Influence of growth rate on the accumulation of ergosterol in yeast-cells in a phosphate limited continuous culture

Summary The influence of the growth rate on the accumulation of ergosterol inSaccharomyces cerevisiae was studied with glucose and ethanol as substrates under P-limitation in chemostat experiments. In cultures with glucose as carbon source a decrease in ergosterol content with dilution rates up to 0.08 h^–1 was observed, whereas above this dilution rate an increase in ergosterol content occurred. Similar but less marked effects were attained with ethanol as carbon source. A maximum specific rate of ergosterol synthesis of about 2.4 mg per h and g dry cell mass was calculated for phosphorus limited cultures.     

Ergosterol as an indicator of mould growth on wood in relation to culture age, humidity stress and nutrient level

Changes in ergosterol content in cultures of Penicillium brevicompactum and Aspergillus versicolor on wood with time, changes in humidity or addition of glucose solutions to wood were studied with HPLC. Lowering of the humidity level caused a very large decline in ergosterol content of cultures of P. brevicompactum on wood over a 10 day period, although small amounts remained after this time. After an initial increase, up to an inoculation time of 45 days, reductions were also observed in control samples maintained at 100% RH, but these were smaller. The amount of ergosterol decreased to very low levels in wood impregnated with low levels of glucose during a 93 day incubation period. Ergosterol concentration in hyphae produced in surface liquid cultures was shown to be higher in mycelia growing on media enriched with nitrogen or with more available nutrients. The concentration of ergosterol in the mycelia of P. brevicompactum in surface liquid cultures varied by a factor of 5 from 2 to 10 mg g⁻. The results clearly show that ergosterol present in solid materials in mainly related to active biomass. With certain prerequisites, ergosterol determinations could also be used for total fungal biomass estimations on wood.     

Ergosterol contents of some wood-rotting basidiomycete fungi grown in liquid and solid culture conditions

Ergosterol contents of six wood-rotting basidiomycetes were analyzed under different cultivation conditions. Four white-rot and two brown-rot fungi were cultivated in liquid synthetic medium with low nutrient nitrogen (2 mM) and 0.1% glucose, and ergosterol in mycelial biomasses were measured weekly for 35 days. The highest ergosterol content per fungal dry mass in the white-rot fungi was found in Phanerochaete chrysosporium being 2100 μg g⁻¹, while in Ceriporiopsis subvermispora it was 1700 μg g⁻¹, Phlebia radiata 700 μg g⁻¹, and Physisporinus rivulosus 560 μg g⁻¹. In brown-rot fungi the ergosterol content was in Poria placenta 2868 μg g⁻¹ and in Gloeophyllum trabeum 3915 μg g⁻¹. On agar media, P. chrysosporium and P. radiata reached the highest ergosterol value in 7 days, while in wood block cultures the ergosterol contents were quite stable. The conversion factors for ergosterol-to-fungal biomass varied from 48 and 243, which were lower than values for ascomycetous soil fungi reported in the literature.     

The Use of Ergosterol as a Quantitative Measure of the Resistance of Cultured Tissues of Brassica napus ssp. oleifera to Leptosphaeria maculans

A simplified method for the quantitative assessment of the fungal lipid ergosterol was used to assess the levels of infection in tissue cultures of oilseed rape (Brassica napus ssp. oleifera) inoculated with Leptosphaeria maculans. The growth of L. maculans in liquid culture throughout a 36-day period correlated well (r = 0·92) with the amount of ergosterol extracted from the mycelium. There were significant differences (P < 0·05) in the amount of ergosterol extracted from infected thin cell layer (TCL) explants and callus tissue of two resistant and three susceptible cultivars of oilseed rape. Amounts of ergosterol extracted from resistant cultivars were < 100 (g and from susceptible > 100 (g. The mean amounts of ergosterol extracted from shoot cultures of two resistant and four susceptible cultivars were similar to those for TCL explants and callus tissue, although the values obtained were variable. This technique can be used in in vitro breeding programmes to accurately assess the resistance of tissue cultures of B. napus to L. maculans and could also have value in conventional breeding programmes.     

