Experimental Study On Improving Lipid Productivity Of Chlorella Vulgaris By Supplementation Phytohormones And Light Quality Shift

The development and utilization of bioenergy has been regarded as an effective way to deal with environmental and energy crises.Therefore,it has attracted much attention in the past half century.However,it has no cost advantage compared with traditional fossil energy.Therefore,the large-scale application of bioenergy has not yet been realized.However,with the introduction of the national eco-economic strategy of“carbon peak and carbon neutrality”,the development of biomass energy has also ushered in unprecedented opportunities.Microalgae,as a representative of the third-generation bioenergy raw material,can use CO₂ produced in the industrial production process as a raw material for photosynthesis to synthesize its own energy substances,and further can be processed into biodiesel.In addition,the absorbed CO₂can be sold in the"carbon trading market".However,in present,the cultivation density and lipid content of microalgae are still relatively low,which greatly restricts the industrialization process of microalgae bioenergy.Therefore,this study used auxin indole acetic acid（IAA）and LED light shift and their combination as the main strategy,supplemented by the optimization of nutrient concentration to increase the lipid yield of autotrophic and mixotrophic Chlorella vulgaris.At the same time,the growth kinetics and bioenergetics parameters of biomass,the content and productivity of the three major components（protein,poly-saccharide and lipid）,and nutrient consumption,chlorophyll and cell element content were investigated.The main conclusions finally obtained are as follows:（1）Increasing the concentration of NaNO₃in the autotrophic culture system within a certain concentration range can help increase the dry weight of Chlorella vulgaris but is not conducive to the accumulation of lipid.When the concentration of NaNO₃ was 0.75 g/L,Chlorella obtained the highest lipid yield（31.36 mg/（L·d））.In the mixotrophic system,when the concentration of glucose and NaNO₃were 10 g/L and 2.25 g/L,respectively,Chlorella vulgaris obtained the largest dry cell weight（2.783 g/L）and lipid yield（85.08 mg/（L·d））.Therefore,in the subsequent experiments,0.75 g/L NaNO₃was used as the autotrophic nitrogen source,and 10 g/L glucose and 2.25 g/L NaNO₃ were used as mixotrophic carbon source and nitrogen source,and they were called the optimized medium.（2）The auxin IAA and LED light shift and their combination all affect the lipid productivity.The effect of IAA on the growth of Chlorella vulgaris is obviously dose-dependent,and its optimal concentration is 10 mg/L under different culture system.The dry cell weight and lipid yield obtained by autotrophic Chlorella vulgaris at 10 mg/L were 37.46%and 48.72%higher than those of the control group（optimized medium）.During mixotrophic culture,the dry cell weight and lipid productivity increased by 10.07%and 23.38%,respectively,compared with the control group.LED light shift strategy can also effectively increase the dry cell weight and lipid productivity of Chlorella vulgaris.Among them,the Chlorella lipid productivity is highest when blue light was used as the culture light source,which was increased by 33.58%（autotrophic）and 34.42%（mixotrophic）,respectively,compared with the control group.In the combined IAA and LED light shift experiment,the maximum lipid yield of Chlorella vulgaris under red light culture was 55.72 mg/（L·d）（autotrophic）and 129.89 mg/（L·d）（mixotrophic）.It is increased by 77.68%and 46.81%,respectively,compared with the control group.（3）Through real-time monitoring of the autotrophic culture process,it is found that the appropriate IAA concentration promotes the absorption of NaNO₃ by Chlorella vulgaris,improves the carbon fixation efficiency of Chlorella vulgaris,and plays a positive role in the accumulation of chlorophyll.Red light increases the rate of NaNO₃ consumption and sulfur absorption of Chlorella vulgaris,but blue light is not conducive to the absorption of NaNO₃.At the same time,both red light and blue light culture increased the CO₂ fixation rate of Chlorella vulgaris and the ability of chlorophyll synthesis in the later stage of cell growth.（4）Through real-time monitoring of the mixotrophic culture process,it was found that the addition of IAA alleviated the rapid decline in chlorophyll of mixotrophic Chlorella vulgaris when the nutritional mode changes,and increased the chlorophyll content in the later period of the culture.At the same time,it also improves the absorption of carbon and nitrogen by Chlorella vulgaris.In the LED light shift experiment,red light and blue light also showed positive effects on chlorophyll synthesis and N absorption.In the combined IAA and LED light shift experiment,it was found that red light is more conducive to the utilization of glucose by microalgae,while blue light relieves the"light-shading effect"of high-density algae cells.Therefore,both red light and blue light increased the lipid productivity of Chlorella vulgaris.In conclusion,the combined strategy of adding IAA and LED dimming during inoculation can maximize the goal of increasing the lipid yield of Chlorella vulgaris under both autotrophic and mixotrophic conditions,which saved the costs of microalgae cultivation and drove the commercialization of microalgae biodiesel.