Studies on marine occurring yeasts: Respiration, fermentation and salt tolerance

The Effects of NaCl and Temperature on Growth and Survival of Yeast Strains Isolated from Danish Cheese Brines

Yeasts play an important role in cheese making, by contributing to microbial community establishment and improving flavor. This study aimed at investigating the impact of NaCl and temperature on growth and survival of 20 strains belonging to the yeast species Candida intermedia (2 strains), Debaryomyces hansenii (11), Kluyveromyces lactis (1), Papiliotrema flavescens (1), Rhodotorula glutinis (1), Sterigmatomyces halophilus (2) and Yamadazyma triangularis (2) isolated from Danish cheese brines. All yeasts could grow in Malt Yeast Glucose Peptone (MYGP) medium with low NaCl (≤ 4%, w/v) concentrations at 25 °C and 16 °C. Further, none of the strains, except for one strain of D. hansenii (KU-9), were able to grow under a condition mimicking cheese brine (MYGP with 23% (w/v) NaCl and 6.3 g/L lactate) at 25 °C, while all yeasts could grow at 16 °C, except for the two strains of C. intermedia. In the survival experiment, D. hansenii, S. halophilus and Y. triangularis survived in MYGP with 23% (w/v) NaCl throughout 13.5 days at 25 °C, with Y. triangularis and S. halophilus being the most NaCl tolerant, while the remaining yeasts survived for less than 7 days. These results enable the selection of relevant yeasts from cheese brines for potential use in the cheese industry.

Contribution of the mitogen-activated protein kinase Hog1 to the halotolerance of the marine yeast Debaryomyces hansenii

Halotolerant species are adapted to dealing continually with hyperosmotic environments, having evolved strategies that are uncommon in other organisms. The HOG pathway is the master system that regulates the cellular adaptation under these conditions; nevertheless, apart from the importance of Debaryomyces hansenii as an organism representative of the halotolerant class, its HOG1 pathway has been poorly studied, due to the difficulty of applying conventional recombinant DNA technology. Here we describe for the first time the phenotypic characterisation of a null HOG1 mutant of D. hansenii. Dhhog1Δ strain was found moderately resistant to 1 M NaCl and sensitive to higher concentrations. Under hyperosmotic shock, DhHog1 fully upregulated transcription of DhSTL1 and partially upregulated that of DhGPD1. High osmotic stress lead to long-term inner glycerol accumulation that was partially dependent on DhHog1. These observations indicated that the HOG pathway is required for survival under high external osmolarity but dispensable under low and mid-osmotic conditions. It was also found that DhHog1 can regulate response to alkali stress during hyperosmotic conditions and that it plays a role in oxidative and endoplasmic reticulum stress. Taken together, these results provide new insight into the contribution of this MAPK in halotolerance of this yeast.

Overlapping responses between salt and oxidative stress in Debaryomyces hansenii

Debaryomyces hansenii is a halotolerant yeast of importance in basic and applied research. Previous reports hinted about possible links between saline and oxidative stress responses in this yeast. The aim of this work was to study that hypothesis at different molecular levels, investigating after oxidative and saline stress: (i) transcription of seven genes related to oxidative and/or saline responses, (ii) activity of two main anti-oxidative enzymes, (iii) existence of common metabolic intermediates, and (iv) generation of damages to biomolecules as lipids and proteins. Our results showed how expression of genes related to oxidative stress was induced by exposure to NaCl and KCl, and, vice versa, transcription of some genes related to osmotic/salt stress responses was regulated by H₂O₂. Moreover, and contrary to S. cerevisiae, in D. hansenii HOG1 and MSN2 genes were modulated by stress at their transcriptional level. At the enzymatic level, saline stress also induced antioxidative enzymatic defenses as catalase and glutathione reductase. Furthermore, we demonstrated that both stresses are connected by the generation of intracellular ROS, and that hydrogen peroxide can affect the accumulation of in-cell sodium. On the other hand, no significant alterations in lipid oxidation or total glutathione content were observed upon exposure to both stresses tested. The results described in this work could help to understand the responses to both stressors, and to improve the biotechnological potential of D. hansenni.

