Biological removal of inhibitors leads to the improved lipid production in the lipid fermentation of corn stover hydrolysate by Trichosporon cutaneum

Microbial Lipid Based Biorefinery Concepts: A Review of Status and Prospects

The use of lignocellulosic biomass as a raw material for the production of lipids has gained increasing attention, especially in recent years when the use of food in the production of biofuels has become a current technology. Thus, the competition for raw materials for both uses has brought the need to create technological alternatives to reduce this competition that could generate a reduction in the volume of food offered and a consequent commercial increase in the value of food. Furthermore, the use of microbial oils has been studied in many industrial branches, from the generation of renewable energy to the obtainment of several value-added products in the pharmaceutical and food industries. Thus, this review provides an overview of the feasibility and challenges observed in the production of microbial lipids through the use of lignocellulosic biomass in a biorefinery. Topics covered include biorefining technology, the microbial oil market, oily microorganisms, mechanisms involved in lipid-producing microbial metabolism, strain development, processes, lignocellulosic lipids, technical drawbacks, and lipid recovery.

Low acyl gellan gum immobilized Lactobacillus bulgaricus T15 produce D-lactic acid from non-detoxified corn stover hydrolysate

Straw biorefinery offers economical and sustainable production of chemicals. The merits of cell immobilization technology have become the key technology to meet D-lactic acid production from non- detoxified corn stover. In this paper, Low acyl gellan gum (LA-GAGR) was employed first time for Lactobacillus bulgaricus T15 immobilization and applied in D-lactic acid (D-LA) production from non-detoxified corn stover hydrolysate. Compared with the conventional calcium alginate (E404), LA-GAGR has a hencky stress of 82.09 kPa and excellent tolerance to 5-hydroxymethylfurfural (5-HMF), ferulic acid (FA), and vanillin. These features make LA-GAGR immobilized T15 work for 50 days via cell-recycle fermentation with D-LA yield of 2.77 ± 0.27 g/L h, while E404 immobilized T15 can only work for 30 days. The production of D-LA from non-detoxified corn stover hydrolysate with LA-GAGR immobilized T15 was also higher than that of free T15 fermentation and E404 immobilized T15 fermentation. In conclusion, LA-GAGR is an excellent cell immobilization material with great potential for industrial application in straw biorefinery industry.

An Approach for Incorporating Glycerol as a Co-Substrate into Unconcentrated Sugarcane Bagasse Hydrolysate for Improved Lipid Production in Rhodotorula glutinis

Sugarcane bagasse is a potential raw material for microbial lipid production by oleaginous yeasts. Due to the limited sugar concentrations in bagasse hydrolysate, increasing carbon the concentration is necessary in order to improve lipid production. We aimed to increase carbon concentration by incorporating glycerol as a co-substrate into unconcentrated bagasse hydrolysate in the cultivation of Rhodotorula glutinis TISTR 5159. Cultivation in hydrolysate without nitrogen supplementation (C/N = 42) resulted in 60.31% lipid accumulation with 11.45 ± 0.75 g/L biomass. Nitrogen source supplementation increased biomass to 26.29 ± 2.05 g/L without losing lipid accumulation at a C/N of 25. Yeast extract improved lipid production in the hydrolysate due to high growth without altering the lipid content of the cells. Mixing glycerol up to 10% v/v into the unconcentrated hydrolysate improved biomass and lipid production. A further increase in glycerol concentrations drastically decreased growth and lipid accumulation by the yeast. By maintaining C/N at 27 using yeast extract as the sole nitrogen source, hydrolysate mixed with 10% v/v glycerol resulted in the highest lipid yield, at 19.57 ± 0.53 g/L with 50.55% lipid content, which was a 2.8-fold increase compared to using the hydrolysate alone. In addition, yeast extracts were superior for promoting growth and lipid production compared to inorganic nitrogen sources.

A novel fermentation strategy for efficient xylose utilization and microbial lipid production in lignocellulosic hydrolysate

The sugar utilization efficiency and the tolerance of microorganism to inhibitors are essential for lipid production from lignocellulosic biomass. In this study, the sugar consumption and inhibitor tolerance characteristics of Trichosporon dermatis 32,903 were investigated. The results showed that the lipid yield on xylose was much lower than that on glucose, while these substrates exhibited comparative efficiency for cell growth. High inoculum size improved the tolerance of T. dermatis 32,903 to inhibitors. Based on these characteristics, sugar-targeted-utilization and cyclic fermentation strategy was developed. The tolerance of high inoculum size to inhibitors was utilized, glucose was targeted for lipid fermentation and xylose was targeted for cell growth. As a result, the lipid production efficiency was greatly enhanced. The lipid titer in hydrolysate of DLCA (Densifying Lignocellulosic biomass with Chemicals followed by Autoclave) pretreated rice straw was improved to as high as 38.4 g/L with lipid yield of 0.207 g/g consumed sugar.

Dual-stage biorefinery to convert spentwash hydrolysate into oleochemicals using Trichosporon cutaneum and Yarrowia lipolytica

Yeast lipids from low-cost renewable feedstock are valuable resources for oleochemicals thus enabling circular chemistry. Current study focuses on lipid and volatile fatty acid (VFA) production through dual-stage fermentation of spentwash in a biorefinery framework with Trichosporon cutaneum (Tc) and Yarrowia lipolytica (Yl). During cell proliferation phase, Tc and Yl accumulated 2.9 and 2.5 g/L of dry biomass respectively in acid-hydrolysed spentwash (AHSW) and produced 16 and 5.5 g/L of total VFA respectively. Lipid yields (29.8%) and lipid titres (0.89 g/L) were higher in Tc/AHSW, when compared to Yl indicating the efficacy of Tc in spentwash bioremediation. Lipid accumulation was enhanced to 35% in Tc/AHSW, in presence of 0.05% NH₄Cl due to oxidative stress of ammonium ions. Analysis of fatty acid composition revealed the presence of higher oleic acid, which is ideal for biodiesel production. The results demonstrate a sustainable biorefinery model for bioremediation of spentwash and its value addition.

