Dissolved oxygen is a critical factor for heterotrophic cell growth and metabolite production. The aim of this study was to investigate the effects of an oxygen-involved protein on cell growth and fatty acid and astaxanthin production in the biologically important thraustochytrid Aurantiochytrium sp. The hemoglobin of the Vitreoscilla stercoraria (VHb) gene was fused upstream with a zeocin resistance gene (ble) and driven by the Aurantiochytrium tubulin promoter. The expression construct was introduced into two strains of Aurantiochytrium sp. by electroporation. Transgenic Aurantiochytrium sp. strains MP4 and SK4 expressing the heterologous VHb achieved significantly higher maximum biomass than their corresponding controls in microaerobic conditions. Furthermore, the transformants of Aurantiochytrium sp. SK4 produced 44% higher total fatty acid and 9-fold higher astaxanthin contents than the wild type control in aerobic conditions. The present study highlights the biotechnological application of VHb in high-cell density fermentation for enhanced biomass production as well as high-value metabolites.

Glucose induces the green alga Chlorella zofingiensis to accumulate high amounts of astaxanthin and triacylglycerol (TAG). Here we used comparative transcriptome analysis to identify the genes and their expressions for the biosynthesis of the important metabolites. Transcriptome data revealed a great number of gene coding sequences involved in most primary and secondary metabolisms, including full coding sequences of all the genes for astaxanthin and TAG biosynthesis. The biosynthesis of IPP (isopentenyl diphosphate) by the MEP (methylerythritol 4-phosphate) pathway was found to be correlated to astaxanthin biosynthesis. The enhanced astaxanthin synthesis induced by glucose was closely related to the up-regulation of astaxanthin pathway genes coupled with the repression of side pathway genes. Most of the genes involved in fatty acid and TAG biosynthesis consist of more than one copy. Their global up-regulation triggered off the accumulation of fatty acids and TAG in C. zofingiensis. In addition, glucose up-regulated DGAT2 (diacylglycerol acyltransferase 2) but down-regulated PDAT (phospholipid: diacylglycerol acyltransferase), suggesting that the acyl-CoA-dependent pathway attributed to the accumulation of TAG. Astaxanthin and TAG accumulations were found to be coordinated. Our transcriptome datasets can serve as a valuable and informative platform for functional studies on carotenoids, lipids and other important metabolites.