A heterotrophic microalga, strain SD116, with the ability to produce high concentrations of docosahexaenoic acid (DHA, C22:6n-3) was isolated from Shuidong Bay, Guangdong Province, China. Nucleotide sequence analysis of the 18S rDNA of SD116 showed that the strain has a close phylogenetic relationship to Aurantiochytrium species. The highest rates for growth and DHA accumulation for SD116 were obtained in 6.0% glucose, 2.0% yeast extract, and 50% artificial seawater (ASW) at a pH of 7 at 28°C. The maximum total lipid content reached 56.3% of the dry cell weight (DCW), and the maximum DHA content accounted for 50.9% of the total fatty acid (TFA) content. It was further found that urea may be a potential nitrogen source for industrial fermentation because of its cheap price and ability to induce a relatively high biomass and lipid production capacity. Using 5 L fermenters, the DCW, total lipid content, and DHA yield were found to be 70.43 g L–1, 71.09% of the DCW, and 17.42 g L–1 (34.79% of the TFA), respectively. The results show that Aurantiochytrium sp. SD116 is a promising candidate for commercial DHA production and could be useful for the synthesis of biomass-related products.

Aurantiochytrium is a promising docosahexaenoic acid (DHA) production candidate due to its fast growth rate and high proportions of lipid and DHA content. In this study, high-throughput RNA sequencing technology was employed to explore the acclimatization of this DHA producer under cold stress at the transcriptional level. The overall de novo assembly of the cDNA sequence data generated 29,783 unigenes, with an average length of 1,200 bp. In total, 13,245 unigenes were annotated in at least one database. A comparative genomic analysis between normal conditions and cold stress revealed that 2,013 genes were differentially expressed during the growth stage, while 2,071 genes were differentially expressed during the lipid accumulation stage. Further functional categorization and analyses showed some differentially expressed genes were involved in processes crucial to cold acclimation, such as signal transduction, cellular component biogenesis and carbohydrate and lipid metabolism. A brief survey of the transcripts obtained in response to cold stress underlines the survival strategy of Aurantiochytrium; of these transcripts, many directly or indirectly influence the lipid composition. This is the first study to perform a transcriptomic analysis of the Aurantiochytrium under low temperature conditions. Our results will help to enhance DHA production by Aurantiochytrium in the future.

Aurantiochytrium is an important docosahexaenoic acid (DHA) producer containing two kinds of fatty acid synthesis pathways, that is, the fatty acid synthase pathway (FAS) for saturated fatty acid synthesis and the polyketide synthase pathway (PKS) for polyunsaturated fatty acid synthesis. To understand the regulation mechanism between the two pathways, the impacts of six short-chain fatty acids on the fatty acid synthesis of Aurantiochytrium sp. SD116 were studied. All short-chain fatty acids showed little effect on the cell growth, but some of them significantly affected lipid accumulation and fatty acid composition. Pentanoic acid and isovaleric acid greatly inhibited the synthesis of saturated fatty acids, whereas the polyunsaturated fatty acid synthesis was not affected. Analysis of malic enzyme activity, which supplied NADPH for saturated fatty acids biosynthesis, indicated that the two fatty acid synthesis pathways can utilize different substrates and possess independent sources of NADPH.