Effects of phosphorous deficiency of a microalga Nannochloropsis oculata on its fatty acid profiles and intracellular structure and the effectiveness in rotifer nutrition

The microalga Nannochloropsis oculata is used as a diet for rotifers and can enrich the highly unsaturated fatty acid (HUFA) proportion of rotifer polar lipids (PL-HUFA). For efficient enrichment of PL-HUFAs in rotifers, we evaluated nutrient–sufficient or –deficient cell populations to clarify when the alga should be harvested and fed to rotifers. The nutrient conditions for N. oculata were spectrophotometrically monitored during cultivation and cells with nutrient sufficiency (NS) and phosphorous deficiency (PD) were harvested. The PD cells exhibited reduced chlorophyll a content, photosynthetic activity, and growth rate and accumulated total lipids as non-polar lipids (NLs). A decrease in the proportion of HUFA to total lipids between two nutrient states corresponded to a similar phenomenon observed in NLs. On the other hand, PLs, including glycolipids and phospholipids, showed different trends. Glycolipids did not change fatty acid composition between two states. Phospholipids, however, showed significantly higher contents of arachidonic acid (ArA) and eicosapentaenoic acid (EPA) only in the early periods of PD culture. It was concluded that phosphorous deficiency stimulated cells to increase the fatty acid desaturation rate of non-plastidial membranes. Mitochondria in the PD cells decreased in number but increased in size and mass. Such adaptation of mitochondria can be associated with high desaturation of non-plastidial membranes. When the early P–limited phase culture was used for the enrichment of rotifers, PL-ArA and EPA of the rotifers were enhanced by twice as much as those fed on NS cells. This study provides evidence that the beginning of phosphorous deficiency is the optimum time to harvest N. oculata cells for enhancing rotifer PL-HUFAs.