Studies on lactose utilization by the microalga Graesiella emersonii in the 3N-BBM-V model medium

Currently, commercial microalgal biomass production is limited mainly due to high production costs. The use of cheap alternative media, such as dairy industry by-products and wastewater, can facilitate the commercialization of prospective microalgal strains. However, the use of dairy by-products is hampered by poor understanding of mechanisms of lactose utilization, i.e. the main carbon (C) source in such substrates, leading to inconsistent performance of microalgae. This study focuses on the ability of an isolated freshwater axenic microalgal strain Graesiella emersonii MSCL 1718 to utilize lactose and its monomers, i.e. glucose and galactose, in modified Bold’s Basal Medium with triple nitrogen and vitamins (3 N-BBM-V). It aims to develop a better understanding of microalgal interactions with C sources available in dairy by-products. The strain can successfully utilize lactose under mixotrophic and heterotrophic conditions when pH is maintained at approximately 7 and efficiently grow in lactose concentrations up to 30 g l⁻¹. Under mixotrophic cultivation conditions, G. emersonii was capable of utilizing all three tested C sources. However, under heterotrophic cultivation conditions, G. emersonii could not utilize galactose as the main C added to 3 N-BBM-V, highlighting the importance of selecting appropriate cultivation conditions. Additionally, the enzyme β-galactosidase, responsible for lactose hydrolysis, was detected in the microalgal biomass, with activity up to 17.03 ± 1.23 U l⁻¹. FTIR analysis of biomass demonstrated an increase in total lipid content and polyunsaturated fatty acids specifically in the mixotrophic group. Overall, G. emersonii demonstrated features suitable for bioconversion of dairy by-products which should be investigated in further studies.