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

Abstract

CO 2 sequestration using microalgae has been deemed as a promising way, which can fix CO 2 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 CO 2 fixation. Here, a new mixotrophic culture strategy using both CO 2 and acetic acid was developed and evaluated for CO 2 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 CO 2 , respectively. The averaged CO 2 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 CO 2 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 CO 2 sequestration. Recently, C. vulgaris has become the second largest cultured algal in China [14], and it is normally cultured in open raceway ponds using acetic acid [15,16], where acetic acid act as a carbon source and also keeps the pH within neutral range [17]. Compared with acetic acid, CO 2 can also be used by C. vulgaris as carbon source with the advantages of higher CO 2 fixation efficiency, and lower-risk of contamination [18]. However, C. vulgaris cells cultured only with CO 2 are inferior to those with acetic acids or glucose in terms of cell size and density, and are difficult to harvest [19]. Therefore, improvement of biomass production and CO 2 sequestration rate is an area of great

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