Freshwater microalgae strain selection for industrial triacylglycerol production – a scale-down approach

Microalgae are a promising source of sustainable lipids, particularly triacylglycerols (TAGs), but large-scale production remains economically unviable and depends on strain selection. In this study, four microalgal strains were selected based on their ability to accumulate TAG: Tetradesmus obliquus wild type (wt), Tetradesmus obliquus starchless mutant 1 (slm1), Chromochloris zofingiensis, and Neochloris oleoabundans. These strains were compared on growth rate, areal fatty acid productivity, and photosynthetic parameters under diurnal light and temperature cycles and nitrogen-sufficient and nitrogen-deficient conditions. This scale-down approach integrates laboratory screening under environmental fluctuations, providing novel insight into strain performance relevant for large-scale TAG production. Under nitrogen sufficiency, T. obliquus wt and slm1 showed the highest growth, reaching 10.0 ± 0.2 (n = 2) and 9.7 ± 0.2 (n = 2) g m⁻² day⁻¹. Under nitrogen starvation, only slm1 maintained growth at 8.4 ± 1.6 g m⁻² day⁻¹, as productivity in other strains declined significantly. Additionally, T. obliquus slm1 reached the highest total fatty acid content with 44.7 ± 3.7%, with 38.8 ± 2.8% of TAG. Therefore, T. obliquus slm1 emerged as the highest TAG producer and reached an average areal productivity of 7.3 ± 1.1 g_TAG m⁻² day⁻¹ over the second week of starvation. The potential of this strain was demonstrated in a 275 L vertically stacked tubular photobioreactor, achieving an average volumetric biomass productivity of 0.53 ± 0.06 g L⁻¹ day⁻¹ during the growth and 0.28 ± 0.06 g L⁻¹ day⁻¹ during the stress phase. These values correspond to areal biomass productivities of 25.2 ± 2.8 g m⁻² day⁻¹ and 13.48 ± 3.01 g m⁻² day⁻¹. After 21 days of stress, the lipid content reached 42.9 ± 0.9% TFA and 32.5 ± 0.5% TAG.