Elevated carbon dioxide stimulates efficient organic carbon consumption for the unicellular alga Galdieria

Unicellular algae are appealing for nutritional and biotechnological utility, but show wide variation across strains and can be challenging to produce. Species in the thermo-acidophilic genus Galdieria use diverse organic carbon sources for fermentative growth, which can include waste-stream feedstocks, while having complete amino acid compositions for human nutrition. Here, we investigated the metabolic dynamics of Galdieria to catalog organic carbon conversion to biomass. Tested strains had enhanced growth upon 3% CO₂ supplementation, triggering efficient glucose uptake to reach ~5 ± 0.3 g dry biomass l^-1. Stable-isotope analysis revealed that organic carbon uptake dominated CO₂ fixation in darkness under mixotrophy, with CO₂ an apparent metabolic trigger. Galdieria yellowstonensis 5587.1 was able to consume up to 8.3 g carbon l^-1 day^-1 from industrial confectionery waste, with C-phycocyanin (C-PC) reaching 3.8% of dry biomass and remaining thermostable at 72°C. Thus, this framework can be used to optimize Galdieria-based bioprocesses for the conversion of inexpensive waste into high-value biomass, and identifies CO₂ as a trigger of organic carbon assimilation, even in heterotrophic conditions.