Biotransformation of lactose-free acid whey by Chlorella vulgaris and Tetradesmus obliquus into value-added products

Lactose-free acid whey (LFAW), an abundant byproduct from the production of skyr, was evaluated as a novel substrate for the mixotrophic cultivation of Chlorella vulgaris and two strains of Tetradesmus obliquus (CCAP 2761A and CCPA276/3A). Nutrient profiling revealed that LFAW contained macronutrients such as calcium, magnesium, and potassium but was deficient in nitrate, phosphate, and trace elements, prompting the formulation of a supplemented medium (S-LFAW). Cultivation trials under mixotrophic conditions in 1 L photobioreactors demonstrated that S-LFAW significantly enhanced biomass productivity for C. vulgaris (140 mg/L/day) and T. obliquus 3A (79 mg/L/day), outperforming both standard Bold's Basal Medium (BBM) and LFAW alone. C. vulgaris grown in S-LFAW exhibited improved protein (13 mg/L/day) and total fatty acid content (31 mg/L/day), along with high carotenoid yields, indicating phototrophic metabolic dominance. T. obliquus 3A showed moderate performance with strain-specific sensitivity to nutrient composition, while T. obliquus 1A had limited growth in all LFAW-based media. The amplicon sequencing (i.e.,16S and 18S rRNA genes) of prokaryotic and eukaryotic contaminants revealed that the addition of limiting nutrients to the culture medium reduces the level of contamination. This study demonstrates the effective valorization of LFAW for sustainable microalgal biomass production, contributing to circular bioeconomy strategies and nutrient recovery from dairy side-streams.