Characteristics Of Carbohydrate Accumulation In Chlorella Vulgaris Mutated By ARTP For Bioethanol Production

Microalgae biomass is a kind of bioenergy with broad prospects,and has received extensive attention in recent years.In this study,Chlorella vulgaris FACHB-25 was irradiated by atmospheric and room temperature plasma（ARTP）.The dominant strains with fast growth rate and high carbohydrate content were selected and compared with the original strains in terms of photosynthetic performance and cell composition.Transcriptome sequencing technology was used to annotate the gene function and analyze the gene expression on each algae strain at the gene level.The optimal plan for flocculation and harvesting of Chlorella was determined.And an experiment of ethanol production by fermentation was carried out,which provided a reference for the subsequent industrial operation of ethanol production by microalgae.The main results are as follows:Using ARTP to mutate Chlorella vulgaris,according to the lethal rate,the final power was determined as 100 w and 120 w.Under this condition,taking growth rate and polysaccharide yield as screening criteria,three dominant algae strains were screened,numbered S100-7,S120-4 and S120-9 respectively.Among them,the growth rate of S120-9 algae strains was faster,the biomass reached 820 mg/L,and the polysaccharide yield increased by 34%compared with the original strain to 237.98 mg/L.The polysaccharide content of S120-4 algae strain reached 37.55%,which increased by43.48%compared with the original strain.In the comparison of photosynthetic performance,the photosynthetic efficiency of S120-9 algae strains is higher,and the apparent electron transfer rate（ETR）of S100-7 and S120-9 algae strains is higher.The proportion of carbon element in the dry weight of algae cells was significantly different,and the proportion of S120-4 reached 47.57%,which was higher than that of the original strain by 42.72%.（1）Transcriptomics was used to study the gene of original strain,S120-4 strain with high carbohydrate content and S120-9 strain with fast growth rate.The gene expression range and quantity of the three strains were compared,and the difference between S120-4 strain and the original strain was significant.By comparing GO,KEGG databases,27167 genes were annotated in go database,and 8849 unigenes were annotated into 35 KEGG pathways.Compared with the original strain,the activities of malate dehydrogenase and pyruvate dehydrogenase in S120-4 cells were decreased,while the activities of alpha-amylase in S120-9 cells were inhibited.（2）Microalgae self flocculation experiment showed that the size of algal cell and EPS could affect the self flocculation effect.Using ferric salt,aluminum salt,polymer flocculant and alkaline flocculant to collect Chlorella.Considering the flocculation efficiency and cost,Fe Cl₃was finally determined as the dominant flocculant.The flocculation efficiency reached 80.20%at the dosage of 0.1 g/L,and the cost of collecting one kilogram Chlorella was only 0.83 yuan.At the same time,it was found that there was no significant difference in the content of microalgae polysaccharide before and after flocculation,indicating that it is feasible to use Fe Cl₃to harvest microalgae with high carbohydrate content.（3）Chlorella was pretreated with acid and alkali.Compared with 0.3mol/L Na OH,5%H₂SO₄had more obvious cell fragmentation and more complete polysaccharide release.Four strains of algae separately take 10 g of algae powder for simultaneous saccharification and fermentation.The ethanol yield of the S120-4 strain with the highest carbohydrate content reached 1.58g,but the biomass accumulation was less,only 500 mg/L.Combining the growth of each algae strain during the culture period,S120-9 algae strain has the highest biomass and polysaccharide production.It is estimated that the ethanol production can reach 0.1033 g/L,which is 1.32 times that of the original algae strain 0.0784 g/L.