The transition to a sustainable bioeconomy relies on efficient conversion of biomass and industrial side-streams into value-added products. This study evaluates the heterotrophic cultivation of the extremophilic microalga Galdieria sulphuraria using a decentralized, 200 L-scale bioreactor system, testing alternative nutrient sources including glycerol, molasses, and hydrolysates from brewer's spent grains. The growth performance, yield, and compositional robustness under these complex substrates were compared with a defined medium containing glucose, ammonium, and phosphate. G. sulphuraria demonstrated robust growth across all tested substrates, reaching biomass concentrations between 10 and 26 g L⁻¹ in semi-continuous operation for over 40 days under acidic and thermophilic conditions. While defined medium supported higher productivity (0.4 kg day⁻¹) and consistent nutrient supply, complex substrates derived from agro-industrial residues enabled substantial biomass formation (0.1–0.2 kg day⁻¹) but required careful management of nutrient balance and potential inhibitory by-products such as acetic and lactic acids from molasses. Biomass analysis revealed variable macromolecular content depending on the substrate, with protein levels between 25 % and 42 % (w/w) and carbohydrates and lipids ranging broadly. The findings highlight the technical feasibility and adaptability of G. sulphuraria for decentralized conversion of waste streams, supporting more resilient and distributed models of resource valorization within the bioeconomy. Further optimization of substrate processing and system integration is recommended to improve productivity and scalability.