Fermentation of levoglucosan with oleaginous yeasts for lipid production

Bioupgrading of the aqueous phase of pyrolysis oil from lignocellulosic biomass: a platform for renewable chemicals and fuels from the whole fraction of biomass

Pyrolysis, a thermal decomposition without oxygen, is a promising technology for transportable liquids from whole fractions of lignocellulosic biomass. However, due to the hydrophilic products of pyrolysis, the liquid oils have undesirable physicochemical characteristics, thus requiring an additional upgrading process. Biological upgrading methods could address the drawbacks of pyrolysis by utilizing various hydrophilic compounds as carbon sources under mild conditions with low carbon footprints. Versatile chemicals, such as lipids, ethanol, and organic acids, could be produced through microbial assimilation of anhydrous sugars, organic acids, aldehydes, and phenolics in the hydrophilic fractions. The presence of various toxic compounds and the complex composition of the aqueous phase are the main challenges. In this review, the potential of bioconversion routes for upgrading the aqueous phase of pyrolysis oil is investigated with critical challenges and perspectives.

Rhodotorula sp.-based biorefinery: a source of valuable biomolecules

The development of an effective, realistic, and sustainable microbial biorefinery depends on several factors, including as one of the key aspects an adequate selection of microbial strain. The oleaginous red yeast Rhodotorula sp. has been studied as one powerful source for a plethora of high added-value biomolecules, such as carotenoids, lipids, and enzymes. Although known for over a century, the use of Rhodotorula sp. as resource for valuable products has not yet commercialized. Current interests for Rhodotorula sp. yeast have sparked from its high nutritional versatility and ability to convert agro-food residues into added-value biomolecules, two attractive characteristics for designing new biorefineries. In addition, as for other yeast-based bioprocesses, the overall process sustainability can be maximized by a proper integration with subsequent downstream processing stages, for example, by using eco-friendly solvents for the recovery of intracellular products from yeast biomass. This review intends to reflect on the current state of the art of microbial bioprocesses using Rhodotorula species. Therefore, we will provide an analysis of bioproduction performance with some insights regarding downstream separation steps for the extraction of high added-value biomolecules (specifically using efficient and sustainable platforms), providing information regarding the potential applications of biomolecules produced by Rhodotorula sp, as well as detailing the strengths and limitations of yeast-based biorefinery approaches. Novel genetic engineering technologies are further discussed, indicating some directions on their possible use for maximizing the potential of Rhodotorula sp. as cell factories. KEY POINTS: • Rhodotorula sp. are valuable source of high value-added compounds. • Potential of employing Rhodotorula sp. in a multiple product biorefinery. • Future perspectives in the biorefining of Rhodotorula sp. were discussed.

Both levoglucosan kinase activity and transport capacity limit the utilization of levoglucosan in Saccharomyces cerevisiae

Manufacturing fuels and chemicals from cellulose materials is a promising strategy to achieve carbon neutralization goals. In addition to the commonly used enzymatic hydrolysis by cellulase, rapid pyrolysis is another way to degrade cellulose. The sugar obtained by fast pyrolysis is not glucose, but rather its isomer, levoglucosan (LG). Here, we revealed that both levoglucosan kinase activity and the transportation of levoglucosan are bottlenecks for LG utilization in Saccharomyces cerevisiae, a widely used cell factory. We revealed that among six heterologous proteins that had levoglucosan kinase activity, the 1,6-anhydro-N-acetylmuramic acid kinase from Rhodotorula toruloides was the best choice to construct levoglucosan-utilizing S. cerevisiae strain. Furthermore, we revealed that the amino acid residue Q341 and W455, which were located in the middle of the transport channel closer to the exit, are the sterically hindered barrier to levoglucosan transportation in Gal2p, a hexose transporter. The engineered yeast strain expressing the genes encoding the 1,6-anhydro-N-acetylmuramic acid kinase from R. toruloides and transporter mutant Gal2p^Q341A or Gal2p^W455A consumed ~ 4.2 g L⁻¹ LG in 48 h, which is the fastest LG-utilizing S. cerevisiae strain to date.

Rhodotorula toruloides: an ideal microbial cell factory to produce oleochemicals, carotenoids, and other products

Requirement of clean energy sources urges us to find substitutes for fossil fuels. Microorganisms provide an option to produce feedstock for biofuel production by utilizing inexpensive, renewable biomass. Rhodotorula toruloides (Rhodosporidium toruloides), a non-conventional oleaginous yeast, can accumulate intracellular lipids (single cell oil, SCO) more than 70% of its cell dry weight. At present, the SCO-based biodiesel is not a price-competitive fuel to the petroleum diesel. Many efforts are made to cut the cost of SCO by strengthening the performance of genetically modified R. toruloides strains and by valorization of low-cost biomass, including crude glycerol, lignocellulosic hydrolysates, food and agro waste, wastewater, and volatile fatty acids. Besides, optimization of fermentation and SCO recovery processes are carefully studied as well. Recently, new R. toruloides strains are developed via metabolic engineering and synthetic biology methods to produce value-added chemicals, such as sesquiterpenes, fatty acid esters, fatty alcohols, carotenoids, and building block chemicals. This review summarizes recent advances in the main aspects of R. toruloides studies, namely, construction of strains with new traits, valorization of low-cost biomass, process detection and optimization, and product recovery. In general, R. toruloides is a promising microbial cell factory for production of biochemicals.

