Effects of Oxygen Availability on Acetic Acid Tolerance and Intracellular pH in Dekkera bruxellensis

Hyper-Osmotic Stress Elicits Membrane Depolarization and Decreased Permeability in Halotolerant Marine Debaryomyces hansenii Strains and in Saccharomyces cerevisiae

The use of seawater and marine microorganisms can represent a sustainable alternative to avoid large consumption of freshwater performing industrial bioprocesses. Debaryomyces hansenii, which is a known halotolerant yeast, possess metabolic traits appealing for developing such processes. For this purpose, we studied salt stress exposure of two D. hansenii strains isolated from marine fauna. We found that the presence of sea salts during the cultivation results in a slight decrease of biomass yields. Nevertheless, higher concentration of NaCl (2 M) negatively affects other growth parameters, like growth rate and glucose consumption rate. To maintain an isosmotic condition, the cells accumulate glycerol as compatible solute. Flow cytometry analysis revealed that the osmotic adaptation causes a reduced cellular permeability to cell-permeant dye SYBR Green I. We demonstrate that this fast and reversible phenomenon is correlated to the induction of membrane depolarization, and occurred even in presence of high concentration of sorbitol. The decrease of membrane permeability induced by osmotic stress confers to D. hansenii resistance to cationic drugs like Hygromycin B. In addition, we describe that also in Saccharomyces cerevisiae the exposure to hyper-osmotic conditions induced membrane depolarization and reduced the membrane permeability. These aspects are very relevant for the optimization of industrial bioprocesses, as in the case of fermentations and bioconversions carried out by using media/buffers containing high nutrients/salts concentrations. Indeed, an efficient transport of molecules (nutrients, substrates, and products) is the prerequisite for an efficient cellular performance, and ultimately for the efficiency of the industrial process.

Nitrate boosts anaerobic ethanol production in an acetate-dependent manner in the yeast Dekkera bruxellensis

In the past few years, the yeast Dekkera bruxellensis has gained much of attention among the so-called non-conventional yeasts for its potential in the biotechnological scenario, especially in fermentative processes. This yeast has been regarded as an important competitor to Saccharomyces cerevisiae in bioethanol production plants in Brazil and several studies have reported its capacity to produce ethanol. However, our current knowledge concerning D. bruxellensis is restricted to its aerobic metabolism, most likely because wine and beer strains cannot grow in full anaerobiosis. Hence, the present work aimed to fulfil a gap regarding the lack of information on the physiology of Dekkera bruxellensis growing in the complete absence of oxygen and the relationship with assimilation of nitrate as nitrogen source. The ethanol strain GDB 248 was fully capable of growing anaerobically and produces ethanol at the same level of S. cerevisiae. The presence of nitrate in the medium increased this capacity. Moreover, nitrate is consumed faster than ammonium and this increased rate coincided with a higher speed of glucose consumption. The profile of gene expression helped us to figure out that even in anaerobiosis, the presence of nitrate drives the yeast cells to an oxidative metabolism that ultimately incremented both biomass and ethanol production. These results finally provide the clues to explain most of the success of this yeast in industrial processes of ethanol production.

First aspects on acetate metabolism in the yeast Dekkera bruxellensis: a few keys for improving ethanol fermentation

Dekkera bruxellensis is continuously changing its status in fermentation processes, ranging from a contaminant or spoiling yeast to a microorganism with potential to produce metabolites of biotechnological interest. In spite of that, several major aspects of its physiology are still poorly understood. As an acetogenic yeast, minimal oxygen concentrations are able to drive glucose assimilation to oxidative metabolism, in order to produce biomass and acetate, with consequent low yield in ethanol. In the present study, we used disulfiram to inhibit acetaldehyde dehydrogenase activity to evaluate the influence of cytosolic acetate on cell metabolism. D. bruxellensis was more tolerant to disulfiram than Saccharomyces cerevisiae and the use of different carbon sources revealed that the former yeast might be able to export acetate (or acetyl-CoA) from mitochondria to cytoplasm. Fermentation assays showed that acetaldehyde dehydrogenase inhibition re-oriented yeast central metabolism to increase ethanol production and decrease biomass formation. However, glucose uptake was reduced, which ultimately represents economical loss to the fermentation process. This might be the major challenge for future metabolic engineering enterprises on this yeast.

Brettanomyces bruxellensis population survey reveals a diploid-triploid complex structured according to substrate of isolation and geographical distribution

Brettanomyces bruxellensis is a unicellular fungus of increasing industrial and scientific interest over the past 15 years. Previous studies revealed high genotypic diversity amongst B. bruxellensis strains as well as strain-dependent phenotypic characteristics. Genomic assemblies revealed that some strains harbour triploid genomes and based upon prior genotyping it was inferred that a triploid population was widely dispersed across Australian wine regions. We performed an intraspecific diversity genotypic survey of 1488 B. bruxellensis isolates from 29 countries, 5 continents and 9 different fermentation niches. Using microsatellite analysis in combination with different statistical approaches, we demonstrate that the studied population is structured according to ploidy level, substrate of isolation and geographical origin of the strains, underlying the relative importance of each factor. We found that geographical origin has a different contribution to the population structure according to the substrate of origin, suggesting an anthropic influence on the spatial biodiversity of this microorganism of industrial interest. The observed clustering was correlated to variable stress response, as strains from different groups displayed variation in tolerance to the wine preservative sulfur dioxide (SO₂). The potential contribution of the triploid state for adaptation to industrial fermentations and dissemination of the species B. bruxellensis is discussed.

Single and combined effects of acetic acid, furfural, and sugars on the growth of the pentose-fermenting yeast Meyerozyma guilliermondii

The tolerance of the pentose-fermenting yeast Meyerozyma guilliermondii to the inhibitors released after the biomass hydrolysis, such as acetic acid and furfural, was surveyed. We first verified the effects of acetic acid and cell concentrations and initial pH on the growth of a M. guilliermondii strain in a semi-synthetic medium containing acetic acid as the sole carbon source. Second, the single and combined effects of furfural, acetic acid, and sugars (xylose, arabinose, and glucose) on the sugar uptake, cell growth, and ethanol production were also analysed. Growth inhibition occurred in concentrations higher than 10.5 g l-1 acetic acid and initial pH 3.5. The maximum specific growth rate (µ) was 0.023 h-1 and the saturation constant (ks) was 0.75 g l-1 acetic acid. Initial cell concentration also influenced µ. Acetic acid (initial concentration 5 g l-1) was co-consumed with sugars even in the presence of 20 mg l-1 furfural without inhibition to the yeast growth. The yeast grew and fermented sugars in a sugar-based medium with acetic acid and furfural in concentrations much higher than those usually found in hemicellulosic hydrolysates.