Ultra-centrifugation force in adaptive evolution changes the cell structure of oleaginous yeast Trichosporon cutaneum into a favorable space for lipid accumulation

Microbial lipid production from lignocellulose biomass provides an essential option for sustainable and carbon neutral supply of future aviation fuels, biodiesel, as well as various food and nutrition products. Oleaginous yeast is the major microbial cell factory but its lipid producing performance is far below the requirements of industrial application. Here we show an ultra-centrifugation fractionation in adaptive evolution (UCF) of Trichosporon cutaneum based on the minor cell density difference. The lightest cells with the maximum intracellular lipid content were isolated by ultra-centrifugation fractionation in the long-term adaptive evolution. Significant changes occurred in the cell morphology with a fragile cell wall wrapping and enlarged intracellular space (two orders of magnitude increase in cell size). Complete and coordinate assimilations of all non-glucose sugars derived from lignocellulose were triggered and fluxed into lipid synthesis. Genome mutations and significant transcriptional regulations of the genes responsible for cell structure were identified and experimentally confirmed. The obtained Trichosporon cutaneum MP11 cells achieved a high lipid production of wheat straw, approximately five-folds greater than that of the parental cells. The study provided an effective method for screening the high lipid containing oleaginous yeast cells as well as the intracellular products accumulating cells in general. This article is protected by copyright. All rights reserved.

Overexpression of Oxidoreductase YghA Confers Tolerance of Furfural in Ethanologenic Escherichia coli Strain SSK42

Furfural is a common furan inhibitor formed due to dehydration of pentose sugars, like xylose, and acts as an inhibitor of microbial metabolism. Overexpression of NADH-specific FucO and deletion of NADPH-specific YqhD had been a successful strategy in the past in conferring tolerance against furfural in Escherichia coli, which highlights the importance of oxidoreductases in conferring tolerance against furfural. In a screen consisting of various oxidoreductases, dehydrogenases, and reductases, we identified the yghA gene as an overexpression target to confer tolerance against furfural. YghA preferably used NADH as a cofactor and had an apparent Km value of 0.03 mM against furfural. In the presence of 1 g liter-1 furfural and 10% xylose (wt/vol), yghA overexpression in an ethanologenic E. coli strain SSK42 resulted in an ethanol efficiency of ∼97%, with a 5.3-fold increase in ethanol titers compared to the control. YghA also exhibited activity against the less toxic inhibitor 5-hydroxymethyl furfural, which is formed due to dehydration of hexose sugars, and thus is a formidable target for overexpression in ethanologenic strain for fermentation of sugars in biomass hydrolysate. IMPORTANCE Lignocellulosic biomass represents an inexhaustible source of carbon for second-generation biofuels. Thermo-acidic pretreatment of biomass is performed to loosen the lignocellulosic fibers and make the carbon bioavailable for microbial metabolism. The pretreatment process also results in the formation of inhibitors that inhibit microbial metabolism and increase production costs. Furfural is a potent furan inhibitor that increases the toxicity of other inhibitors present in the hydrolysate. Thus, it is desirable to engineer furfural tolerance in E. coli for efficient fermentation of hydrolysate sugars.

A novel oleaginous yeast Saccharomyces cerevisiae CU-TPD4 for lipid and biodiesel production

This study reports on the novel Saccharomyces cerevisiae CU-TPD4 that was isolated from coconut waste residues obtained from a coconut factory in Thailand. The CU-TPD4 isolate was confirmed to be a S. cerevisiae by molecular analysis and to be an oleaginous yeast with more than 20% (w/w) of the cell dry weight (CDW) present in the form of lipids. The lipid content and lipid yield of CU-TPD4 (52.96 ± 1.15% of CDW and 1.78 ± 0.06 g/L, respectively) under optimized growth conditions were much higher than those under normal growth conditions (22.65 ± 1.32% of CDW and 1.24 ± 0.12 g/L, respectively). The major fatty acids produced by CU-TPD4 were oleic (C18:1), palmitoleic (C16:1), stearic (C18:0), and palmitic (C16:0) acids. Mathematical estimation of the physical properties of the biodiesel obtained by transesterification of the extracted lipid suggested it was suitable as biodiesel with respect to the ASTM D6751 and EN 14214 international standards. Consequently, S. cerevisiae CU-TPD4 is expected to emerge as a promising alternative for biodiesel production.

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.

Recent advances in lipid metabolic engineering of oleaginous yeasts

With the increasing demand to develop a renewable and sustainable biolipid feedstock, several species of non-conventional oleaginous yeasts are being explored. Apart from the platform oleaginous yeast Yarrowia lipolytica, the understanding of metabolic pathway and, therefore, exploiting the engineering prospects of most of the oleaginous species are still in infancy. However, in the past few years, enormous efforts have been invested in Rhodotorula, Rhodosporidium, Lipomyces, Trichosporon, and Candida genera of yeasts among others, with the rapid advancement of engineering strategies, significant improvement in genetic tools and techniques, generation of extensive bioinformatics and omics data. In this review, we have collated these recent progresses to make a detailed and insightful summary of the major developments in metabolic engineering of the prominent oleaginous yeast species. Such a comprehensive overview would be a useful resource for future strain improvement and metabolic engineering studies for enhanced production of lipid and lipid-derived chemicals in oleaginous yeasts.

Immobilization of Laccase on Magnetic Nanoparticles and Application in the Detoxification of Rice Straw Hydrolysate for the Lipid Production of Rhodotorula glutinis

The production of microbial lipid using lignocellulosic agroforestry residues has attracted much attention. But, various inhibitors such as phenols and furans, which are produced during lignocellulosic hydrolysate preparation, are harmful to microbial lipid accumulation. Herein, we developed a novel detoxification strategy of rice straw hydrolysate using immobilized laccase on magnetic Fe₃O₄ nanoparticles for improving lipid production of Rhodotorula glutinis. Compared with free laccase, the immobilized laccase on magnetic nanoparticles showed better stability, which still retained 76% of original activity at 70 °C and 56% at pH 2 for 6 h. This immobilized laccase was reused to remove inhibitors in acid-pretreated rice straw hydrolysate through recycling with external magnetic field. The results showed that most of phenols, parts of furans, and formic acids could be removed by immobilized laccase after the first batch. Notably, the immobilized laccase exhibited good reusability in repeated batch detoxification. 78.2% phenols, 43.8% furfural, 30.4% HMF, and 16.5% formic acid in the hydrolysate were removed after the fourth batch. Furthermore, these detoxified rice straw hydrolysates, as substrates, were applied to the lipid production of Rhodotorula glutinis. The lipid yield in detoxified hydrolysate was significantly higher than that in undetoxified hydrolysate. These findings suggest that the immobilized laccase on magnetic nanoparticles has a potential to detoxify lignocellusic hydrolysate for improving microbial lipid production.