Microbial lipid biosynthesis from lignocellulosic biomass pyrolysis products

Lipids are a biorefinery platform to prepare fuel, food and health products. They are traditionally obtained from plants, but those of microbial origin allow for a better use of land and C resources, among other benefits. Several (thermo)chemical and biochemical strategies are used for the conversion of C contained in lignocellulosic biomass into lipids. In particular, pyrolysis can process virtually any biomass and is easy to scale up. Products offer cost-effective, renewable C in the form of readily fermentable molecules and other upgradable intermediates. Although the production of microbial lipids has been studied for 30 years, their incorporation into biorefineries was only described a few years ago. As pyrolysis becomes a profitable technology to depolymerize lignocellulosic biomass into assimilable C, the number of investigations on it raises significantly. This article describes the challenges and opportunities resulting from the combination of lignocellulosic biomass pyrolysis and lipid biosynthesis with oleaginous microorganisms. First, this work presents the basics of the individual processes, and then it shows state-of-the-art processes for the preparation of microbial lipids from biomass pyrolysis products. Advanced knowledge on separation techniques, structure analysis, and fermentability is detailed for each biomass pyrolysis fraction. Finally, the microbial fatty acid platform comprising biofuel, human food and animal feed products, and others, is presented. Literature shows that the microbial lipid production from anhydrosugars, like levoglucosan, and short-chain organic acids, like acetic acid, is straightforward. Indeed, processes achieving nearly theoretical yields form the latter have been described. Some authors have shown that lipid biosynthesis from different lignin sources is biochemically feasible. However, it still imposes major challenges regarding strain performance. No report on the fermentation of pyrolytic lignin is yet available. Research on the microbial uptake of pyrolytic humins remains vacant. Microorganisms that make use of methane show promising results at the proof-of-concept level. Overall, despite some issues need to be tackled, it is now possible to conceive new versatile biorefinery models by combining lignocellulosic biomass pyrolysis products and robust oleaginous microbial cell factories.

Lipid production by oleaginous yeasts

Microbial lipid production has been studied extensively for years; however, lipid metabolic engineering in many of the extraordinarily high lipid-accumulating yeasts was impeded by inadequate understanding of the metabolic pathways including regulatory mechanisms defining their oleaginicity and the limited genetic tools available. The aim of this review is to highlight the prominent oleaginous yeast genera, emphasizing their oleaginous characteristics, in conjunction with diverse other features such as cheap carbon source utilization, withstanding the effect of inhibitory compounds, commercially favorable fatty acid composition-all supporting their future development as economically viable lipid feedstock. The unique aspects of metabolism attributing to their oleaginicity are accentuated in the pretext of outlining the various strategies successfully implemented to improve the production of lipid and lipid-derived metabolites. A large number of in silico data generated on the lipid accumulation in certain oleaginous yeasts have been carefully curated, as suggestive evidences in line with the exceptional oleaginicity of these organisms. The different genetic elements developed in these yeasts to execute such strategies have been scrupulously inspected, underlining the major types of newly-found and synthetically constructed promoters, transcription terminators, and selection markers. Additionally, there is a plethora of advanced genetic toolboxes and techniques described, which have been successfully used in oleaginous yeasts in the recent years, promoting homologous recombination, genome editing, DNA assembly, and transformation at remarkable efficiencies. They can accelerate and effectively guide the rational designing of system-wide metabolic engineering approaches pinpointing the key targets for developing industrially suitable yeast strains.

Fast pyrolysis as a tool for obtaining levoglucosan after pretreatment of biomass with niobium catalysts

Levoglucosan (LGA) is a promising chemical platform derived from the pyrolysis of biomass that offers access to a variety of value-added products. We report an efficient route to produce LGA via the pretreatment of biomass with niobium compounds (oxalate, chloride and oxide) followed by fast pyrolysis coupled with gas chromatography-mass spectrometry (Py-GC-MS) at temperatures of 350-600 °C. Catalytic pretreatment reduces the quantity of lignin in the biomass, concentrates the cellulose and enhance LGA formation during fast pyrolysis. The pretreatment also removes alkaline metals, preventing competitive side reactions. The effect of several parameters such as catalyst weight, time, temperature, and solvent, with the optimal pretreatment conditions determined to be 3 (wt.%) niobium oxalate for 1 h at 23 °C in water. Pretreatment increased the LGA yields by 6.40-fold for sugarcane bagasse, 4.15-fold for elephant grass, 4.13-fold for rice husk, 2.86-fold for coffee husk, and 1.86-fold for coconut husk as compared to the raw biomasses. These results indicate that biomass pretreatment using niobium derivates prior fast pyrolysis can be a promising technique for biomass thermochemical conversion in LGA and others important pyrolytic products.

Catalytic Hydrotreatment of the Pyrolytic Sugar and Pyrolytic Lignin Fractions of Fast Pyrolysis Liquids Using Nickel Based Catalysts

