# Nutritional and Medicinal Properties of Microbial Oil
## 3 Microbial Oil Production  
While edible and non-edible plant oils were the source of the first- and second-generation biodiesel, respectively, the greater use of microbes as an oil source gave rise to the third generation of biodiesel [24]. Microbial oils have a similar fatty acid composition to plant oils; therefore, they can be used as an alternative to them [25]. Microbial oils contain unsaturated fatty acids, which are highly beneficial for human health. Additionally, they can be used in the diets of vegan and vegetarian individuals. Studies have shown that the advantage of microbial oil production lies in the oleaginous microorganisms' ability to accumulate a high diversity of lipids in significant amounts with a low input (Figure 2). Microbial oil production occurs independently of climate conditions compared to plants. For example, many adverse conditions, such as the impact of poor climate conditions on plant growth, may affect the oil production from plants. However, such hindrances are minimal in microbial oil production. It requires no land area and ensures high efficiency [26]. In recent years, there has been a significant increase in research efforts to advance microbial lipid technology as a sustainable source of oil. The aim is to use it to replace unsustainable oils, such as palm oil, and to create advanced biofuels. However, the  
Figure 2: Advantages and disadvantages of microbial oil production. commercialization of this new technology is facing some challenges, such as scalability and economic and ecological sustainability. The production costs of the lipids are still high, and consumers are reluctant to accept GMO-derived oil [2; 3]. Some ideas to help overcome these challenges will be discussed in the upcoming section. Bacterial cells face two major limitations when it comes to producing lipids. They tend to have lower biomass yields compared to other groups, resulting in fewer lipids per gram of biomass. Additionally, lipid extraction can be difficult, due to their adhesion to cell membranes [27]. Yeasts are also used in the research on the biosynthesis of microbial oil. Oleaginous yeasts include representatives of such species as _Rhodosporidium toruloides, Rhodotorula glutritis, Trichosporo oleaginous, Lipomyces starkeyi_, and _Y. lipolytica_, the last of which being considered a model organism for studying the mechanisms involved in lipid metabolism covering lipid uptake, their storage, and their deposition or mobilization [27; 28; 29]. As previously mentioned, the biosynthesis of microbial lipids can take place in two different biochemical pathways, i.e., de novo and ex novo, which involve the synthesis of fatty acid precursors from glucose or the uptake of fatty substances from the environment and their accumulation in lipid bodies, respectively; therefore, the fatty acid composition is largely dependent on the substrate used [30]. The wide range of substrates used, often waste ones, allows for the simultaneous management of industrial waste and the production of valuable metabolites [31; 32]. Filamentous fungi cannot be ignored when discussing oleaginous microorganisms. At least two fungal cell oils have been commercialized so far, i.e., microbial oil from the culture of _M. circinelloides_ with a high level of \(\gamma\)-linolenic acid and arachidonic-rich oil from _M. alpine_[27; 33]. In contrast to bacteria and fungi, microalgae are autotrophic microorganisms and convert sunlight and CO\({}_{2}\) into biomass with the simultaneous biosynthesis of lipids, pigments, and carbohydrates [34]. Typical subjects to research on microbial oil biosynthesis are the following algae species: _Schizochytrium_ sp., _Nannochloropsis oceanica_, _Chlamydomonas reinhardtii_, _Cryptheodimium colnuti_, _Chlorella vulgaris_, and _Dunaliella salina_[35]. There are two stages in the production of SCOs by oleaginous microorganisms: biomass growth and lipid accumulation. The first stage involves a nutrient-rich medium that supports cell proliferation and biomass growth [26].