NITROGEN LIMITATION INCREASES BREVETOXINS IN KARENIA BREVIS (DINOPHYCEAE): IMPLICATIONS FOR BLOOM TOXICITY1

Laboratory and field measurements of the toxin content in Karenia brevis cells vary by >4‐fold. These differences have been largely attributed to genotypic variations in toxin production among strains. We hypothesized that nutrient limitation of growth rate is equally or more important in controlling the toxicity of K. brevis, as has been documented for other toxic algae. To test this hypothesis, we measured cellular growth rate, chlorophyll a, cellular carbon and nitrogen, cell volume, and brevetoxins in four strains of K. brevis grown in nutrient‐replete and nitrogen (N)‐limited semi‐continuous cultures. N‐limitation resulted in reductions of chlorophyll a, growth rate, volume per cell and nirtogen:carbon (N:C) ratios as well as a two‐fold increase (1%–4% to 5%–9%) in the percentage of cellular carbon present as brevetoxins. The increase in cellular brevetoxin concentrations was consistent among genetically distinct strains. Normalizing brevetoxins to cellular volume instead of per cell eliminated much of the commonly reported toxin variability among strains. These results suggest that genetically linked differences in cellular volume may affect the toxin content of K. brevis cells as much or more than innate genotypic differences in cellular toxin content per unit of biomass. Our data suggest at least some of the >4‐fold difference in toxicity per cell reported from field studies can be explained by limitation by nitrogen or other nutrients and by differences in cell size. The observed increase in brevetoxins in nitrogen limited cells is consistent with the carbon:nutrient balance hypothesis for increases in toxins and other plant defenses under nutrient limitation.     

Isolation of ergosterol peroxide from Nomuraea rileyi infected larvae of tobacco cutworm

In the search for potential cytotoxic substances produced by Nomuraea rileyi, an active compound was isolated from mycosed insects through an activity guided fractionation process. The compound, cytotoxic against the Sf9 insect cell line, was identified to be ergosterol peroxide (5α, 8α-epidioxy-24(R)-methylcholesta-6, 22-dien-3β-ol) using nuclear magnetic resonance techniques, infrared spectrometry, and mass spectroscopy. Anticancer screens demonstrated that ergosterol peroxide at micromolar concentrations inhibited the growth of hormone-dependent breast cancer cell line (T47D), hormone-independent breast cancer cell line (MDA-MB-231), human epidermoid carcinoma in mouth cell line (KB), human cervical carcinoma cell line (HeLa), lung cancer cell line (H69AR) and human cholangiocarcinoma cell line (HuCCA-1). Ergosterol peroxide showed moderate effects against Spodoptera litura larvae; 46.7% mortality via topical application after 7 day post-treatment whereas the insect’s death was not found in per os application. The amounts of ergosterol peroxide produced by N. rileyi cultures under in vitro and in vivo were determined. The physiological levels of ergosterol peroxide detected in mycosed and mummified cadavers were very low (0.011 and 0.386 μg/larva) less then levels that either inhibited insect cell proliferation or caused insecticidal activity.     

Comparative utilization of inorganic and organic compounds as sole nitrogen sources by the submergent duckweed,Lemna trisulca L.

The capability of the submergentLemna trisulca L. to utilize various inorganic and organic sources of nitrogen was studied using both non-axenic and axenic cultures. When doubling time for frond production was measured, the nitrogen sources in order of effectiveness were urea, aspartic acid, nitrate, glutamic acid, arginine, ammonium and casein hydrolysate. Nitrite supported a relatively rapid growth rate after an initial lag of 7 days. Other parameters of growth such as fresh or dry weight per frond or ohlorophyll content did not oorrelate well with rate of frond production. Casein hydrolysate and urea were found to elicit a morphology different from that seen in cultures containing the other nitrogen sources. These preferences for source of nitrogen were different than those known for the emergent species ofLemnaceae. The unique value ofL. trisulca as a subject for plant physiological research is discussed. This study also provides a possible explanation for the existence of nutritional niches existing in aquatio ecosystems containing several different species ofLemnaceae.     

Petroleum Bioremediation in Seawater Using Guano as the Fertilizer

The biodegradation of hydrocarbon pollutants in open systems, such as oceans, is generally limited by the availability of utilizable nitrogen and phosphorus sources. Here the authors demonstrate the potential of overcoming this problem with guano as the fertilizer. In the first set of experiments, the principle and conditions for growing bacteria on a water insoluble fertilizer was established, using uric acid as the nitrogen source and a pure culture of an isolated hydrocarbon-degrading bacterium, Alcanivorax sp. OK2. Using a simulated open system, it was demonstrated that uric acid (the major nitrogen component of guano) binds to crude oil and is available for the growth of strain OK2 and petroleum degradation. In the second set of experiments, using a simulated open system, it was demonstrated that commercial guano was an effective source of nitrogen and phosphorus for the growth of marine bacteria on crude oil. Bacterial cultures reached over 10⁸ cells per ml and 70% of the crude oil was degraded. Controls using ammonium sulfate and phosphate in place of guano in the simulated open system reached only 10⁶ cells per ml and showed no detectable hydrocarbon degradation. Isolation and characterization of the bacteria in the crude oil/guano cultures indicated that they were primarily strains of Alcanivorax and Alteromonas.     