Catalytic hydrotreatment is recognized as an efficient method to improve the properties of pyrolysis liquids (PO) to allow co-feeding with fossil fuels in conventional refinery units. The promising catalyst recipes identified so far are catalysts with high nickel contents (38 to 57 wt.%), promoted by Cu, Pd, Mo and/or a combination, and supported by SiO2, SiO2-ZrO2, SiO2-ZrO2-La2O3 or SiO2-Al2O3. To gain insights into the reactivity of the pyrolytic sugar (PS) and pyrolytic lignin (PL) fraction of PO, hydrotreatment studies (350 °C, 120 bar H2 pressure (RT) for 4 h) were performed in a batch autoclave. Catalyst performance was evaluated by considering the product properties (H/C ratio, the charring tendency (TGA) and molecular weight distribution (GPC)) and the results were compared with a benchmark Ru/C catalyst. All Ni based catalysts gave products oils with a higher H/C compared to Ru/C. The Mo promoted catalyst performed best, giving a product with the highest H/C ratio (1.54) and the lowest TG residue (0.8 wt.% compared to 12 wt.% for the fresh PS). The results further revealed that the PS fraction is highly reactive and full conversion was achieved at 350 °C. In contrast, the PL fraction was rather inert, and only part of the PL fraction was converted. The fresh and spent catalysts after the hydrotreatment of the PS and PL fractions were characterized by elemental analysis, powder X-Ray diffraction (XRD) and high resolution transmission electron microscopy (HRTEM-EDX). The results revealed that the use of PS as the feed leads to higher amounts of coke deposits on the catalysts, and higher levels of Ni agglomeration when compared to experiments with PL and pure PO. This proofs that proper catalyst selection for the PS fraction is of higher importance than for the PL fraction. The Mo promoted Ni catalysts showed the lowest amount of coke and the lowest tendency for Ni nanoparticle agglomeration compared to the monometallic Ni and bimetallic Ni-Cu catalysts.

Oil-lipids, carotenoids and fatty acids simultaneous production by Rhodotorula mucilaginosa CCT3892 using sugarcane molasses as carbon source

Abstract One of the most important classes of nutritional biomolecules is the oleaginous compounds group, which specially includes the oil-lipids, the carotenoids and the fatty acids. These biocompounds present a wide range of industrial applications because their ability to act as an energy source, antioxidants and metabolic agents for the human body. Therefore, the food industry, mainly focusing on food supplementation, is always searching for new sources of them. In this context, the present study evaluated the total lipids, carotenoids and fatty acids simultaneous production by the Rhodotorula mucilaginosa CCT3892 yeast, using residual sugarcane molasses as carbon source. The results obtained demonstrated that the cultivation of yeast in molasses medium (MC) produced the same content of total lipids and carotenoids (16.50% ± 0.68% and 0.053 ± 0.001 mg g-1, respectively) as the obtained from a synthetic medium (SC) (15.36% ± 1.36% and 0.051 ± 0.001 mg g-1 0.005). Concerning the fatty acids biosynthesis, the MC cultivation generated the most interesting profile once it presented a greater content of oleic acid (74.05%), an unsaturated compound with high nutritional value. The cultivation carried out with the molasses and yeast extract supplementation (MYEC) did not provide an improvement in microbial oil production, what indicated that in this condition there was a predominance of others sorts of substrate metabolization by the yeast cells, as confirmed by the microbial kinetics study.

Heterologous rhamnolipid biosynthesis by P. putida KT2440 on bio-oil derived small organic acids and fractions

In many cases in industrial biotechnology, substrate costs make up a major part of the overall production costs. One strategy to achieve more cost-efficient processes in general is to exploit cheaper sources of substrate. Small organic acids derived from fast pyrolysis of lignocellulosic biomass represent a significant proportion of microbially accessible carbon in bio-oil. However, using bio-oil for microbial cultivation is a highly challenging task due to its strong adverse effects on microbial growth as well as its complex composition. In this study, the suitability of bio-oil as a substrate for industrial biotechnology was investigated with special focus on organic acids. For this purpose, using the example of the genetically engineered, non-pathogenic bacterium Pseudomonas putida KT2440 producing mono-rhamnolipids, cultivation on small organic acids derived from fast pyrolysis of lignocellulosic biomass, as well as on bio-oil fractions, was investigated and evaluated. As biosurfactants, rhamnolipids represent a potential bulk product of industrial biotechnology where substitution of traditional carbon sources is of conceivable interest. Results suggest that maximum achievable productivities as well as substrate-to-biomass yields are in a comparable range for glucose, acetate, as well as the mixture of acetate, formate and propionate. Similar yields were obtained for a pretreated bio-oil fraction, which was used as reference real raw material, although with significantly lower titers. As such, the reported process constitutes a proof-of-principle for using bio-oil as a potential cost-effective alternative carbon source in a future bio-based economy.

A comparative investigation of fast pyrolysis with enzymatic hydrolysis for fermentable sugars production from cellulose

In this study, ball milling and ionic liquid pretreatments were utilized to alter cellulose structure prior to fast pyrolysis and enzymatic hydrolysis. The variations in the products distribution of cellulose fast pyrolysis, and their dependence on the structure of cellulose, and the temperature of fast pyrolysis were illustrated. Fast pyrolysis of pretreated cellulose yielded more levoglucosan than crystalline cellulose (14.7%) at 300 °C. Nevertheless, the levoglucosan achieved higher yield (64.3%) from crystalline cellulose at 400 °C. At last, a comparison between fast pyrolysis and enzymatic hydrolysis for cellulose saccharifaction was made. Fast pyrolysis was a promising alternative to liberate levoglucosan from cellulose. Further investigation and development were required to maximize the levoglucosan production.

