Yeasts are microscopic fungi inhabiting all Earth environments, including those inhospitable for most life forms, considered extreme environments. According to their habitats, yeasts could be extremotolerant or extremophiles. Some are polyextremophiles, depending on their growth capacity, tolerance, and survival in the face of their habitat’s physical and chemical constitution. The extreme yeasts are relevant for the industrial production of value-added compounds, such as biofuels, lipids, carotenoids, recombinant proteins, enzymes, among others. This review calls attention to the importance of yeasts inhabiting extreme environments, including metabolic and adaptive aspects to tolerate conditions of cold, heat, water availability, pH, salinity, osmolarity, UV radiation, and metal toxicity, which are relevant for biotechnological applications. We explore the habitats of extreme yeasts, highlighting key species, physiology, adaptations, and molecular identification. Finally, we summarize several findings related to the industrially-important extremophilic yeasts and describe current trends in biotechnological applications that will impact the bioeconomy.

Ustilago maydis is an important model to study intermediary and mitochondrial metabolism, among other processes. U. maydis can grow, at very different rates, on glucose, lactate, glycerol, and ethanol as carbon sources. Under nitrogen starvation and glucose as the only carbon source, this fungus synthesizes and accumulates neutral lipids in the form of lipid droplets (LD). In this work, we studied the accumulation of triacylglycerols in cells cultured in a medium containing acetate, a direct precursor of the acetyl-CoA required for the synthesis of fatty acids. The metabolic adaptation of cells to acetate was studied by measuring the activities of key enzymes involved in glycolysis, gluconeogenesis, and the pentose phosphate pathways. Since growth on acetate induces oxidative stress, the activities of some antioxidant enzymes were also assayed. The results show that cells grown in acetate plus nitrate did not increase the amount of LD, but increased the activities of glutathione reductase, glutathione peroxidase, catalase, and superoxide dismutase, suggesting a higher production of reactive oxygen species in cells growing in acetate. The phosphofructokinase-1 (PFK1) was the enzyme with the lowest specific activity in the glycolytic pathway, suggesting that PFK1 controls the flux of glycolysis. As expected, the activity of the phosphoenolpyruvate carboxykinase, a gluconeogenic enzyme, was present only in the acetate condition. In summary, in the presence of acetate as the only carbon source, U. maydis synthesized fatty acids, which were directed into the production of phospholipids and neutral lipids for biomass generation, but without any excessive accumulation of LD.

Respiratory supercomplexes are found in mitochondria of eukaryotic cells and some bacteria. A hypothetical role of these supercomplexes is electron channeling, which in principle should increase the respiratory chain efficiency and ATP synthesis. In addition to the four classic respiratory complexes and the ATP synthase, U. maydis mitochondria contain three type II NADH dehydrogenases (NADH for reduced nicotinamide adenine dinucleotide) and the alternative oxidase. Changes in the composition of the respiratory supercomplexes due to energy requirements have been reported in certain organisms. In this study, we addressed the organization of the mitochondrial respiratory complexes in U. maydis under diverse energy conditions. Supercomplexes were obtained by solubilization of U. maydis mitochondria with digitonin and separated by blue native polyacrylamide gel electrophoresis (BN-PAGE). The molecular mass of supercomplexes and their probable stoichiometries were 1200 kDa (I₁:IV₁), 1400 kDa (I₁:III₂), 1600 kDa (I₁:III₂:IV₁), and 1800 kDa (I₁:III₂:IV₂). Concerning the ATP synthase, approximately half of the protein is present as a dimer and half as a monomer. The distribution of respiratory supercomplexes was the same in all growth conditions. We did not find evidence for the association of complex II and the alternative NADH dehydrogenases with other respiratory complexes.

It has been shown that the alternative oxidase in mitochondria of fungi and plants has important functions in the response against stress conditions, although their role in some organisms is still unknown. This is the case of Ustilago maydis. There is no evidence of the participation of the U. maydis Aox1 in stressful conditions such as desiccation, high or low temperature, and low pH, among others. Therefore, in this work, we studied the role of the U. maydis Aox1 in cells exposed to oxidative stress induced by methyl viologen (paraquat). To gain insights into the role of this enzyme, we took advantage of four strains: the FB2 wild-type, a strain without the alternative oxidase (FB2aox1Δ), other with the Aox1 fused to the Gfp under the control of the original promoter (FB2aox1-Gfp), and one expressing constitutively de Aox1-Gfp (FB2Potef:aox1-Gfp). Cells were incubated for various times in the presence of 1 mM paraquat and growth, replicative capacities, mitochondrial respiratory activity, Aox1 capacity, and the activities of several antioxidant enzymes (catalase, glutathione peroxidase, glutathione reductase, and superoxide dismutase) were assayed. The results show that (1) the response of U. maydis against oxidative stress was the same in the presence or absence of the Aox1; (2) the activities of the antioxidant enzymes remained constant despite the oxidative stress; and (3) there was a decrease in the GSH/GSSG ratio in U. maydis cells incubated with paraquat.

Lactate is considered to be a waste metabolite produced during muscle fatigue. In contrast with this simplistic point of view, in this review we provide evidence of the multiple and complex functions of this metabolite. We show that: 1) lactate is the final product of the glycolysis regardless the oxygen concentration in the cell 2) lactate is part of two types of shuttle, one that functions in the intermembrane space of the mitochondrion, and another intercellular, which is responsible for feeding lactate to certain cell types, such as neurons or heart muscle, 3) in sperm,lactate is transported directly to the mitochondrial matrix and there it is oxidized to produce pyruvate and NADH, 4) in the liver, lactate participates in the oxidation of ethanol through the generation of hydrogen peroxide, 5) Depending on the cell line, lactate can function as anti-inflammatory agent (endocrine) and/or a regulator of gene expression. Keywords: Lactic fermentation; hypoxia; muscle fatigue; gene regulator

The respiratory activities of mitochondrial complexes I, II, and IV were analyzed in permeabilized Rhodotorula mucilaginosa cells and isolated mitochondria, and the kinetic parameters K_0.5 and V_max were obtained. No difference in substrate affinities were found between mitochondria and permeabilized cells. The activities of the components of the mitochondrial respiratory chain of the Antarctic yeast R. mucilaginosa M94C9 were identified by in-gel activity and SDS-PAGE. The mitochondria exhibited activity for the classical components of the electron transport chain (Complexes I, II, III, and IV), and supercomplexes were formed by a combination of the respiratory complexes I, III, and IV. Unfortunately, the activities of the monomeric and dimeric forms of the F1F0-ATP synthase were not revealed by the in-gel assay, but the two forms of the ATP synthase were visualized in the SDS-PAGE. Furthermore, two alternative pathways for the oxidation of cytosolic NADH were identified: the alternative NADH dehydrogenase and the glycerol-3-phosphate dehydrogenase. In addition, an NADPH dehydrogenase and a lactate cytochrome b2 dehydrogenase were found. The residual respiratory activity following cyanide addition suggests the presence of an alternative oxidase in cells.