Maximum CO2 Utilization by Nutritious Microalgae

Abstract

High-protein microalgae are a promising alternative to soy for more rapidly and sustainably produced protein-rich animal feed. However, there are still significant barriers to be overcome in growing nutritious microalgae, recovering nutrients from wastewater, and fixing CO₂ from flue gas in full-scale sustainable operations. Currently, it is generally assumed that nutritious microalgae, including Scenedesmus obliquus, are inhibited by CO₂ levels characteristic of industrial flue gases. Experiments in a 2 L photobioreactor with the ability to control CO₂ concentrations and pH demonstrated that the inhibition of S. obliquus was not important until 10% CO₂ and was not prohibitively reduced even at 35% CO₂. The rate of growth exceeded all values in the literature for S. obliquus at concentrations greater than 2.5% CO₂, and the amino acid content of the microalgae was equal or superior to that of soy. A substrate inhibition model indicated that CO₂ levels comparable to flue gases do not substantially inhibit S. obliquus growth, with careful pH control. The model indicated maximum biomass productivity of 640 ± 100 mg L–¹ d–¹ at 4.5% CO₂ (Kₘ of 0.8 ± 0.4% CO₂, Kᵢ of 26 ± 9% CO₂, and vₘₐₓ of 860 ± 120 mg L–¹ d–¹), which exceeds previously measured biomass productivity values at inhibitory CO₂ concentrations. Protein contents of S. obliquus and soy were comparable.