초록
<P><B>Background</B></P><P>Unlike the well-studied backer yeast where catabolite repression represents a burden for mixed substrate fermentation, <I>Yarrowia lipolytica</I>, an oleaginous yeast, is recognized for its potential to produce single cell oils and citric acid from different feedstocks. These versatilities of <I>Y. lipolytica</I> with regards to substrate utilization make it an attractive host for biorefinery application. However, to develop a commercial process for the production of citric acid by <I>Y. lipolytica</I>, it is necessary to better understand the primary metabolism and its regulation, especially for growth on mixed substrate.</P><P><B>Results</B></P><P>Controlling the dissolved oxygen concentration (pO<SUB>2</SUB>) in <I>Y. lipolytica</I> cultures enhanced citric acid production significantly in cultures grown on glucose in mono- or dual substrate fermentations, whereas with glycerol as mono-substrate no significant effect of pO<SUB>2</SUB> was found on citrate production. Growth on mixed substrate with glucose and glycerol revealed a relative preference of glycerol utilization by <I>Y. lipolytica</I>. Under optimized conditions with pO<SUB>2</SUB> control, the citric acid titer on glucose in mono- or in dual substrate cultures was 55 and 50 g/L (with productivity of 0.6 g/L*h in both cultures), respectively, compared to a maximum of 18 g/L (0.2 g/L*h) with glycerol in monosubstrate culture. Additionally, in dual substrate fermentation, glycerol limitation was found to trigger citrate consumption despite the presence of enough glucose in pO<SUB>2</SUB>-limited culture. The metabolic behavior of this yeast on different substrates was investigated at transcriptomic and <SUP>13</SUP>C-based fluxomics levels.</P><P><B>Conclusion</B></P><P>Upregulation of most of the genes of the pentose phosphate pathway was found in cultures with highest citrate production with glucose in mono- or in dual substrate fermentation with pO<SUB>2</SUB> control. The activation of the glyoxylate cycle in the oxygen limited cultures and the imbalance caused by glycerol limitation might be the reason for the re-consumption of citrate in dual substrate fermentations. This study provides interesting targets for metabolic engineering of this industrial yeast.</P><P><B>Electronic supplementary material</B></P><P>The online version of this article (doi:10.1186/s12934-017-0690-0) contains supplementary material, which is available to authorized users.</P>