Heterotrophic microalgal production is often constrained by heavy reliance on glucose, of which the supply chain embodies significant carbon emissions. Here, the acidophilic microalga Galdieria sulphuraria is studied in growth media prepared from starch through microwave-assisted hydrolysis. The results imply that short-chain sugars with 8+ glucose units were the dominating species in the resultant hydrolysates after reactions at 140–160 °C in the presence of 10 mM acetic acid catalyst. Despite the low glucose levels (< 0.05 g/L) in these hydrolysates, the microalgal biomass development was comparable with that in the control group in 2 g/L standard glucose, suggesting the potential utilization of oligosaccharides by G. sulphuraria. The absence of intracellular oligosaccharides rules out the path of direct cellular uptake through transport proteins. In a culture supernatant, i.e., an environment free of living cells, oligosaccharides were found to depolymerize. Further investigations of transcriptomics and extracellular proteomics indicate the possible secretion of GH15 glucoamylase by G. sulphuraria, which promoted the extracellular hydrolysis of oligosaccharides into glucose that can be readily transported into cells for metabolism. Energy saving of up to c.a. 60 % can be achieved by adopting microwave-assisted hydrolysis in place of the conventional approach. The findings highlight short-chain sugars as an effective glucose reservoir for G. sulphuraria, which could largely streamline the preparation of organic carbon feed for heterotrophic farming. This study presents a green, low-impact pathway to recover carbon – channeling it from organic wastes to value-added algal biomass.