Abstract
The polyextremophilic red microalga Galdieria sulphuraria represents a promising platform for waste-to-biomass bioprocesses, as it can grow heterotrophically under acidic and thermophilic conditions that intrinsically limit microbial contamination. However, quantitative data on the combined effects of temperature and pH under continuous heterotrophic operation remain scarce. In this work, G. sulphuraria SAG 21.92 was cultivated heterotrophically in laboratory-scale chemostats operated as CSTRs and fed with a glucose-based medium. Steady-state experiments were performed by varying temperature (30–55 °C) and inlet medium pH (1–5) at fixed residence time (2 d) and inlet organic carbon concentration (6 gC L?¹). Biomass concentration and productivity were determined from dry-weight measurements, while carbon-based mass balances quantified respiratory carbon losses. A broad thermal optimum was observed between 35 and 45 °C, with biomass productivities up to 3.0 gX L?¹ d?¹. Inlet pH acted as an effective control handle for environmental pH, which decreased at steady state due to biological acidification. When analysed as a function of environmental pH, biomass productivity showed a narrow optimum around pH 1.9, corresponding to maximum biomass concentrations (7.7 gX L?¹) and minimised respiratory carbon losses. Overall, these results identify favourable operating windows for maximising growth performance and carbon use efficiency, providing quantitative guidelines for process optimisation and scale-up.
