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 heavily 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, as well as nitrogen-sufficient and nitrogen-deficient cultivation conditions. 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. Combining growth rate with TAG content, T. obliquus slm1 emerged as the highest TAG producer and reached an average areal productivity of 7.3±1.1 gTAG m-2 day-1 over the second week of starvation. The potential of this strain was demonstrated in a 275L vertically stacked tubular photobioreactor, achieving an average volumetric biomass productivity of 0.53±0.06 g L-1 day-1 during the growth phase 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, respectively. After 21 days of stress, the lipid content reached 42.9±0.9% TFA and 32.5±0.5% TAG.