Metabolic response of Botryococcus braunii to high bicarbonate dosages and other conditions: analysis of photosynthetic performance, productivity, and lipidomic profile

In this study the metabolic responses of Botryococcus braunii were analyzed upon different inorganic carbon dosages and nutrient limitation conditions in terms of lipid and biomass productivity, as well as photosynthetic performance. The nutritional schemes evaluated included different levels of sodium bicarbonate and nitrogen and phosphorus starvation, which were contrasted against standard cultures fed with CO2. Bicarbonate was found to be an advantageous carbon source since high dosages caused a significant increase in biomass and lipid productivity, in addition to an enhanced photosynthetic quantum yield and neutral lipids abundance. This contrasts to the commonly used approach of microalgae nutrient limitation, which leads to high lipid accumulation at the expense of impaired cellular growth, causing a decline in overall lipid productivity. The lipidome analysis served to hypothesize about the influence of the nutritional context on B. braunii structural and storage lipid metabolism, besides the adaptive responses exhibited by cells that underwent nutrient stress.

Influence of Nutrient Manipulation on Growth and Biochemical Constituent in Anabaena variabilis and Nostoc muscorum to Enhance Biodiesel Production

The present study aims to improve biomass and biochemical constituents, especially lipid production of Anabaena variabilis and Nostoc muscorum by formulating an optimal growth condition using various concentrations of nutrients (NO3−, PO43− and CO32−) for biodiesel production. The supplementation of the three nutrients by +50% showed the maximum dry weight and biomass productivity, while the macromolecule contents were varied. The depletion of N-NO3− by 50% N-NO3− showed the maximum lipid yield (146.67 mg L−1) in A. variabilis and the maximum carbohydrate contents (285.33 mg L−1) in N. muscorum with an increase of 35% and 30% over control of the synthetic medium, respectively. However, variation in P-PO43− and C-CO32− showed insignificant improving results for all biochemical compositions in both cyanobacteria. A. variabilis was the superior species for lipid and protein accumulation; however, N. muscorum showed the maximum carbohydrate content. Accordingly, A. variabilis was selected for biodiesel production. In A. variabilis, −50% N-NO3− resulted in 35% higher lipid productivity compared to the control. Furthermore, the fatty acid profile and biodiesel quality-related parameters have improved under this condition. This study has revealed the strategies to improve A. variabilis lipid productivity for biodiesel production for small-scale in vitro application in terms of fuel quality under low nitrate levels.

Citrate-Mediated Acyl-CoA Synthesis Is Required for the Promotion of Growth and Triacylglycerol Production in Oleaginous Yeast Lipomyces starkeyi

The oleaginous yeast Lipomyces starkeyi is an excellent producer of triacylglycerol (TAG) as a feedstock for biodiesel production. To understand the regulation of TAG synthesis, we attempted to isolate mutants with decreased lipid productivity and analyze the expression of TAG synthesis-related genes in this study. A mutant with greatly decreased lipid productivity, sr22, was obtained by an effective screening method using Percoll density gradient centrifugation. The expression of citrate-mediated acyl-CoA synthesis-related genes (ACL1, ACL2, ACC1, FAS1, and FAS2) was decreased in the sr22 mutant compared with that of the wild-type strain. Together with a notion that L. starkeyi mutants with increased lipid productivities had increased gene expression, there was a correlation between the expression of these genes and TAG synthesis. To clarify the importance of citrate-mediated acyl-CoA synthesis pathway on TAG synthesis, we also constructed a strain with no ATP-citrate lyase responsible for the first reaction of citrate-mediated acyl-CoA synthesis and investigated the importance of ATP-citrate lyase on TAG synthesis. The ATP-citrate lyase was required for the promotion of cell growth and TAG synthesis in a glucose medium. This study may provide opportunities for the development of an efficient TAG synthesis for biodiesel production.