Abstract
BackgroundThe heterotrophic and mixotrophic culture of oleaginous microalgae is a promising process to produce biofuel feedstock due to the advantage of fast growth. Various organic carbons have been explored for this application. However, despite being one of the most abundant and economical sugar resources in nature, D-xylose has never been demonstrated as a carbon source for wild-type microalgae. The purpose of the present work was to identify the feasibility of D-xylose utilization by the oleaginous microalga Chlorella sorokiniana.ResultsThe sugar uptake kinetic analysis was performed with 14C-labeled sugars and the data showed that the D-glucose induced algal cells (the alga was heterotrophically grown on D-glucose and then harvested as D-glucose induced cells) exhibited a remarkably increased D-xylose uptake rate. The maximum D-xylose transport rate was 3.8 nmol min−1 mg−1 dry cell weight (DCW) with Km value of 6.8 mM. D-xylose uptake was suppressed in the presence of D-glucose, D-galactose and D-fructose but not L-arabinose and D-ribose. The uptake of D-xylose activated the related metabolic pathway, and the activities of a NAD(P)H-linked xylose reductase (XR) and a unique NADP+-linked xylitol dehydrogenase (XDH) were detected in C. sorokiniana. Compared with the culture in the dark, the consumption of D-xylose increased 2 fold under light but decreased to the same level with addition of 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU), indicating that extra chemical energy from the light-dependent reaction contributed the catabolism of D-xylose for C. sorokiniana.ConclusionsAn inducible D-xylose transportation system and a related metabolic pathway were discovered for microalga for the first time. The transportation of D-xylose across the cell membrane of C. sorokiniana could be realized by an inducible hexose symporter. The uptake of D-xylose subsequently activated the expression of key catalytic enzymes that enabled D-xylose entering central metabolism. Results of this research are useful to better understand the D-xylose metabolic pathway in the microalga C. sorokiniana and provide a target for genetic engineering to improve D-xylose utilization for microalgal lipid production.
Highlights
The heterotrophic and mixotrophic culture of oleaginous microalgae is a promising process to produce biofuel feedstock due to the advantage of fast growth
D-xylose uptake in C. sorokiniana The consumption of D-xylose by C. sorokiniana was observed in the D-glucose induced algal cells
Non-linear regression analysis showed that the maximum D-xylose transport rate was 3.8 nmol min−1 mg−1 dry cell weight (DCW)with Km value of 6.8 mM
Summary
The heterotrophic and mixotrophic culture of oleaginous microalgae is a promising process to produce biofuel feedstock due to the advantage of fast growth. Microalgal biomass is considered as potential feedstock for making biofuels and high-value chemicals [1,2]. Based on the analysis of energetics and carbon metabolism during microalgae growth, Yang et al [6] demonstrated that heterotrophic and mixotrophic cultures produced more ATP and biomass from the energy supplied than the phototrophic culture. Some low value materials have been explored in order to reduce the cost, such as crude glycerol, acetate, food waste and cassava starch [4,7,8], the availability and sustainability of the organic carbon supplies remain limiting factors for large-scale applications
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