Lipid Production by Rhodotorula glutinis in Continuous Cultivation with a Gravity Sedimentation System

Lipid accumulation is generally believed to be a partially growth-coupled biochemical process that results in differences in lipid content between different cells. To separate lipid-rich cells and increase the cellular biomass in bioreactors during the cultivation of the oleaginous yeasts, a gravity sedimentation system (GSS) is coupled to a bioreactor. The dilution rate (D) and the ratio of the outflow rate from the outlet of the GSS to the inflow rate into the bioreactor (B) were evaluated. The biomass in the bioreactor with GSS increased by 16.3% and 30.6% at D values of 0.05 h-1 (B = 0.25) and 0.02 h-1 (B = 0.5), respectively. Interestingly, cells containing 39.3% lipids were obtained from the outlet of the GSS (D = 0.02 h-1, B = 0.5), and the lipid content increased by 7.8% compared to that of the bioreactor. The results indicated that use of a GSS is a very effective method for increasing the cell concentration and separation of lipid-rich cells.

Utilization of Wheat Bran Acid Hydrolysate by Rhodotorula mucilaginosa Y-MG1 for Microbial Lipid Production as Feedstock for Biodiesel Synthesis

The lignocellulosic hydrolysate was used as the fermentation feedstock of Rhodotorula mucilaginosa Y-MG1 for the production of microbial lipids as the potential raw material for biodiesel synthesis. On synthetic media and under nitrogen-limiting condition, the Y-MG1 strain produces 2.13 g/L of lipids corresponding to 32.7% of lipid content. This strain was able to assimilate a wide range of substrates, especially C5 and C6 sugars as well as glycerol and sucrose. Fatty acid composition shows a divergence depending on the nature of used carbon source with a predominance of oleic acid or linoleic acid. An effective hydrolysis process, based on diluted acid treatment, was established for providing the maximum of fermentable sugars from different characterized lignocellulosic wastes. The highest yield of reducing sugars (56.6 g/L) could be achieved when wheat bran was used as the raw material. Hydrolysate detoxification step was not required in this study since the Y-MG1 strain was shown to grow and produce lipids in the presence of inhibitors and without the addition of external elements. Operating by controlled fed-batch fermentation yielded a dry biomass and oil yield of up to 11 g/L and 38.7% (w/w), respectively. The relative fatty acid composition showed the presence of increased levels of monounsaturated (66.8%) and saturated (23.4%) fatty acids in lipids of Y-MG1 grown on wheat bran. The predictive determination of biodiesel properties suggests that this oil may effectively be used for biodiesel production.

A novel method for real-time estimation of insoluble solids and glucose concentrations during enzymatic hydrolysis of biomass

The study describes a novel method using instantaneous mixing torque and rotational speed to estimate insoluble solids and glucose concentrations during enzymatic hydrolysis of biomass. This method is cost-effective for real-time monitoring and control of enzymatic hydrolysis and potentially scalable. The model was developed using biomass slurries at three solids loading (20, 30 and 45%) at various rotational speeds from 50 to 400 rpm. The results showed a significant drop in mixing torque at 12 h with high solids loading. Maximum glucose concentration (205 g/l) during hydrolysis was achieved at 45% solids loading. Insoluble solids and glucose concentration as a function of torque and rotational speeds were modeled using a modified Herschell-Bulkley model. The model describes the experimental observations with high fidelity (R² = 0.84) and can be used for real time monitoring of many multiphase reaction systems as enzymatic hydrolysis of lignocellulosic biomass and dry grind corn ethanol processes.

Repeated cultures of Saccharomyces cerevisiae SC90 to tolerate inhibitors generated during cassava processing waste hydrolysis for bioethanol production

Large amount of cassava pulp is produced as by-product of industrial tapioca production. The value-added process of this low-cost waste is to use it as a substrate for bioethanol production. However, during the pulp pretreatment by acidification combined with steam explosion, many yeast inhibitors including acetic acid, formic acid, levulinic acid, furfural and 5-hydroxymethylfurfural are generated and these compounds have negative effects on the subsequent fermentation step. Therefore, the objective of this study was to investigate whether the repeated cultures of Saccharomyces cerevisiae SC90 could alleviate this problem. To obtain the inhibitor tolerable cells, the repeated culture was performed by growing yeast cells to a specific growth rate (µ) of 0.22 h-1 or higher (80% of the µ in control) and then transferring them to progressively higher concentrations of hydrolysate ranging from 20 to 100% (v/v). The results showed a tendency of longer lag phase as well as time to reach maximum cell number (t maxc) with an increase in hydrolysate concentration. However, the repeated culture at the same hydrolysate concentration could shorten both lag period and t maxc. Interestingly, the growth and fermentation efficiency of adapted cells in 100% hydrolysate were significantly higher (p ≤ 0.05) than those of non-adapted cells by 38% and 27%, respectively.

Phosphate removal combined with acetate supplementation enhances lipid production from water hyacinth by Cutaneotrichosporon oleaginosum

BACKGROUND

Microbial lipids derived from various lignocellulosic feedstocks have emerged as a promising candidate for the biodiesel industry and a potential substitute for high value-added fats. However, lignocellulosic biomass, especially herbaceous biomass, such as water hyacinth, contains high concentrations of nitrogenous components. These compounds impede microbial lipid production, as lipid biosynthesis is commonly induced by imposing a nutrient deficiency, especially nitrogen starvation. Novel strategies and bioprocesses are pivotal for promoting lipid production from nitrogen-rich biomass.

