Effect of incident light and light gradients on eicosapentaenoic acid distribution between lipid classes in Nannochloropsis oceanica

Commercial production of eicosapentaenoic acid (EPA) from photoautotrophic microalgae like Nannochloropsis oceanica requires higher productivity and larger scales to reduce costs. Improving productivity can be achieved by increasing biomass concentrations, which creates light gradients in the reactor depending on the culture’s acclimation and the reactor geometry. These light gradients affect physiology, lipid synthesis, but also the distribution of fatty acids between lipid classes. In this study we evaluated the combined effect of the incident light intensity and light gradient on growth, biochemical composition, and fatty acid distribution between lipid classes. A total of 13 cultivations were performed in continuous mode using three different incident light intensities (200, 670, and 1550 μmol photons m⁻² s⁻¹) and four dilution rates (from 0.29 to 0.75 day⁻¹). Reducing dilution rates resulted in higher biomass concentrations, steeper light gradients, and lower average light intensities. Increasing incident light intensity improved biomass productivity from 0.5 to 1.8 g L⁻¹ day⁻¹, while the biomass yield on light decreased from 1.05 to 0.44 g mol⁻¹. Lowering average light intensities decreased the triglyceride content from 11.1 to 1.5% w/w, and increased the galactolipid content, mainly monogalactosyldiacylglycerol, up from 3.1 to 5.1% w/w. Total EPA contents did not decrease at low incident light but decreased by 28% at highly saturating light, both relative to medium incident light. The EPA content in polar lipids increased at lower average light intensities, and decreased in the neutral fraction simultaneously. These results highlight the tight regulation of EPA content between lipid polar and neutral fractions under different light regimes.