Identification, functional characterization, and crystal structure determination of bacterial levoglucosan dehydrogenase

Levoglucosan is the 1,6-anhydrosugar of d-glucose formed by pyrolysis of glucans and is found in the environment and industrial waste. Two types of microbial levoglucosan metabolic pathways are known. Although the eukaryotic pathway involving levoglucosan kinase has been well-studied, the bacterial pathway involving levoglucosan dehydrogenase (LGDH) has not been well-investigated. Here, we identified and cloned the lgdh gene from the bacterium Pseudarthrobacter phenanthrenivorans and characterized the recombinant protein. The enzyme exhibited high substrate specificity toward levoglucosan and NAD+ for the oxidative reaction and was confirmed to be LGDH. LGDH also showed weak activities (∼4%) toward l-sorbose and 1,5-anhydro-d-glucitol. The reverse (reductive) reaction using 3-keto-levoglucosan and NADH exhibited significantly lower Km and higher kcat values than those of the forward reaction. The crystal structures of LGDH in the apo and complex forms with NADH, NADH + levoglucosan, and NADH + l-sorbose revealed that LGDH has a typical fold of Gfo/Idh/MocA family proteins, similar to those of scyllo-inositol dehydrogenase, aldose-aldose oxidoreductase, 1,5-anhydro-d-fructose reductase, and glucose-fructose oxidoreductase. The crystal structures also disclosed that the active site of LGDH is distinct from those of these enzymes. The LGDH active site extensively recognized the levoglucosan molecule with six hydrogen bonds, and the C3 atom of levoglucosan was closely located to the C4 atom of NADH nicotinamide. Our study is the first molecular characterization of LGDH, providing evidence for C3-specific oxidation and representing a starting point for future biotechnological use of LGDH and levoglucosan-metabolizing bacteria.

The Engineering Potential of Rhodosporidium toruloides as a Workhorse for Biotechnological Applications

Moving our society towards a bioeconomy requires efficient and sustainable microbial production of chemicals and fuels. Rhodotorula (Rhodosporidium) toruloides is a yeast that naturally synthesizes substantial amounts of specialty chemicals and has been recently engineered to (i) enhance its natural production of lipids and carotenoids, and (ii) produce novel industrially relevant compounds. The use of R. toruloides by companies and research groups has exponentially increased in recent years as a result of recent improvements in genetic engineering techniques and the availability of multiomics information on its genome and metabolism. This review focuses on recent engineering approaches in R. toruloides for bioproduction and explores its potential as a biotechnological chassis.

Effect of Glycerol Pretreatment on Levoglucosan Production from Corncobs by Fast Pyrolysis

In this manuscript, glycerol was used in corncobs' pretreatment to promote levoglucosan production by fast pyrolysis first and then was further utilized as raw material for chemicals production by microbial fermentation. The effects of glycerol pretreatment temperatures (220⁻240 °C), time (0.5⁻3 h) and solid-to-liquid ratios (5⁻20%) were investigated. Due to the accumulation of crystalline cellulose and the removal of minerals, the levoglucosan yield was as high as 35.8% from corncobs pretreated by glycerol at 240 for 3 h with a 5% solid-to-liquid ratio, which was obviously higher than that of the control (2.2%). After glycerol pretreatment, the fermentability of the recovered glycerol remaining in the liquid stream from glycerol pretreatment was evaluated by Klebsiella pneumoniae. The results showed that the recovered glycerol had no inhibitory effect on the growth and metabolism of the microbe, which was a promising substrate for fermentation. The value-added applications of glycerol could reduce the cost of biomass pretreatment. Correspondingly, this manuscript offers a green, sustainable, efficient and economic strategy for an integrated biorefinery process.

Conversion and assimilation of furfural and 5-(hydroxymethyl)furfural by Pseudomonas putida KT2440

The sugar dehydration products, furfural and 5-(hydroxymethyl)furfural (HMF), are commonly formed during high-temperature processing of lignocellulose, most often in thermochemical pretreatment, liquefaction, or pyrolysis. Typically, these two aldehydes are considered major inhibitors in microbial conversion processes. Many microbes can convert these compounds to their less toxic, dead-end alcohol counterparts, furfuryl alcohol and 5-(hydroxymethyl)furfuryl alcohol. Recently, the genes responsible for aerobic catabolism of furfural and HMF were discovered in Cupriavidus basilensis HMF14 to enable complete conversion of these compounds to the TCA cycle intermediate, 2-oxo-glutarate. In this work, we engineer the robust soil microbe, Pseudomonas putida KT2440, to utilize furfural and HMF as sole carbon and energy sources via complete genomic integration of the 12 kB hmf gene cluster previously reported from Burkholderia phytofirmans. The common intermediate, 2-furoic acid, is shown to be a bottleneck for both furfural and HMF metabolism. When cultured on biomass hydrolysate containing representative amounts of furfural and HMF from dilute-acid pretreatment, the engineered strain outperforms the wild type microbe in terms of reduced lag time and enhanced growth rates due to catabolism of furfural and HMF. Overall, this study demonstrates that an approach for biological conversion of furfural and HMF, relative to the typical production of dead-end alcohols, enables both enhanced carbon conversion and substantially improves tolerance to hydrolysate inhibitors. This approach should find general utility both in emerging aerobic processes for the production of fuels and chemicals from biomass-derived sugars and in the biological conversion of high-temperature biomass streams from liquefaction or pyrolysis where furfural and HMF are much more abundant than in biomass hydrolysates from pretreatment.

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HMF and furfural are common microbial inhibitors in biomass conversion.

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HMF and furfural gene cluster was isolated from Burkholderia phytofirmans..

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We heterologously express the HMF/furfural gene cluster in Pseudomonas putida..

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Expression enables cultivation on HMF and furfural as a sole carbon source.

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Expression also enables enhanced conversion on lignocellulosic hydrolysate.

