Low-temperature effects on docosahexaenoic acid biosynthesis inSchizochytriumsp. TIO01 and its proposed underlying mechanism

Background Schizochytriumspecies are known for their abundant production of docosahexaenoic acid (DHA). Low temperatures can promote the biosynthesis of polyunsaturated fatty acids (PUFAs) in many species. This study investigates low-temperature effects on DHA biosynthesis inSchizochytriumsp. TIO01 and its underlying mechanism. Results TheSchizochytriumfatty acid biosynthesis pathway was evaluated based on de novo genome assembly (contig N50 = 2.86 Mb) and iTRAQ-based protein identification. Our findings revealed that desaturases, involved in DHA synthesis via the fatty acid synthase (FAS) pathway, were completely absent. The polyketide synthase (PKS) pathway and the FAS pathway are, respectively, responsible for DHA and saturated fatty acid synthesis inSchizochytrium. Analysis of fatty acid composition profiles indicates that low temperature has a significant impact on the production of DHA inSchizochytrium, increasing the DHA content from 43 to 65% of total fatty acids. However, the expression levels of PKS pathway genes were not significantly regulated as the DHA content increased. Further, gene expression analysis showed that pathways related to the production of substrates (acetyl-CoA and NADPH) for fatty acid synthesis (the branched-chain amino acid degradation pathway and the pentose phosphate pathway) and genes related to saturated fatty acid biosynthesis (the FAS pathway genes and malic enzyme) were, respectively, upregulated and downregulated. These results indicate that low temperatures increase the DHA content by likely promoting the entry of relatively large amounts of substrates into the PKS pathway. Conclusions In this study, we provide genomic, proteomic, and transcriptomic evidence for the fatty acid synthesis pathway inSchizochytriumand propose a mechanism by which low temperatures promote the accumulation of DHA inSchizochytrium. The high-quality and nearly complete genome sequence ofSchizochytriumprovides a valuable reference for investigating the regulation of polyunsaturated fatty acid biosynthesis and the evolutionary characteristics ofThraustochytriidaespecies.