RESULTS

Here a combined strategy of phosphate removal and acetate supplementation was described for enhanced microbial lipid production on water hyacinth hydrolysates by Cutaneotrichosporon oleaginosum (formerly Cryptococcus curvatus). Lipid production was significantly improved, when the phosphorus limitation and sugars/acetate co-utilization strategies were used separately. In this case, acetate and glucose were consumed simultaneously. Lipid production was observed by the combination of phosphate removal with acetate supplementation. Lipid titer, content, and yield were determined to be 7.3 g/L, 59.7% and 10.1 g/100 g raw water hyacinth, respectively. These data were increased by 4.2, 4.6, and 4.3 times, respectively, compared to those from the unprocessed hydrolysates. The fatty acid compositions of the resulting lipids bear a marked resemblance to those of rapeseed oil, indicating their applicability to the biodiesel industry.

CONCLUSIONS

The combination of phosphate removal and acetate supplementation was successful in significantly enhancing microbial lipid production. This strategy offers a valuable solution for nitrogen-rich lignocellulosic feedstocks utilization, which should foster more economical nitrogen-rich biomass-to-lipid bioprocesses.

Lipid production via simultaneous conversion of glucose and xylose by a novel yeast, Cystobasidium iriomotense

The yeast strains IPM32-16, ISM28-8sT, and IPM46-17, isolated from plant and soil samples from Iriomote Island, Japan, were explored in terms of lipid production during growth in a mixture of glucose and xylose. Phylogenetically, the strains were most closely related to Cystobasidium slooffiae, based on the sequences of the ITS regions and the D1/D2 domain of the LSU rRNA gene. The strains were oleaginous, accumulating lipids to levels > 20% dry cell weight. Moreover, kinetic analysis of the sugar-to-lipid conversion of a 1:1 glucose/xylose mixture showed that the strains consumed the two sugars simultaneously. IPM46-17 attained the highest lipid content (33%), mostly C16 and C18 fatty acids. Thus, the yeasts efficiently converted lignocellulosic sugars to lipids, aiding in biofuel production (which benefits the environment, promotes rural jobs, and strengthens fuel security). The strains constituted a novel species of Cystobasidium, for which we propose the name Cystobasidium iriomotense (type strain ISM28-8sT = JCM 24594T = CBS 15015T).

Agro-industrial waste recycling by Trichosporon fermentans: conversion of waste sweetpotato vines alone into lipid

Agro-industrial waste can be used to replace traditional carbohydrates, such as sucrose, starch, and glucose in many industrial fermentation processes. This study investigated the conversion of pre-treated waste sweetpotato vines (SV) into lipid by Trichosporon fermentans under the separate hydrolysis and fermentation (SHF) and the simultaneous saccharification and fermentation (SSF) processes. The results showed that SV autoclaving significantly increased the lipid accumulation of T. fermentans compared with acid or alkaline hydrolysis. The effects of different pre-treatments on SV were also studied by scanning electron microscopy and Fourier transform infrared spectroscopy, which showed the partial removal of the aliphatic fractions, hemicelluloses, and lignin during pre-treatment. Moreover, the lipid yield of T. fermentans in SSF was 6.98 g L^-1, which was threefold higher than that (2.79 g L^-1) in SHF, and the lipid contents of yeast in SSF and SHF were 36 and 25%, respectively. Overall, this study indicated that SSF using autoclaved SV could increase the growth and lipid production of T. fermentans and provided an efficient way to realize the resource utilization of waste SV.

Amaranth seeds (Amaranthus palmeri L.) as novel feedstock for biodiesel production by oleaginous yeast

The potential of lipid accumulation by oleaginous yeast Cryptococcus vishniaccii grown on amaranth seed aqueous extract (AAE) media was assessed. Maximum cell biomass productivity of 104 mg/L/h, lipid productivity of 54 mg/L/h, and lipid content of 52.31% were recorded on AAE when carbon to nitrogen (C:N) ratio increased from 134 to 147 after removal of ammonia nitrogen. The lipid droplet (LD) size (2.32 ± 0.38 μm) was visualized by fluorescence microscopy using Nile red stain indicating maximum accumulated triacylglycerol (TAG) at C:N 147. Fatty acid methyl ester (FAME) profile obtained after transesterification of extracted lipid revealed the presence of palmitic acid (16:0), palmitoleic acid (16:1), stearic acid (18:0), oleic acid (18:1), and linoleic acid (18:2). Data showed the presence of high monounsaturated fatty acid (MUFA) content (68.17%) depicting improved winter operating conditions of biodiesel. Various quality parameters of biodiesel were evaluated and compared to the American and European biodiesel standards specifications. Based on the lipid productivity, distribution of fatty acids, and evaluated properties obtained; the lipid accumulation by C. vishniaccii utilizing amaranth seeds as substrate could serve as a feasible feedstock for biodiesel production.

Fostering triacylglycerol accumulation in novel oleaginous yeast Cryptococcus psychrotolerans IITRFD utilizing groundnut shell for improved biodiesel production

The investigation was carried out to examine the potential of triacylglycerol (TAG) accumulation by novel oleaginous yeast isolate Cryptococcus psychrotolerans IITRFD on utilizing groundnut shell acid hydrolysate (GSH) as cost-effective medium. The maximum biomass productivity and lipid productivity of 0.095±0.008g/L/h and 0.044±0.005g/L/h, respectively with lipid content 46% was recorded on GSH. Fatty acid methyl ester (FAME) profile obtained by GC-MS analysis revealed oleic acid (37.8%), palmitic (29.4%) and linoleic (32.8%) as major fatty acids representing balance between oxidative stability (OS) and cold flow filter properties (CFFP) for improved biodiesel quality. The biodiesel property calculated were correlated well with the fuel standards limits of ASTM D6751, EN 14214 and IS 15607. The present findings raise the possibility of using agricultural waste groundnut shell as a substrate for production of biodiesel by novel oleaginous yeast isolate C. psychrotolerans IITRFD.