Specific production rate of VHH antibody fragments by Saccharomyces cerevisiae is correlated with growth rate, independent of nutrient limitation

Saccharomyces cerevisiae carrying a multicopy integrated expression vector containing the gene encoding a Llama antibody fragment, has been cultivated in continuous cultures both under carbon and nitrogen limiting conditions with galactose as the sole carbon source. VHH-R2 expression was under control of the inducible GAL7 promoter. Induction however, was independent of the galactose consumption rate and maximal at all growth rates. VHH-R2 was secreted with 70% efficiency at all growth rates and under both limitations. The specific production rate increased linear with increasing growth rate in a growth-associated manner. However, when grown under nitrogen limitation at growth rates above 0.09 h(-1), the extracellular VHH-R2 was less active or part of the VHH-R2 was in an inactive form. From our results we conclude that to obtain a maximal amount of VHH per kilogram biomass per hour, VHH production should be done in carbon limited continuous cultures at high specific growth rates.     

Ammonium influence on nitrogen assimilating enzymes and protein accumulation in suspension cultures of Paul's Scarlet rose

The influence of NH₄⁺ on protein accumulation was examined by growing suspension cultures of Rosa cv. Paul's Scarlet on two defined media. Both contained 1920 μmol of NO₃^? but only one contained 72.8 μmol of NH₄⁺. At the conclusion of a 14-day growth period, cultures grown with NH₄⁺ possessed twice as much protein as cultures grown without NH₄⁺. The influence of NH₄⁺ did not appear to be a substrate effect, since the amount of NH₄⁺ provided accounted for only 10% of the nitrogen recovered in protein. The provision of NH₄⁺ in the starting medium increased the activity (μmol substrate. h^?1· g^?1 fr wt) of glutamate dehydrogenase and glutamate synthase, and reduced the activity of glutamine synthetase. A comparison of the total activity per culture for each of these enzymes with the rate of nitrogen incorporation into protein showed that the enzymatic potential of glutamine synthetase and glutamate dehydrogenase greatly exceeded the actual in vivo rate of nitrogen assimilation through the respective pathways. Thus it was concluded that the availability of either of these enzymes does not limit nitrogen assimilation in rose cells and the fluctuations in their level brought about by NH₄⁺ was of no physiological importance. The activity of glutamate synthase per culture approximated the rate of nitrogen incorporation into protein during early stages of growth, and for that reason may have limited nitrogen assimilation or caused a diversion of nitrogen through the alternative pathway to glutamate catalyzed by glutamate dehydrogenase.     

Transparent exopolymer particle production and aggregation by a marine planktonic diatom (Thalassiosira weissflogii) at different growth rates

Transparent exopolymer particles (TEP) play an important role in the ocean carbon cycle as they are sticky and affect particle aggregation and the biological carbon pump. We investigated the effect of growth rate on TEP production in nitrogen limited semi‐continuous cultures of the diatom Thalassiosira weissflogii (Grunow) G. Fryxell & Hasle. Steady‐state diatom concentrations and other indicators of biomass (chl a, and total carbohydrate) were inversely related to growth rate, while individual cell volume increased with growth rate. There was no change in total TEP area with growth rate; however, individual TEP were larger at high growth rates and the number of individual TEP particles was lower. TEP concentration per cell was higher at higher growth rates. SYTOX Green staining showed that <5% of the diatom population had permeable cell membranes, with the proportion increasing at low growth rates. However, TEP production rates were greater at high growth rates, refuting our hypothesis that TEP formation is dependent on dying cells with compromised cell membranes in a diatom population. Measurements of particle size distribution in the cultures using laser scattering showed that they were most aggregated at high growth rates. These results indicate a coupling between TEP production and growth rate in diatoms under N limitation, with fast growing T. weissflogii producing more TEP and aggregates.     

