Simultaneous saccharification and microbial lipid fermentation of corn stover by oleaginous yeast 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.

Combination of simultaneous saccharification and fermentation of corn stover with consolidated bioprocessing of cassava starch enhances lipid production by the amylolytic oleaginous yeast Lipomyces starkeyi

Highly integrated processes are crucial for the commercial success of microbial lipid production from low-cost substrates. Here, combination of simultaneous saccharification and fermentation (SSF) of corn stover with consolidated bioprocessing (CBP) of cassava starch by Lipomyces starkeyi was firstly developed as a novel strategy for lipid production. Starch was quickly hydrolyzed within 24 h by the amylolytic enzymes secreted by L. starkeyi to provide adequate fermentable sugars at the initial stage of culture, which eliminated the pre-hydrolysis step. More interestingly, synergistic effect for achieving higher lipid production by combined utilization of corn stover and cassava starch at relatively low enzyme dosage was realized, in comparison with the separate utilization of these two substrates. The fatty acid profiles indicated that lipid prepared by the combination strategy was suitable precursor for biodiesel production. The combined SSF&CBP strategy offers a simplified, highly-efficient, and economical route for co-valorization of low-cost substrates into lipids.

Optimization of Pretreatment Conditions and Enzymatic Hydrolysis of Corn Cobs for Production of Microbial Lipids by Trichosporon oleaginosus

Microbial lipids produced from lignocellulosic biomass are sustainable alternative feedstock for biodiesel production. In this study, corn cobs were used as a carbon source for lipid production and growth of oleaginous yeast Trichosporon oleaginosus. Lignocellulosic biomass was subjected to alkali and acid pretreatment using sulfuric acid and sodium hydroxide under different temperatures, catalyst concentrations and treatment times. Pretreatment of corn cobs was followed by cellulase hydrolysis. Hydrolysis of alkali pretreated (2% NaOH at 50 °C for 6 h, 1% NaOH at 50 °C for 16 h, 2% NaOH at 121 °C for 1 h, 1% NaOH at 121 °C for 2 h) and acid pretreated (1% H2SO4 120 °C for 20 min, and 2% H2SO4 120 °C for 10 min) corn cobs resulted in more than 80% of the theoretical yield of glucose. The effect of substrate (5, 10, 15 and 20%, g g−1) and cellulase loading (15 and 30 Filter Paper Units per gram of glucan, FPU g−1) on fermentable sugar yield was also studied. The maximal glucose concentration of 81.64 g L−1 was obtained from alkali-pretreated corn cobs (2% NaOH at 50 °C for 6 h) at 20% substrate loading and 30 FPU of Cellic CTec2 g−1 of glucan. Enzymatic hydrolysates of pretreated biomasses and filtrates of lignocellulosic slurries obtained after pretreatment were used for growth and lipid synthesis by T. oleaginosus. The highest lipid concentration of 18.97 g L−1 was obtained on hydrolysate of alkali-pretreated corn cobs (with 1% NaOH at 50 °C for 16 h) using a 15% (g g−1) substrate loading and 15 FPU g−1 of cellulase loading. Significant lipid accumulation was also achieved using undetoxified filtrates of pretreated slurries as substrates. Results showed that pretreated corn cobs and undetoxified filtrates are suitable carbon sources for the growth and efficient accumulation of lipids in T. oleaginosus.

Photosynthetic bacteria with iron oxide nanoparticles as catalyst for cooking oil removal and valuable products recovery with heavy metal co-contamination

Waste cooking oil discharge causes environmental pollution in receiving waters, particularly when associated with heavy metals that can lead to formation of hazardous organometallic compounds. This study combined iron oxide nanomaterial and the anoxygenic photosynthetic bacterium Rhodopseudomonas faecalis PA2 for removal of cooking oil in the presence of heavy metals. R. faecalis PA2, with known capability to generate beneficial substances from several wastes, was capable of cooking oil removal with production of valuable products. Oil removal, biomass, protein, and carotenoid production were 82.38%, 1.48 g/L, 1,600.19 mg/L, and 1,046.33 mg/L, respectively, under optimal conditions (cooking oil as carbon source and 30% inoculum density). Iron (Fe) stimulates growth of R. faecalis; in this study, Fe₃O₄ nanoparticles were synthesized and used as a catalyst to facilitate interaction and high reactivity between Fe and R. faecalis PA2. Size measurement by transmission electron microscopy (17.44 nm), X-ray diffraction peaks, and magnetic susceptibility confirmed that the synthesized nanoparticles were magnetite Fe₃O₄. Biomass, protein, and carotenoid production of the Fe₃O₄ supplemented experiment increased by 61.56%, 70.78%, and 57.2%, respectively, when compared with the control. When different concentrations of heavy metals (Pb, Ni, Co, and Zn) were supplemented in the media containing cooking oil, Fe₃O₄ addition increased heavy metal tolerance, improved bacterial growth, and enhanced valuable products when compared with the non-supplemented group. This study reports the positive impact of nanoparticle application as a catalyst for valorization of cooking oil waste with heavy metal co-contamination by the photosynthetic bacterium R. faecalis PA2.