Potential of oleaginous yeast Trichosporon sp., for conversion of sugarcane bagasse hydrolysate into biodiesel

This study reports production of microbial oil from a yeast strain Trichosporon sp., (RW) isolated from decayed wood. Preliminary analysis based on fluorescence microscopy and spectroscopy of Nile red stained yeast cells showed accumulation of lipid globules. The potential of the yeast to produce lipids was evaluated on glucose, glycerol and acid hydrolysate of sugarcane bagasse, where Trichosporon sp. (RW) was found to accumulate 21.45 (59.6%), 18.41 (56%) and 10.25g/l (40.5%) of the lipids after 120h of fermentation at 30°C. FAME analysis of lipids by GC-FID and NMR revealed oleic acid (18:1) as the major constituent, corresponding to 50.05, 46.48 and 54.66% of the accumulated lipids in glucose, glycerol and hydrolysate grown cultures, respectively. Other accumulated lipids included palmitic (16:0), linoleic (18:2) and stearic acids (18:0) in that order. The cetane number of the lipids ranged from 52.39 to 59.57 indicating suitability for biodiesel production.

Irradiation pretreatment facilitates the achievement of high total sugars concentration from lignocellulose biomass

This study evaluated the two hydrolysis strategies, involving one thermal and one dilute acid/enzymatic hydrolysis, to produce high xylose and glucose concentrations from lignocellulose assisted with irradiation pretreatment. Prior to hydrolysis, lignocellulose was pretreated by γ-irradiation at 800KGy. The merits of irradiation pretreatment on lignocellulose were contributed to size-reduced particle distributions and low shear rate of material, which allowed high biomass loadings up to 30-40%(w/v, equals to 23-29wt.%) for the consequent hydrolysis process. Results showed that hemicellulose fraction could achieve ∼84g/L of total sugars containing ∼55g/L xylose and ∼21g/L glucose through this two steps hydrolysis. Cellulose fraction would release ∼251g/L of total sugars consisting of ∼235g/L glucose and ∼16g/L xylose in the ultimate enzymatic hydrolysate. To the best of our knowledge, it was the first report of achieving 235g/L glucose in cellulose enzymatic hydrolysate derived from lignocellulose.

Co-utilization of corn stover hydrolysates and biodiesel-derived glycerol by Cryptococcus curvatus for lipid production

In the present study, synergistic effects were observed when glycerol was co-fermented with glucose and xylose for lipid production by the oleaginous yeast Cryptococcus curvatus. Glycerol was assimilated simultaneously with sugars at the beginning of the culture without adaption time. Furthermore, better lipid production results, i.e., lipid yield and lipid productivity of 18.0g/100g and 0.13g/L/h, respectively, were achieved when cells were cultured in blends of corn stover hydrolysates and biodiesel-derived glycerol than those in the hydrolysates alone. The lipid samples had fatty acid compositional profiles similar to those of vegetable oils, suggesting their potential for biodiesel production. This co-utilization strategy provides an extremely simple solution to advance lipid production from both lignocelluloses and biodiesel-derived glycerol in one step.

Converting paper mill sludge into neutral lipids by oleaginous yeast Cryptococcus vishniaccii for biodiesel production

Paper mill sludge (PMS) was assessed as cheap renewable lignocellulosic biomass for lipid production by the oleaginous yeast Cryptococcus vishniaccii (MTCC 232). The sonicated paper mill sludge extract (PMSE) exhibited enhanced lipid yield and lipid content 7.8±0.57g/l, 53.40% in comparison to 5.5±0.8g/l, 40.44% glucose synthetic medium, respectively. The accumulated triglycerides (TAG) inside the lipid droplets (LDs) were converted to biodiesel by transesterification and thoroughly characterized using GC-MS technique. The fatty acid methyl ester (FAME) profile obtained reveals elevated content of oleic acid followed by palmitic acid, linoleic acid and stearic acid with improved oxidative stability related to biodiesel quality.

Biorefinery of corn cob for microbial lipid and bio-ethanol production: An environmental friendly process

Microbial lipid and bio-ethanol were co-generated by an integrated process using corn cob bagasse as raw material. After pretreatment, the acid hydrolysate was used as substrate for microbial lipid fermentation, while the solid residue was further enzymatic hydrolysis for bio-ethanol production. The effect of acid loading and pretreatment time on microbial lipid and ethanol production were evaluated. Under the optimized condition for ethanol production, ∼131.3g of ethanol and ∼11.5g of microbial lipid were co-generated from 1kg raw material. On this condition, ∼71.6% of the overall fermentable sugars in corn cob bagasse could be converted into valuable products. At the same time, at least 33% of the initial COD in the acid hydrolysate was depredated.

Co-fermentation of acetate and sugars facilitating microbial lipid production on acetate-rich biomass hydrolysates

The process of lignocellulosic biomass routinely produces a stream that contains sugars plus various amounts of acetic acid. As acetate is known to inhibit the culture of microorganisms including oleaginous yeasts, little attention has been paid to explore lipid production on mixtures of acetate and sugars. Here we demonstrated that the yeast Cryptococcus curvatus can effectively co-ferment acetate and sugars for lipid production. When mixtures of acetate and glucose were applied, C. curvatus consumed both substrates simultaneously. Similar phenomena were also observed for acetate and xylose mixtures, as well as acetate-rich corn stover hydrolysates. More interestingly, the replacement of sugar with equal amount of acetate as carbon source afforded higher lipid titre and lipid content. The lipid products had fatty acid compositional profiles similar to those of cocoa butter, suggesting their potential for high value-added fats and biodiesel production. This co-fermentation strategy should facilitate lipid production technology from lignocelluloses.

Acceleration of biodetoxification on dilute acid pretreated lignocellulose feedstock by aeration and the consequent ethanol fermentation evaluation

BACKGROUND

Biodetoxification by the fungus Amorphotheca resinae ZN1 provides an effective way of inhibitor removal from pretreated lignocellulose feedstock and has been applied in the process of ethanol, biolipids, and lactic acid production. However, the long-time used and the consumption of considerable xylose in the pretreated materials reduced the process efficiency. The improvements of biodetoxification should be made to enhance the production of biochemical from lignocellulosic materials.

