Autotrophic and mixotrophic biomass production of the acidophilic Galdieria sulphuraria ACUF 64

Abstract Galdieria sulphuraria is an acidophilic microalga isolated in proximity of sulfuric ponds where pH is below 3 and most organisms cannot grow. We cultivated G. sulphuraria ACUF 64 free of contamination for over 2 months in a medium containing organic carbon at pH 1.7 with continuous, high intensity, lighting. We compared biomass productivity of chemostat and repeated batch cultivations. The optimal biomass density in autotrophic and mixotrophic cultures was identified. In autotrophy biomass productivity was 28.3 g_x·m⁻²·day⁻¹, 1.8 to 7.7-fold higher than previously reported. Autotrophy was compared to ‘oxygen balanced’ mixotrophy where intracellular recirculation of O₂ and CO₂ take place. Aeration was not needed and 92% of the substrate carbon was converted into biomass. In mixotrophy biomass productivity was 1.8 times higher than autotrophic culture and linear growth was maintained at high biomass concentration (9.7 g_x·L⁻¹). Light tolerance and high productivity in dense culture make our strain promising for mixotrophic outdoor cultivation.