Evaluation of Microalgae Cultivation at Air-CO2 Equilibrium pH for Improving Carbon Utilization Efficiency: Article No. 104392

Microalgae are often cultivated at near-neutral pH to optimize growth. However, this results in significant CO2 loss through outgassing in open-pond cultivation systems, leading to low CO2 utilization efficiency and higher biomass production costs. One potential solution is algal cultivation at air-CO2 equilibrium pH, which minimizes CO2 outgassing but may inhibit growth due to stresses at higher pH levels. In this study, we evaluated the viability of this approach by growing Picochlorum celeri and Tetraselmis striata under outdoor relevant conditions at equilibrium pH. Compared to pH 7 cultures, biomass productivity declined by 35 % for P. celeri and 57 % for T. striata. Although CO2 outgassing could be minimized, the significant loss in productivity led to a higher overall minimum biomass selling price (MBSP). Increased ammonia toxicity at the higher pH was found to be one of the growth-limiting factors and was mitigated by reducing ammonium fertilizer concentration. This adjustment resulted in a more moderate productivity decline of 13 % (instead of 35 %) for T. striata, and consequently, a lower MBSP was achieved. These findings suggest that equilibrium pH cultivation may be a viable method for reducing CO2 loss without significantly compromising biomass productivity. Depending on the strain, strategies to mitigate stressors, such as ammonia toxicity, may be necessary.