| Microalgae have been praised as one of the most potential feedstock for biodiesel production due to its fast growth rate,high lipid content and no competition with farmland.However,the low lipid productivity can't meet the industrialization of microalgae biodiesel due to the imbalance of biomass and lipid content.Therefore,how to improve algal lipid productivity to achieve low-cost commercialization of biodiesel production is receiving considerable attention.In this paper,the effects of batch nitrogen-starvation strategy(i.e.nitrogen-sufficient and nitrogen-starvation two-step strategy)and continuous nitrogen-limitation strategy(low-nitrogen one-step strategy)on the growth and lipid content of Chlorella vulgaris were investigated.Further,the effects of different light intensities on the growth and lipid content of Chlorella vulgaris were investigated to obtain the optimum combination cultivation mode of light and nitrogen and to improve the lipid productivity.The major results were as follows:(1)In N-sufficiency stage of batch nitrogen-starvation strategy,the maximum biomass was achieved by 4.88 g/L.When transferred to N-starvation stage of batch nitrogen-starvation strategy,the biomass was decreased from 4.88 g/L to 3.0 g/L,while the lipid content was increased from 18.88 % to 50.5 %.The maximum biomass and lipid content of continuous nitrogen-limitation strategy was 4.61 g/L and 52.5 %,respectively.The maximal lipid productivity was 286.15 mg/L/d,which presented1.59-fold more than that of batch nitrogen-starvation strategy.Moreover,the dominant FAs of batch nitrogen-starvation and continuous nitrogen-limitation strategy were palmitic acid,palmitoleic acid,hexadecadienoic acid,hexadecatrienoic acid,oleic acid,octadecanoic acid,linolenic acid and linoleic acid.Furthermore,the content of oleic acid in continuous nitrogen-limitation treated cells(37.42 %)was much higher than that of batch nitrogen-starvation strategy(29.02 %).The summation of SFAs and MUFA in continuous nitrogen-limitation treated cells(59.53 %)was just higher than that of batch nitrogen-starvation strategy(50.66%).This indicates that continuous nitrogen-limitation strategy is more favorable for the preparation of microalgae biodiesel.(2)The effects of different light intensities on the growth and lipid content ofChlorella were remarkable.The maximal biomasses and lipid content of medium light intensity(100 ?mol/m~2/s,ML)were 6.39 g/L and 56.5 %,which was 1.49-9.8-fold and 1.18-1.38-fold more than that of low light mode(50 ?mol/m~2/s,LL)and high light mode(300 ?mol/m~2/s,HL),respectively.The maximum lipid productivity in ML mode was 601.938 mg/L/d,which was 2.16 and 8.12 times higher than that of LL and HL models.These results indicate that the ML mode stimulated lipid accumulation and effectively increasd lipid productivity.(3)A total of 45 differentially expressed metabolites were detected in moderate light intensity continuous limitation-nitrogen treated cells.Among them,sucrose,lactic acid,glutamic acid,proline,sn-glycerol-3-phosphocholine and alanine 6metabolites were significantly different(VIP > 1,p < 0.05),which were identified as a biomarker for the accumulation of lipid synthesis in Chlorella under this strategy.In particular,sucrose significantly decreased from 40.29 mg/g to 21.55 mg/g,lactate decreased from 4.150 mg/g to 0.6841 mg/g,alanine decreased from 2.948 mg/g to1.684 mg/g,sn-glycero-3-p increased from 0.07 to 0.42 mg/g.This indicating that the glycolysis process was significantly enhanced so that pyruvate is accumulated.Pyruvate was further converted into acetyl-CoA.Then acetyl-CoA is mainly used for TCA cycle or lipid synthesis.The up-regulations of proline(from 0.14 to 0.53 mg/g)and glutamic acid(7.47 to 9.11 mg/g)indicated that the two amino acids might be important regulatory factors to protect C.vulgaris cells from light and nutrition-limitation stress.The down-regulation of metabolites associated with TCA cycling(malate,alpha-ketoglutarate,succinate,fumarate)promoted energy and metabolic flux into fatty acids,which enhanced the synthesis and accumulation of lipid. |
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