Anaerobic batch conversion of pine wood torrefaction condensate

Organic compound rich torrefaction condensate, owing to their high water content and acidic nature, have yet to be exploited for practical application. In this study, microbial conversion of torrefaction condensate from pine wood through anaerobic batch digestion (AD) to produce methane was evaluated. Torrefaction condensate exhibited high methane potentials in the range of 430-492mL/g volatile solids (VS) and 430-460mL/gVS under mesophilic and thermophilic conditions, respectively. Owing to the changes in the composition, the methane yields differed with the torrefaction condensates produced at different temperatures (225, 275 and 300°C), with a maximum of 492±18mL/gVS with the condensate produced at 300°C under mesophilic condition. The cyclic batch AD experiments showed that 0.1VS_substrate:VS_inoculum is optimum, whereas the higher substrate loading (0.2-0.5) resulted in a reversible inhibition of the methane production. The results suggest that torrefaction condensate could be practically valorized through AD.

Effect of acid additives on sugarcane bagasse pyrolysis: Production of high yields of sugars

The aim of this work was to improve sugarcane bagasse thermochemical conversion to pyrolytic sugars production, particularly to levoglucosan. The experiments were carried out evaluating the effect of acid washing with HNO₃ (0.1wt.%) followed by H₂SO₄ addition (0.1, 0.2 and 0.3wt.%) at pyrolysis temperatures of 350, 400, 450, 500, 550 and 600°C was studied by Py-GC/MS. The experimental results showed that HNO₃ washing, followed by H₂SO₄ concentration of 0.2wt.% at 350°C resulted in an increase in levoglucosan yield between 5 and 7 times the yield obtained when the raw bagasse was processed. Thus, these results are very attractive to improve pyrolytic sugars production in sugarcane bagasse by previously acid treatment to pyrolysis technology.

Comparison of ethanol production from corn cobs and switchgrass following a pyrolysis-based biorefinery approach

BACKGROUND

One of the main obstacles in lignocellulosic ethanol production is the necessity of pretreatment and fractionation of the biomass feedstocks to produce sufficiently pure fermentable carbohydrates. In addition, the by-products (hemicellulose and lignin fraction) are of low value, when compared to dried distillers grains (DDG), the main by-product of corn ethanol. Fast pyrolysis is an alternative thermal conversion technology for processing biomass. It has recently been optimized to produce a stream rich in levoglucosan, a fermentable glucose precursor for biofuel production. Additional product streams might be of value to the petrochemical industry. However, biomass heterogeneity is known to impact the composition of pyrolytic product streams, as a complex mixture of aromatic compounds is recovered with the sugars, interfering with subsequent fermentation. The present study investigates the feasibility of fast pyrolysis to produce fermentable pyrolytic glucose from two abundant lignocellulosic biomass sources in Ontario, switchgrass (potential energy crop) and corn cobs (by-product of corn industry).

RESULTS

Demineralization of biomass removes catalytic centers and increases the levoglucosan yield during pyrolysis. The ash content of biomass was significantly decreased by 82-90% in corn cobs when demineralized with acetic or nitric acid, respectively. In switchgrass, a reduction of only 50% for both acids could be achieved. Conversely, levoglucosan production increased 9- and 14-fold in corn cobs when rinsed with acetic and nitric acid, respectively, and increased 11-fold in switchgrass regardless of the acid used. After pyrolysis, different configurations for upgrading the pyrolytic sugars were assessed and the presence of potentially inhibitory compounds was approximated at each step as double integral of the UV spectrum signal of an HPLC assay. The results showed that water extraction followed by acid hydrolysis and solvent extraction was the best upgrading strategy. Ethanol yields achieved based on initial cellulose fraction were 27.8% in switchgrass and 27.0% in corn cobs.

CONCLUSIONS

This study demonstrates that ethanol production from switchgrass and corn cobs is possible following a combined thermochemical and fermentative biorefinery approach, with ethanol yields comparable to results in conventional pretreatments and fermentation processes. The feedstock-independent fermentation ability can easily be assessed with a simple assay.

The integration of dilute acid hydrolysis of xylan and fast pyrolysis of glucan to obtain fermentable sugars

BACKGROUND

Fermentable sugars are important intermediates in the biological conversion of biomass. Hemicellulose and amorphous cellulose are easily hydrolyzed to fermentable sugars in dilute acid, whereas crystalline cellulose is more difficult to be hydrolyzed. Cellulose fast pyrolysis is an alternative method to liberate valuable fermentable sugars from biomass. The amount of levoglucosan generated from lignocellulose by fast pyrolysis is usually lower than the theoretical yield based on the cellulose fraction. Pretreatment is a promising route to improve the yield of levoglucosan from lignocellulose.

RESULTS

The integration of dilute sulfuric acid hydrolysis and fast pyrolysis to obtain fermentable sugars was evaluated in this study. Dilute sulfuric acid hydrolysis could remove more than 95.1 and 93.4 % of xylan (the main component of hemicellulose) from sugarcane bagasse and corncob with high yield of xylose. On the other hand, dilute sulfuric acid hydrolysis was also an effective pretreatment to enhance levoglucosan yield from lignocellulose. Dilute acid hydrolysis could accumulate glucan (the component of cellulose) and remove most of the alkali and alkaline earth metals which were powerful catalysts during fast pyrolysis. Further increase in dilute acid concentration (from 0 to 2 %) in pretreatment could promote the yield of levoglucosan in fast pyrolysis. The acid pretreated sugarcane bagasse and corncob gave levoglucosan yields of 43.8 and 35.2 % which were obvious higher than those of raw sugarcane bagasse (12.0 %) and corncob (7.0 %).