Effects of non-steady-state iron limitation on nitrogen assimilatory enzymes in the marine diatom thalassiosira weissflogii (BACILLARIOPHYCEAE)

Since the recognition of iron‐limited high nitrate (or nutrient) low chlorophyll (HNLC) regions of the ocean, low iron availability has been hypothesized to limit the assimilation of nitrate by diatoms. To determine the influence of non‐steady‐state iron availability on nitrogen assimilatory enzymes, cultures of Thalassiosira weissflogii (Grunow) Fryxell et Hasle were grown under iron‐limited and iron‐replete conditions using artificial seawater medium. Iron‐limited cultures suffered from decreased efficiency of PSII as indicated by the DCMU‐induced variable fluorescence signal (F_v/F_m). Under iron‐replete conditions, in vitro nitrate reductase (NR) activity was rate limiting to nitrogen assimilation and in vitro nitrite reductase (NiR) activity was 50‐fold higher. Under iron limitation, cultures excreted up to 100 fmol NO₂^?·cell^?1·d^?1 (about 10% of incorporated N) and NiR activities declined by 50‐fold while internal NO₂^? pools remained relatively constant. Activities of both NR and NiR remained in excess of nitrogen incorporation rates throughout iron‐limited growth. One possible explanation is that the supply of photosynthetically derived reductant to NiR may be responsible for the limitation of nitrogen assimilation at the NO₂^? reduction step. Urease activity showed no response to iron limitation. Carbon:nitrogen ratios were equivalent in both iron conditions, indicating that, relative to carbon, nitrogen was assimilated at similar rates whether iron was limiting growth or not. We hypothesize that, diatoms in HNLC regions are not deficient in their ability to assimilate nitrate when they are iron limited. Rather, it appears that diatoms are limited in their ability to process photons within the photosynthetic electron transport chain which results in nitrite reduction becoming the rate‐limiting step in nitrogenassimilation.     

CHANGES IN CELL COMPOSITION AND LIPID METABOLISM MEDIATED BY SODIUM AND NITROGEN AVAILABILITY IN THE MARINE DIATOM PHAEODACTYLUM TRICORNUTUM (BACILLARIOPHYCEAE)1

The effects of nitrogen starvation in the presence or absence of sodium in the culture medium were monitored in batch cultures of the marine diatom Phaeodactylum tricornutum Bohlin. During nitrogen starvation in the presence of sodium, cell nitrogen and chlorophyll a decreased, mainly as a consequence of continued cell division. These decreases were accompanied by decreases in the rates of photosynthesis and respiration. There was no change in either cell volume or carbohydrate, but both carbon and lipid increased. During nitrogen starvation in the absence of sodium, cell division ceased. Cell nitrogen and chlorophyll a remained constant, and respiration did not decrease, but the changes in the photosynthetic rate and the lipid content per cell were similar to cultures that were nitrogen-starved in the presence of sodium. The carbon-to-nitrogen ratio increased in both cultures. Nitrogen, in the form of nitrate, and sodium were resupplied to cultures that had been preconditioned in nitrogen- and sodium-deficient medium for 5 d. Control cultures to which neither nitrate or sodium were added remained in a static state with respect to cell number, volume, and carbohydrate but showed slight increases in lipid. Cells in cultures to which 10 mM nitrate alone was added showed a similar response to cultures where no additions were made. Cells in cultures to which 50 mM sodium alone was added divided for 2 d, with concomitant small decreases in all measured constituents. Cell division resumed in cultures to which both sodium and nitrate were added. The lipid content fell dramatically in these cells and was correlated to metabolic oxidation via measured increases in the activity of the glyoxylate cycle enzyme, isocitrate lyase. We conclude that lipids are stored as a function of decreased growth rate and are metabolized to a small extent when cell division resumes. However, much higher rates of metabolism occur if cell division resumes in the presence of a nitrogen source.     

Control of nitrogenase in chemostat cultures of Rhodobacter capsulatus grown on ammonium at different illuminations