Vacuolar control of subcellular cation distribution is a key parameter in the adaptation of Debaryomyces hansenii to high salt concentrations

Debaryomyces hansenii is a halotolerant and Na⁺-includer yeast that can be isolated from different food and low-water activity products. It has also been defined as a marine-occurring yeast but key aspects for this salt tolerant behavior are far from being understood. Here, we searched for clues helping to elucidate the basis of this ability. Our results on growth, Rb⁺ transport, total K⁺ and Na⁺ content and vacuolar fragmentation are compatible with a yeast species adapted to cope with salt stress. On the other hand, we confirmed the existence of D. hansenii strategies that are generally observed in sensitive organisms, such as the production of glycerol as a compatible solute and the efficient vacuolar sequestration of Na⁺. We propose a striking role of D. hansenii vacuoles in the maintenance of constant cytosolic K⁺ values, even in the presence of extracellular Na⁺ concentration values more than two orders of magnitude higher than extracellular K⁺. Finally, the ability to deal with cytosolic Na⁺ levels significantly higher than those found in S. cerevisiae, shows the existence of important and specific salt tolerance mechanisms and determinants in D. hansenii.

The halotolerant Debaryomyces hansenii, the Cinderella of non-conventional yeasts

Debaryomyces hansenii is a halotolerant yeast with a high biotechnological potential, particularly in the food industry. However, research in this yeast is limited by its molecular peculiarities. In this review we summarize the state of the art of research in this microorganisms, describing both pros and cons. We discuss (i) its halotolerance, (ii) the molecular factors involved in saline and osmotic stress, (iii) its high gene density and ambiguous CUG decoding, and (iv) its biotechnological and medical interests. We trust that all the bottlenecks in its study will soon be overcome and D. hansenii will become a fundamental organism for food biotechnological processes. Copyright © 2016 John Wiley & Sons, Ltd.

Advances in yeast systematics and phylogeny and their use as predictors of biotechnologically important metabolic pathways

Detection, identification and classification of yeasts have undergone a major transformation in the last decade and a half following application of gene sequence analyses and genome comparisons. Development of a database (barcode) of easily determined DNA sequences from domains 1 and 2 (D1/D2) of the nuclear large subunit rRNA gene and from ITS now permits many laboratories to identify species quickly and accurately, thus replacing the laborious and often inaccurate phenotypic tests previously used. Phylogenetic analysis of gene sequences is leading to a major revision of yeast systematics that will result in redefinition of nearly all genera. This new understanding of species relationships has prompted a change of rules for naming and classifying yeasts and other fungi, and these new rules are presented in the recently implemented International Code of Nomenclature for algae, fungi, and plants (Melbourne Code). The use of molecular methods for species identification and the impact of Code changes on classification will be discussed, as will use of phylogeny for prediction of biotechnological applications.

Study of the potential of the air lift bioreactor for xylitol production in fed-batch cultures by Debaryomyces hansenii immobilized in alginate beads

Cell immobilization has shown to be especially adequate for xylitol production. This work studies the suitability of the air lift bioreactor for xylitol production by Debaryomyces hansenii immobilized in Ca-alginate operating in fed-batch cultures to avoid substrate inhibition. The results showed that the air lift bioreactor is an adequate system since the minimum air flow required for fluidization was even lower than that leading to the microaerobic conditions that trigger xylitol accumulation by this yeast, also maintaining the integrity of the alginate beads and the viability of the immobilized cells until 3 months of reuses. Maximum productivities and yields of 0.43 g/l/h and 0.71 g/g were achieved with a xylose concentration of 60 g/l after each feeding. The xylose feeding rate, the air flow, and the biomass concentration at the beginning of the fed-batch operation have shown to be critical parameters for achieving high productivities and yields. Although a maximum xylitol production of 139 g/l was obtained, product inhibition was evidenced in batch experiments, which allowed estimating at 200 and 275 g/l the IC50 for xylitol productivity and yield, respectively. The remarkable production of glycerol in the absence of glucose was noticeable, which could not only be attributed to the osmoregulatory function of this polyol in conditions of high osmotic pressure caused by high xylitol concentrations but also to the role of the glycerol synthesis pathway in the regeneration of NAD(+) in conditions of suboptimal microaeration caused by insufficient aeration or high oxygen demand when high biomass concentrations were achieved.

