Influence of irradiance on the growth and biochemical composition of Nitzschia aff. pellucida

In this paper the influence of irradiance and the operation modes on biomass production and biochemical composition, especially the fatty acid profile, on the diatom Nitzschia aff. pellucida was studied. Experiments were carried out under laboratory conditions, but simulating those outdoors, at irradiances ranging from 100 to 1000 µmol photons m⁻² s⁻¹, both in batch and semi-continuous mode. The data show that the growth rate increased with irradiance, following a hyperbolic relationship with the average irradiance (Eₐᵥ). However, at irradiances higher than 450 µmol photons m⁻² s⁻¹, photo-inhibition was observed, with Fᵥ/Fₘ values notably decreasing at irradiances above 1000 µmol photons m⁻² s⁻¹. To take account of this phenomenon, a growth model is proposed that considers the saturation constant modification (Eₖ) as a function of available irradiance. The data indicate that N. aff. pellucida is better adapted to low irradiances, below 300 µmol photons m⁻² s⁻¹. Regarding the biochemical composition, the highest content of lipids (~ 20%) and carbohydrates (~ 40%) was achieved at high irradiances (≥ 450 µmol photons m⁻² s⁻¹), while the highest protein content (> 8%) was found at low irradiances (≤ 200 µmol photons m⁻² s⁻¹). Elevated amounts of saturated fatty acids (SFA) (> 60%) were measured at high irradiances, while the highest content of unsaturated fatty acids (USFA) (> 35%) was found at low irradiances. Eicosapentaenoic acid (C20:5 n-3) (EPA) and arachidonic acid (C20:4 n-6) (ARA) were only detected at 200 µmol m⁻² s⁻¹ but not at higher irradiances. The results confirm the effects of irradiance and the cultivation mode on the growth and biochemical composition of N. aff. pellucida. The model is an important tool when using this valuable strain to produce compounds of biological interest.