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.

Strategies for Improvement of Lipid Production by Yeast Trichosporon oleaginosus from Lignocellulosic Biomass

Microbial lipids have similar fatty acid composition to plant oils, and therefore, are considered as an alternative feedstock for biodiesel production. Oleaginous yeasts accumulate considerable amounts of lipids intracellularly during growth on low-cost renewable feedstocks such as lignocellulosic biomass. In this study, we cultivated yeast Trichosporon oleaginosus on hydrolysate of alkaline pretreated corn cobs. Different process configurations were evaluated and compared, including separate hydrolysis and fermentation (SHF) with cellulase recycle and simultaneous saccharification and fermentation (SSF) in batch and fed-batch mode. At low enzyme loading, the highest lipid concentration of 26.74 g L⁻¹ was reached in fed-batch SSF fed with 2.5% (g g⁻¹) substrate. Batch SHF was conducted for four rounds with recycling the cellulase adsorbed on unhydrolyzed lignocellulosic biomass. Thirty percent of cellulase saving was achieved for rounds 2–4 without compromising productivity and lipid yield. The addition of Tween 80 to lignocellulosic slurry improved the hydrolysis rate of structural carbohydrates in pretreated lignocellulosic biomass. Furthermore, supplementing the growth medium with Tween 80 improved lipid yield and productivity without affecting yeast growth. Oleaginous yeast T. oleaginosus is a promising strain for the sustainable and efficient production of lipids from renewable lignocellulosic feedstock.

Tolerance of Trichosporon cutaneum to lignin derived phenolic aldehydes facilitate the cell growth and cellulosic lipid accumulation

Phenolic aldehydes are the major inhibitors from lignocellulose pretreatment. Previous studies show that oleaginous yeasts are difficult to survive in lignocellulosic hydrolysates even after the removal of furan aldehydes and organic acids inhibitors. This study investigated the cell viability, sugar consumption and lipid accumulation of the major oleaginous yeasts including Trichosporon cutaneum, Rhodosporidium toruloides, Rhodotorula glutinis, Yarrowia lipolytica in wheat straw hydrolysate containing only phenolic aldehydes after furan aldehydes and organic acids were selectively degraded by microorganisms. The results confirmed that the existence of residual phenolic aldehydes was the major reason for poor cell growth and metabolism of oleaginous yeasts. Only T. cutaneum demonstrated the higher tolerance by biodegrading phenolic aldehydes and the satisfactory cell growth and lipid production were obtained. This study revealed that T. cutaneum might be one of the promising cell factories for microbial lipid production from lignocellulosic feedstock.

Biodiesel Production From Lignocellulosic Biomass Using Oleaginous Microbes: Prospects for Integrated Biofuel Production

Biodiesel is an eco-friendly, renewable, and potential liquid biofuel mitigating greenhouse gas emissions. Biodiesel has been produced initially from vegetable oils, non-edible oils, and waste oils. However, these feedstocks have several disadvantages such as requirement of land and labor and remain expensive. Similarly, in reference to waste oils, the feedstock content is succinct in supply and unable to meet the demand. Recent studies demonstrated utilization of lignocellulosic substrates for biodiesel production using oleaginous microorganisms. These microbes accumulate higher lipid content under stress conditions, whose lipid composition is similar to vegetable oils. In this paper, feedstocks used for biodiesel production such as vegetable oils, non-edible oils, oleaginous microalgae, fungi, yeast, and bacteria have been illustrated. Thereafter, steps enumerated in biodiesel production from lignocellulosic substrates through pretreatment, saccharification and oleaginous microbe-mediated fermentation, lipid extraction, transesterification, and purification of biodiesel are discussed. Besides, the importance of metabolic engineering in ensuring biofuels and biorefinery and a brief note on integration of liquid biofuels have been included that have significant importance in terms of circular economy aspects.

Consolidated bioprocesses for efficient bioconversion of palm biomass wastes into biodiesel feedstocks by oleaginous fungi and yeasts

Consolidated bioprocesses for bioconversion of lignocellulosic biomass into biodiesel feedstocks were developed. Palm empty fruit bunch (EFB) was biologically pretreated coupling with fungal lipid production (121.4 ± 2.7 mg/g-EFB) by lignocellulolytic oleaginous fungi prior to lipid production by oleaginous yeasts. In subsequent separate hydrolysis and fermentation (SHF) of fungal pretreated EFB (FPEFB), the oleaginous yeast with the maximum lipid yield of 37.0 ± 0.1 mg/g-FPEFB was screened. While a higher lipid yield of 47.9 ± 1.5 mg/g-FPEFB was achieved in simultaneous saccharification and fermentation (SSF) with less enzyme requirement. Fed-batch SSF of non-sterile FPEFB was proven as a practical and efficient strategy to increase lipid yield up to 53.4 ± 0.5 mg/g-FPEFB. Total lipid yield by both fungi and yeast was 165.0 ± 4.4 mg/g-EFB. Interestingly, the consolidated bioprocesses of enzyme and lipid production also achieved comparable total lipid yield of 149.3 ± 6.6 mg/g-EFB. These strategies may contribute greatly to cost-effective and sustainable bioconversion of lignocellulosic biomass into biodiesel feedstocks.

