Rationally leveraging mixotrophic growth of microalgae in different photobioreactor configurations for reducing the carbon footprint of an algal biorefinery: a techno-economic perspective

Mixotrophy, the ability of an organism to simultaneously assimilate carbon dioxide (CO 2 ) and organic carbon, is of prime importance to algal biorefineries because it boosts algal biomass productivities, associated-CO 2 capture rates and intercellular lipid content; along with wastewater remediation. In the present study, the effect of light, CO 2 and glucose, in various permutations, on the growth of Chlorella vulgaris , cultivated in flat panel and bubble column photobioreactors, was investigated. The average specific growth rate of Chlorella vulgaris in light-sufficient conditions when supplied with (−glucose, −CO 2 ); (−glucose, +CO 2 ); (+glucose, −CO 2 ) and (+glucose, +CO 2 ) were 0.048 h −1 , 0.075 h −1 , 0.080 h −1 and 0.084 h −1 , respectively in flat panel reactors. The corresponding specific growth rates in bubble column reactors were 0.042 h −1 , 0.070 h −1 , 0.082 h −1 and 0.083 h −1 . Chlorella vulgaris was found to require light for assimilation of glucose because it did not grow in the dark under glucose-sufficient conditions. Furthermore, carbon balance revealed that the mixotrophic cultivation of Chlorella vulgaris had a positive carbon footprint (pseudo-mixotroph). Literature review revealed that the carbon footprint of mixotrophic algal cultivation may be negative (true-mixotroph) or positive depending upon the predominant mode of nutrition and the cultivation conditions. The novelty of the present study lies in the formulation of a strategy to identify true-and-pseudo-mixotrophs and their subsequent utilization for design of low-cost, minimal-resource and high-throughput cultivation strategies for biomass generation and CO 2 sequestration in algal biorefineries. Experimental set-up to study mixotrophy in Chlorella vulgaris .