BackgroundThe oleaginous yeast, Yarrowia lipolytica, has been utilized as an industrial host for about 60 years for various applications. Recently, the metabolic engineering of this host has become increasingly popular due to its ability to accumulate lipids as well as improvements made toward developing new genetic tools. Y. lipolytica can robustly metabolize glucose, glycerol, and even different lipid classes. However, little is known about its xylose metabolizing capability. Given the desirability of having a robust xylose utilizing strain of Y. lipolytica, we performed a comprehensive investigation and elucidation of the existing components of its xylose metabolic pathway.ResultsA quick and efficient means of determining functionality of the candidate xylose pathway genes (XYR, XDH, and XKS) from Y. lipolytica was desirable. We challenged Escherichia coli mutants lacking either the xylose isomerase (xylA) gene or the xylulose kinase (xylB) gene to grow on xylose minimal media by expressing the candidate genes from Y. lipolytica. We showed that the XKS of Y. lipolytica is able to rescue xylose growth of E. coli ΔxylB, and the XDH enabled growth on xylitol, but not on xylose, of E. coli ΔxylA. Overexpression of XKS and XDH in Y. lipolytica improved growth on xylitol, indicating that expression of the native enzymes was limiting. Overexpression of XKS and XDH in Y. lipolytica also enables robust growth on xylose under high nitrogen conditions without the need for adaptation. These results prove that a complete xylose pathway exists in Y. lipolytica, but the pathway is poorly expressed. To elucidate the XYR gene, we applied the E. coli ΔxylA xylose growth challenge with 14 candidate XYR genes and XDH. The XYR2 candidate was able to rescue growth of E. coli ΔxylA xylose on minimal media.ConclusionsWhile a native xylose pathway exists in Y. lipolytica, the microorganism’s inability to grow robustly on xylose is an effect of cryptic genetic circuits that control expression of key enzymes in the metabolic pathway. We have characterized the key enzymes associated with xylose metabolism and demonstrated that gene regulatory issues can be overcome using strong hybrid promoters to attain robust growth on xylose without adaptation.Electronic supplementary materialThe online version of this article (doi:10.1186/s13068-016-0562-6) contains supplementary material, which is available to authorized users.

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Fatty acid has recently received considerable interest as a possible precursor for producing renewable hydrocarbon. In this study, we investigated hydrothermal catalytic deoxygenation of palmitic acid to produce paraffin over a Ni/ZrO 2 catalyst with no or low-pressure (100 psi) external supply of H 2. The results show that the presence of water greatly improved conversion of palmitic acid and paraffin yield. Significant improvement was attributed to the formation of in-situ H 2. Without an external H 2 supply, a 64.2 C% conversion of palmitic acid was achieved in the presence of water, while only a 17.2 C% conversion was achieved without water. The results also show that the presence of water suppressed the side reactions of palmitic acid, specifically ketonization and esterification. We concluded that, compared with decarboxylation and hydrodeoxygenation, decarbonylation was the major route for palmitic acid deoxygenation catalyzed by Ni/ZrO 2. Varieties of shorter-chain paraffin (C 8-C 14) were formed through hydrogenolysis, which also produced a considerable amount of CH 4. A viable reaction pathway for hydrothermal catalytic deoxygenation of palmitic acid in the presence of Ni/ZrO 2 was suggested. The results show that hydrogenolysis and decarbonylation were the major reactions that occurred. This study demonstrates that this hydrothermal catalytic process is a promising approach for producing liquid paraffin (C 8-C 15) from fatty acids under no or low-pressure H 2 .

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Microalgae have attracted growing attention due to their potential in biofuel feedstock production. However, current understanding of the regulatory mechanisms for lipid biosynthesis and storage in microalgae is still limited. This study revealed that the microalga Chlorella sorokiniana showed sequential accumulation of starch and lipids. When nitrogen was replete and/or depleted over a short period, starch was the predominant carbon storage form with basal levels of lipid accumulation. After prolonged nitrogen depletion, lipid accumulation increased considerably, which was partially due to starch degradation, as well as the turnover of primary metabolites. Lipid accumulation is also strongly dependent on the linear electron flow of photosynthesis, peaking at lower light intensities. Collectively, this study reveals a relatively clear regulation pattern of starch and lipid accumulation that is basically controlled by nitrogen levels. The mixotrophic growth of C. sorokiniana shows promise for biofuel production in terms of lipid accumulation in the final biomass.