In 1920, Kurt Noack of Germany compared the metabolic rates of thermophilic and mesophilic fungi at different temperatures. Using the volume of carbon dioxide evolved over time as a measure of the metabolic rate, he compared Thermoascus aurantiacus (a thermophilic fungus) with Penicillium glaucum (a mesophilic fungus), both grown in identical media. The quantity of carbon dioxide released by the mesophilic fungus in 24 hours was equivalent to 67% of its dry weight at 15°C and 133% at 25°C. Noack reasoned that if this fungus could grow at 45 °C, the extrapolated value of carbon dioxide according to the van't Hoff rule would be 532%. However, the actual value for the thermophilic fungus used at 45°C was 310%. From this, Noack inferred that at a given temperature the metabolism of a thermophilic fungus is actually slower than what is expected based on the van't Hoff rule.
Subsequent measurements of oxygen uptake of mycelial suspensions by the Warburg method show that at their respective temperature optima, thermophilic fungi have a respiratory rate comparable to the mesophilic fungi (Prasad et al., 1979; Rajasekaran and Maheshwari, 1993). An unexpected observation was that the respiratory rate of mesophilic fungi is temperature-compensated over a broad range of temperatures (Figure 10.5). All major chemical components—proteins, lipids and nucleic acids—have their structural and functional properties altered by changes in temperature. How the mesophilic fungi achieve the observed metabolic homeostasis at a broad range of temperatures is not known. The adaptive modification of their basic biochemical structures and functions to exploit the natural environments continues to be the focus of investigations on biochemical adaptation.
Figure 10.5 Respiration of mycelia of thermophilic (Thermomyces lanuginosus, Penicillium duponti) and mesophilic (Trichoderma viride, Aspergillus phoenicis) fungi measured by Warburg manometric method. The data is given as Arrhenius plots of the logarithm of respiratory rate (QO2) of shaker-grown mycelia against reciprocal of absolute temperature. (From Rajasekaran and Maheshwari (1993).)
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