Enhancing Carbon Fixation And Astaxanthin Content Of Haematococcus Pluvialis By Nuclear Irradiation And High CO₂ Stress

Microalgae have recently received growing attention given its prospects as a source of renewable energy and its potential for CO2 capture.High-value unicellular microalgae Haematococcus pluvialis was mutated with ? irradiation and gradually domesticated with high concentration of CO2 to promote its tolerance to the high concentration of CO2 in flue gas.The growth rate and the astaxanthin content of the mutated strain were further enhanced by 15%CO2,and the feasibility of the poly-generation of astaxanthin and biodiesel was discussed in this study.The growth rate and astaxanthin content of H.pluvialis mutant were significantly increased by 60Co-? irradiation and 15%CO2 domestication.The biomass productivity of the strain mutated with y irradiation increased from 0.07 g/L/d to 0.09 g/L/d,which was 28.6%higher than that of the wild strain.The astaxanthin and lipids content of the mutant were 46.0 mg/L ? 45.9%,respectively,which were 1.36-fold and 27%higher than those of the wild strain.With the aeration of 15%CO2,the growth rate of the green domesticated mutant increased to 0.13 g/L/d,which was 1.3 times higher than the undomesticated mutant.The biomass dry weight of the red cyst under 15%CO2 was 1.6 times higher than that under air.The astaxanthin content increased from 15 mg/L to 87.4 mg/L under 15%CO2,and the astaxanthin content in biomass increased by 50%compared with that under air.Based on the in-situ Raman analysis,the kinetic model of lipids and astaxanthin evolution in cells under 15%CO2 showed that there was a positive correlation between the accumulation of lipids and astaxanthin.The biosynthesis of lipids happened earlier than that of astaxanthin and the accumulation rate was much higher.To elucidate the molecular mechanism underlying the enhanced growth rate.and astaxanthin content in the H.pluvialis mutant under 15%CO2,transcription sequencing was conducted.Significant up-regulation were found in the expression of unigenes related to carbon fixation,glycolysis pathways,pyruvate metabolism,fatty acid biosynthesis and carotenoid biosynthesis.(1)Carbon metabolism:the electron transfer of photoreaction was enhanced due to the 18.5-fold up-regulation of PsbO in PSII,thus increasing the photosynthetic efficiency.The Calvin cycle was enhanced due to the 19.0-fold up-regulation of FBA in C3 photosynthetic pathway.The 2.7-fold up-regulation of ppdk increased the formation of phosphoenol pyruvate,which is the initial acceptor of CO2 in the C4 photosynthetic pathway.The enhancement of both C3 and C4 photosynthetic pathway leaded to the increasing efficiency of carbon fixation.The 3.5-fold up-regulation of PK enhanced the accumulation of pyruvate,thus promoting the reallocation of organic carbon from carbohydrate to lipids and carotenoid via pyruvate metabolism.(2)Lipids and astaxanthin biosynthesis:,the acetyl-CoA synthetase,acetyl-CoA carboxylase and FabH up-regulated by 2.9-,3.6-and 3.1-fold,respectively,thus significantly enhancing the fatty acid biosynthesis.The astaxanthin biosynthesis was significantly enhanced by the 10.4-,5.8-and 18.4-fold up-regulation of PSY,1cyB and CrtZ,respectively.Biodiesel was transesterified from the lipids derived from wet biomass of H.pluvialis.With astxanthin in lipids,the transesterification efficiency can be increased by 43%,and the crystallization temperature of biodiesel can be reduced to-7?.There was 63.6%of saturated fatty acid and monounsaturated fatty acid in lipids from H.pluvialis.The higher heating value of the H.pluvialis biodiesel was 41MJ/kg,which was similar to diesel.Carbon fixation and astaxanthin production with H.pluvialis under high concentration of CO2 were conducted in a industrialised demonstration project.The cultural condition was optimized via controlling pH or CO2 concentration.The highest CO2 utilization ratio of green cells in plate bioreactor was 77%.The CO2 utilization ratio of red cells in tubular bioreactor was between 65%to 100%.The astaxanthin and lipids content in biomass of H.pluvialis cultured in outdoor tubular bioreactor were 3.6%and 31.8%,respectively.