CO2 gradient domestication improved high-concentration CO2 tolerance and photoautotrophic growth of Euglena gracilis

In order to improve CO2 biofixation efficiency of microalgae cultivated with coal-chemical flue gas, CO2 gradient do-mestication was employed to improve high-concentration CO2 tolerance and photoautotrophic growth of acid-tolerant Euglena gracilis. The dried biomass yield of photoautotrophic growth of E.gracilis increased from 1.09 g/L (wild-type strain) by 21 % to 1.32 g/L with CO2 gradient domestication to 15 % CO2. The RuBisCO activity and biomass produc-tion of E.gracilis strain domesticated to 99 % CO2 were 2.63 and 3.4 times higher, respectively, than those of wild-type strain. The chlorophyll a and b contents were 2.52 and 1.79 times higher, respectively, than those of wild-type strain. Superoxide dismutase and catalase activities of 99 % CO2-domesticated strain increased to 1.24 and 6 times, which reduced peroxide damage under high carbon stress and resulted in lower apoptotic and necrotic rates of domesticated strain. Thus, this work provides valuable guidance for CO2 fixation and adaptive evolution of E. gracilis in industrial flue gas.