Rhodobacter capsulatus strain 37b4 was grown phototrophically in chemostat cultures with 2 mM of ammonium chloride and 30 mM of malate at a constant dilution rate of 0.075 h^-1. When illumination was raised from 3000 to 30000 lx, steady state biomass levels as well as malate uptake increased linearly with increasing illumination. Yet, in no case external ammonium could be detected in the culture fluid. Specific nitrogenase activity increased by a factor of ten between 3000 and 15000 lx and approached constancy above 15 000 lx. When samples were anaerobically withdrawn from the chemostat and subsequently grown in batch cultures under saturating light conditions, biomass increased to a constant level, independently of the illumination used in the previous chemostat culture. In fact, the specific nitrogen contents of cells were 0.195 and 0.154 (g of N per g of protein) with chemostat cultures adapted to 3000 and 30000 lx, respectively. With the former cultures, specific nitrogen contents decreased to 0.142 g of nitrogen per g of cell protein upon incubation in a batch system. This suggests the existence of free nitrogen compounds in cells of chemostat cultures, the concentrations of which decrease while protein levels increase with increasing energy supply. Intracellular amino acid pools revealed slightly elevated levels of major amino acids in low-light cultures as compared to high-light cultures. On the basis of intracellular levels of ammonium, however, no significant differences could be detected. Since, in addition, malate consumption increased linearly with increasing illumination, it is proposed that light controls nitrogenase in Rhodobacter capsulatus via the C/N ratio, as represented by malate and ammonium consumption, rather than directly.     

Photosynthesis and nitrogen utilization in exponentially growing nitrogen-limited cultures of Lemna gibba

The photosynthetic performance and nitrogen utilization of Lemna gibba L. G3 adapted to limited nitrogen supply was studied. The plants were adapted to two levels of nitrogen limitation where the nitrogen addition rates were calculated to sustain relative growth rates (RGR) of 0.15 day^?1 and 0.25 day^?1, respectively. The photosynthetic performance of these cultures was compared to nitrogen-sufficient cultures with an average RGR of 0.32 day^?1. Plants transferred from nitrogen-sufficient conditions attained RGR values corresponding to the nitrogen addition rates after 6 to 10 days. Light-saturated net photosynthesis declined during adaptation according to the drop in growth rate, and a concomitant decrease in the respiration rate was recorded. The efficiency of net photosynthesis on a dry weight basis increased with increased nitrogen supply, whereas it was the same in all cultures when expressed on a chlorophyll basis. The light compensation point was unaffected by the nitrogen regime. Limited nitrogen supply resulted in an increased proportion of dry matter in the roots, which led to decreased leaf area ratios. The net assimilation rates also decreased, but not to the same extent as the leaf area ratios. Growth-limiting amounts of nitrogen were added to the cultures once daily, and the net influx of N was higher than the requirement for N, also in adapted cultures with a steady growth rate. This resulted in transient, periodic fluctuations in the NO₃^?, NH₄⁺ and amino acid pools. Also the rates of NO₃^? reduction and NH₄⁺ assimilation fluctuated as did the amino acid assimilation which paralleled NH₄⁺ assimilation. The role of flux rates over the plasmalemma and tonoplast for control of nitrogen assimilation rates are discussed.     

The extraction and quantification of ergosterol from ectomycorrhizal fungi and roots

Recently, ergosterol analysis has been used to quantify viable fungal biomass in resynthesized ectomycorrhizae. An objective of our study was to quantify ergosterol in a range of ectomycorrhizal isolates under differing growth conditions. In addition, we tested the applicability of the method on field-collected roots of ectomycorrhizal and vesicular-arbuscular (VA) mycorrhizal plants. Quantification of sitosterol as a biomass indicator of plant roots was also undertaken. Ergosterol was not detected in roots of uninoculated Betula populifolia seedlings, and sitosterol was not detected in an ectomycorrhizal fungal isolate but was present in birch roots. Ergosterol was produced in all isolates examined, which represented the major orders of ectomycorrhizal fungi. The range of values obtained, from 3 to nearly 18 g ergosterol mg^-1 dry mass, agrees well with reported values for other mycorrhizal and decomposer fungi. Hyphal ergosterol was the same during growth on phytic acid and KH₂PO₄. Reduction of growth temperature from 25° C to 15° C had little effect on ergosterol content of cultures harvested at similar growth stages. Ergosterol and sitosterol were detected in field-collected ectomycorrhizae of B. populifolia and Pinus sylvestris and VA mycorrhizae of Acer rubrum and Plantago major. Both ergosterol content and ergosterol to sitosterol ratios were significantly lower in VA mycorrhizae than ectomycorrhizae. Calculations of viable fungal biomass associated with field-collected roots were in agreement with those reported by others using the method on resynthesized ectomycorrhizae. Estimates of total mass could be obtained for field-collected B. populifolia roots by a simultaneously using ergosterol to estimate fungal biomass and sitosterol to estimate root mass. Some potential applications and limitations of sterol quantification in studies of mycorrhizal physiology and ecology are discussed.     

