Study Of Lipid Accumulation In Tetradesmus Obliquus Induced By Carbon Source

As a renewable and clean energy source,microalgae biodiesel is considered as an ideal alternative to traditional fossil fuels.As a key nutrient for microalgae,carbon accounts for about 50%of the total cell content and greatly affects the growth metabolism of microalgae.Most studies have focused on the effect of a single carbon source on microalgae growth,while a comprehensive assessment of microalgae growth and harvesting efficiency is lacking.Cost-effectiveness is a key factor in measuring whether microalgae can be commercialized on a large scale.In this study,the lipid-rich Tetradesmus obliquus were used as the object of study.Firstly,we investigated the consumption and fixation efficiency of sufficient single carbon sources in a light/dark cycle,and analyzed the effect on the growth and flocculation efficiency of microalgae.Secondly,the effect of mixed carbon sources on the accumulation of biomass recovery efficiency was discussed,and the auto-flocculation mechanism of microalgae cells was also explored.Finally,in order to achieve further optimization of cultivation cost,the microalgae cultivation was coupled with wastewater purification,and the effect of mixed carbon source addition method on the biomass recovery efficiency of microalgae and wastewater purification efficiency was analyzed.The optimal addition strategy of carbon source was applied to the closed column reactor for the expanded cultivation of microalgae to verify the feasibility of the strategy.The main findings of the study are as follows.（1）Various single carbon sources have different effects on the growth and flocculation efficiency of microalgae.The fixation of organic carbon sources occurred mainly in the dark environment,with glucose having the highest carbon fixation efficiency and biomass of 1.22 g C·L^-1 and 1.99 g·L^-1,respectively.However,the lower flocculation efficiency（13.97%）resulted in poor biomass recovery efficiency(0.28g·L^-1).The highest biomass recovery efficiency of 0.36 g·L^-1 was achieved when glycerol was used as the carbon source,mainly due to the high microalgal self-flocculation efficiency（89.63%）.（2）There was a synergistic effect between the utilization of mixed carbon sources glycerol and glucose by microalgae,and the consumption of both could simultaneously provide intermediates of metabolic pathways for each other,thus providing more precursors of acetyl coenzyme A for lipid accumulation.The exponential period was dominated by the consumption of glucose（65.5%）,and eventually reached the optimal lipid content,lipid productivity and biomass recovery efficiency of 34.11%,25.28mg·g^-1·d^-1 and 0.91 g·L^-1,respectively.（3）The auto-flocculation effect of microalgae was mainly attributed to the charge neutralization and bridging between cells.The carbon source stimulated the secretion of tryptophan-like proteins and hydrophobic fraction C—（C/H）in the bound EPS（LB-EPS and TB-EPS）,which are the key substances to enhance the flocculation efficiency.The results showed that the experimental group with the highest flocculation efficiency（86.30%）was sodium acetate mixed sodium bicarbonate,whose maximum fluorescence intensity and hydrophobic component C—（C/H）content were the highest,at 8.90*10³R.U.·g^-1 and 55.72%,respectively.The C=O and S=O functional groups are important binding sites for the self-flocculation effect,and the addition of organic carbon sources obviously stimulated the accumulation of these substances.The proportion of C=O reached 8.20%-12.70%,and the amplitude of S=O functional groups was also significantly enhanced.The peak was most significant in the glycerol+glucose group,and the scanning electron micrographs showed that the microalgae cells were adhered to each other by the large amount of secretion covering the surface,thus forming agglomerates.（4）The optimal nutrient introduction strategy was to add carbon at 1-day intervals.The best microalgal growth and wastewater purification efficiency was obtained by Tetradesmus obliquus after 9 days.The highest biomass,lipid content,lipid productivity and biomass recovery efficiency were achieved at 1.53 g·L^-1,38.04%,29.76 mg·g^-1·d^-1 and 1.21 g·L^-1,respectively.The total nitrogen and total phosphorus in the wastewater achieved high removal efficiencies of 91.65%and 97.42%,with final concentrations of only 8.74 mg·L^-1 and 0.19 mg·L^-1,respectively.（5）Applying the carbon source optimization strategy in the closed column reactor,the final biomass(1.49 g·L^-1),lipid productivity(28.45 mg·g^-1·d^-1),and biomass recovery efficiency(1.13 g·L^-1)of Tetradesmus obliquus were similar to those cultured in flasks.Meanwhile,the wastewater purification efficiency was also high,and the final total nitrogen(4.28 mg·L^-1)and total phosphorus(0.13 mg·L^-1)concentrations met the Class IA standard of the Emission Standards for Pollutants from Urban Wastewater Treatment Plants.In addition,the C16-18 fatty acid content（97.41%）of the microalgae cells obtained from the optimized culture was abundant,and all the biodiesel indexes met the quality standards of ASTM D6751 in the United States and EN 14214 in Europe.