Mixotrophic cultivation of Galdieria sulphuraria for C-phycocyanin and protein production

G. sulphuraria is a polyextremophilic microalga that can tolerate low pH, high temperature and high osmotic pressure. We cultivated G. sulphuraria ACUF 064 in chemostat at a biomass concentration of 134 to 243 g·m⁻² aiming for maximal pigment content without compromising biomass productivity. Autotrophy was compared to ‘oxygen balanced’ mixotrophy with intracellular recirculation of oxygen and carbon dioxide. No differences were found in C-phycocyanin (C-PC) and protein content between autotrophic and mixotrophic cultures. In mixotrophy the biomass productivity and concentration were doubled compared to the photoautotrophic counterpart. In mixotrophy aeration was not needed and 89% of the substrate carbon was converted into biomass. Mixotrophically grown biomass contained 10% w/w C-PC which, combined with its high areal biomass productivity (49 g·m⁻²·day⁻¹), sums up as one of the highest C-PC areal productivities ever reported (5 g·m⁻²·day⁻¹) under 24 h/24 h illumination. C-PC extracted from G. sulphuraria was more stable than the currently used C-PC extracts from Spirulina. No significant loss of color was observed down to a pH of 3 and up to a temperature of 55 °C. G. sulphuraria had a protein content of 62% w/w and compared favorably with FAO dietary recommendation of adults regarding amino acid composition. G. sulphuraria contains a high proportion of essential, sulfur amino acids compared to Chlorella, Spirulina and soybean protein. Due to its attractive amino acid profile and high protein content, G. sulphuraria is a good candidate for food and feed applications to overcome sulfur amino acid deficiencies. In addition, oxygen balanced mixotrophy allows for efficient and productive cultivation of G. sulphuraria biomass.