Biomass pretreatment affects Ustilago maydis in producing itaconic acid

BACKGROUND

In the last years, the biotechnological production of platform chemicals for fuel components has become a major focus of interest. Although ligno-cellulosic material is considered as suitable feedstock, the almost inevitable pretreatment of this recalcitrant material may interfere with the subsequent fermentation steps. In this study, the fungus Ustilago maydis was used to produce itaconic acid as platform chemical for the synthesis of potential biofuels such as 3-methyltetrahydrofuran. No studies, however, have investigated how pretreatment of ligno-cellulosic biomass precisely influences the subsequent fermentation by U. maydis. Thus, this current study aims to first characterize U. maydis in shake flasks and then to evaluate the influence of three exemplary pretreatment methods on the cultivation and itaconic acid production of this fungus. Cellulose enzymatically hydrolysed in seawater and salt-assisted organic-acid catalysed cellulose were investigated as substrates. Lastly, hydrolysed hemicellulose from fractionated beech wood was applied as substrate.

RESULTS

U. maydis was characterized on shake flask level regarding its itaconic acid production on glucose. Nitrogen limitation was shown to be a crucial condition for the production of itaconic acid. For itaconic acid concentrations above 25 g/L, a significant product inhibition was observed. Performing experiments that simulated influences of possible pretreatment methods, U. maydis was only slightly affected by high osmolarities up to 3.5 osmol/L as well as of 0.1 M oxalic acid. The production of itaconic acid was achieved on pretreated cellulose in seawater and on the hydrolysed hemicellulosic fraction of pretreated beech wood.

CONCLUSION

The fungus U. maydis is a promising producer of itaconic acid, since it grows as single cells (yeast-like) in submerged cultivations and it is extremely robust in high osmotic media and real seawater. Moreover, U. maydis can grow on the hemicellulosic fraction of pretreated beech wood. Thereby, this fungus combines important advantages of yeasts and filamentous fungi. Nevertheless, the biomass pretreatment does indeed affect the subsequent itaconic acid production. Although U. maydis is insusceptible to most possible impurities from pretreatment, high amounts of salts or residues of organic acids can slow microbial growth and decrease the production. Consequently, the pretreatment step needs to fit the prerequisites defined by the actual microorganisms applied for fermentation.

Glycolytic sequence and respiration of Debaryomyces hansenii as compared to Saccharomyces cerevisiae

The fermentation and respiration activities of Debaryomyces hansenii were compared with those of Saccharomyces cerevisiae grown to stationary phase with high respiratory activity. It was found that: (a) glucose consumption, fermentation and respiration were lower than for S. cerevisiae; (b) fasting produced a much smaller decrease of respiration; (c) glucose consumed and not transformed to ethanol was higher; (d) in S. cerevisiae, full oxygenation prevented ethanol production but this effect was reversed by CCCP, whereas D. hansenii still showed some ethanol production under aerobiosis, which was moderately increased by CCCP. ATP levels were similar in the two yeasts. Levels of glycolytic intermediaries after glucose addition, and enzyme activities, indicated that the main difference and limiting step to explain the lower fermentation of D. hansenii is phosphofructokinase activity. Respiration and fermentation, which are lower in D. hansenii, compete for the re-oxidation of reduced nicotinamide adenine nucleotides; this competition, in turn, seems to play a role in defining the fermentation rates of the two yeasts. The effect of CCCP on glucose consumption and ethanol production also indicates a role of ADP in both the Pasteur and Crabtree effects in S. cerevisiae but not in D. hansenii. D. hansenii shows an alternative oxidase, which in our experiments did not appear to be coupled to the production of ATP.

