Mutual influence of light and CO ₂ on carbon sequestration via cultivating mixotrophic alga Auxenochlorella protothecoides UMN280 in an organic carbon-rich wastewater

This study focusses on the assimilation of carbon in concentrated municipal wastewater rich in organic carbon using the mixotrophic microalga Auxenochlorella protothecoides UMN280 with the addition of supplemental CO ₂. The entire growth period of A. protothecoides UMN280 can be characterized by three phases: first, a phase where algae grew in a mixotrophic-dominated mode; second, a transition phase; and last, a phase where algae grew in a photoautotrophic-dominated mode. In this study, it was found that light intensity had a strong effect on algal biomass production; the culture system would transfer from a mixotrophic-dominated mode to a photoautotrophic-dominated mode quicker under higher light intensities. The addition of CO ₂ exhibited an important role in the photoautotrophic-dominated cultivation stage. At certain level of irradiance and certain range of CO ₂ injection rate, higher CO ₂ injection rate would result in a higher level of carbon fixation. It is clearly beneficial to inject exogenous CO ₂ in the mixotrophic wastewater algae production system when a light source is available, such as during daylight hours.