Genetic modification by overexpressing Vitreoscilla haemoglobin (VHb) is an efficient and economic method for improving O₂ utilization for microbial fermentation. In this study, VHb was expressed in oleaginous yeast, Yarrowia lipolytica, and its effect on lipid accumulation was analysed. During fermentation, the dissolved oxygen (DO) concentration was controlled at 5, 10, 20 and 30 of full O₂ saturation and also uncontrolled by varying the stirring speed and aeration rate. Yarrowia lipolytica harbouring the VHb gene (VHb⁺ strain) displayed more pseudomycelium than the control strain (VHb^- strain), and VHb expression also enhanced the cell density. When DO level was controlled at 30%, the biomass of VHb⁺ strains reached about 19 g l^-1 and increased by 27% compared to VHb^- strains. Total fatty acid contents, although, were higher in VHb⁺ strains than in VHb^- strains under all DO levels, and maximally increased by c. 40% (from 10·5 to 14·5% of the biomass) when the DO concentration was controlled at 30%. VHb overexpression, however, markedly suppressed citrate secretion. In addition, expression of VHb also induced significant changes in fatty acid composition and increased the oleic acid content. SIGNIFICANCE AND IMPACT OF THE STUDY: Genetic modification by overexpressing Vitreoscilla haemoglobin (VHb) is an efficient and economic method for improving O₂ utilization for microbial fermentation. In this study, VHb was expressed in Yarrowia lipolytica, which is a most attractive model for microbial oil production because of its excellent lipid accumulation capacity and convenient genetic tools. Expression of VHb resulted in lipid accumulation increased and cell morphology shift, however, markedly suppressed citrate secretion. This study provided the new strategy for fermentation technology to improve lipid production in oleaginous micro-organisms.

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