Lignocellulosic Biomass as a Substrate for Oleaginous Microorganisms: A Review

Microorganisms capable of accumulating lipids in high percentages, known as oleaginous microorganisms, have been widely studied as an alternative for producing oleochemicals and biofuels. Microbial lipid, so-called Single Cell Oil (SCO), production depends on several growth parameters, including the nature of the carbon substrate, which must be efficiently taken up and converted into storage lipid. On the other hand, substrates considered for large scale applications must be abundant and of low acquisition cost. Among others, lignocellulosic biomass is a promising renewable substrate containing high percentages of assimilable sugars (hexoses and pentoses). However, it is also highly recalcitrant, and therefore it requires specific pretreatments in order to release its assimilable components. The main drawback of lignocellulose pretreatment is the generation of several by-products that can inhibit the microbial metabolism. In this review, we discuss the main aspects related to the cultivation of oleaginous microorganisms using lignocellulosic biomass as substrate, hoping to contribute to the development of a sustainable process for SCO production in the near future.

Enhanced biohydrogen production from cotton stalk hydrolysate of Enterobacter cloacae WL1318 by overexpression of the formate hydrogen lyase activator gene

BACKGROUND

Biohydrogen production from lignocellulose has become an important hydrogen production method due to its diversity, renewability, and cheapness. Overexpression of the formate hydrogen lyase activator (fhlA) gene is a promising tactic for enhancement of hydrogen production in facultative anaerobic Enterobacter. As a species of Enterobacter, Enterobacter cloacae was reported as a highly efficient hydrogen-producing bacterium. However, little work has been reported in terms of cloning and expressing the fhlA gene in E. cloacae for lignocellulose-based hydrogen production.

RESULTS

In this study, the formate hydrogen lyase activator (fhlA) gene was cloned and overexpressed in Enterobacter cloacae WL1318. We found that the recombinant strain significantly enhanced cumulative hydrogen production by 188% following fermentation of cotton stalk hydrolysate for 24 h, and maintained improved production above 30% throughout the fermentation process compared to the wild strain. Accordingly, overexpression of the fhlA gene resulted in an enhanced hydrogen production potential (P) and maximum hydrogen production rate (R _m), as well as a shortened lag phase time (λ) for the recombinant strain. Additionally, the recombinant strain also displayed improved glucose (12%) and xylose (3.4%) consumption and hydrogen yield Y(H₂/S) (37.0%) compared to the wild strain. Moreover, the metabolites and specific enzyme profiles demonstrated that reduced flux in the competitive branch, including succinic, acetic, and lactic acids, and ethanol generation, coupled with increased flux in the pyruvate node and formate splitting branch, benefited hydrogen synthesis.

CONCLUSIONS

The results conclusively prove that overexpression of fhlA gene in E. cloacae WL1318 can effectively enhance the hydrogen production from cotton stalk hydrolysate, and reduce the metabolic flux in the competitive branch. It is the first attempt to engineer the fhlA gene in the hydrogen-producing bacterium E. cloacae. This work provides a highly efficient engineered bacterium for biohydrogen production from fermentation of lignocellulosic hydrolysate in the future.

Fungi (Mold)-Based Lipid Production

There is an increasing need for the development of alternative energy sources with a focus on reducing greenhouse gas emissions and striving toward a sustainable economy. Bioethanol and biodiesel are currently the primary choices of alternative transportation fuels. At present, biodiesel is not competitive with conventional fuel due to its high price, and the only way to compete with conventional fuel is to improve the quality, reduce the costs, and coproduce value-added products. With the high demand for lipids in the energy sector and other industrial applications, microbial lipids accumulated from microorganisms, especially oleaginous fungi and yeasts have been the important topic of many recent research studies. This chapter summarizes the current status of knowledge and technology about lipid production by oleaginous fungi and yeasts for biofuel applications and other value-added products. The chapter focuses on several aspects such as the most promising oleaginous strains, strain development, improvement of lipid production, methods and protocols to cultivate oleaginous fungi, substrate utilization, fermentation process design, and downstream processing. The feasibility and challenges during the large-scale commercial production of microbial lipids as fuel sources are also discussed. It provides an overview of microbial lipid production biorefinery and also future development directions.