Heterologous Expression of Glycerol 3-Phosphate Dehydrogenase Gene [DhGPD1] from the Osmotolerant Yeast Debaryomyces hansenii in Saccharomyces cerevisiae

The role for the gene encoding glycerol 3-phosphate dehydrogenase (DhGPD1) from the osmotolerant yeast Debaryomyces hansenii, in glycerol production and halotolerance, was studied through its heterologous expression in a Saccharomyces cerevisiae strain deficient in glycerol synthesis (gpd1Delta). The expression of the DhGPD1 gene in the gpd1Delta background restored glycerol production and halotolerance to wild type levels, corroborating its role in the salt-induced production of glycerol. Although the gene was functional in S. cerevisiae, its heterologous expression was not efficient, suggesting that the regulatory mechanism may not be shared by these two yeasts.

Metabolic flux response to salt-induced stress in the halotolerant yeast Debaryomyces hansenii

The toxic effect of NaCl and KCl on growth of the marine yeast Debaryomyces hansenii on glucose or glycerol was studied. Above a threshold value, both salts reduced the specific growth rate, specific glucose and glycerol respiration rates and specific glucose fermentation rate, as well as biomass yields. The exponential inhibition constant, k, and minimum toxic concentration, Cmin were similar for all physiological parameters assayed. The effect of either salt on the specific activity of several glycolytic enzymes showed a similar inhibition pattern, although at much lower salt concentrations compared with the physiological parameters. In agreement with published results on glycerol phosphate dehydrogenase stimulation by salt, we present evidence that a general glycolytic flux deviation could occur naturally during salt stress, due to the intrinsic sensitivity of the glycolytic enzymes to intracellular ion concentrations.

Osmoregulation of the salt-tolerant yeast Debaryomyces hansenii grown in a chemostat at different salinities

The intracellular solute composition of the salt-tolerant yeast Debaryomyces hansenii was studied in glucose-limited chemostat cultures at different concentrations of NaCl (4 mM, 0.68 M, and 1.35 M). A strong positive correlation between the total intracellular polyol concentration (glycerol and arabinitol) and medium salinity was demonstrated. The intracellular polyol concentration was sufficient to balance about 75% of the osmotic pressure of the medium in cultures with 0.68 and 1.35 M NaCl. The intracellular concentration of K+ and Na+, which at low external salinity gave a considerable contribution to the intracellular water potential, was only slightly enhanced with raised medium salinity. However, the ratio of intracellular K+ to Na+ decreased; but this decrease was less drastic in the cells than in the surrounding medium, i.e., the cells were able to select for K+ in favor of Na+. The turgor pressure, which was estimated on the basis of intracellular solute concentrations, was 2,200 kPa in cultures with 4 mM NaCl and decreased when the external salinity was raised, resulting in a value of about 500 kPa in cultures with 1.35 M NaCl. The maintenance of a positive turgor pressure at high salinity was mainly due to an increased production and accumulation of glycerol.

Microcalorimetric monitoring of growth of Saccharomyces cerevisiae: osmotolerance in relation to physiological state

The importance of the physiological state of a culture of Saccharomyces cerevisiae for tolerance to sudden osmotic dehydration was studied, and it was investigated whether specific osmotolerance factors were demonstrable. The microcalorimeter was used to monitor growth, and different physiological states of the culture were selected and their osmotolerance was tested. In addition to cells in the stationary phase, cells from the transition phase between respirofermentative and respiratory catabolism were osmotolerant. S. cerevisiae exhibited ever-changing metabolism during batch growth on either glucose or ethanol as the carbon source. Instantaneous heat production per biomass formation (dQ/dX) and specific activity of sn-glycerol 3-phosphate dehydrogenase (GPDH) (EC 1.1.1.8) were shown to differ for different physiological states. Neither high respiratory activity nor low total cellular activity, nor factors involved in osmoregulation, i.e., intracellular glycerol or activity of GPDH, correlated with the osmotolerant phenotype.

