Screening Of Heterotrophic Energy Microalgae And Their Utilization Of Volatile Fatty Acids

The energy crisis caused by the increasing depletion of fossil fuels and the ecological degradation caused by global warming are regarded as the two major problems that need to be solved in the world today.Fossil fuels are the most important cause of global warming due to the large amount of carbon dioxide released during the combustion process.More and more people believe that traditional fossil fuels should be replaced by new renewable energy sources.Microalgae have the advantages of fast growth rate,easy cultivation and high lipid content,which are the raw materials for the third generation biodiesel.Microalgae can not only produce biodiesel but also treat wastewater beside reduce CO₂emission,which is regarded as a potential green bio-resource.The high production cost and low lipid yield are considered as the main limitation bottleneck for biodiesel production from microalgae.Therefore,an effective way to achieve green energy production and waste recycling might be solved by screening energy microalgae with high lipid yield and by seeking available waste carbon sources to cultivate microalgae.Volatile fatty acids（VFAs）,which are intermediate products from the anaerobic fermentation of organic wastes,were selected as carbon sources in this study.Nine strains of microalgae with high environmental adaptability and lipid content were initially selected.Firstly,the growth and nitrogen and phosphorus removal capacity of the nine strains were investigated by VFAs dosing,and a dominant energy microalgae,Chlorella sorokinensis（C.sorokinensis）was selected.Then,the optimal VFAs ratio and the optimal environmental conditions for lipid accumulation under the carbon and nitrogen regulation of microalgae were investigated.Finally,the growth and wastewater treatment of C.sorokinensis in acid-producing wastewater were studied.The main conclusions of this paper are as follows:（1）In this paper,the dominant energy microalgae were screened by the addition of VFAs under heterotrophic cultivation.The highest biomass concentration of 0.77 g L^-1,specific growth rate of 0.25 d^-1and biomass productivity of 91.43 mg L^-1d^-1were obtained by C.sorokinensis,a dominant energy microalgae with high tolerance to VFAs.In addition,C.sorokiniana showed the highest removal efficiency of total nitrogen and total phosphorus with 95.96%and 93.42%,respectively.C.sorokiniana showed good ability of high adaptation and utilization of volatile acids.（2）The optimal VFAs ratio and carbon to nitrogen ratio for heterotrophic culture growth of C.sorokiniana were investigated using different mixed VFAs ratios as substrates.Among them,the highest biomass concentration(0.80 g L^-1),specific growth rate(0.26 d^-1),biomass yield(95.00 mg L^-1d^-1),nitrogen（97.44%）and phosphorus（91.02%）removal efficiency were obtained for C.sorokiniana at 8:1:1 ratio of acetic acid（AA）:propionic acid（PA）:butyric acid（BA）.C.sorokiniana showed the highest utilization of AA,followed by BA and PA.Studies have shown that the intracellular carbon flow of C.sorokiniana flows to various metabolic pathways to generate various energy storage substances under nitrogen stress.Total sugar was rapidly accumulated as the primary energy storage substance,and lipid was slowly accumulated as the second energy storage substance.The highest lipid content of 45.98%was obtained under nitrogen-free conditions.The degradation of total sugar can provide both metabolites for fatty acid synthesis and energy for protein synthesis.In addition,the nitrogen concentration affected the utilization pathway of volatile acids by C.sorokiniana,and most of the AA absorbed in the absence of nitrogen was involved in the lipid metabolism pathway of microalgae,which promoted the biosynthesis of microalgae lipids and facilitated the accumulation of lipids.（3）Acid-producing wastewater was used as an alternative medium to cultivate C.sorokiniana,which was able to accumulate biomass,lipids,total sugars and proteins in all three utilizations of wastewater.The nutrients and small molecule organic acids in the wastewater were efficiently utilized by C.sorokiniana,and the total removal rates of COD,TP,NH₃-N and TP were 74.44%,88.05%,79.08%and 82.69%,respectively.AA,PA and BA were utilized efficiently with utilization rate of 58.99%,70.54%and81.52%,respectively.High lipid content,protein content and total sugar content were obtained in all three utilizations,and the values were all above 15%.The highest lipid content was 39.15%,protein content was 42.43%and total sugar content was 46.40%.Comparing the microbial community structure and abundance of acid-producing wastewater before and after the addition of C.sorokiniana,it was found that the algal-bacterial symbiotic system played a positive role.The dominant genera were altered due to the enriched growth of the beneficial bacteria Protebacteria,Bacteroidota,Hydrogenophaga and Algoriphagus.