Xiaochen Yu

Oil production by oleaginous yeasts using the hydrolysate from pretreatment of wheat straw with dilute sulfuric acid

This paper explores the use of the hydrolysate from the dilute sulfuric acid pretreatment of wheat straw for microbial oil production. The resulting hydrolysate was composed of pentoses (24.3g/L) and hexoses (4.9 g/L), along with some other degradation products, such as acetic acid, furfural, and hydroxymethylfurfural (HMF). Five oleaginous yeast strains, Cryptococcus curvatus, Rhodotorula glutinis, Rhodosporidium toruloides, Lipomyces starkeyi, and Yarrowia lipolytica, were evaluated by using this hydrolysate as substrates. The results showed that all of these strains could use the detoxified hydrolysate to produce lipids while except R. toruloides non-detoxified hydrolysate could also be used for the growth of all of the selective yeast strains. C. curvatus showed the highest lipid concentrations in medium on both the detoxified (4.2g/L) and non-detoxified (5.8 g/L) hydrolysates. And the inhibitory effect studies on C. curvatus indicated HMF had insignificant impacts at a concentration of up to 3g/L while furfural inhibited cell growth and lipid content by 72.0% and 62.0% at 1g/L, respectively. Our work demonstrates that lipid production is a promising alternative to utilize hemicellulosic sugars obtained during pretreatment of lignocellulosic materials.

Feasibility of filamentous fungi for biofuel production using hydrolysate from dilute sulfuric acid pretreatment of wheat straw

BackgroundLipids produced from filamentous fungi show great promise for biofuel production, but a major limiting factor is the high production cost attributed to feedstock. Lignocellulosic biomass is a suitable feedstock for biofuel production due to its abundance and low value. However, very limited study has been performed on lipid production by culturing oleaginous fungi with lignocellulosic materials. Thus, identification of filamentous fungal strains capable of utilizing lignocellulosic hydrolysates for lipid accumulation is critical to improve the process and reduce the production cost.ResultsThe growth performances of eleven filamentous fungi were investigated when cultured on glucose and xylose. Their dry cell weights, lipid contents and fatty acid profiles were determined. Six fungal strains with high lipid contents were selected to culture with the hydrolysate from dilute sulfuric acid pretreatment of wheat straw. The results showed that all the selected fungal strains were able to grow on both detoxified liquid hydrolysate (DLH) and non-detoxified liquid hydrolysate (NDLH). The highest lipid content of 39.4% was obtained by Mortierella isabellina on NDLH. In addition, NDLH with some precipitate could help M. isabellina form pellets with an average diameter of 0.11 mm.ConclusionThis study demonstrated the possibility of fungal lipid production from lignocellulosic biomass. M. isabellina was the best lipid producer grown on lignocellulosic hydrolysates among the tested filamentous fungi, because it could not only accumulate oils with a high content by directly utilizing NDLH to simplify the fermentation process, but also form proper pellets to benefit the downstream harvesting. Considering the yield and cost, fungal lipids from lignocellulosic biomass are promising alternative sources for biodiesel production.

Lignocellulosic biomass as a carbohydrate source for lipid production by Mortierella isabellina

Various carbon sources including monosugars, disaccharides and carboxymethyl-cellulose (CMC) were used for single-cell oil production by the filamentous fungus Mortierella isabellina. In addition, the inhibitory effects of lignocellulose-derived compounds (lignin aldehydes, furan aldehydes and weak acid) were investigated. C6 sugars were preferably used for growth compared to C5 sugars. CMC was not an usable substrate, implying the absence of a cellulase system in this fungus. Lignin derivatives showed the most inhibitory effects, but acetic and formic acids at concentrations of 4 g/L improved lipid production, achieving 6.81 ± 0.07 g/L and 6.66 ± 0.33 g/L respectively, which was twice as high as that of the control. A 16.8% lipid yield from hydrolysate suggested that this fungus could be useful for microbial lipid production.

Microbial lipid production from xylose by Mortierella isabellina.

