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Effects of elevated CO_2 (5000μl/L) on sensitivity comparison of six species of algae and interspecific competition of three species of algae were investigated. The results showed that, the cell densities of six species of algae grown in elevated CO_2 significantly increased compared to those in ambient CO_2 (360μl/L), and with the time prolonged, the increasing extent increased. Therefore, elevated CO_2 can promote the growth of six species of algae. However, there were differences in sensitivity between six species of algae. Based on the effects of elevated CO_2 on biomass, the sensitive order (from high to low) was Platymanas sp., Platymanas subcordiformis, Nitzschia closterium, Isochrysis galbana Parke 8701, Dunaliella salina, Chlorella sp., on the condition of solitary cultivation. Compared to ambient CO_2, elevated CO_2 promoted the growth of three species of algae, Platymanas subcordiformis, Nitzschia closterium and Isochrysis galbana Parke 8701 under the condition of mixed cultivation. The sensitivity of the three species to elevated CO_2 in mixed cultivation changed a lot compared to the condition of solitary cultivation. When grown in elevated CO_2 under the condition of mixed cultivation, the sensitive order from high to low were Nitzschia clostertium, Platymonas subcordiformis and Isochrysis galbana Parke 8701. However, under the condition of solitary cultivation, the sensitive order in elevated CO_2 was Isochrysis galbana Parke 8701, Nitzschia clostertium, Platymonas subcordiformis, from sensitive to less sensitive. On the day 21, the dominant algae, the sub-dominant algae and inferior algae grown in elevated CO_2 did not change. However, the population increasing dynamic and composition proportion of three algal species have significantly changed. [ABSTRACT FROM AUTHOR]