The effect of nitrogen deprivation on the growth, oil yields and fatty acid production of the diatoms Phaeodactylum tricornutum and Nitzchia sp. in laboratory cultures

Microalgal production of triglycerides and fatty acids plays an important role in some energy and pharmacological applications. Studies indicate that nitrogen deprivation of algal cultures could result in higher accumulation of nitrogen-poor carbon compounds such as polysaccharides and fats, a reduction in the rate of photosynthesis and decline in nitrogenous photosynthetic pigments or the reduction in thylakoid stacking and absorption. The aims of this study are to determine if changes in the nitrogen availability in the culture medium will affect the growth rate, oil yields and fatty acid composition of two species of diatoms Phaeodactylum tricornutum and Nitzchia sp. Media were prepared with three different concentrations of nitrogen (as NaNO3), being 0.882 mM (100% of recommended concentration), 0.661 mM (75%) and 0.441 mM (50%). The 75% N-treated Phaeodactylum tricornutum cultures had lower maximum cell densities (11,496 * 103), but reached maximum cell density in a shorter time (8.33 days), had higher total oil yields (104.6 mg) and higher oil content as a percentage of cell mass (50.75%) than the 100% treatment. Similarity analysis and PERMANOVA indicate nitrogen had a significant effect on fatty acid profiles, particularly between the 50% and 100% N treatments. One of the more abundant fatty acids of existing industrial significance is eicosapentaenoic acid (EPA). Production of this compound declined significantly with declining nitrogen availability. Due to low growth rates in all Nitzchia sp. cultures, insufficient biomass could be produced to provide multiple replicate extractions for quantitative statistical comparisons and replicates were pooled to provide a single extract for gas chromatograph analysis. The 100% N-treated culture of Nitzchia sp. reached highest maximum cell densities (4,965 *103) and highest percentages of oil per cell mass (37.62%). However, the 50% N-treated Nitzchia sp. cultures produced higher total oil yields (176.9 mg) than both 100% and 75% N-treated Nitzchia sp. cells (161.0 mg and 135.3, respectively). The 50% N-treated Nitzchia sp. were the only cultures to produce 11, 14, 17- eicosatrienoic (0.0122 mg *mL-1 n-hexane), lignoceric acid (0.0149 mg*mL-1 n-hexane) and tridecanoic acid (0.00183 mg *mL-1 n-hexane). As in Phaeodactylum tricornutum, the 100% Ntreated Nitzchia sp. cells produced the highest concentration of EPA (1.68 mg*mL-1 n-hexane). The results show that, depending on the objectives of the culturing and the nature of the industry, total oil production could be increased by some reduction in the nitrogen levels of the culture medium. This could also influence the financial feasibility of such projects. However, nitrogen limitation reduces production of some oils of industrial significance and reduces the biomass of cells that are maintained by the culture medium. Optimising oil production will therefore require a balance between supporting cell growth and stimulating total oil yields, but for some particular oils, maximising growth will also maximise yield.