CONCLUSIONS

Obtaining fermentable sugars by combination dilute acid hydrolysis of xylan and fast pyrolysis of glucan could make full utilization of biomass, and get fermentable sugars economically from biomass for bio-refinery.

Engineering levoglucosan metabolic pathway in Rhodococcus jostii RHA1 for lipid production

Oleaginous strains of Rhodococcus including R. jostii RHA1 have attracted considerable attention due to their ability to accumulate triacylglycerols (TAGs), robust growth properties and genetic tractability. In this study, a novel metabolic pathway was introduced into R. jostii by heterogenous expression of the well-characterized gene, lgk encoding levoglucosan kinase from Lipomyces starkeyi YZ-215. This enables the recombinant R. jostii RHA1 to produce TAGs from the anhydrous sugar, levoglucosan, which can be generated efficiently as the major molecule from the pyrolysis of cellulose. The recombinant R. jostii RHA1 could grow on levoglucosan as the sole carbon source, and the consumption rate of levoglucosan was determined. Furthermore, expression of one more copy of lgk increased the enzymatic activity of LGK in the recombinant. However, the growth performance of the recombinant bearing two copies of lgk on levoglucosan was not improved. Although expression of lgk in the recombinants was not repressed by the glucose present in the media, glucose in the sugar mixture still affected consumption of levoglucosan. Under nitrogen limiting conditions, lipid produced from levoglucosan by the recombinant bearing lgk was up to 43.54 % of the cell dry weight, which was comparable to the content of lipid accumulated from glucose. This work demonstrated the technical feasibility of producing lipid from levoglucosan, an anhydrosugar derived from the pyrolysis of lignocellulosic materials, by the genetically modified rhodococci strains.

Lipid accumulation from pinewood pyrolysates by Rhodosporidium diobovatum and Chlorella vulgaris for biodiesel production

This study evaluated the suitability of pinewood pyrolysates as a carbon source for lipid production and cultivation of the oleaginous yeast Rhodosporidium diobovatum and the microalgae Chlorella vulgaris. Thermal decomposition of pinewood and fractional condensation were used to obtain an oil rich in levoglucosan which was upgraded to glucose by acid hydrolysis. Blending of pyrolytic sugars with pure glucose in both nitrogen rich and nitrogen limited conditions was studied for R. diobovatum, and under nitrogen limited conditions for C. vulgaris. Glucose consumption rate decreased with increasing proportions of pyrolytic sugars increasing cultivation time. While R. diobovatum was capable of growth in 100% (v/v) pyrolytic sugars, C. vulgaris growth declined rapidly in blends greater than 20% (v/v) until no growth was detected in blends >40%. Finally, the effects of pyrolysis sugars on lipid composition was evaluated and biodiesel fuel properties were estimated based on the lipid profiles.

Bioconversion of anhydrosugars: Emerging concepts and strategies

As methods for the use of anhydrosugars in chemical and biofuel production continue to develop, our collective knowledge of anhydrosugar processing enzymes continues to improve, including their mechanistic details, structural dynamics and modes of substrate binding. Of particular interest, anhydrosugar kinases, such as levoglucosan kinase (LGK) and 1,6-anhydro-N-acetylmuramic acid kinase (AnmK), utilize an unusual mechanism whereby the sugar substrate is both cleaved and phosphorylated. The phosphorylated sugar can then be routed to other metabolic pathways, thereby allowing its further bioconversion. Advanced engineering efforts to improve the catalytic efficiency and stability of LGK have been steadily progressing. Other enzymes that cleave the glycosidic bond of disaccharide sugars containing an anhydrosugar component are also being identified and characterized. Accordingly, the potential future use of these enzymes in large-scale production strategies is becoming increasingly viable. Here, a mini-review of the observed characteristics of anhydrosugar processing enzymes is presented along with recent developments in the bioconversion of these sugars. © 2016 IUBMB Life 68(9):700-708, 2016.

Rhodotorula glutinis-potential source of lipids, carotenoids, and enzymes for use in industries

Rhodotorula glutinis is capable of synthesizing numerous valuable compounds with a wide industrial usage. Biomass of this yeast constitutes sources of microbiological oils, and the whole pool of fatty acids is dominated by oleic, linoleic, and palmitic acid. Due to its composition, the lipids may be useful as a source for the production of the so-called third-generation biodiesel. These yeasts are also capable of synthesizing carotenoids such as β-carotene, torulene, and torularhodin. Due to their health-promoting characteristics, carotenoids are commonly used in the cosmetic, pharmaceutical, and food industries. They are also used as additives in fodders for livestock, fish, and crustaceans. A significant characteristic of R. glutinis is its capability to produce numerous enzymes, in particular, phenylalanine ammonia lyase (PAL). This enzyme is used in the food industry in the production of l-phenylalanine that constitutes the substrate for the synthesis of aspartame—a sweetener commonly used in the food industry.

