Optimization Of Photosynthetic Carbon Sequestration Processes And Improvement Of Egg Quality By Feeding Algal Powder Of Two Microalgae

Human activities lead to massive emissions of CO₂and other greenhouse gases,resulting in an increasingly serious greenhouse effect and a serious threat to human food security.How to combine the CO₂emission reduction with the food and feed production to realize the green recycling of CO₂resources has increasingly become an international research hotspot.Microalgae can efficiently fix CO₂through photosynthesis and produce highly nutritious biomass,which can be used as feed raw materials to enhance the production performance of farmed animals and improve the quality of animal products.In this paper,Arthrospira platensis and Auxenochlorella pyrenoidosa were taken as the research objects to systematically explore the application of these two species of microalgae in efficient photosynthetic carbon fixation and algal meal as feed additives for laying hens to improve their egg quality.The main findings are as follows:1.Six strains of Arthrospira platensis were photoautotrophically cultivated in an air shaker,and Arthrospira platensis CDB0201 was determined to be the best algal strain after primary screening and re-screening.The highest biomass concentration that can be obtained is 1.55 g/L,The CO₂fixation rate was 338.66 mg/L/d,the protein productivity was 126.85mg/L/d,and the phycocyanin productivity was 26.50 mg/L/d.The algal strain CDB0201 was photoautotrophically cultivated in a CO₂shaker with light,and the addition amount of Na HCO₃（5.6-16.8 g/L）in the medium was optimized under constant 2%CO₂conditions.It was confirmed that with 14 g/L Na HCO₃and 2%CO₂combined carbon source for 7 days,the biomass concentration and CO₂fixation rate reached the maximum values（2.04 g/L and 469.68 mg/L/d）,which were higher than the same light and temperature in the air shaker,they were significantly improved by 33.93%and 41.65%respectively（p<0.05）.This indicated that the above-mentioned combined carbon sources had the dual functions of in-situ carbon supplementation and p H stabilization and significantly improved the biomass production and photosynthetic carbon fixation performance of Arthrospira platensis（p<0.05）.The protein content of the produced Arthrospira platensis biomass is as high as 63.87%of its dry weight,and the phycocyanin content is 136.15 mg/g,which is a high-quality Arthrospira platensis.The optimized process of algal strain CDB0201 was established in a 5-L external light fermenter:with the increase of biomass concentration,the proportion of CO₂was adjusted from 2%to 2.4%to retain the p H value in 8.65;The light intensity remained stable after adjusting from 55 to 350μmol/m²/s;the stirring speed was adjusted from 100 to 150 r/min,and the ventilation rate was maintained at 2000 m L/min.Under this condition for 9 days,the highest biomass concentration was 2.44 g/L,which was 19.33%higher than the highest value in the above shake flask system（p<0.05）while the CO₂fixation rate（432.25 mg/L/d）was slightly lower than the optimal result of the above shake flask,indicating that it has the stable amplification effect in the 5-L light fermenter system.The protein content of the produced Arthrospira platensis biomass is as high as 60.77%of its dry weight,and the content of phycocyanin,total chlorophyll and total carotenoid reaches 125.11 mg/g,6.65 mg/g and 1.56mg/g,respectively.It is high quality Arthrospira platensis.After feeding 2%commercial Arthrospira platensis powder in the basal diet of Sanhuang chickens for 15 days,The egg yellowness（15）,carotenoid content（129.85 mg/kg）andβ-carotene content（328.89μg/kg）were increased by 2 times,2.10 times and 4.62 times respectively compared with the control group;the cholesterol content（1213 mg/100g）and fatty acidω-6/ω-3 ratio（4.10）in the egg yolk were significantly decreased by 65.37%and90.96%compared with the control group（p<0.01）.The amino acid content in egg white was11.89%of the wet weight of egg white,which was 17.14%（p<0.01）higher than that of the control group;the total amount of eight essential amino acids was 4.60%of the wet weight of egg white,which was significantly higher than that of the control group by 18.25%（p<0.01）.The results indicated that commercial available Arthrospira powder as a feed additive for laying hens could significantly improve the egg quality（p<0.01）.2.Shake-flask photoautotrophic culture of Auxenochlorella pyrenoidosa was carried out in a CO₂-illuminated shaker,and the light intensity was adjusted from 100 to 240μmol/m²/s,and an optimized carbon source combination（0.5 g/L Na HCO₃and 2%CO₂was used initially,and the CO₂ratio was adjusted to 2.5%on the 7th day of cultivation）,after the 1st day of culture,3.75 g/L sodium nitrate was added,30℃,160 r/min for 10 days,and the highest biomass concentration（10.17 g/L）and CO₂fixation rate（1746.67 mg/L/d）could be achieved,which were 4.71 times and 9.11 times higher than those in the air shaker under the constant light and temperature conditions for 8 days,respectively（p<0.01）;compared with the constant carbon source combination（0.5 g/L Na HCO₃and 2%CO₂）,the light intensity was adjusted from 100 to 160μmol/m²/s for 10 days,which were significantly increased by44.67%and 60.43%,respectively（p<0.01）;Compared with the group supplemented with sodium nitrate on the 4th day of cultivation,it was significantly increased by 6.49%and5.53%,respectively（p<0.05）.These result indicate that the growth rate and CO₂fixation rate of Auxenochlorella pyrenoidosa can be significantly increased by the optimal culture conditions in the shaking flask system（p<0.01）.The protein content of the harvested Chlorella biomass reached 54.82%dry weight,and the total chlorophyll and total carotenoid contents reached 50.12 mg/g and 7.39 mg/g,respectively,which was a nutrient-rich Chlorella biomass.The scaled-up culture process of Auxenochlorella pyrenoidosa was optimized and established in a 5-L light fermenter:constant 2%CO₂was introduced,the light intensity was adjusted from 100 to 650μmol/m²/s,and the aeration volume was adjusted from 2.0 to 4.5L/min.After 12 days of culture under this process,the biomass concentration reached 6.00g/L,the CO₂fixation rate reached 805.40 mg/L/d,and the protein productivity reached252.98 mg/L/d.The first two were 2.37 times and 3.66 times significantly higher than the results of culturing for 8 days under the constant light and temperature conditions in an air shaker（p<0.01）.This indicated that the above optimized results of shake flasks can greatly improve the biomass production and carbon sequestration performance of Auxenochlorella pyrenoidosa in a 5-L light fermenter.The harvested Chlorella biomass is rich in protein and pigment,the protein content is 55.39%of dry weight,and the total chlorophyll and total carotenoid content are as high as 57.94 mg/g and 10.13 mg/g,respectively,which is a high-quality Chlorella biomass.Egg quality was analyzed after feeding Sanhuang chickens with Chlorella powder for 15days.The results showed that the optimum amount of Chlorella powder added was 2.5%,the egg yolk color（9）,total carotenoid content（73.35 mg/kg）,polyunsaturated fatty acids（10.35%of total fatty acids）were higher than those in the control group,respectively.Improved by 50%,74.85%and 24.10%（p<0.01）;the cholesterol content in egg yolk was3388 mg/100g,which was 3.28%lower than the control group（p<0.05）.At the same time,the amino acid content（10.86%of egg white wet weight）and the eight essential amino acids（4.17%of egg white wet weight）in egg white were 7.00%and 7.20%higher than those in the control group,respectively（p<0.05）.The results indicated that commercial available Chlorella powder as a feed additive for laying hens could significantly improve egg quality（p<0.05）.