Effects Of Different Carbon Sources And GABA On The Synthesis Of α-Linolenic Acid And Lutein In Acutodesmus Sp. HLGY

Under the background of"Carbon Peaking and Carbon Neutrality",the resource utilization of CO₂ has become a common topic for mankind.Microalgae have attracted much attention due to their ability to combine biological carbon sequestration with the production of high-value added products.Alpha-linolenic acid（ALA）and lutein can be used in food,medicine,healthcare products and other industries,so they have commercial value.Microalgae,a biological resource rich in fatty acids and pigments,are potential sources ofα-linolenic acid and lutein.In addition,the cultivation process of microalgae has many advantages,such as artificial control,no competition for land with grain,and fast growth rate.However,the industrial production of microalgae faces the bottleneck of high cultivation cost and low product yield.Combined with carbon source optimization,the use of plant growth regulators is an effective means to accelerate the production of metabolites in microalgae.γ-Aminobutyric acid（GABA）is a four-carbon non-protein amino acid,which is involved in the regulation of plant growth and metabolism.However,the role and related regulatory mechanism of GABA in microalgae culture are not clear,so further research is necessary.In this study,Acutodesmus sp.HLGY was used to investigate the effects of different carbon sources（CO₂,glucose,glycerol）on its growth andα-linolenic acid（ALA）accumulation.Meanwhile,the consumption rates of the three carbon sources and the expression of genes related to algal growth and oil synthesis were analyzed.In addition,the effect of CO₂ combined with GABA on lutein synthesis in Acutodesmus sp.HLGY was investigated.Combined with 3-mercaptopropionic acid（3-MP）,an inhibitor of GABA synthase,the mechanism of GABA on the growth and lutein synthesis was revealed.This study provided theoretical references for the production of a variety of high-value metabolites in microalgae.The specific research results are as follows:（1）The addition of 1.25%CO₂ significantly promoted biomass accumulation and increased ALA yield and productivity to 929.77 mg L^-1 and 132.40 mg L^-1 d^-1,respectively,which were 1.27 times and 1.35 times of the control.5 g L^-1 glucose significantly increased the yield and productivity of ALA to 344.40 mg L^-1 and 250.44mg L^-1 d^-1,respectively,which were 1.88 times and 2.81 times of the control group.Glycerol 1.25 g L^-1 also promoted algal growth and ALA accumulation,and the yield and productivity reached 620.32 mg L^-1 and 156.08 mg L^-1 d^-1,which were 1.18 and 1.75times of the control,respectively.（2）CO₂,glucose and glycerol reduced the pH of microalgae medium.They upregulated the transcript levels of growth-and lipogenesis-related genes in Acutodesmus sp.HLGY,thereby promoting microalgae growth and unsaturated fatty acid accumulation,with the maximum photochemical efficiency Fv/Fm of PSII increased significantly.In addition,both ALA and lutein could be efficiently accumulated by CO₂ and glycerol,with lutein content reaching 30.98 mg g^-1 and 30.49mg g^-1 under 1.25%CO₂ and 1.25 g L^-1 glycerol,respectively.（3）A strategy was established for CO₂ combined with GABA to induce efficient lutein accumulation in Acutodesmus sp.HLGY.On the basis of 1.25%CO₂,250μM GABA promoted the biomass accumulation and further enhanced lutein biosynthesis.The content and productivity of lutein were 36.85 mg g^-1 and 20.56 mg L^-1 d^-1,respectively,which were 1.32 and 1.51 times of the control.Exogenous GABA significantly up-regulated the expression levels of genes related to carbon sequestration and lutein accumulation in Acutodesmus sp.HLGY,and activated its antioxidant system.（4）On the basis of 1.25%CO₂,3-MP reduced intracellular lutein content and ascorbic acid content,downregulated the expression of antioxidant enzyme genes,and provoked a significant rise in reactive oxygen species（ROS）levels during the later phase of cultivation.The results showed that GABA could enhance the growth and lutein accumulation of microalgae by up-regulating the transcription of genes involved in carbon fixation and lutein synthesis.