Production of biorenewable styrene: utilization of biomass-derived sugars and insights into toxicity

Fermentative production of styrene from glucose has been previously demonstrated in Escherichia coli. Here, we demonstrate the production of styrene from the sugars derived from lignocellulosic biomass depolymerized by fast pyrolysis. A previously engineered styrene-producing strain was further engineered for utilization of the anhydrosugar levoglucosan via expression of levoglucosan kinase. The resulting strain produced 240 ± 3 mg L−1 styrene from pure levoglucosan, similar to the 251 ± 3 mg L−1 produced from glucose. When provided at a concentration of 5 g L−1, pyrolytic sugars supported styrene production at titers similar to those from pure sugars, demonstrating the feasibility of producing this important industrial chemical from biomass-derived sugars. However, the toxicity of contaminant compounds in the biomass-derived sugars and styrene itself limit further gains in production. Styrene toxicity is generally believed to be due to membrane damage. Contrary to this prevailing wisdom, our quantitative assessment during challenge with up to 200 mg L−1 of exogenously provided styrene showed little change in membrane integrity; membrane disruption was observed only during styrene production. Membrane fluidity was also quantified during styrene production, but no changes were observed relative to the non-producing control strain. This observation that styrene production is much more damaging to the membrane integrity than challenge with exogenously supplied styrene provides insight into the mechanism of styrene toxicity and emphasizes the importance of verifying proposed toxicity mechanisms during production instead of relying upon results obtained during exogenous challenge.

Utilization of pyrolytic substrate by microalga Chlamydomonas reinhardtii: cell membrane property change as a response of the substrate toxicity

Acetic acid derived from fast pyrolysis of lignocellulosic biomass is a promising substrate for microalgae fermentation for producing lipid-rich biomass. However, crude pyrolytic acetic acid solution contains various toxic compounds inhibiting algal growth. It was hypothesized that such an inhibition was mainly due to the cell membrane damage. In this work, the cell membrane property of algal cells was evaluated at various conditions to elucidate the mechanisms of inhibition caused by the pyrolytic substrate solution. It was found that acetic acid itself served a carbon source for boosting algal cell growth but also caused cell membrane leakage. The acetic acid concentration for highest cell density was 4 g/L. Over-liming treatment of crude pyrolytic acetic acid increased the algal growth with a concurrent reduction of cell membrane leakage. Directed evolution of algal strain enhanced cell membrane integrity and thus increased its tolerance to the toxicity of the crude substrate. Statistical analysis shows that there was a significant correlation between the cell growth performance and the cell membrane integrity (leakage) but not membrane fluidity. The addition of cyto-protectants such as Pluronic F68 and Pluronic F127 enhanced the cell membrane integrity and thus, resulted in enhanced cell growth. The transmission electron microscopy (TEM) of algal cells visually confirmed the cell membrane damage as the mechanism of the pyrolytic substrate inhibition. Collectively, this work indicates that the cell membrane is one major reason for the toxicity of pyrolytic acetic acid when being used for algal culture. To better use this pyrolytic substrate, cell membrane of the microorganism needs to be strengthened through either strain improvement or addition of membrane protectant reagents.

Identification of Soil Microbes Capable of Utilizing Cellobiosan

Approximately 100 million tons of anhydrosugars, such as levoglucosan and cellobiosan, are produced through biomass burning every year. These sugars are also produced through fast pyrolysis, the controlled thermal depolymerization of biomass. While the microbial pathways associated with levoglucosan utilization have been characterized, there is little known about cellobiosan utilization. Here we describe the isolation and characterization of six cellobiosan-utilizing microbes from soil samples. Each of these organisms is capable of using both cellobiosan and levoglucosan as sole carbon source, though both minimal and rich media cellobiosan supported significantly higher biomass production than levoglucosan. Ribosomal sequencing was used to identify the closest reported match for these organisms: Sphingobacterium multivorum, Acinetobacter oleivorans JC3-1, Enterobacter sp SJZ-6, and Microbacterium sps FXJ8.207 and 203 and a fungal species Cryptococcus sp. The commercially-acquired Enterobacter cloacae DSM 16657 showed growth on levoglucosan and cellobiosan, supporting our isolate identification. Analysis of an existing database of 16S rRNA amplicons from Iowa soil samples confirmed the representation of our five bacterial isolates and four previously-reported levoglucosan-utilizing bacterial isolates in other soil samples and provided insight into their population distributions. Phylogenetic analysis of the 16S rRNA and 18S rRNA of strains previously reported to utilize levoglucosan and our newfound isolates showed that the organisms isolated in this study are distinct from previously described anhydrosugar-utilizing microbial species.

Conversion of levoglucosan and cellobiosan by Pseudomonas putida KT2440

Pyrolysis offers a straightforward approach for the deconstruction of plant cell wall polymers into bio-oil. Recently, there has been substantial interest in bio-oil fractionation and subsequent use of biological approaches to selectively upgrade some of the resulting fractions. A fraction of particular interest for biological upgrading consists of polysaccharide-derived substrates including sugars and sugar dehydration products such as levoglucosan and cellobiosan, which are two of the most abundant pyrolysis products of cellulose. Levoglucosan can be converted to glucose-6-phosphate through the use of a levoglucosan kinase (LGK), but to date, the mechanism for cellobiosan utilization has not been demonstrated. Here, we engineer the microbe Pseudomonas putida KT2440 to use levoglucosan as a sole carbon and energy source through LGK integration. Moreover, we demonstrate that cellobiosan can be enzymatically converted to levoglucosan and glucose with β-glucosidase enzymes from both Glycoside Hydrolase Family 1 and Family 3. β-glucosidases are commonly used in both natural and industrial cellulase cocktails to convert cellobiose to glucose to relieve cellulase product inhibition and to facilitate microbial uptake of glucose. Using an exogenous β-glucosidase, we demonstrate that the engineered strain of P. putida can grow on levoglucosan up to 60 g/L and can also utilize cellobiosan. Overall, this study elucidates the biological pathway to co-utilize levoglucosan and cellobiosan, which will be a key transformation for the biological upgrading of pyrolysis-derived substrates.

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Levoglucosan kinase is engineered into Pseudomonas putida KT2440.