Valorization of palm biomass wastes for biodiesel feedstock and clean solid biofuel through non-sterile repeated solid-state fermentation

Palm biomass wastes are currently considered as promising solid biofuels. However, their high potassium content leads to formation of slag in combustion chambers and causes frequent power-plant shutdowns for maintenance. Therefore, this study aimed to develop a low-cost practical biological pretreatment for these wastes. Oleaginous fungi Aspergillus tubingensis TSIP9, which originates from palm wastes, was used to pretreat biomass wastes and simultaneously produce oils through non-sterile solid state fermentation (SoSF). The operating conditions were optimized through response surface methodology. The fungi could grow and produce oils with good biodiesel fuel properties. After SoSF, potassium content in biomass wastes was reduced by 90% and cellulose content increased to >57%, making it suitable as clean solid biofuel. Repeated-SoSF with 90% substrate replacement was highly effective in continuously pretreating biomass wastes and producing fungal oils. This study demonstrates the cost-effective and environmentally friendly process for production of clean renewable energy through zero-waste strategy.

Microbial lipid production from dilute acid and dilute alkali pretreated corn stover via Trichosporon dermatis

Microbial lipid production from lignocellulosic biomass has attracted much attention recently. In this study, T. dermatis 32903 was selected from eleven promising oleaginous yeast strains. Carbon to nitrogen ratio (C/N) was investigated and optimized to maximize lipid production. Dilute acid (DA) pretreated corn stover (CS) and dilute alkali (AL) pretreated CS were then used for microbial lipid production, resulting in lipid concentrations of 7.46 g/L and 6.81 g/L, with sugar to lipid yields reached 0.104 g/g and 0.101 g/g, respectively. Washing of DA-CS and AL-CS enhanced lipid production to 11.43 g/L and 20.36 g/L with sugar to lipid yields improved to 0.156 g/g and 0.186 g/g, respectively. As degradation products in pretreated biomass showed severe inhibition on lipid fermentation, eight typical degradation products were further investigated for their effects on lipid fermentation. T. dermatis 32903 exhibited high tolerance to furan derivatives and week acids, but lower tolerance to phenolic compounds.

Deficiency of β-Glucosidase Beneficial for the Simultaneous Saccharification and Lipid Production by the Oleaginous Yeast Lipomyces starkeyi

It is inevitably for cellobiose to be co-generated during enzymatic hydrolysis of cellulose, especially when the cellulase is lack of β-glucosidase activity. In the present study, cellobiose was found superior to glucose for cell growth by L. starkeyi, regardless of the sugar concentrations. Glucose was assimilated preferentially when cellobiose and glucose were co-fermented. Deficiency of β-glucosidase was observed to be beneficial for the simultaneous saccharification and lipid production (SSLP). High lipid titer and cellulose conversion of 9.1 g/L and 92.4%, respectively, were achieved when cellulase with low β-glucosidase activity was supplemented. The SSLP achieved higher lipid titer of 9.5 g/L when a pre-hydrolysis process was introduced. The glucosidase generated by L. starkeyi was primarily cell-bound, which contributed significantly to the cellobiose utilization and the high lipid production. These results provided a novel scheme for enhanced lipid production from lignocellulosic biomass with reduced enzyme usage, which is believed to facilitate the design of a more cost-effective lignocellulose-to-lipid route.

Microbial Lipid Production from Corn Stover by the Oleaginous Yeast Rhodosporidium toruloides Using the PreSSLP Process

Dry acid pretreatment and biodetoxification (DryPB) has been considered as an advanced technology to treat lignocellulosic materials for improved downstream bioconversion. In this study, the lipid production from DryPB corn stover was investigated by the oleaginous yeast Rhodosporidium toruloides using a new process designated prehydrolysis followed by simultaneous saccharification and lipid production (PreSSLP). The results found that prehydrolysis at 50 °C and then lipid production at 30 °C improved lipid yield by more than 17.0% compared with those without a prehydrolysis step. The highest lipid yield of 0.080 g/g DryPB corn stover was achieved at a solid loading of 12.5%. The fatty acid distribution of lipid products was similar to those of conventional vegetable oils that are used for biodiesel production. Our results suggested that the integration of DryPB process and PreSSLP process can be explored as an improved technology for microbial lipid production from lignocellulosic materials.

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.

Triacylglycerols accumulation and glycolipids secretion by the oleaginous yeast Rhodotorula babjevae Y-SL7: Structural identification and biotechnological applications

The newly isolated oleaginous yeast, Rhodotorula babjevae Y-SL7, was shown to accumulate high intracellular content of microbial oil (mainly triacylglycerols) and to secret, under the same culture conditions, an atypical glycolipid. This unusual behavior was induced when the strain was subjected to nitrogen limitation and high amount of carbon. A series of analytical methods was adopted in order to structurally characterize the secreted glycolipid. The latter consists of a mixture of 9 molecules formed by a polyol head group, bound through the carboxyl end of an acetylated 3-hydroxy fatty acid with C18 or C16 chain length. In addition of their physicochemical properties such as emulsifying activity on hydrophobic substrates, Y-SL7 glycolipids have shown a therapeutically promising cytotoxic effect against different cancer cell lines. In fact, Y-SL7 strain can be used for the production of triacylglycerols as energetic molecules and for the secretion of a biosurfactant of therapeutic and environmental interest.

