Substrate and nutrient manipulation during continuous cultivation of extremophilic algae, Galdieria spp. RTK 37.1, substantially impacts biomass productivity and composition

Abstract

The extremophilic nature and metabolic flexibility of Galdieria spp. highlights their potential for biotechnological application. However, limited research into continuous cultivation of Galdieria spp. has slowed progress towards the commercialization of these algae. The objective of this research was to investigate biomass productivity and growth yields during continuous photoautotrophic, mixotrophic and heterotrophic cultivation of Galdieria sp. RTK371; a strain recently isolated from within the Taupō Volcanic Zone in Aotearoa-New Zealand. Results indicate Galdieria sp. RTK371 grows optimally at pH 2.5 under warm white LED illumination. Photosynthetic O₂ production was dependent on lighting intensity with a maximal value of (133.5 ± 12.1 nmol O₂ mg_biomass⁻¹ h⁻¹) achieved under 100 μmol m⁻² s⁻¹ illumination. O₂ production rates slowed significantly to 42 ± 1 and <0.01 nmol O₂ mg_biomass⁻¹ h⁻¹ during mixotrophic and heterotrophic growth regimes respectively. Stable, long-term chemostat growth of Galdieria sp. RTK371 was achieved during photoautotrophic, mixotrophic and heterotrophic growth regimes. During periods of ammonium limitation, Galdieria sp. RTK371 increased its intracellular carbohydrate content (up to 37% w/w). In contrast, biomass grown in ammonium excess was composed of up to 65% protein (w/w). Results from this study demonstrate that the growth of Galdieria sp. RTK371 can be manipulated during continuous cultivation to obtain desired biomass and product yields over long cultivation periods.