Mechanisms of halotolerance in microorganisms

Microorganisms have the ability to adapt to a wide range of NaCl concentrations. In general the NaCl tolerance shown by microbes far exceeds the salt tolerance of any other organism, procryote or eukaryote. There are at least three mechanisms available for adaptation to different salt concentrations. The first would be a passive one in which the cytoplasmic ion content would always equal that in the medium. A second mechanism which is used by many organisms involves concentrating compatible solutes to create an osmotic balance between the cytoplasm and the external environment. The third mechanism involves changing the cell physiology to control the movement of water allowing the cell to exist with an ionically dilute cytoplasm. This article will review the major developments and discuss the implications of increasing knowledge about salt tolerance in microorganisms.

On the mechanism of salt tolerance. Production of glycerol and heat during growth of Debaryomyces hansenii

As glycerol was suggested as an osmotic agent in the salt tolerant Debaryomyces hansenii the concentrations of total, intracellular, and extracellular glycerol produced by this yeast was followed during growth in 4 mM, 0.68 M, and 2.7 M NaCl media. The total amount of glycerol was not directly proportional to biomass production but to the cultural salinity with maximum concentrations just prior to or at the beginning of the stationary phase. In all cultures the cells lost some glycerol to the media, at 2.7 M NaCl the extracellular glycerol even amounted maximally to 80% of the total. A distinct maximum of intracellular glycerol, related to dry weight or cell number, appeared during the log phase at all NaCl concentrations. As the intracellular calculated glycerol concentrations amounted to 0.2 M, 0.8 M, and 2.6 M in late log phase cells at 4mM, 0.68 M, and 2.7 M NaCl, respectively, whereas the corresponding analysed values for the glycerol concentrations of the media were 0.7 mM, 2.5 mM, and 3.0 mM, glycerol contributes to the osmotic balance of the cells. During the course of growth all cultures showed a decreasing heat production related to cell substance produced, most pronounced at 2.7 M NaCl. At 2.7 M NaCl the total heat production amounted to--1690 kJ per mole glucose consumed in contrast to--1200 and--1130 kJ at 4 mM and 0.68 M NaCl, respectively. The Ym-values were of an inverse order, being 129, 120, and 93 at 4 mM, 0.68 M, and 2.7 M NaCl respectively.

Membrane permeability and the loss of germination factor from Neurospora crassa at low water activities

Neurospora crassa conidia incubating in buffer at low water activities (a(w)) release a germination-essential component as well as 260-nm absorbing and ninhydrin-positive materials, regardless of whether an electrolyte or non-electrolyte is used to reduce a(w). Chloroform and antibiotics known to increase cell-membrane permeability have a similar effect. This suggests that membrane damage occurs in media of low a(w) and that an increase in permeability is responsible for the release of cellular components. The damage caused in media of low a(w) is nonlethal in most cases, and the conidia recover when transferred to nutrient medium.

Regulation of the potassium to sodium ratio and of the osmotic potential in relation to salt tolerance in yeasts

By using the isotope pairs (22)Na-(24)Na and (42)K-(86)Rb, the uptake and retention of Na and K was studied in the salt-tolerant Debaryomyces hansenii and in the less tolerant Saccharomyces cerevisiae at NaCl levels of 4 mm and 0.68, 1.35, and 2.7 m in the medium. The ratio of K to Na is much higher in the cells than in the media, and higher in D. hansenii than in S. cerevisiae under comparable conditions. The difference between the two species is due to a better Na extrusion and a better uptake of K in D. hansenii. The kinetics of ion transport show that at about the time when extrusion of Na could be demonstrated in D. hansenii, K-Rb previously lost to an easily washable compartment of the cells was reabsorbed in both organisms. More H(+) was given off from S. cerevisiae than from D. hansenii in the course of these events. The findings fit the working hypothesis tested, which regards salt tolerance as partly dependent on the ability to mobilize energy to extrude Na from the cells and to take up K. The volume changes in S. cerevisiae are greater and are more slowly overcome than those in D. hansenii. The total salt level of the cells is not sufficient to counteract the osmotic potential of the medium, so that additional osmoregulatory mechanisms must be involved in determining halotolerance.