Potential of aquatic oomycete as a novel feedstock for microbial oil grown on waste sugarcane bagasse

Biodiesel production from vegetable oils is not sustainable and economical due to the food crisis worldwide. The development of a cost-effective non-edible feedstock is essential. In this study, we proposed to use aquatic oomycetes for microbial oils, which are cellulolytic fungus-like filamentous eukaryotic microorganisms, commonly known as water molds. They differ from true fungi as cellulose is present in their cell wall and chitin is absent. They show parasitic as well as saprophytic nature and have great potential to utilize decaying animal and plant debris in freshwater habitats. To study the triacylglycerol (TAG) accumulation in the aquatic oomycetes, the isolated water mold Achlya diffusa was cultivated under semi-solid-state conditions on waste sugarcane bagasse, which was compared with the cultivation in Czapek (DOX) medium. A. diffusa grown on waste sugarcane bagasse showed large lipid droplets in its cellular compartment and synthesized 124.03 ± 1.93 mg/gds cell dry weight with 50.26 ± 1.76% w/w lipid content. The cell dry weight and lipid content of this water mold decreased to 89.54 ± 1.21 mg/gds and 38.82% w/w, respectively, when cultivated on standard medium Czapek-Dox agar (CDA). For the fatty acid profile of A. diffusa grown in sugarcane bagasse and CDA, in situ transesterification (IST) and indirect transesterification (IDT) approaches were evaluated. The lipid profile of this mold revealed the presence of C_12:0, C_14:0, C_16:0, C_18:0, C_18:1, C_18:2, C_20:0, and C_21:0 fatty acids, which is similar to vegetable oils. The biodiesel properties of the lipids obtained from A. diffusa satisfied the limits as determined by international standards ASTM-D6751 and EN-14214 demonstrating its suitability as a fuel for diesel engines.

Cascade hydrolysis and fermentation of corn stover for production of high titer gluconic and xylonic acids

Simultaneous saccharification and fermentation (SSF) is an efficient fermentation operation in lignocellulose biorefining. However, SSF may not be applicable when the pH values of hydrolysis and fermentation do not match, or the strong intermediate inhibitors on cellulase activity are generated. This study proposed a cascade hydrolysis and fermentation (CHF) process for cellulosic gluconic acid fermentation to overcome the inhibition of the intermediate glucono-γ-lactone on cellulase activity. The pretreated and detoxified corn stover feedstock was enzymatically hydrolyzed into hydrolysate slurry, then gluconic acid and xylonic acid fermentations were directly conducted by inoculating Gluconobacter oxydans strain without solid/liquid separation. The sugar loss and energy consumption were effectively avoided by moving the solid/liquid separation into the fermentation stage. The experiments and the techno-economic analysis show that the CHF is simple and cost effective fermentation operation when SSF is not applicable.

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.

Efficient bioconversion from acid hydrolysate of waste oleaginous yeast biomass after microbial oil extraction to bacterial cellulose by Komagataeibacter xylinus

Biomass acid hydrolysate of oleaginous yeast Trichosporon cutaneum after microbial oil extraction was applied as substrate for bacterial cellulose (BC) production by Komagataeibacter xylinus (also named as Gluconacetobacter xylinus previously) for the first time. BC was synthesized in static culture for 10 days, and the maximum BC yield (2.9 g/L) was got at the 4th day of fermentation. Most carbon sources in the substrate (glucose, mannose, formic acid, acetic acid) can be utilized by K. xylinus. The highest chemical oxygen demand (COD) removal (40.7 ± 3.0%) was obtained at the 6th day of fermentation, and then the COD increased possibly due to the degradation of BC. The highest BC yield on COD consumption was 38.7 ± 4.0% (w/w), suggesting that this is one efficient bioconversion for BC production. The BC structure was affected little by the substrate by comparison with that generated in classical HS medium using field-emission scanning electron microscope (FE-SEM), Fourier transform infrared, and X-ray diffraction. Overall, this technology can both solve the issue of waste oleaginous yeast biomass and produce valuable biopolymer (BC).

Single cell oil production by Trichosporon cutaneum from steam-exploded corn stover and its upgradation for production of long-chain α,ω-dicarboxylic acids

BACKGROUND

Single cell oil (SCO) production from lignocelluloses by oleaginous microorganisms is still high in production cost, making the subsequent production of biofuels inviable economically in such an era of low oil prices. Therefore, how to upgrade the final products of lignocellulose-based bioprocess to more valuable ones is becoming a more and more important issue.

