Simultaneous utilization of glucose and xylose byCandida curvata D in continuous culture

Single Cell Oil Production by Oleaginous Yeasts Grown in Synthetic and Waste-Derived Volatile Fatty Acids

Four yeast isolates from the species—Apiotrichum brassicae, Candida tropicalis, Metschnikowia pulcherrima, and Pichia kudriavzevii—previously selected by their oleaginous character and growth flexibility in different carbon sources, were tested for their capacity to convert volatile fatty acids into lipids, in the form of single cell oils. Growth, lipid yields, volatile fatty acids consumption, and long-chain fatty acid profiles were evaluated in media supplemented with seven different volatile fatty acids (acetic, butyric, propionic, isobutyric, valeric, isovaleric, and caproic), and also in a dark fermentation effluent filtrate. Yeasts A. brassicae and P. kudriavzevii attained lipid productivities of more than 40% (w/w), mainly composed of oleic (>40%), palmitic (20%), and stearic (20%) acids, both in synthetic media and in the waste-derived effluent filtrate. These isolates may be potential candidates for single cell oil production in larger scale applications by using alternative carbon sources, combining economic and environmental benefits.

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.

New kids on the block: emerging oleaginous yeast of biotechnological importance

There is growing interest in using oleaginous yeast for the production of a variety of fatty acids and fatty acid-derived oleochemicals. This is motivated by natural propensity for high flux through lipid biosynthesis that has naturally evolved, making them a logical starting point for additional genetic engineering to improve titers and productivities. Much of the academic and industrial focus has centered on yeast that have significant genetic engineering tool capabilities, such as Yarrowia lipolytica, and those that have naturally high lipid accumulation, such as Rhodosporidium toruloides and Lipomyces starkeyi; however, there are oleaginous yeast with phenotypes better aligned with typically inhibitory process conditions, such as high salt concentrations and lignocellulosic derived inhibitors. This review addresses the foundational work in characterizing two emerging oleaginous yeast of interest: Debaryomyces hansenii and Trichosporon oleaginosus. We focus on the physiological and metabolic properties of these yeast that make each attractive for bioprocessing of lignocellulose to fuels and chemicals, discuss their respective genetic engineering tools and highlight the critical barriers facing the broader implementation of these oleaginous yeast.

Characterization of newly isolated oleaginous yeasts - Cryptococcus podzolicus, Trichosporon porosum and Pichia segobiensis

The yeast strains Cryptococcus podzolicus, Trichosporon porosum and Pichia segobiensis were isolated from soil samples and identified as oleaginous yeast strains beneficial for the establishment of microbial production processes for sustainable lipid production suitable for several industrial applications. When cultured in bioreactors with glucose as the sole carbon source C. podzolicus yielded 31.8% lipid per dry biomass at 20°C, while T. porosum yielded 34.1% at 25°C and P. segobiensis 24.6% at 25°C. These amounts correspond to lipid concentrations of 17.97 g/L, 17.02 g/L and 12.7 g/L and volumetric productivities of 0.09 g/Lh, 0.1 g/Lh and 0.07 g/Lh, respectively. During the culture of C. podzolicus 30 g/l gluconic acid was detected as by-product in the culture broth and 12 g/L gluconic acid in T. porosum culture. The production of gluconic acid was eliminated for both strains when glucose was substituted by xylose as the carbon source. Using xylose lipid yields were 11.1 g/L and 13.9 g/L, corresponding to 26.8% and 33.4% lipid per dry biomass and a volumetric productivity of 0.07 g/Lh and 0.09 g/Lh, for C. podzolicus and T. porosum respectively. The fatty acid profile analysis showed that oleic acid was the main component (39.6 to 59.4%) in all three strains and could be applicable for biodiesel production. Palmitic acid (18.4 to 21.1%) and linolenic acid (7.5 to 18.7%) are valuable for cosmetic applications. P. segobiensis had a considerable amount of palmitoleic acid (16% content) and may be suitable for medical applications.

