Oxygen stress mitigation for microalgal biomass productivity improvement in outdoor raceway ponds

Dissolved oxygen (DO) concentrations at supersaturated levels are commonly observed in microalgae cultures due to photosynthesis. Unlike cultivation in closed photobioreactors, the impact of excessive oxygen and the mitigation methods in open systems such as raceway ponds have hardly been studied. In this study, photosyn-thetic rates of Monoraphidium minutum 26B-AM and Scenedesmus obliquus UTEX393, two of the top DISCOVR strains for outdoor raceway cultivation, were compared at different DO levels. Increasing DO from 0 to 30 mg L-1 caused a 36.6 % and 26.1 % reduction in photosynthetic rate for M. minutum 26B-AM and S. obliquus UTEX393, respectively. To improve the outdoor pond biomass productivity of each strain, three oxygen stress mitigation methods were tested. The first method raised the dissolved bicarbonate concentration by an order of magnitude to favor bicarbonate transport into the cell, increasing the CO2:O2 ratio near RuBisCO, thus reducing photo -respiration. Although this method increased the observed photosynthesis rate of M. minutum 26B-AM by 26 % and S. obliquus UTEX393 by 13 % in laboratory testing, it failed to improve biomass productivity under outdoor pond conditions for both strains. The second method added sodium sulfite as an oxygen scavenger to chemically lower the DO concentration in the culture. Chemical reduction of dissolved molecular oxygen did not signifi-cantly improve the biomass productivity of M. minutum 26B-AM; however, a 10 % improvement in biomass productivity for S. obliquus UTEX393 was observed. The final method sparged cultures with air to strip oxygen out of the liquid medium. This did not improve the biomass productivity of M. minutum 26B-AM, likely due to unfavorable weather conditions during cold season cultivation; however, an improvement of 36 % in biomass productivity was achieved for S. obliquus UTEX393 during the warm season cultivation. Of the three tested methods, air-sparging was the most effective and practical for improving outdoor biomass production.