Enhancing cell growth and phycocyanin production in heterotrophic and mixotrophic cultures of Galdieria sulphuraria through modulation of the C/N ratio and organic carbon sources

The growth and accumulation of metabolites in the polyextremophile microalga Galdieria sulphuraria CCMEE 5587.1 were evaluated under mixotrophic and heterotrophic conditions, using glucose and glycerol as carbon sources with varying C/N ratios. The results showed that glycerol resulted in lower biomass productivity (P_X = 0.32 to 0.61 g L⁻¹ d⁻¹) and phycocyanin production (P_PC = 1.7 to 20.9 mg L⁻¹ d⁻¹) compared to glucose (P_X = 0.57 to 1.38 g L⁻¹ d⁻¹ and P_PC = 1.4 to 31.3 mg L⁻¹ d⁻¹). A lower C/N ratio enhanced the synthesis of phycobiliproteins and supported cell growth. In contrast, an increase in the C/N ratio led to the accumulation of nitrogen compounds alongside a rise in carbohydrate accumulation. When considering culture mode, biomass yield on carbon was significatively higher in mixotrophic conditions compared to heterotrophic conditions. This indicates that cells derive energy from both photosynthesis and respiration. Among the conditions assessed, mixotrophy combined with a low C/N ratio using glucose is recommended to produce phycocyanin from G. sulphuraria.