INFORM January 2024

HEALTH & NUTRITION

inform January 2024, Vol. 35 (1) • 21

growing on glucose-based media. However, the large-scale production of microbial oil with glucose as a raw material faces the puzzle of competing with a human food source. Therefore, it becomes necessary to search for other suitable raw mate rials. Ideally, manufacturers seek a source that reduces costs, accounts for the food and energy crises that plague the planet, and adheres to product quality standards. Fortunately, species that belong to Mucorales (such as Cunninghamella echinulata , Thamnidium elegans , Umbelopsis isabellina ) can produce appreciable amounts of lipids during growth on low-cost substrates such as molasses, biodiesel derived glycerol, and plant oils. More importantly, Mucorales include species that produce SCO containing gamma-linolenic acid (GLA) at considerable levels. This PUFA exhibits interest ing anti-cancer activity against several cell lines when adminis tered in the form of FA lithium or potassium salts ( https://doi. org/10.1002/elsc.201400208). Recently, some cheap and commonly available raw mate rials, such as lignocellulose and agro-industrial wastes have been considered as feedstock for chemical and biotechno logical applications (Table 1). The oil produced by oleaginous fungi using lignocellulosic biomass as a substrate is a two steps process: the conversion of biomass to monosaccharides by heat-acid treatment or enzyme hydrolysis and then sugar fermentation by the microorganisms . Unfortunately, the first step of the process produces compounds, such as furan alde hydes, weak acids, and aromatic compounds, that inhibit fun gal growth . However, researchers have found solutions to deal with this issue. For instance, inoculating growth media based on hydrolysate of alkaline pretreated corn cobs with vegetative mycelium of U. isabellina instead of spores not only led cells to overcome the inhibition effect by 5-hydroxymethylfurfural and furan in the medium, but the fungus also produced signifi cant amounts of biomass and lipids ( https://doi.org/10.1016/j. procbio.2021.06.021). Another cheap substrate for microbial oil production is crude glycerol, a by-product of the biodiesel industry, which is

usually treated as commercial waste. Glycerol is not the most favorable carbon source for fungal growth. However, recently, we discovered that it may play a key-role in suppressing lipid degradation during the late oleaginous phase without alter ing fatty acid composition, as shown when U. isabellina was cultivated on blends of glucose and glycerol at various concen trations (https://doi.org/10.1016/j.crcon.2023.03.008 ). In any case, it must be noted that the chemical composition of the selected substrate affects the lipid production, in a species-re lated manner. ALONG WITH THE FEASTS CAME THE YEASTS Yeast species are important microorganisms for various bio technological applications, as they are able to adapt their phys iological traits to specific niches and environmental conditions through the passage of time. Intriguingly, according to some estimations, almost 10% of all yeast species developed the ability to store high amounts of intracellular lipids as one of these adaptations. Oleaginous yeasts are found mostly in the phylum Ascomycota. These yeasts not only have the unusual capacity to use a wide range of agricultural and industrial wastes, but also possess some noteworthy advantages com pared to filamentous fungi, such as easier cultivation, higher cell density, and better productivity rates. Researchers have proposed that oleaginous yeasts might serve as an alternative to oil crops since their lipid profile is chemically similar (mainly containing oleic, palmitic, lin oleic, stearic acids) and can produce more exotic oils, like cocoa-butter substitutes from low-cost industrial fats (https:// doi.org/10.1007/s00284-002-3833-3). Oleaginous yeasts are not able to synthesize PUFAs, although genetically manipu lated strains have been employed for industrial applications. For instance, DuPont (USA) generated two Yarrowia lipolytica strains which can produce eicosapentaenoic and docosapen taenoic acid at considerable levels (https://doi.org/10.1038/ s41467-019-12025-8).

FIG. 1. Lipid droplets morphology in the mycelia and the chlamydospores of the fungus Umbelopsis isabellina after Nile Red staining under (a) optical and (b) fluorescence microscopy. Source: Dritsas & Aggelis, CRC, 6, 326-333, 2023.