Maximum CO2 Utilization by Nutritious Microalgae
Hannah R. Molitor, Emily J. Moore and Jerald L. Schnoor
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.