RESULTS

This study reported an acceleration method of A. resinae ZN1-based biodetoxification on the corn stover (CS) feedstock pretreated using dry dilute acid pretreatment. Under proper aeration and well-mixing condition, the conversion rate of furfural, 5-hydroxymethylfurfural (HMF), acetic acid, and typical phenolic compounds were significantly accelerated by more than twofolds faster, which resulted in the reduction of biodetoxification time from 96 h in the conventional process to 36 h. Simultaneous saccharification and ethanol fermentation assay on accelerated biodetoxification of the dry dilute acid pretreated CS feedstock achieved the similar ethanol titer (48.56 g/L of 36 h' accelerated biodetoxification vs. 50.00 g/L of 4 days' conventional biodetoxification) and yield (58.10 vs. 59.63 %). Substrate priority of inhibitors to sugars by A. resinae ZN1 was discovered and considerable xylose was reserved in the accelerated biodetoxification. Cell growth of A. resinae fungus in liquid medium and on pretreated CS solids revealed that the enhanced aeration enhanced the biodetoxification rate rather than the cell growth rate. Accelerated inhibitor conversion might come from the increased supply of cofactors of nicotinamide adenine dinucleotide or nicotinamide adenine dinucleotide phosphate from the step of aldehyde inhibitors to the corresponding acids, instead of cell mass increase.

CONCLUSION

Accelerated biodetoxification reduced the period of biodetoxification and retained the xylose components in the pretreated CS, which provided a practical method on improving process efficiency for cellulosic ethanol production from severe pretreated lignocellulose feedstock.

Organic fraction of municipal solid waste as a suitable feedstock for the production of lipid by oleaginous yeast Cryptococcus aerius

The detoxified pre-hydrolysate and enzymatic hydrolysate of OFMSW were used as substrates for lipid production by Cryptococcus aerius. Factorial experimental designs were employed for the optimization of dilute acid pre-hydrolysis, detoxification by over-liming, enzymatic hydrolysis, and lipid production. OFMSW pre-hydrolysis with 3% H2SO4 for 45 min was found to be the optimal treatment, resulted in total sugar concentration of 65.5 g/L (32.8% yield, based on grams of total reducing sugar per gram of OFMSW). The optimal detoxification conditions of the pre-hydrolysate by over-liming was incubation at 30 °C and pH 11 for 24h, resulted in the reduction of total nitrogen, total phenolic compounds, and furans by 51.3%, 45.1%, and 100%, respectively. The residual solid was subjected to enzymatic hydrolysis, and the highest sugar concentration of 30.5 g/L was obtained. At optimal conditions, the yeast cultivation on the detoxified pre-hydrolysate and enzymatic hydrolysate resulted in the lipid production of 3.9 g/L (12.8% yield, based on g lipid per g consumed sugar) and 4.3g/L (17.1% yield, based on g lipid per g consumed sugar), respectively. The elemental analysis showed the presence of heavy metals including iron (925 mg/l), zinc (59 mg/l), lead (4.7 mg/l), and nickel (3.5mg/l) in the pre-hydrolysate, which were significantly reduced by the over-liming detoxification.

Transcriptional analysis of Amorphotheca resinae ZN1 on biological degradation of furfural and 5-hydroxymethylfurfural derived from lignocellulose pretreatment

BACKGROUND

Furfural and 5-hydroxymethylfurfural (HMF) are the two major inhibitor compounds generated from lignocellulose pretreatment, especially for dilute acid, steam explosion, neutral hot water pretreatment methods. The two inhibitors severely inhibit the cell growth and metabolism of fermenting strains in the consequent bioconversion step. The biodetoxification strain Amorphotheca resinae ZN1 has demonstrated its extraordinary capacity of fast and complete degradation of furfural and HMF into corresponding alcohol and acid forms. The elucidation of degradation metabolism of A. resinae ZN1 at molecular level will facilitate the detoxification of the pretreated lignocellulose biomass and provide the metabolic pathway information for more powerful biodetoxification strain development.

RESULTS

Amorphotheca resinae ZN1 was able to use furfural or HMF as the sole carbon source for cell growth. During the detoxification process, A. resinae ZN1 firstly reduced furfural or HMF into furfuryl alcohol or HMF alcohol, and then oxidized into furoic acid or HMF acid through furan aldehyde as the intermediate at low concentration level. The cell mass measurement suggested that furfural was more toxic to A. resinae ZN1 than HMF. In order to identify the degradation mechanism of A. resinae ZN1, transcription levels of 137 putative genes involved in the degradation of furfural and HMF in A. resinae ZN1 were investigated using the real-time quantitative PCR (qRT-PCR) method under the stress of furfural and HMF, as well as the stress of their secondary metabolites, furfuryl alcohol and HMF alcohol. Two Zn-dependent alcohol dehydrogenase genes and five AKR/ARI genes were found to be responsible for the furfural and HMF conversion to their corresponding alcohols. For the conversion of the two furan alcohols to the corresponding acids, three propanol-preferring alcohol dehydrogenase genes, one NAD(P)(+)-depending aldehyde dehydrogenase gene, or two oxidase genes with free oxygen as the substrate were identified under aerobic condition.

CONCLUSIONS

The genes responsible for the furfural and HMF degradation to the corresponding alcohols and acids in A. resinae ZN1 were identified based on the analysis of the genome annotation, the gene transcription data and the inhibitor conversion results. These genetic resources provided the important information for understanding the mechanism of furfural and HMF degradation and modification of high tolerant strains used for biorefinery processing.

