In the late nineteenth century, greenhouse gases have increased the earth average temperature and the seawater level upto 0.3-0.6℃ and 10-25mm, respectively. Increased concentration of $CO_2$ in the atmosphere is considered to be one of the main causes of the greenhouse effect. In recent years, many attempts, especially the fixation using microalgae, have been made to reduce the quantity of $CO_2$. Many factors influence microalgal productivity such as available light energy, supply of nutrients, a favorable temperature and a supply of $CO_2$(Grobbelaar J. U. et al., 1995). With many environmental factors influencing microalgal productivity, the desirable cultivation factors were investigated using a green alga, Euglena gracilis, since it has much protein and vitamin E to be used as fodder. In this study, the optimum cultivation conditions such as HRT, $CO_2$ concentration, light intensity for the continuous $CO_2$ fixation were investigated. For the mass cultivation, the continuous and semi-continuous cultivation methods were employed.
The optimum hydraulic retention time (HRT) for the continuous cultivation was 4 days at carbon dioxide concentration of 10%. In this condition, the final cell number was $3.57×10^6/㎖$. The growth of Euglena gracilis Z increased according to the light intensity.
