Study On Improving DHA Productivity Of Schizochytrium Sp. By Stepwise Cultivation With Shifting Dissolved Oxygen Level

Docosahexaenoic acid (DHA), which belongs to ω-3polyunsaturated fatty acids, has attracted much attention because of its beneficial effect on human health. In last years, the demand for DHA was increasing rapidly in food and pharmaceutical industry. At present, DHA is mainly derived from deep sea fish oil, but the production is limited. Besides, the quality of the deep sea fish oil is variable and far away to meet the expanding market. However, marine microorganisms, such as marine fungi-Schizochytrium sp., are considered as an alternative source of DHA. The process of producing DHA with marine microorganism is more sustainable and controllable. In this work, intensifying of Schizochytrium sp. culture and DHA production was investigated, aiming at increasing the DHA productivity and reducing the cost.Firstly, in order to increase DHA productivity, culture conditions, which include carbon source, nitrogen source, inoculum age, inoculum size, salinity, temperature, initial pH, liquid medium volume and culture time, were systematically investigated. The results showed that the optimized medium was composed of70g L-1glucose,15g L-1peptone,20g L-1seawater crystal. The optimized culture condition would be:36h inoculum age,5%inoculum size, initial pH6,50mL/250mL liquid medium volume,25℃,170rpm and flask culture for120h. Under this condition, the DHA productivity reached to16.27mg L-1h-1Secondly, a membrane bioreactor was made to enhance aeration and the dissolved oxygen level was increased. Schizochytrium sp. culture for DHA was evaluated by measuring biomass production and DHA content in two different culture protocols, including single shake and enhanced aeration using membrane modes. The results showed:1) during single shake flask culture process, the biomass of Schizochytrium sp. increased slowly, but the DHA content was higher;2) during membrane bioreactor culture process, the biomass of Schizochytrium sp. increased rapidly, but the DHA content was lower.In order to improve DHA productivity further, a culture strategy of two-stage process was proposed:Schizochytrium sp. was cultured with enhanced aeration mode (0.2L min-1) for60hours, and shifted to shake flask culture to96hours. Final DHA productivity reached to21.26mg L-1h-1. It was30.3%higher than that in shake flask culture.Finally, a culture strategy of three-stage process was attempted. In the third stage, the dissolved oxygen level was decreased by lower the rotational speed. However, due to the decreasing activity of cells and the inadequate of carbon source, the DHA productivity was just increased slightly.