# Nutritional and Medicinal Properties of Microbial Oil
## 3 Microbial Oil Production
Oil or storage lipid accumulation occurs under nutrient-limited conditions. The fermentation techniques used for yeast SCO production often control the C:N ratio by limiting the amount of nitrogen and providing excess carbon (often in the form of glucose or some other sugar substrate). Without available nitrogen, the metabolism shifts to triacylglycerol (TAG) synthesis for energy storage until favorable growing conditions return [36]. Nitrogen limitation has been found to profoundly influence the lipid accumulation in yeasts, mainly because of a shift in its metabolic flux leading to the cessation of growth and triggering lipid accumulation [16; 37; 38]. It is noteworthy that phosphorus limitation may further improve the efficiency of microbial lipid biosynthesis, which was proved by [PERSON] et al. [32]. Still, some balance needs to be achieved, as the simultaneous limitation of both phosphorus and nitrogen sources promotes lipid accumulation in cells, creating unfavorable conditions for biomass growth. Remarkably, oleaginous and non-oleaginous organisms do not differ in the biosynthesis pathways of lipids; however, there is a need for a continuous supply of the triacylglycerol building block--acetyl-CoA--and NADPH production. The continuous production of acetyl-CoA in oleaginous microorganisms is achieved by a cascade of enzyme reactions triggered by a nutrient limitation, among which ATP-citrate lyase seems unique. It cleaves the acetyl-CoA from the citrate in the cytosol and disrupts the Krebs cycle. The malic enzyme and the pentose phosphate pathway are crucial to the NADPH supply [37; 39]. De novo and ex novo synthesis are the two available routes for lipid accumulation in oleaginous yeast cells. The above-mentioned mechanisms are connected to de novo lipid biosynthesis, where fatty acid precursors, such as acetyl-CoA and malonyl-CoA, are generated and incorporated into lipid storage biosynthesis. For ex novo synthesis, the lipid accumulation is initiated independently from nitrogen availability in the hydrophobic medium, and it is generated simultaneously with cell growth [30; 40]. It is necessary to alter a microorganism's metabolic pathways to cause lipid buildup by preventing cell division beyond a particular point. Although it is not always the case, nitrogen limiting is the recommended approach. However, the culture medium also requires an abundant supply of carbon, which is typically provided by glucose, though alternative carbohydrate feedstocks may be utilized if they are more affordable [41]. Usually, nitrogen sulfate or yeast extracts are applied as a nitrogen source, but more varied carbon sources such as glycerol, monosaccharides, lignocellulosic, or oily wastes could also be used. SCO can be produced using different modes of cultures, including SmF (submerged) and SSF (solid-state fermentation). Oleaginous microorganisms can be cultivated in batch, fed-batch, and continuous cultures. Usually, flask cultures are less effective than stirred-tank bioreactors [32; 39]. The volumetric productivity practically obtainable in the yeast lipid fermentation process is generally limited (less than 100 g/L). Therefore, the industrial production processes tend to involve the processing of significant volumes of culture broth to harvest cell mass and recover lipids [38]. Extensive studies have been conducted on improving the heterotrophic lipid productivity of yeast species like _R. toruloides, R. glutinis, and Y. lipolytica_[42]. For engineered _Y. lipolytica_ yeast strains, it is possible to achieve 1.2 g/L/h productivity in a glucose-based medium [43]. Several papers have detailed pilot-scale cultivations of oleaginous yeast in up to 300-L bioreactors, but still, there is no clear indication of whether a heterotrophic process is economically feasible. On the other hand, autotrophic microalgae can produce lipids whose price can amount to USD 1.7 to USD 5.9, according to some techno-economic studies, but they can hardly compete with plant oils whose process varies between USD 0.5 and USD 1.9 [42].