41%,35.48%,33.11%and 34.33%,respectively. |