RESULTS

Differently sourced cellulases were compared in the enzymatic hydrolysis of the steam-exploded corn stover (SECS) and the cellulase from the mixed culture of Trichoderma reesei and Aspergillus niger was found to have the highest enzymatic hydrolysis yield 86.67 ± 4.06%. Three-stage enzymatic hydrolysis could greatly improve the efficiency of the enzymatic hydrolysis of SECS, achieving a yield of 74.24 ± 2.69% within 30 h. Different bioprocesses from SECS to SCO were compared and the bioprocess C with the three-stage enzymatic hydrolysis was the most efficient, producing 57.15 g dry cell biomass containing 31.80 g SCO from 327.63 g SECS. An efficient and comprehensive process from corn stover to long-chain α,ω-dicarboxylic acids (DCAs) was established by employing self-metathesis, capable of producing 6.02 g long-chain DCAs from 409.54 g corn stover and 6.02 g alkenes as byproducts.

CONCLUSIONS

On-site cellulase production by the mixed culture of T. reesei and A. niger is proven the most efficient in providing cellulase to the lignocellulose-based bioprocess. Three-stage enzymatic hydrolysis was found to have very good application value in SCO production by Trichosporon cutaneum from SECS. A whole process from corn stover to long-chain DCAs via a combination of biological and chemical approaches was successfully established and it is an enlightening example of the comprehensive utilization of agricultural wastes.

Efficient Conversion of Fructose-Based Biomass into Lipids with Trichosporon fermentans Under Phosphate-Limited Conditions

Limiting nitrogen supply has been routinely used as the master regulator to direct lipid biosynthesis. However, this strategy does not work with nitrogen-rich substrates, such as Jerusalem artichoke (JA), a fructose-based biomass, while it is difficult to obtain a high carbon-to-nitrogen (C/N) molar ratio. In this study, an alternative strategy to promote lipid accumulation by the oleaginous yeast Trichosporon fermentans CICC 1368 was developed by limiting phosphorous supply, and this strategy was implemented with JA hydrolysate as substrate. We showed that lipid accumulation was directly correlated with the C/P ratio of the culture media for T. fermentans. The time course of cell growth and lipid production was analyzed in a media with an initial C/P ratio of 6342, and the cellular lipid content could reach up to 48.5% of dry biomass. Moreover, JA hydrolysates were used as substrate for microbial lipid accumulation, under high C/P molar ratio condition, lipid yield, lipid content, and lipid coefficient increased by 10, 30, and 34%, respectively. It showed that by limiting phosphorus, the conversion of sugar into lipids can be improved effectively. Limiting phosphorus provides a promising solution to the problem of microbial lipid production with nitrogen-rich natural materials.

Inhibitor degradation and lipid accumulation potentials of oleaginous yeast Trichosporon cutaneum using lignocellulose feedstock

Oleaginous yeast Trichosporon cutaneum is robust to high levels of lignocellulose derived inhibitor compounds with considerable lipid accumulation capacity. The potential of lipid accumulation of T. cutaneum ACCC 20271 was investigated using corn stover hydrolysates with varying sugar and inhibitor concentrations. Biodiesel was synthesized using the extracted lipid and the product satisfied the ASTM standards. Among the typical inhibitors, T. cutaneum ACCC 20271 is relatively sensitive to furfural and 4-hydroxybenzaldehyde, but strongly tolerant to high titers of formic acid, acetic acid, levulinic acid, HMF, vanillin, and syringaldehyde. It is capable of complete degradation of formic acid, acetic acid, vanillin and 4-hydroxybenzaldehyde. Finally, the inhibitor degradation pathways of T. cutaneum ACCC 20271 were constructed based on the newly sequenced whole genome information and the experimental results. The study provided the first insight to the inhibitor degradation of T. cutaneum and demonstrated the potentials of lipid production from lignocellulose.

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.

Production of single cell oil from Lipomyces starkeyi ATCC 56304 using biorefinery by-products

Single cell oil (SCO) is a valuable noncrop-based renewable oil source. Hemicellulose derived sugars can be utilized to produce SCO using the oleaginous yeast Lipomyces starkeyi ATCC 56304. Bran by-products were tested as hemicellulose-rich feedstocks for the production of SCO. Whole and destarched corn and wheat bran hydrolysates were produced using hydrothermal and dilute sulfuric acid (0%, 0.5%, 1.0%, v/v) pretreatment along with enzymatic hydrolysis. Whole bran hydrolysates produced from hydrothermal pretreatment generated the highest average oil yields of 126.7 and 124.3 mg oil/g sugar for both wheat and corn bran, respectively. 1.0% acid pretreatment was effective for the destarched bran generating a hemicellulose hydrolysis efficiency of 94% and 84% for wheat and corn bran, respectively, resulting in the highest oil yield of 70.7 mg oil/g sugar. The results indicate pretreated corn and wheat bran hydrolysates can serve as viable feedstocks for oleaginous yeast SCO bioconversion.

