Complete Conversion of Organic Acids into Protein-Rich Biomass: Discovering the Photoformatotrophic Capacity of Galdieria sulphuraria

Galdieria sulphuraria (GS) SAG 108.79 was cultivated at pH < 2 using formic (FA) and acetic acid (AA) as carbon sources in mixotrophy. When the supply rate of FA and AA matched the CO2 fixation rate of the photoautotrophic reference, complete conversion to biomass occurred. However, if the organic acid supply exceeded the carbon fixation rate, biomass yields decreased to 90% Cgx·Cgs–1. This decrease was followed by increased biomass productivity with AA but not with FA, indicating that FA can be efficiently used as a carbon source in mixotrophy but is a poor energy source. This study provides the first proof of the effective growth of a wild-type microalgae species using FA. Formate dehydrogenase, the key enzyme for FA oxidation, was found in GS genome, a rare trait among microalgae. FA and AA did not affect the protein content (52% w/w) and amino acid profile, which complied with FAO recommendations for adults and confirmed the potential of GS as a novel protein source. Given the high biomass yield and the potential for efficient FA and AA production through CO2 electrocatalysis, mixotrophic cultivation using those organic acids emerges as a promising alternative to concentrated CO2 or air in GS.