Production of Fatty Acid-derived valuable chemicals in synthetic microbes

Fatty acid derivatives, such as hydroxy fatty acids, fatty alcohols, fatty acid methyl/ethyl esters, and fatty alka(e)nes, have a wide range of industrial applications including plastics, lubricants, and fuels. Currently, these chemicals are obtained mainly through chemical synthesis, which is complex and costly, and their availability from natural biological sources is extremely limited. Metabolic engineering of microorganisms has provided a platform for effective production of these valuable biochemicals. Notably, synthetic biology-based metabolic engineering strategies have been extensively applied to refactor microorganisms for improved biochemical production. Here, we reviewed: (i) the current status of metabolic engineering of microbes that produce fatty acid-derived valuable chemicals, and (ii) the recent progress of synthetic biology approaches that assist metabolic engineering, such as mRNA secondary structure engineering, sensor-regulator system, regulatable expression system, ultrasensitive input/output control system, and computer science-based design of complex gene circuits. Furthermore, key challenges and strategies were discussed. Finally, we concluded that synthetic biology provides useful metabolic engineering strategies for economically viable production of fatty acid-derived valuable chemicals in engineered microbes.

Microbial lipid-based lignocellulosic biorefinery: feasibility and challenges

Although single-cell oil (SCO) has been studied for decades, lipid production from lignocellulosic biomass has received substantial attention only in recent years as biofuel research moves toward producing drop-in fuels. This review gives an overview of the feasibility and challenges that exist in realizing microbial lipid production from lignocellulosic biomass in a biorefinery. The aspects covered here include biorefinery technologies, the microbial oil market, oleaginous microbes, lipid accumulation metabolism, strain development, process configurations, lignocellulosic lipid production, technical hurdles, lipid recovery, and technoeconomics. The lignocellulosic SCO-based biorefinery will be feasible only if a combination of low- and high-value lipids are coproduced, while lignin and protein are upgraded to high-value products.

Analysis of biodegradation performance of furfural and 5-hydroxymethylfurfural by Amorphotheca resinae ZN1

BACKGROUND

Furfural and 5-hydroxymethylfurfural (HMF) are the degradation products of lignocellulose during pretreatment operations and significantly inhibit the consequent enzymatic hydrolysis and fermentation processes. The biodetoxification fungus Amorphotheca resinae ZN1 had demonstrated its excellent capacity on degrading lignocellulose derived inhibitors and helped the fermentation processes to achieve high yield of ethanol and biochemicals. Analysis of the biological degradation performance of furfural and HMF by A. resinae ZN1 will provide essential information for their fast and complete removal from the pretreated lignocellulose materials and facilitate the consequent ethanol fermentation.

RESULTS

The degradation performance of furfural and HMF by A. resinae ZN1 was investigated by capturing intermediate metabolic products at various culture conditions. A. resinae ZN1 converts furfural/HMF into furfuryl/HMF alcohols and furoic/HMF acids simultaneously at aerobic condition, and only the corresponding furfuryl/HMF alcohols are obtained at anaerobic condition. The existence of glucose accelerates the degradation rate of furfural and HMF by A. resinae ZN1 and the cell mass growth rate aerobically. Remarkably, glucose is not consumed before furfural or HMF is degraded to a low threshold concentration. The finding suggests that furfural or HMF has a substrate priority of utilization by A. resinae ZN1 than glucose. This property may help the detoxification of furfural and HMF to be operated without consuming glucose.

CONCLUSIONS

The biological degradation performance of furfural and HMF by A. resinae ZN1 was investigated experimentally. Oxygen supply is important on the complete biodegradation of furfural and HMF by A. resinae ZN1. Furfural or HMF has the priority of substrate utilization than glucose by A. resinae ZN1. This study provided important information for detoxification enhancement and strain modification.

Helically agitated mixing in dry dilute acid pretreatment enhances the bioconversion of corn stover into ethanol

BACKGROUND

Dry dilute acid pretreatment at extremely high solids loading of lignocellulose materials demonstrated promising advantages of no waste water generation, less sugar loss, and low steam consumption while maintaining high hydrolysis yield. However, the routine pretreatment reactor without mixing apparatus was found not suitable for dry pretreatment operation because of poor mixing and mass transfer. In this study, helically agitated mixing was introduced into the dry dilute acid pretreatment of corn stover and its effect on pretreatment efficiency, inhibitor generation, sugar production, and bioconversion efficiency through simultaneous saccharification and ethanol fermentation (SSF) were evaluated.

RESULTS

The overall cellulose conversion taking account of cellulose loss in pretreatment was used to evaluate the efficiency of pretreatment. The two-phase computational fluid dynamics (CFD) model on dry pretreatment was established and applied to analyze the mixing mechanism. The results showed that the pretreatment efficiency was significantly improved and the inhibitor generation was reduced by the helically agitated mixing, compared to the dry pretreatment without mixing: the ethanol titer and yield from cellulose in the SSF reached 56.20 g/L and 69.43% at the 30% solids loading and 15 FPU/DM cellulase dosage, respectively, corresponding to a 26.5% increase in ethanol titer and 17.2% increase in ethanol yield at the same fermentation conditions.

CONCLUSIONS

The advantage of helically agitated mixing may provide a prototype of dry dilute acid pretreatment processing for future commercial-scale production of cellulosic ethanol.

Lipid production from corn stover by the oleaginous yeast Cryptococcus curvatus

BACKGROUND

Microbial lipids produced from lignocellulosic biomass hold great promise for the biodiesel industry. These lipids usually consist of three major processes: pretreatment, enzymatic hydrolysis and lipid production. However, the conventional strategy of using biomass hydrolysates as the feedstock for lipid production suffers from low lipid coefficient and prohibitively high costs. More cost-effective and integrated processes are required to advance lignocellulosic biomass-based microbial lipid technology.

RESULTS

Three different strategies were tested using the oleaginous yeast Cryptococcus curvatus ATCC 20509 as a lipid producer and alkaline-pretreated corn stover as a model material. It was found that the separate hydrolysis and enhanced lipid production process required more cellulolytic enzymes yet afforded a low lipid coefficient of 115.6 mg/g pretreated corn stover. When biomass hydrolysis and lipid production were integrated, the amounts of cellulase and xylanase were reduced and no β-glucosidase was required. The simultaneous saccharification and lipid production process gave a lipid coefficient of 129.4 mg/g pretreated corn stover. A higher lipid coefficient of 159.4 mg/g pretreated corn stover was obtained using the simultaneous saccharification and enhanced lipid production (SSELP) process. Furthermore, cellulolytic enzymes were found recoverable and reusable upon recycling the spent supernatants of the SSELP process, which could reduce enzyme consumption and wastewater discharge.

