Fig. 1: Optimization of the irradiation protocol to maximize lipid yield in Chlorella sorokiniana cultures.

a Experimental design of the 3-step optimization. Time points of irradiation and analysis are marked with arrows on the schematic culture growth curve. Cultures were irradiated with different doses (Gy) and dose rates (Gy/min) of X-radiation after 3 or 20 days (early exponential or early stationary phase). At each step the selection of conditions were narrowed to those that delivered positive changes. b–e Effects of irradiation applied in early exponential phase measured over 30 days after irradiation (Step 1). The number of biologically independent experiments were n = 12 for controls and n = 3 for treatments. b Cell density by optical density at 750 nm (OD₇₅₀). Sigmoidal data fitting (all R² > 0.95) is shown. c Growth rates during exponential phase. d Dry weight biomass. e Lipid yield in biomass at day-30 after irradiation determined by Nile Red fluorescence (a.u., arbitrary units). f–j Effects of irradiation applied in early stationary phase measured 10 days later (Step 2). f Cell density. g Biomass. The number of biologically independent experiments were n = 8 for controls and n = 6–10 for treatments. h Lipid yield in biomass. The number of biologically independent experiments were n = 16 for controls and n = 5–14 for treatments. i Cell volume. j Relative starch yield in biomass. The number of biologically independent experiments were n = 7 for controls and n = 3–7 for treatments. All data are means ± standard error. Significant difference compared to control (non-irradiated) treatments was evaluated using non-parametric two-tailed Mann–Whitney U tests and is indicated at p < 0.05 (*).