Abstract
In humans, dietary polyunsaturated fatty acids (PUFAs) are involved in therapeutic processes such as prevention and treatment of cardiovascular diseases, neuropsychiatric disorders, and dementia. We examined the physiology, PUFA accumulation and glycerol lipid biosynthesis in the marine microalga Nannochloropsis salina in response to constant suboptimal temperature (<20 °C). As expected, N. salina exhibited significantly reduced growth rate and photosynthetic activity compared to optimal cultivation temperature. Total fatty acid contents were not significantly elevated at reduced temperatures. Cultures grown at 5 °C had the highest quantity of eicosapentanoic acid (EPA) (C20:5n3) and the lowest growth rate. Additionally, we monitored broadband lipid composition to model the occurrence of metabolic alteration and remodeling for various lipid pools. We focused on triacylglycerol (TAG) with elevated PUFA content. TAGs with EPA at all three acyl positions were higher at a cultivation temperature of 15 °C. Furthermore, monogalactosyldiacylglycerol and digalactosyldiacylglycerol, which are polar lipids associated with chloroplast membranes, decreased with reduced cultivation temperatures. Moreover, gene expression analysis of key genes involved in Kennedy pathway for de novo TAG biosynthesis revealed bimodal variations in transcript level amongst the temperature treatments. Collectively, these results show that Nannochloropsis salina is a promising source of PUFA containing lipids.
Highlights
In humans, deficiency of long chain polyunsaturated fatty acids (PUFAs) increases the risk of cardiovascular disease, hypertension, inflammatory disease, and neuropsychiatric disorders, such as depression and dementia [1,2]
Cold stress or suboptimal temperature is a major trigger for plastic metabolism of lipids in microalgae, as plants and green algae acclimate to cold environment by changing their metabolic pathways [30]
Cold stress instigates at 15 ◦ C after 48 h of experimental conditions, where photosynthetic measurements (Fv/Fm values) were significantly decreased (p < 0.05) compared to optimal temperature treatment
Summary
Deficiency of long chain polyunsaturated fatty acids (PUFAs) increases the risk of cardiovascular disease, hypertension, inflammatory disease, and neuropsychiatric disorders, such as depression and dementia [1,2]. Lipid accumulation in microalgae occurs mainly in three forms; triacylglycerides (TAGs), saturated fatty acids, and PUFAs [15]. TAGs are accumulated in lipid bodies, which are single-layer, membrane-wrapped, protein embedded organelles that are 0.2–2.5 μm in diameter that consist of ~90% TAGs and 10% free fatty acids. These lipid bodies are present in the cytoplasm of most plant and algal cells [26,27]. We selected Nannochloropsis salina (CCMP 1776), a marine microalga found in the Eustigmatophyceae class, for its ability to grow in a high saline environment (e.g., brackish water) and for the demonstrated PUFA accumulation of this species [29]. We define the effect of continuous, reduced cultivation temperature on physiological parameters, metabolic pool alteration, lipid metabolism, and Kennedy pathway gene expression to understand the mechanism of EPA incorporation in triglycerides
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