Nutrient Availability Shifts Biomass and Lipid Allocation in Scenedesmus obliquus: Dose Responses to Nitrogen, Phosphate and Iron

Graphical Abstract Graphical Abstract Nutrient-dose studies often identify a preferred microalgal cultivation condition from biomass concentration or lipid percentage alone. Because these responses can move in opposite directions, either measure may give an incomplete picture of lipid amount per culture volume. This study therefore aimed to compare biomass, lipid content and calculated endpoint lipid concentration across separate nitrogen, phosphate and iron dose series in Scenedesmus obliquus, and to determine whether the dose favoured depended on the endpoint assessed. Nitrogen (5–20 mg L⁻¹), phosphate (1–4 mg L⁻¹) and iron (0.5–2.0 mg L⁻¹) were each tested at four doses, with three independently cultivated cultures per dose. Biomass concentration and lipid content were measured; lipid concentration was calculated from their paired values for each culture. Within-series differences were assessed by one-way ANOVA and Tukey’s test. Biomass increased with dose, reaching 2.877 ± 0.025, 2.777 ± 0.025 and 2.863 ± 0.015 g L⁻¹ at the highest nitrogen, phosphate and iron doses, respectively. Lipid content declined from 77.03 ± 1.17 to 51.58 ± 0.73%, 74.83 ± 0.70 to 52.20 ± 0.36%, and 73.00 ± 0.20 to 50.13 ± 0.25% across the respective series. The highest observed calculated lipid concentrations were 1.656 ± 0.048 g L⁻¹ at 5 mg L⁻¹ nitrogen, 1.544 ± 0.026 g L⁻¹ at 1 mg L⁻¹ phosphate and 1.555 ± 0.017 g L⁻¹ at 1.0 mg L⁻¹ iron. None was statistically distinct from all neighbouring lower-dose treatments. The dose that maximized biomass did not maximize calculated lipid concentration in any series. The findings support endpoint-specific dose selection, while the pairwise comparisons do not establish a unique lipid-concentration optimum. Calculated endpoint concentration does not establish lipid productivity or recovered product output.