CONCLUSIONS

The SSELP process was superior to other processes in terms of converting alkaline-pretreated corn stover into lipids by C. curvatus, as it required less cellulolytic enzymes and had a higher lipid coefficient. Moreover, the process facilitated easy enzyme recycling that should lead to further reduction of enzyme consumption. These results provide valuable information for cost-effective lipid production from lignocelluloses, which should be particularly important in achieving a sustainable production of biodiesel.

Lipid fermentation of corncob residues hydrolysate by oleaginous yeast Trichosporon cutaneum

Corncob residues (CCR) are cellulose residues of corncob after xylan (hemicellulose) is extracted for production of xylitol. Here, an oleaginous yeast Trichosporon cutaneum ACCC 20271 was screened for lipid fermentation using CCR hydrolysate. The initial carbon-to-nitrogen molar ratio (C/N ratio) and the initial sugar concentration of the CCR hydrolysate were investigated in the lipid fermentation of T. cutaneum ACCC 20271. A C/N ratio gradient was generated by changing the corn steep liquor (CSL) addition and an optimal C/N ratio of 49.3 was obtained. The different initial sugar concentration was obtained by changing the cellulase amount and the lipid titer was enhanced by the increased sugar concentration. To our knowledge, this is the first report on using CCR as the feedstock for lipid fermentation. The lipid titer of 12.3g/L and dry cell weight (DCW) of 38.4 g/L were the highest values among the studies using lignocellulose for lipid production.

Evaluating the effect of medium composition and fermentation condition on the microbial oil production by Trichosporon cutaneum on corncob acid hydrolysate

The effect of medium composition and cultural condition on the growth and lipid accumulation of oleaginous yeast Trichosporon cutaneum on corncob acid hydrolysate was systematically investigated. Glucose, xylose, and cellobiose were shown to be promising sugar for lipid production by T. cutaneum. Adding other nitrogen sources into the hydrolysate was not beneficial for the lipid production possibly due to the existence of other nitrogen sources in it. Interestingly, adding MgSO4·7H2O, CuSO4·5H2O, MnSO4·H2O, and KCl (optimal concentration were 0.3, 3.0×10(-3), 3.0×10(-3), and 0.4 g/L, respectively) could stimulate the lipid production by T. cutaneum. Additionally, inoculum concentration, temperature, and initial pH (optimal value were 5%, 28 °C, and 6.0, respectively) showed influence on the lipid production of T. cutaneum. Under the optimum conditions, the biomass (22.9 g/L) had a weak increase (3.6%), while the lipid content (45.4%) and lipid coefficient (22.9%) increased obviously (about 26.5% and 31.6%) compared with the initial conditions.

Efficient conversion of biomass into lipids by using the simultaneous saccharification and enhanced lipid production process

BACKGROUND

Microbial lipid production by using lignocellulosic biomass as the feedstock holds a great promise for biodiesel production and biorefinery. This usually involves hydrolysis of biomass into sugar-rich hydrolysates, which are then used by oleaginous microorganisms as the carbon and energy sources to produce lipids. However, the costs of microbial lipids remain prohibitively high for commercialization. More efficient and integrated processes are pivotal for better techno-economics of microbial lipid technology.

RESULTS

Here we describe the simultaneous saccharification and enhanced lipid production (SSELP) process that is highly advantageous in terms of converting cellulosic materials into lipids, as it integrates cellulose biomass hydrolysis and lipid biosynthesis. Specifically, Cryptococcus curvatus cells prepared in a nutrient-rich medium were inoculated at high dosage for lipid production in biomass suspension in the presence of hydrolytic enzymes without auxiliary nutrients. When cellulose was loaded at 32.3 g/L, cellulose conversion, cell mass, lipid content and lipid coefficient reached 98.5%, 12.4 g/L, 59.9% and 204 mg/g, respectively. Lipid yields of the SSELP process were higher than those obtained by using the conventional process where cellulose was hydrolyzed separately. When ionic liquid pretreated corn stover was used, both cellulose and hemicellulose were consumed simultaneously. No xylose was accumulated over time, indicating that glucose effect was circumvented. The lipid yield reached 112 mg/g regenerated corn stover. This process could be performed without sterilization because of the absence of auxiliary nutrients for bacterial contamination.

CONCLUSIONS

The SSELP process facilitates direct conversion of both cellulose and hemicellulose of lignocellulosic materials into microbial lipids. It greatly reduces time and capital costs while improves lipid coefficient. Optimization of the SSELP process at different levels should further improve the efficiency of microbial lipid technology, which in turn, promote the biotechnological production of fatty acid-derived products from lignocellulosic biomass.

Exometabolomics approaches in studying the application of lignocellulosic biomass as fermentation feedstock

Lignocellulosic biomass is the future feedstock for the production of biofuel and bio-based chemicals. The pretreatment-hydrolysis product of biomass, so-called hydrolysate, contains not only fermentable sugars, but also compounds that inhibit its fermentability by microbes. To reduce the toxicity of hydrolysates as fermentation media, knowledge of the identity of inhibitors and their dynamics in hydrolysates need to be obtained. In the past decade, various studies have applied targeted metabolomics approaches to examine the composition of biomass hydrolysates. In these studies, analytical methods like HPLC, RP-HPLC, CE, GC-MS and LC-MS/MS were used to detect and quantify small carboxylic acids, furans and phenols. Through applying targeted metabolomics approaches, inhibitors were identified in hydrolysates and their dynamics in fermentation processes were monitored. However, to reveal the overall composition of different hydrolysates and to investigate its influence on hydrolysate fermentation performance, a non-targeted metabolomics study needs to be conducted. In this review, a non-targeted and generic metabolomics approach is introduced to explore inhibitor identification in biomass hydrolysates, and other similar metabolomics questions.