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Cellobiosan can be cleaved to levoglucosan and glucose by β-glucosidases.

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This provides a path forward to co-utilize levoglucosan and cellobiosan.

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These transformations will be important for hybrid processing applications.

The comparison of obtaining fermentable sugars from cellulose by enzymatic hydrolysis and fast pyrolysis

Sugars are one of intermediates in the biological and chemical conversion of biomass. The objective of this study was to make comparison of obtaining fermentable sugars by enzymatic hydrolysis and fast pyrolysis of ball milling pretreated cellulose. After ball milling pretreatment for 0-18h, with the accumulation of alkali and alkali earth metals (from 50.8 to 276.4ppm) and decrease of the crystalline structure (from 89.8% to 10.1%), the hydrolysis yields increased from 23.6% to 56.0% in enzymatic saccharification, while the yields of levoglucosan diminished from 61.5% to 45.6% gradually in fast pyrolysis. Both enzymatic saccharification and fast pyrolysis had unique attractive features and unfavorable limitations. The present research provided a concept for considering choices among the technologies and feedstocks currently available.

Bioconversion of Raw Glycerol Generated from the Synthesis of Biodiesel by Different Oleaginous Yeasts: Lipid Content and Fatty Acid Profile of Biomass

In this work, 12 different yeast strains were evaluated to gauge their ability to accumulate lipids using raw glycerol as the main carbon source. Lipomyces lipofer NRRL Y-1155 stood out above the other strains, achieving 9.48 g/l biomass, 57.64 % lipid content and 5.46 g/l lipid production. The fatty acid profile was similar to vegetable oils commonly used in the synthesis of biodiesel, with the predominance of polyunsaturated acids, especially linoleic acid, reaching 68.3 % for Rhodotorula glutinis NRRL YB-252. The occurrence of palmitic acid (39.3 % for Lipomyces starkeyi NRRL Y-11557) was also notable. Thus, yeast biomass with high lipid content can be a sustainable and renewable alternative as a raw material for the biodiesel industry.

Mathematical modeling of the fermentation of acid-hydrolyzed pyrolytic sugars to ethanol by the engineered strain Escherichia coli ACCC 11177

Pyrolysate from waste cotton was acid hydrolyzed and detoxified to yield pyrolytic sugars, which were fermented to ethanol by the strain Escherichia coli ACCC 11177. Mathematical models based on the fermentation data were also constructed. Pyrolysate containing an initial levoglucosan concentration of 146.34 g/L gave a glucose yield of 150 % after hydrolysis, suggesting that other compounds were hydrolyzed to glucose as well. Ethyl acetate-based extraction of bacterial growth inhibitors with an ethyl acetate/hydrolysate ratio of 1:0.5 enabled hydrolysate fermentation by E. coli ACCC 11177, without a standard absorption treatment. Batch processing in a fermenter exhibited a maximum ethanol yield and productivity of 0.41 g/g and 0.93 g/L·h(-1), respectively. The cell growth rate (r x ) was consistent with a logistic equation [Formula: see text], which was determined as a function of cell growth (X). Glucose consumption rate (r s ) and ethanol formation rate (r p ) were accurately validated by the equations [Formula: see text] and [Formula: see text], respectively. Together, our results suggest that combining mathematical models with fermenter fermentation processes can enable optimized ethanol production from cellulosic pyrolysate with E. coli. Similar approaches may facilitate the production of other commercially important organic substances.

Direct conversion of inulin into cell lipid by an inulinase-producing yeast Rhodosporidium toruloides 2F5

In this study, an inulinase-producing yeast strain 2F5 of Rhodosporidium toruloides was obtained. It was found that the yeast strain 2F5 could produce higher amount of oil from inulin and larger lipid bodies in its cells than any other yeast strains tested in this study. Under the optimal conditions, 62.14% (w/w) of lipid based on cell dry weight and 15.82g/l of the dry cell mass were produced from 6.0% (w/v) inulin at flask level, leaving 0.92% (w/v) of total sugar in the fermented medium. During 2-l fermentation, 70.36% (w/w) of lipid based on cell dry weight and 15.64g/l of the dry cell mass were produced from 6.0% (w/v) inulin. Over 99.09% of the fatty acids from the yeast strain 2F5 grown on inulin was C16:0, C18:0, C18:1 and C18:2, especially C18:1 (52.2%). The biodiesel prepared using the lipids produced by the yeast strain 2F5 could be burnt well.

Nanoemulsification of pseudo-ceramide by molecular association with mannosylerythritol lipid

Ceramide molecules in water-based solutions readily attract each other to form molecular crystals, which seriously hampers to diversify their formulations. This paper describes a facile method that allows fabrication of stable ceramide emulsions through an effective molecular association with a lipid having an asymmetric molecular geometry. The lipid considered in this study is mannosylerythritol lipid (MEL). MEL is specialized in having a unique molecular structure containing sugar alcohol erythritol as a hydrophilic part and two alkyl chains with different number of carbons as hydrophobic moieties. Our particular interest has been focused on experimentally demonstrating how MEL interacts with pseudo-ceramide molecules by observing phase properties, emulsion morphology, and suspension stability. The pseudo-ceramide emulsions prepared with MEL show remarkably improved dispersion stability without either formation of molecular crystals or changes in particle sizes even after storing them for a long time. This suggests that MEL readily associates with the pseudo-ceramide due to the hydrophobic interaction, while it makes a break in the continuity of the molecular assembly of the pseudo-ceramide molecules themselves due to the geometric hindrance coming from MEL's asymmetric molecular structure.