The biological carbon dioxide fixation using microalgae has been known as an effective carbon dioxide reduction technology. With many environmental factors influencing microalgal productivity, the desirable cultivation factors were investigated using a green alga, Euglena gracilis. It has the high protein and vitamin E to be used as fodder. In batch culture, initial pH, temperature, carbon dioxide concentration and light intensity in the optimum cultivation condition were 3.5, 27℃, 5-10% and 520μmol/㎡/s, respectively. When the tap water and freshwater were used as the medium to cultivative Euglena, tap water which was not sterilized was effective. In case of freshwater, pH control was necessary. Half-velocity coefficient (KI) in Monod model under photoautotrophic condition was 86.68μmol/㎡/s In this study, the optimum hydraulic retention time (HRT) for the continuous carbon dioxide fixation was 3 days at the optimum cultivation condition. For the experiment to assess the potential of Euglena as a biosensor, leachates from three different landfills were used. Value of $96h-EC_50$ was measured to indicate the toxicity of leachate. As a result of this experiment, $96h-EC_50$ of the leachate from open dumping site was showed the lower value than that from sanitary landfill and leachate from the young landfill was more toxic than that from the old landfill.
