| Today the world is facing the challenge of diminishing fossil fuel resources. People are seeking alternative sources of energy. Bioenergy has attracted considerable attentions from researchers as one of the most viable long-term alternatives for fossil fuels. Microalgae are by far among the most promising materials to produce biodiesel due to its high content of oil, large variety of species, and eco-friendly nature. The key to biodiesel production is maximizing the biomass and fatty acid content of microalgae. Nitrogen or phosphorus starvation is commonly used to improve fatty acid content of microalgae. However, the starvation causes cell growth inhibition and death resulting in a loss of biomass.This paper studies the effect of nitrogen starvation, phosphorus concentrations, and the addition of sodium acetate on the growth, biomass, and fatty acid composition and content in a model green microalga Chlamydomonas reinhardtii. By this way, we aim to find a way to optimize the culture conditions of the microalga with increasing fatty acid contents, and to investigate the metabolic mechanism of the fatty acids under different culture conditions.First we studied the changes of fatty acid contents in different concentrations of phosphorus(1X phosphorus, 2X phosphorus, and no phosphorus) under nitrogen starvation. Nitrogen and phosphorus starvation was shown to inhibit significantly the growth of C. reinhardtii. The biomass significantly decreased under nitrogen and phosphorus deficiency — about 31.7% lower than the control group. However, we found that nitrogen starvation improved unit fatty acid content, especially under nitrogen and phosphorus starvation, for the unit fatty acid content was about 77.6% higher than the control group.Based on the positive results mentioned above, we proceeded to study the effect of sodium acetate on C. reinhardtii under nitrogen and phosphorus starvation. It turned out that the addition of sodium acetate can boost microalga growth, but reduces the unit fatty acid content at the same time. It was showed that sodium acetate can significantly increase total fatty acid content. When the sodium acetate concentration was 4g/L, the total fatty acid was at its highest with a 70.4% increase than the control group.In total 16 types of fatty acids were detected in C. reinhardtii. 7 of them accounted for more than 90% of the total fat content, which were C16:0, C17:0, C18:0, C18:1n9t, C18:1n9c, C18:2n6c and C18:3n6. Nitrogen and sodium acetate were also shown to affect the quality of the fatty acids(based on cetane, iodine, and saponification values), but the cetane value of fatty acid in different treatment were all in the viable ranges.In this study, a metabolic profile was created based on metabolite changes from Day 3 to Day 6. The analysis of the metabolic profile showed that regardless of the treatments, the difference in metabolites content within the control group was greatest at Day 3 and Day 4, and this difference diminished at Day 5 and Day 6. Metabolic profile also showed that the overall level of metabolites in nitrogen starvation was significantly lower than TAP control group. However, with the addition of sodium acetate, the overall level of metabolites was higher than the control group. We also noticed that under nitrogen starvation, the contents of metabolites such as Glycerol, Haxadecanoic acid, Phosphate, and Glycine increased, while the contents of Alpha.-Linolenic acid, Acetic acid, Propanoic acid, Naphthalene decreased. On the other hand, with the addition of sodium acetate, the levels of metabolites such as Haxadecanoic acid, Alpha.-Linolenic acid, Propanoic acid, Phosphate, Glycine, Myristic acid and 1,2-Benzenedicarboxylic acid increased. As some of these metabolites play an important role in the metabolic pathway, thus their changes in content could be related to stimulation of cell growth and fatty acid accumulation in C. reinhardtii by sodium acetate.As a summary, in this paper we present a method that can stimulate biomass and fatty acid accumulation at the same time. A metabolic profile was created to help identify the key metabolic pathways and key metabolites involved. These findings can potentially shed new lights on the industrialization of microalgae-derived biofuels. |
PDF Full Text Request