Chemical and Physical Culture Conditions Significantly Influence the Cell Mass and Docosahexaenoic Acid Content of Aurantiochytrium limacinum Strain PKU#SW8

Abstract

Thraustochytrids are well-known unicellular heterotrophic marine protists because of their promising ability to accumulate docosahexaenoic acid (DHA). However, the implications of their unique genomic and metabolic features on DHA production remain poorly understood. Here, the effects of chemical and physical culture conditions on the cell mass and DHA production were investigated for a unique thraustochytrid strain, PKU#SW8, isolated from the seawater of Pearl River Estuary. All the tested fermentation parameters showed a significant influence on the cell mass and concentration and yield of DHA. The addition of monosaccharides (fructose, mannose, glucose, or galactose) or glycerol to the culture medium yielded much higher cell mass and DHA concentrations than that of disaccharides and starch. Similarly, organic nitrogen sources (peptone, yeast extract, tryptone, and sodium glutamate) proved to be beneficial in achieving a higher cell mass and DHA concentration. PKU#SW8 was found to grow and accumulate a considerable amount of DHA over wide ranges of KH2PO4 (0.125–1.0 g/L), salinity (0–140% seawater), pH (3–9), temperature (16–36 °C), and agitation (140–230 rpm). With the optimal culture conditions (glycerol, 20 g/L; peptone, 2.5 g/L; 80% seawater; pH 4.0; 28 °C; and 200 rpm) determined based on the shake-flask experiments, the cell mass and concentration and yield of DHA were improved up to 7.5 ± 0.05 g/L, 2.14 ± 0.03 g/L, and 282.9 ± 3.0 mg/g, respectively, on a 5-L scale fermentation. This study provides valuable information about the fermentation conditions of the PKU#SW8 strain and its unique physiological features, which could be beneficial for strain development and large-scale DHA production.

Nitrogen Starvation Enhances the Production of Saturated and Unsaturated Fatty Acids in Aurantiochytrium sp. PKU#SW8 by Regulating Key Biosynthetic Genes

“…To understand the effect of nitrogen concentration on the biomass and DHA production of PKU#SW8, different concentrations (0–1.2 g/L) of monosodium glutamate (MSG) in the culture medium were tested. MSG has been reported to be one of the optimal nitrogen sources for high yields of DHA (g per g biomass) produced by thraustochytrids [ 7 , 17 , 33 ]. A defined nitrogen source such as MSG can generally be easily utilized by microorganisms for rapid cell growth and the glutamate can be converted into other nitrogen-containing compounds [ 34 ].…”

“…Owing to their high lipid content, thraustochytrids have potential biotechnological applications, particularly in the nutraceutical and biofuel industries [ 3 ]. Certain thraustochytrids, namely the strains that belong to Aurantiochytrium (formerly Schizochytrium ) genus, have shown the unique ability to accumulate large amounts of polyunsaturated fatty acids (PUFAs), especially docosahexaenoic acid (DHA) [ 4 , 5 , 6 , 7 , 8 ]. In addition to PUFAs, some studies have also suggested the high potential of thraustochytrids to accumulate saturated fatty acids (SFAs) [ 9 , 10 , 11 , 12 ].…”

“…The unique fatty acids that are produced by different species or strains of thraustochytrids depend on the culture conditions. For example, nitrogen source plays a crucial role in the regulation of cellular growth and lipid biosynthesis in thraustochytrids [ 7 , 19 , 20 ]. Several studies have shown that a high carbon-to-nitrogen ratio is advantageous for lipid accumulation in thraustochytrids grown in complex media containing yeast extract [ 21 , 22 , 23 ] or corn steep liquor [ 24 ].…”

Production, Biosynthesis, and Commercial Applications of Fatty Acids From Oleaginous Fungi

“…Cellular lipid content and lipid yield were 62.2% and 0.205 g/g glucose, respectively, using a medium with a carbon to nitrogen (C/N) molar ratio of 6.1 and a C/P molar ratio of 9,552 ( 46 ), which means that the accumulation of lipid can also be regulated by limiting phosphorus in the medium. As a consequence, the regulation of phosphorus and sulfur limitation is of great significance for the production of lipids from nitrogen-rich crude materials ( 41 , 45 , 47 , 48 ).…”

“…PKU#Sw8, the increase in DHA production coincided with the up-regulation of gene expression under nitrogen-deficient culture conditions ( 44 ). Chen et al ( 45 ) optimized the culture of Thraustochytrid sp. PKU#SW8 under optimal culture conditions (glycerol, 20 g/L; peptone, 2.5 g/L; 80% seawater; pH 4.0; 28°C), the cell mass, DHA concentration and yield of PKU#SW8 were increased to 7.5 ± 0.05 g/L, 2.14 ± 0.03 g/L and 282.9 ± 3.0 mg/g, respectively, on a 5-L scale fermentation.…”