Effect of organic carbon source on mixotrophic growth and biomass composition in Auxenochlorella protothecoides: implications for sucrose-rich agroindustrial waste valorisation

Abstract

Microalgae are promising sustainable bioresources due to their rapid growth, metabolic flexibility, and capacity to valorise organic substrates under mixotrophic conditions. Comparing different carbon sources is crucial to evaluate algal metabolism and the potential of sucrose-rich industrial wastes as feedstocks. However, the nature of the organic carbon source strongly affects growth performance, carbon conversion efficiency, and biomass composition. This study primarily evaluates the mixotrophic cultivation of Auxenochlorella protothecoides on acetic acid, glucose, and sucrose using an iso‑carbon condition (0.48 gC/L); then, glucose and sucrose concentrations were increased to 1.44 and 2.88 gC/L, testing both balanced and unbalanced C: N ratios. Glucose allowed the highest specific growth rates (μ = 1.70 d⁻¹) and biomass productivity (2.07 g/(L·d)). High C: N ratios strongly favoured lipid accumulation to 38% DW, whereas C: N-balanced media restored the protein synthesis to 37% DW. Acetate favoured efficient carbon conversion (Y_X/S = 1.15 g·g⁻¹) and high ammonium removal (87%). By contrast, sucrose-supplemented cultures supported only low growth performance, due to metabolic constraints in disaccharide utilization. The comparison between experimental (DW_Exp) and a dual stoichiometric model (DW_{Cmax, Th} and DW_{Nmax, Th}) based on empirical biomass composition (CH_1.8O_0.5N_0.1) mathematically proved that sucrose underperformed by 90–95% in the unbalanced C: N ratio for the biomass yields due to sucrose metabolism limitations. These results suggest that while sucrose-rich substrates (such as molasses) can support mixotrophic growth, hydrolysis pretreatment can be highly beneficial for carbon use, biomass productivity, and process sustainability in microalgal bioprocesses.