Culture conditions including nitrogen source and concentration, xylose concentration, and inoculum level were evaluated for the effect on cell growth and lipid production of an oleaginous fungus, Mortierella isabellina, grown on xylose. Yeast extract and ammonium sulfate were found to be the best amongst the organic and inorganic nitrogen sources tested, respectively. Subsequent combination of these two nitrogen sources at a nitrogen ratio of 1:1 further enhanced lipid production. The highest cell biomass 28.8 g L(-1) and lipid 18.5 g L(-1) were obtained on a medium containing 100 g L(-1) xylose and 50.4 mM nitrogen with a spore concentration of 10(8) mL(-1). Specifically, nitrogen concentration and inoculum level were demonstrated to be important for obtaining a high lipid yield on xylose consumed of 0.182 g g(-1). The results suggest that M. isabellina holds great potential to be a candidate for biofuel production from xylose, the second most abundant sugar from lignocellulose.

Induction of D-xylose uptake and expression of NAD(P)H-linked xylose reductase and NADP + -linked xylitol dehydrogenase in the oleaginous microalga Chlorella sorokiniana

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.

Sequential hydrothermal fractionation of yeast Cryptococcus curvatus biomass.

A sequential hydrothermal liquefaction (SEQHTL) process was evaluated in this work for fractionating different component of yeast biomass. Sugar and protein were separated first at a lower temperature, and the remaining biomass was then converted to bio-oil at a higher temperature. The separated aqueous products were investigated to be recycled as a carbon and nitrogen sources for the yeast culture. In the first step of SEQHTL, the temperature effect on the yield of sugar/protein and inhibitory compounds (acetic acid and 5-hydroxymethyl furfural (5HMF)) was investigated. The highest yields of sugar and protein and a minimal level of inhibitory compounds were obtained at 180°C. At the second step of SEQHTL, the highest bio-oil yield was achieved at 240°C. In comparison to the one-step hydrothermal liquefaction process, SEQHTL produced a higher quality bio-oil with higher fatty acid and lower nitrogen contents.

Improved lipid accumulation by morphology engineering of oleaginous fungus Mortierella isabellina

Oleaginous fungi capable of accumulating a considerable amount of lipids are promising sources for lipid‐based biofuel production. The specific productivities of filamentous fungi in submerged fermentation are often correlated with morphological forms. However, the relationship between morphological development and lipid accumulation is not known. In this study, distinct morphological forms of oleaginous fungus Mortierella isabellina including pellets of different sizes, free dispersed mycelia, and broken hyphal fragments were developed by additions of different concentrations of magnesium silicate microparticles. Different morphological forms led to different levels of lipid accumulation as well as different spatial patterns of lipid distribution within pellets/mycelial aggregates. Significant higher lipid content (0.75 g lipid/g cell biomass) and lipid yield (0.18 g lipid/g glucose consumed) were achieved in free dispersed mycelia than in pellets. Moreover, extracellular metabolite analysis showed that production of undesirable by‐product malate was repressed in free dispersed mycelium form. Unveiling the desired morphological form of M. isabellina for lipid accumulation provided insights into molecular mechanism of lipid biosynthesis linked with morphological development, as well as design and optimization of bioprocess to produce lipid‐based biofuels. Biotechnol. Bioeng. 2014;111: 1758–1766.

Selective esterification to produce microalgal biodiesel and enrich polyunsaturated fatty acid using zeolite as a catalyst

Microalgae can be both a promising biofuel feedstock and a source of polyunsaturated fatty acids (PUFA). This paper reports a novel integrated process that simultaneously produces biodiesel and enriches PUFA. It was accomplished by using zeolite as a selective catalyst that preferentially converts shorter-chain fatty acids (SCFA) into fatty acid methyl esters (FAME) (86% conversion for S. limacinum and 65% conversion for N. salina) and enriches high-value PUFA (70% for S. limacinum and 78% for N. salina) in the unreacted free fatty acid (FFA) stream. The esterification reaction rate was affected by acid strength and pore size, while the selectivity of zeolite increased as pore size of zeolite decreased. This approach allows production of high quality biodiesel and efficient PUFA enrichment. The unreacted PUFA can be further refined for nutraceutical or other applications to improve economic viability of microalgal biodiesel production.