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.

Engineering Aspergillus oryzae A-4 through the chromosomal insertion of foreign cellulase expression cassette to improve conversion of cellulosic biomass into lipids

A genetic modification scheme was designed for Aspergillus oryzae A-4, a natural cellulosic lipids producer, to enhance its lipid production from biomass by putting the spotlight on improving cellulase secretion. Four cellulase genes were separately expressed in A-4 under the control of hlyA promoter, with the help of the successful development of a chromosomal genetic manipulation system. Comparison of cellulase activities of PCR-positive transformants showed that these transformants integrated with celA gene and with celC gene had significantly (p<0.05) higher average FPAase activities than those strains integrated with celB gene and with celD gene. Through the assessment of cellulosic lipids accumulating abilities, celA transformant A2-2 and celC transformant D1-B1 were isolated as promising candidates, which could yield 101%-133% and 35.22%-59.57% higher amount of lipids than the reference strain A-4 (WT) under submerged (SmF) conditions and solid-state (SSF) conditions, respectively. Variability in metabolism associated to the introduction of cellulase gene in A2-2 and D1-B1 was subsequently investigated. It was noted that cellulase expression repressed biomass formation but enhanced lipid accumulation; whereas the inhibitory effect on cell growth would be shielded during cellulosic lipids production owing to the essential role of cellulase in substrate utilization. Different metabolic profiles also existed between A2-2 and D1-B1, which could be attributed to not only different transgene but also biological impacts of different integration. Overall, both simultaneous saccharification and lipid accumulation were enhanced in A2-2 and D1-B1, resulting in efficient conversion of cellulose into lipids. A regulation of cellulase secretion in natural cellulosic lipids producers could be a possible strategy to enhance its lipid production from lignocellulosic biomass.

Chemical and thermal characterization of potato peel waste and its fermentation residue as potential resources for biofuel and bioproducts production

The growing demand for renewable fuels has driven the interest in the utilization of alternative waste materials such as potato peel waste (PPW) which contains fermentable carbohydrate. Fermentation of PPW using a mixed microbial consortium yielded about 60% unreacted PPW fermentation residue (PPW-FR). The PPW and PPW-FR were characterized by a combination of Fourier transform infrared (FTIR) and nuclear magnetic resonance (NMR) spectroscopies, gas chromatography-mass spectrometry (GC-MS), and thermogravimetric analysis (TGA) to quantify changes after fermentation. Fermentation of PPW resulted in fermentation of starch and concentrating lignin plus suberin and lipids in PPW-FR. TGA analysis showed that decomposition peaks differed for PPW (423 °C) and PPW-FR (457 °C). Pyrolysis-GC/MS showed an increase in phenolic and long chain fatty acid compounds with a concomitant decrease in carbohydrate derived compounds in the PPW after fermentation. Both the PPW and PPW-FR have shown potential based on properties to be converted into crude biofuel via thermochemical processes.

Identification of oleaginous yeast strains able to accumulate high intracellular lipids when cultivated in alkaline pretreated corn stover

Microbial oil is a potential alternative to food/plant-derived biodiesel fuel. Our previous screening studies identified a wide range of oleaginous yeast species, using a defined laboratory medium known to stimulate lipid accumulation. In this study, the ability of these yeasts to grow and accumulate lipids was further investigated in synthetic hydrolysate (SynH) and authentic ammonia fiber expansion (AFEX™)-pretreated corn stover hydrolysate (ACSH). Most yeast strains tested were able to accumulate lipids in SynH, but only a few were able to grow and accumulate lipids in ACSH medium. Cryptococcus humicola UCDFST 10-1004 was able to accumulate as high as 15.5 g/L lipids, out of a total of 36 g/L cellular biomass when grown in ACSH, with a cellular lipid content of 40 % of cell dry weight. This lipid production is among the highest reported values for oleaginous yeasts grown in authentic hydrolysate. Preculturing in SynH media with xylose as sole carbon source enabled yeasts to assimilate both glucose and xylose more efficiently in the subsequent hydrolysate medium. This study demonstrates that ACSH is a suitable medium for certain oleaginous yeasts to convert lignocellullosic sugars to triacylglycerols for production of biodiesel and other valuable oleochemicals.

Oleaginous fungal lipid fermentation on combined acid- and alkali-pretreated corn stover hydrolysate for advanced biofuel production

A combined hydrolysis process, which first mixed dilute acid- and alkali-pretreated corn stover at a 1:1 (w/w) ratio, directly followed by enzymatic saccharification without pH adjustment, has been developed in this study in order to minimize the need of neutralization, detoxification, and washing during the process of lignocellulosic biofuel production. The oleaginous fungus Mortierella isabellina was selected and applied to the combined hydrolysate as well as a synthetic medium to compare fungal lipid accumulation and biodiesel production in both shake flask and 7.5L fermentor. Fungal cultivation on combined hydrolysate exhibited comparable cell mass and lipid yield with those from synthetic medium, indicating that the integration of combined hydrolysis with oleaginous fungal lipid fermentation has great potential to improve performance of advanced lignocellulosic biofuel production.

