Enhanced biomass and CO2 sequestration of Chlorella vulgaris using a new mixotrophic cultivation method

CO2 sequestration using microalgae has been deemed as a promising way, which can fix CO2 and simultaneously produce valuable bio-products. However, large scale open-pond production of Chlorella vulgaris is normally cultured using acetic acid as a carbon source, which is costly and shows a low efficiency in CO2 fixation. Here, a new mixotrophic culture strategy using both CO2 and acetic acid was developed and evaluated for CO2 sequestration by a C. vulgaris culture in an open pond. Our results show that the growth rate of C. vulgaris under this new mixotrophic condition reaches 0.24 g/L/d, which is higher than the 0.15 and 0.21 g/L/d of photoheterotrophic culture with acetic acids and photoautotrophic culture with CO2, respectively. The averaged CO2 fixation rate was determined as 0.29 g/L/d, which is much higher than heterotrophic method but slightly lower than photoautotrophic method. This result was further confirmed in a 125 m(2) open raceway pond. Physiological and biochemical characterization showed that the cell quality of C. vulgaris under mixotrophic conditions is better than those of photoautotrophic method and photoheterotrophic cultures. The enzyme activity assay and transcriptome sequencing analysis revealed that the metabolism of carbohydrates and amino acids was significantly enhanced under mixotrophic condition compared with other groups, which may attribute to the increased biomass and CO2 sequestration of C. vulgaris. Our results suggest that this mixotrophic strategy can be applied in large-scale cultivation of C. vulgaris for biomass production and CO2 sequestration.