Photoautotrophic organic acid production: Glycolic acid production by microalgal cultivation

Although microalgae produce value-added products, such as lipids, pigments, and polysaccharides using light and carbon dioxide, these intracellular products require costly downstream processes such as extraction and purification. Thus, extracellular products are desirable for economic production. While reported before, the secretion of glycolic acid by microalgal photorespiration has not received attention for industrial applications. We developed a two-stage continuous cultivation system to increase glycolic acid production using a glycolate dehydrogenase (GYD1) deficient mutant of Chlamydomonas reinhardtii which produces high concentrations of glycolic acid. Specifically, 3% CO₂ was supplied in the first-stage culture for the production of biomass and ambient air (0.03% CO₂) was supplied to the second stage for the production of glycolic acid. As a result, overall glycolic acid productivity reached 82.0 mg L^-1 d^-1 at a dilution rate of 0.34 d^-1. However, as the pH of the second stage decreased to 4.7 due to the increased glycolic acid production, we controlled the pH of the second stage at pH 6.0, resulting in 122.6 mg L^-1 d^-1 of glycolic acid productivity. Flux balance analysis revealed that the experimental glycolic acid production rate was 69% of the theoretical glycolic acid production rate. The deviation might be due to the toxicity of glycolic acid. When a techno-economic analysis was conducted based on the experimental results, the minimum glycolic acid production cost was estimated to be $31 kg⁻¹, indicating a potential for industrial production. These findings suggest that microalgae can be utilized for the cost-effective industrial production of glycolic acid.