# Lignocellulosic Biomass as a Substrate for Oleaginous Microorganisms: A Review
## 2 Oleaginous microorganisms
### Sugar Conversion into SCO and Regulatory Mechanisms in Sugar Assimilation  
The conversion of sugars and other hydrophilic substrates into SCO is done through de novo biosynthesis, which comprises three physiological phases, including the growth phase occurring under balanced conditions, the oleaginous phase occurring under nitrogen limited conditions, and the reserve lipid turnover phase occurring after the depletion of the carbon source in the growth medium. During the growth phase, oleaginous microorganisms convert the carbon source into cell mass, rich in proteins and polysaccharides using the glycolytic pathway and the pentose phosphate pathway (PPP), while limited quantities of lipids, mainly polar lipids essential for the construction of cell membranes, are synthesized [44]. Then, in the stationary growth phase, the depletion of at least one essential nutrient (e.g., nitrogen, sulfate, phosphate or magnesium), induces the beginning of the oil accumulation phase [5; 26; 70], which extends until the carbon source is exhausted. Finally, during lipid turnover phase, TAGs are degraded to generate energy for cell maintenance and, under specific circumstances, for growth [57]. The metabolism of sugars, including hexoses and pentoses, has been extensively investigated and their theoretical stoichiometric conversion to lipids has been calculated. Oleaginous microorganisms show an efficient capacity to catabolize glucose and similar sugars to pyruvate, which is used in lipid biosynthesis. 100 g of glucose (about 0.56 mol) yields one mol of pyruvate, which is converted into 1.1 mol of acetyl-coA, corresponding to a maximum theoretical value of 0.32 g of lipids per g of glucose. If xylose is considered, a yield of 1.2 mol of acetyl-coA per 100 g of xylose (0.66 mol) is expected, which corresponds to 0.34 g of lipids per g of substrate [98]. Although the theoretical results offer a good approximation, under real culture conditions, the lipid yield rarely exceeds 0.22 g/g of glucose and 0.23 g/g of xylose [2; 99]. On the other hand, the efficient conversion of sugar compounds, such as cellobiose and molasses, into storage lipids, has also been reported [50; 73]. The yields obtained from oleaginous microorganisms cultivated on a single sugar are not enough to predict the efficient conversion of lignocellulosic biomass into SCO production. Instead, the simultaneous assimilation of hexoses and pentoses contained in raw substrates of plant origin is necessary to achieve sustainable SCO production processes [100]. Numerous studies on oleaginous microorganisms (especially yeasts) growing on sugar mixtures have shown that the assimilation of individual sugars is sequential, starting with the easiest to degrade and passing to the most complex, demonstrating diauxic growth [101; 102; 103; 104; 105; 106]. Commonly, the preferred sugars are hexoses, like glucose [50; 107; 108; 109], although some species are capable in efficiently metabolizing pentoses like xylose [50; 73; 101]. Diauxic behavior is attributed to two different mechanisms that may or may not occur simultaneously. The first is catabolic repression, in which the preferred sugar causes repression in the transcription of the enzymes involved in the catabolism of other sugars, and the second is allosteric competition of the transporters, in which the transporters of the preferred sugar inhibit the transporters of the other sugars. Both phenomena result in an inefficient consumption of lignocellulosic biomass and low rates of lipid accumulation [100]. In contrast, other studies on the growth of oleaginous microorganisms on mixtures of sugars have shown the capacity of some strains to consume sugars simultaneously resulting in high lipid production. Lipid yield for oleaginous yeasts and heterotrophically growing microalgae ranges from 0.15 to 0.26 g per g of sugar, when glucose and xylose (or mannose) are used simultaneously as carbon substrates [50; 104; 105; 106; 107; 108; 109; 110; 111; 112; 113; 114].