Evaluation of microalgae cultivation at air-CO2 equilibrium pH for improving carbon utilization efficiency

Journal Article · Mon Dec 01 00:00:00 UTC 2025 · Algal Research

DOI: https://doi.org/10.1016/j.algal.2025.104392 · OSTI ID:3375939

Gao, Song; Kalamaris, Nicholas N.; Klein, Bruno; Edmundson, Scott J. ; Yadav, Geetanjali; Davis, Ryan; Huesemann, Michael H.

Microalgae are often cultivated at near-neutral pH to optimize growth. However, this results in significant CO2 loss through outgassing in open 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 the 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 minimum biomass selling price (MBSP). The increased ammonia toxicity at the higher pH is one of the growth limiting factors and was mitigated by reducing ammonium fertilizer usage. The adjustment resulted in only a productivity decline of 13% (instead of 35%) for T. striata. Consequently, a lower MBSP was achieved. These findings suggest that the 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.

• Research Organization: Pacific Northwest National Laboratory (PNNL), Richland, WA (United States)
• Sponsoring Organization: USDOE
• Grant/Contract Number: AC05-76RL01830
• OSTI ID: 3375939
• Report Number(s): PNNL-SA-211256
• Journal Information: Algal Research, Journal Name: Algal Research Vol. 92
• Country of Publication: United States
• Language: English