Elevated carbon dioxide stimulates highly efficient organic-carbon consumption and confectionary-waste valorization under mixotrophy in the unicellular alga Galdieria

Unicellular algae are appealing for nutritional and biotechnological utility but have wide variation across strains and can be challenging to produce. The thermo-acidophilic algal genus Galdieria use diverse organic-carbon sources for fermentative growth that can include waste-stream feedstocks and have complete amino-acid compositions for human nutrition. Here, we investigated Galdieria metabolic dynamics to catalog organic-carbon conversion to biomass. Tested strains had enhanced growth upon 3% CO ₂ supplementation, triggering efficient glucose uptake to reach ∼5 ± 0.3 g dry biomass L ^⁻¹ . Stable-isotope analysis revealed that organic-carbon uptake dominates CO₂ fixation in darkness under mixotrophy, with CO₂ an apparent metabolic trigger. Galdieria sulphuraria 5587.1 can consume up to 8.3 g carbon L ⁻¹ day ⁻¹ from industrial confectionery waste, with C-phycocyanin reaching 3.8% of dry biomass and remaining thermostable at 72°C. This framework can optimize Galdieria -based bioprocesses for inexpensive waste conversion into high-value biomass and identifies CO₂ as a trigger of organic-carbon assimilation, even in heterotrophic conditions.