Oxygen Limitation Improves Ganoderic Acid Biosynthesis in Submerged Cultivation of Ganoderma lucidum

The effects of oxygen limitation on the production of ganoderic acid (GA), a secondary metabolite with antitumor activity, and on transcription levels of triterpene biosynthesis genes were investigated in liquid cultures of Ganoderma lucidum. A low oxygen supply level was beneficial to total GA biosynthesis, but negative to the cell growth. The higher GA production was obtained under micro-aerobic conditions (i.e. initial overall kLa values at 0.02 and 0.13/h). The maximum GA production of 272.3 ± 11.5 mg/L was obtained at an initial overall kLa of 0.13/h, which was 1.7-fold that at a normal cultivation condition (an initial kLa of 5.51/h). For four major individual GAs, the production level of GA-Mk, -T, -S, and -Me in the hypoxia-induced cells was increased by 50, 87, 62, and 111%, compared with that of the control, respectively. Meanwhile, the transcriptions of four key genes encoding 3-hydroxy-3-methylglutaryl coenzyme A reductase, mevalonate-5-pyrophosphate decarboxylase, squalene synthase and squalene epoxidase in the triterpene biosynthetic pathway were up-regulated under the hypoxia condition (at an initial kLa of 0.13/h). Reactive oxygen species was generated in response to hypoxia, which seemed to be involved in the regulation of GA biosynthesis. The information obtained provides an insight into the role of oxygen limitation in the GA biosynthesis, and will be helpful for optimizing the fermentation process on a large scale.