Fourier transform infrared spectroscopy as a method to study lipid accumulation in oleaginous yeasts

BACKGROUND

Oleaginous microorganisms, such as different yeast and algal species, can represent a sustainable alternative to plant oil for the production of biodiesel. They can accumulate fatty acids (FA) up to 70% of their dry weight with a predominance of (mono)unsaturated species, similarly to what plants do, but differently from animals. In addition, their growth is not in competition either with food, feed crops, or with agricultural land.Despite these advantages, the exploitation of the single cell oil system is still at an early developmental stage. Cultivation mode and conditions, as well as lipid extraction technologies, represent the main limitations. The monitoring of lipid accumulation in oleaginous microorganisms is consequently crucial to develop and validate new approaches, but at present the majority of the available techniques is time consuming, invasive and, when relying on lipid extraction, can be affected by FA degradation.

RESULTS

In this work the fatty acid accumulation of the oleaginous yeasts Cryptococcus curvatus and Rhodosporidium toruloides and of the non-oleaginous yeast Saccharomyces cerevisiae (as a negative control) was monitored in situ by Fourier Transform Infrared Spectroscopy (FTIR). Indeed, this spectroscopic tool can provide complementary information to those obtained by classical techniques, such as microscopy, flow cytometry and gas chromatography. As shown in this work, through the analysis of the absorption spectra of intact oleaginous microorganisms it is possible not only to monitor the progression of FA accumulation but also to identify the most represented classes of the produced lipids.

CONCLUSIONS

Here we propose FTIR microspectroscopy - supported by multivariate analysis - as a fast, reliable and non invasive method to monitor and analyze FA accumulation in intact oleaginous yeasts. The results obtained by the FTIR approach were in agreement with those obtained by the other classical methods like flow cytometry and gas chromatography. Moreover, the possibility to track lipid production in real time is highly desirable to support the initial screening of strains and media as well as to optimize the scaling up experiments, which are essential for a viable and successful development of an industrial production process.

Lipid accumulation by a cellulolytic strain of Aspergillus niger

Lipid accumulation by a cellulolytic mold, Aspergillus niger, was studied. The amount of lipid accumulated ranged from 13.6-16.6% on various carbon sources, namely glucose, xylose, avicel (microcrystalline cellulose) and bagasse (a natural lignocellulosic substrate). Neutral lipids, phospholipids and glycolipids of the mycelia varied from 41.0-46.2%, 34.9-38.4% and 18.7-22.6% of total lipids, respectively. Unsaturated fatty acids comprised around 80% of total fatty materials with linoleic and oleic acid predominating. Of the four nitrogen sources tested, NH4Cl was the best source for lipid synthesis from cellulose (bagasse). Optimum temperature range for growth and lipid synthesis was 25-30 degrees C.

Conversion of pentoses to ethanol by yeasts and fungi

Fermentation of D-xylose is of interest in enhancing the yield of ethanol obtainable from lignocellulosic hydrolysates. Such hydrolysates can contain both pentoses and hexoses, and while technology to convert hexoses to ethanol is well established, the fermentation of pentoses had been problematical. To overcome the difficulty, yeasts and fungi have been sought and identified in recent years that can convert D-xylose into ethanol. However, operation of their cultures in the presence of the pentose to obtain rapid and efficient ethanol production is somewhat more complex than in the archetype alcoholic fermentation, Saccharomyces cerevisiae on D-glucose. The complexity stems, in part, from the association of ethanol accumulation in cultures where D-xylose is the sole carbon source with conditions that limit growth, by oxygen in particular, although limitation by other nutrients might also be implicated. Aspects of screening for appropriate organisms and of the parameters that play a role in determining culture variables, especially those associated with ethanol productivity, are reviewed. Performance with D-xylose as sole carbon source, in sugar mixtures, and in lignocellulosic hydrolysates is discussed. A model that involves biochemical considerations of D-xylose metabolism is presented that rationalizes the effects of oxygen on cultures where D-xylose is the sole carbon source, notably effects of the specific rate of oxygen use on the rate and extent of ethanol accumulation. Alternate methods to direct fermentation of D-xylose have been developed that depend on its prior isomerization to D-xylose, followed by fermentation of the pentulose by certain yeasts and fungi. Factors involved in the biochemistry, use, and performance of these methods, which with some organisms involves sensitivity to oxygen, are reviewed.