Abstract: Water pollution and climate warming are important environmental problems facing the world today. Microalgae have the ability to purify water and fix CO₂ at the same time. In order to explore the removal rate of three mixed nitrogen sources and the fixed rate of CO₂ by algae, this study took Scenedesmus obliquus, Chlorococcum sphacosum GD and Chlorella vulgaris as experimental objects. Microalgae were inoculated in BG-11 medium with urea nitrogen, nitrate nitrogen and ammonium nitrogen （mass concentration 247 mg/L） as mixed nitrogen sources, and the removal rate of different nitrogen sources and the fixation efficiency of CO₂ were studied under the condition of 300 mL /min air supply carbon source. The results showed that there were differences in the utilization of nitrogen sources among the three microalgae, and the absorption and utilization of ammonium nitrogen were inhibited. Urea nitrogen, nitrate nitrogen, ammonium nitrogen removal rate of Scenedesmus obliquus were 92.90%, 79.28% and 22.86% respectively, and utilization of urea nitrogen was the highest. The nitrate utilization rates of Chlorococcum sphacosum GD were 78%, 80.10% and 39.45%, respectively, and those of Chlorella vulgaris were 62.00%, 81.04% and 34.66%, respectively. The maximum CO₂ fixation efficiency were 0.125, 0.142, and 0.217 g/（L·d）, respectively. The average carbon fixation rate were 0.067, 0.097, and 0.115 g/（L·d）, respectively. The carbon fixation efficiency of the Chlorella vulgaris was the highest, followed by the Chlorococcum sphacosum GD, and that of Scenedesmus obliquus was the lowest.

Key words: microalgae, urea nitrogen, nitrate nitrogen, ammonium nitrogen, removal rate, CO₂ fixation efficiency