Production of added-value microbial metabolites during growth of yeast strains on media composed of biodiesel-derived crude glycerol and glycerol/xylose blends

Abstract

A total of 11 yeast strains of Yarrowia lipolytica, Metschnikowia sp., Rhodotorula sp. and Rhodosporidium toruloides were grown under nitrogen-limited conditions with crude glycerol employed as substrate in shake flasks, presenting interesting dry cell weight (DCW) production. Three of these strains belonging to Metschnikowia sp. accumulated significant quantities of endopolysaccharides (i.e. the strain V.V.-D4 produced 11.0 g/L of endopolysaccharides, with polysaccharides in DCW ≈ 63% w/w). A total of six Y. lipolytica strains produced either citric acid or mannitol. Most of the screened yeasts presented somehow elevated lipid and polysaccharides in DCW values at the early steps of growth despite nitrogen appearance in the fermentation medium. Lipid in DCW values decreased as growth proceeded. R. toruloides DSM 4444 cultivated on media presenting higher glycerol concentrations presented interesting lipid-accumulating capacities (maximum lipid = 12.5 g/L, maximum lipid in DCW = 43.0–46.0% w/w, conversion yield on glycerol = 0.16 g/g). Replacement of crude glycerol by xylose resulted in somehow decreased lipid accumulation. In xylose/glycerol mixtures, xylose was more rapidly assimilated from glycerol. R. toruloides total lipids were mainly composed of triacylglycerols. Total cellular fatty acid composition on xylose presented some differences compared with that on glycerol. Cellular lipids contained mainly oleic and palmitic acid.

Lipid Production by Yeasts Growing on Commercial Xylose in Submerged Cultures with Process Water Being Partially Replaced by Olive Mill Wastewaters

“…This result is in disagreement with recent investigations demonstrating the strain R. toruloides NRRL Y-27012 produced huge quantities of SCO (Y L/X > 40% w/w, in some trials this value was = 54.3% w/w, with an L max value ≈ 12 g/L) on media composed of crude glycerol, the principal waste stream deriving from biodiesel production process [39]. Equally, significant SCO quantities were produced by the strain R. glutinis NRRL YB-252 (Y L/X = 38.2% w/w, corresponding to L max ≈ 7.2 g/L) in similar types of media (crude glycerol) [40], demonstrating that in the above-mentioned yeast, as well as in a plethora of other yeast species and genera, glycerol is a very competitive substrate related with the production of TDCW and added-value extracellular and intracellular metabolites [33,[40][41][42][43]. It is not clearly understandable why such important discrepancies exist between the lipid production process for R. toruloides NRRL Y-27012 and R. glutinis NRRL YB-252 growing on xylose and glycerol, but in any case-and in accordance with the literature [4,5,22,23,44,45]-it appears that the carbon source seems to play a very crucial role in the de novo lipid production process, even if implicated substrates present important similarities among them in molecular and metabolic level.…”

“…Plate application of the sample and the lipid standards and visualization after the development were conducted according to Papanikolaou et al [32]. Glyceryl trioleate (TAG), cholesterol (CL), cholesteryl linoleate (CE), oleic acid (FFA), and monononadecanoin (MAG) [33] were employed as lipid standards for the identification of the main bands on TLC plates.…”

“…These differences could, finally, reflect in the quantity of cellular lipids produced by the microorganisms growing on these substrates; for instance, xylose metabolism implicates in many instances the pentose-phosphate pathway [5,22,23], that is somehow less efficient as regards biomass production compared with the typical EMP glycolysis utilized for the catabolism of hexoses or glycerol [5,22]. On the other hand, xylose metabolism implicates in its first step, the reduction of xylose into xylitol, that in many cases in which oleaginous microorganisms are involved [30,33] is secreted into the medium and it is not re-utilized in order to be converted into SCO. Therefore, there would be a carbon "loss" related with the production of microbial lipids when xylose is used as microbial substrate amenable to be converted into cellular lipid compared with utilization of hexoses or glycerol [22,30,33].…”

“…On the other hand, xylose metabolism implicates in its first step, the reduction of xylose into xylitol, that in many cases in which oleaginous microorganisms are involved [30,33] is secreted into the medium and it is not re-utilized in order to be converted into SCO. Therefore, there would be a carbon "loss" related with the production of microbial lipids when xylose is used as microbial substrate amenable to be converted into cellular lipid compared with utilization of hexoses or glycerol [22,30,33].…”

“…The remaining "red" yeast strain implicated in the current investigation, namely R. toruloides DSM 4444, presented efficient cell growth and non-negligible lipid accumulation (Y L/X ≈ 30% w/w) on media composed of commercial-type xylose ( Table 1). The above-mentioned strain has been revealed capable to produce interesting SCO quantities during growth of glycerol and, to lesser extent glycerol/xylose blends [33], whereas it accumulated indeed impressive quantities of storage lipid during growth on flour-rich waste stream hydrolysates, mostly composed of glucose, during growth in shake-flask or bioreactor experiments [46].…”