Quantitative studies of the development of S. cerevisiae mitochondria

The quantitative changes in mitochondria and cytochromes during transition of Saccharomyces cerevisiae from one steady state to another, while growing in continuous culture under controlled environmental conditions, were followed. No Mitochondria, or mitochondria like structures, were detectable in electron micrographs of permanganate-fixed anaerobic cells. Microaerobiosis (3μM dissolved oxygen) was sufficient to visualize mitochondrial profiles and induce cytochromes and their sections had a reduced number of mitochondrial profiles compared with cells grown in limiting glucose. In the presence of ergosterol and Tween 80 mitochondriogenesis, whether induced by aerobiosis or glucose limitation, involved enhanced definition of crystal and outer mitochondrial membranes and increased number of profiles. Where membrane formation was limited, by the absence of aerobiosis involved eytochrome induction and profile visualization, but limited profile Proliferation; the adapted cells consequently contained fewer, but more eytochrome-enriched, mitochondria than cells adapted in the presence of ergosterol and Tween 80. Increase in dissolved oxygen from 3μM to 52μM further enhanced membrane definition and increased the size, but not the number, of mitochondrial profiles. Evidence, obtained by measurement of eytochrome concentration per unit mitochondrial volume and per unit crystal area, support the concept that mitochondriogensis and cytochrome synthesis are not synchronized process and that cytochromes are added to or depleted from the mitochondrial cristae in response to culture conditions.     

Effect of nitrate-nitrogen limitation on photosynthesis of the diatom Phaeodactylum tricornutum Bohlin (Bacillariophyceae)

Abstract Nitrate limited growth of the diatom Phaeodactylum tricornutum in chemostat cultures produced marked changes in biochemical composition and a six-fold reduction in the specific growth rate. This was associated with a reduction in the carbon and chlorophyll a specific light saturated rates, with little effect on light limited photosynthesis. Variations in specific growth rate were quantitatively related to carbon specific net photosynthesis and maximum chlorophyll a specific light saturated rates were positively correlated with cell nitrogen contents. The correlation between nitrogen content and photosynthesis for P. tricornutum and the differential effect of nitrogen supply on the light response curve of photosynthesis is qualitatively and quantitatively similar to published results for terrestrial vascular plants. There was little change in the photon (quantum) yield of photosynthesis which was not significantly different from 0.125mol O₂ mol photon^-1 the theoretical upper limit based on the Z scheme, even under severe nitrate deficiency. The capacity to maintain a high photon yield under nitrate limitation is discussed in relation to the nitrogen requirements of the stromal and membrane components of the photosynthetic apparatus.     

Nutrient-specific effects in the coordination of cell growth with cell division in continuous cultures of <Emphasis Type="Italic">Saccharomyces cerevisiae</Emphasis>

Cell cycle progression of Saccharomyces cerevisiae cells was monitored in continuous cultures limited for glucose or nitrogen. The G1 cell cycle phase, before initiation of DNA replication, did not exclusively expand when growth rate decreased. Especially during nitrogen limitation, non-G1 phases expanded almost as much as G1. In addition, cell size remained constant as a function of growth rate. These results contrast with current views that growth requirements are met before initiation of DNA replication, and suggest that distinct nutrient limitations differentially impinge on cell cycle progression.     

ERGOSTEROL BIOSYNTHESIS: A FUNGAL PATHWAY FOR LIFE ON LAND?

Sterols, essential lipids of most eukaryotic cells, ensure important structural and signaling functions. The selection pressure that has led to different dominant sterols in the three eukaryotic kingdoms remains unknown. Here, we investigated the influence of the progression in the different steps of the ergosterol biosynthetic pathway (EBP) on the yeast resistance to transitions from aqueous to aerial media, typical perturbations of the higher fungi habitats. Five mutants of the EBP (ergΔ), accumulating different sterol intermediates in the EBP, and the wild‐type (WT) strain were exposed to drying under atmospheric air or nitrogen and wetting. Results show that the progression in the EBP parallels an increase in the yeast resistance to air‐drying with a maximal survival rate for the WT strain. When drying/wetting was performed under nitrogen, yeast survival was higher, particularly for the earlier mutants of the EBP. Thus, ergosterol, through its protective role against mechanical and oxidative stress, might have been selected by the pressure induced by drying/wetting cycles occurring in the fungi habitats. These results support the Bloch hypothesis, which postulates that the properties of sterols are gradually optimized for function along the biosynthetic pathway and provide a response to the enduring question “why ergosterol in fungi?”.