Carbon source utilization and inhibitor tolerance of 45 oleaginous yeast species

Conversion of lignocellulosic hydrolysates to lipids using oleaginous (high lipid) yeasts requires alignment of the hydrolysate composition with the characteristics of the yeast strain, including ability to utilize certain nutrients, ability to grow independently of costly nutrients such as vitamins, and ability to tolerate inhibitors. Some combination of these characteristics may be present in wild strains. In this study, 48 oleaginous yeast strains belonging to 45 species were tested for ability to utilize carbon sources associated with lignocellulosic hydrolysates, tolerate inhibitors, and grow in medium without supplemented vitamins. Some well-studied oleaginous yeast species, as well as some that have not been frequently utilized in research or industrial production, emerged as promising candidates for industrial use due to ability to utilize many carbon sources, including Cryptococcus aureus, Cryptococcus laurentii, Hannaella aff. zeae, Tremella encephala, and Trichosporon coremiiforme. Other species excelled in inhibitor tolerance, including Candida aff. tropicalis, Cyberlindnera jadinii, Metschnikowia pulcherrima, Schwanniomyces occidentalis and Wickerhamomyces ciferrii. No yeast tested could utilize all carbon sources and tolerate all inhibitors tested. These results indicate that yeast strains should be selected based on characteristics compatible with the composition of the targeted hydrolysate. Other factors to consider include the production of valuable co-products such as carotenoids, availability of genetic tools, biosafety level, and flocculation of the yeast strain. The data generated in this study will aid in aligning yeasts with compatible hydrolysates for conversion of carbohydrates to lipids to be used for biofuels and other oleochemicals.

Characterization of fibrolytic and lipid accumulating fungi isolated from fresh cattle feces

To characterize coprophilous fungi for converting lignocellulose into lipids, four fungal strains utilizing cellulose microcrystalline and xylan were screened. The fungi were identified as Cladosporium sp. F1, Circinella sp. F6, Mycocladus sp. F49, and Byssochlamys sp. F52 based on the ITS1-5.8S-ITS2 sequence similarity. The strain F52 accumulated 336.0 mg/L reducing sugars on cottonseed shells treated with ethanol. The combination of F1+F52 increased the reducing sugar accumulating rates. However, the activities of avicelase and xylanase were not correlated with the reducing sugars accumulated by the test strains. Strains F6 and F52 produced higher cellular lipids (above 530.7 mg/L) than other strains. However, the strain F52 could produce more cellular lipids with xylose and mannose as the sole carbon sources. The results indicated that the reducing sugar contents accumulated by the different strains were influenced by the fungal taxa and ligocellulosic types. With fibrolytic and lipid accumulating activities, diverse fungi harboring in herbivore feces need to be further characterized.

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.

An oleaginous endophyte Bacillus subtilis HB1310 isolated from thin-shelled walnut and its utilization of cotton stalk hydrolysate for lipid production

BACKGROUND

Third generation biodiesel processing from microbial lipids using low-cost lignocellulosic feedstocks has attracted much attention. Endophytes isolated from oleaginous plants possibly have the capacity to accumulate lipids similar to the hosts. However, little work has been reported in terms of endophytic bacteria isolation from oleaginous plants and their lipid production using lignocellulosic hydrolysate as substrate.

RESULTS

A new oleaginous endophyte HB1310 has been isolated from the thin-shelled walnut, and identified as Bacillus subtilis on the basis of both 16S rDNA gene sequencing and examination of its physiological and biochemical properties. This strain effectively accumulates cellular lipids using cotton stalk hydrolysate as a substrate. The optimum C/N ratio, culture temperature, and pH value were determined to be 50/1, 30°C, and 6.5, respectively. Batch fermentation was conducted in a bioreactor using these parameters. Satisfactory production, with a maximum lipid productivity of 2.3 g/L, lipid content of 39.8% (w/w), and cell dry weight (CDW) of 5.7 g/L, was obtained at a culture time of 48 h. Variations in the fluorescent intensity and lipid inclusion formation of cells collected at different sampling times illustrate the potential of this bacterium to be useful for cellular lipid production. The fatty acid profile of the produced bacterial lipids showed that the major constituents are myristic, palmitic, stearic, oleic, and linoleic acids with an estimated cetane number of about 61.8, indicating that this strain may be suitable for biodiesel production.

CONCLUSIONS

The present investigation is the first report of an oleaginous endophytic bacterium isolated from the thin-shelled walnut. This strain is capable of producing high lipid contents rapidly using cotton stalk hydrolysate as a substrate, and its lipids are suitable for use as the feedstock for biodiesel production.

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.