The effects of low temperature on docosahexaenoic acid biosynthesis in Schizochytrium sp TIO01 and the underlying mechanism

Abstract Background Schizochytrium are known for their abundant production of docosahexaenoic acid (DHA). Low temperatures can promote the biosynthesis of polyunsaturated fatty acids in many species. In this study, the effects of low temperature on the biosynthesis of DHA in Schizochytrium sp TIO01 and the underlying mechanism was investigated. Results Based on the de novo genome assembly (contig N50=2.86 Mb) and iTRAQ-based protein identification, we first reconstructed the detailed Schizochytrium fatty acid biosynthesis pathway. 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 separately responsible for DHA and saturated fatty acid synthesis in Schizochytrium . Analysis of fatty acid composition profiles indicates that low temperature has a significant impact on the production of DHA in Schizochytrium, increasing the DHA content and overall total fatty acids from 43% to 65%. The increased DHA content, however, was not a result of the expression of the PKS pathway genes. Further gene expression analysis indicated that low temperatures may promote DHA accumulation by the up-regulation of both the pentose phosphate pathway and the branched-chain amino acid degradation pathway (increasing the production of the substrates for polyunsaturated fatty acid synthesis: acetyl-CoA and NADPH). In addition, low temerpatures result in a down-regulation of the FAS pathway (reducing the consumption of the substrates for saturated fatty acid synthesis) and malic enzyme, leading to a decreased saturated fatty acid content. Conclusions These findings elucidate the detailed fatty acid synthesis pathway of Schizochytrium, revealing an underlying mechanism by which low temperatures promote the accumulation of DHA in Schizochytrium. The high-quality and nearly complete genome sequence of Schizochytrium provides a valuable reference for further investigation of the regulation of polyunsaturated fatty acids biosynthesisand the evolutionary characteristics in Thraustochytriidae species.