Studies on the biology of thermophilic fungi

Abstract

Among the eukaryotes, only some fungi possess the ability to grow at temperatures up to 60 °C. The thermophilic fungi, as they are called, are an important component of the microflora which develops in self heating masses of organic matter. They are also widespread in soil. This study was undertaken to obtain some basic information on their growth and physiological characteristics. Such information was expected to prove useful in understanding the ecology and evolution of thermophilic fungi. Growth A thermophilic fungus, Thermomyces lanuginosus, commonly found in compost and soil, was selected for most experiments. Its Tmin_\text{min}min , Topt_\text{opt}opt , and Tmax_\text{max}max for growth were close to 30 °C, 50 °C, and 60 °C, respectively. The values for specific growth rate and the molar growth yield (biomass produced per mole of sugar utilized) of thermophilic fungi studied were comparable to those of mesophilic species when measured at their respective temperature optima. These findings discount the belief that thermophiles grow faster and have a higher maintenance energy requirement than mesophiles. Respiration Measurements of respiratory rates of mycelia showed that thermophilic fungi do not have a faster rate of metabolism than mesophilic fungi. This suggests that an ability to hold their reaction rates steady despite increased kinetic energy of reacting molecules could be the hallmark of thermophilic fungi. Determination of the respiratory quotient indicated that at the high growth temperature (50 °C) the main endogenous substrate was carbohydrate, whereas at low temperature (30 °C) it was lipid. Determination of the C6_66 /C1_11 ratio indicated that the predominant pathway of hexose degradation in T. lanuginosus at 50 °C was the Embden–Meyerhof pathway, and at 30 °C it was the pentose phosphate pathway. This suggested that metabolism in thermophilic fungi readily responds to temperature changes; temperature acts as a switch in regulating metabolism. Lipid Composition Because lipids can exhibit phase transitions as a function of temperature, an inability to adjust the lipid composition of the plasma membrane had been proposed as a reason for the inability of thermophilic fungi to grow at low temperatures. The higher content of unsaturated fatty acids in the phospholipid fraction of T. lanuginosus grown at 30 °C suggested that this is unlikely to be the cause of inhibited growth at low temperatures. Nutrient Uptake System Active transport of glucose in T. lanuginosus was mediated by a substrate specific carrier. The transport seemed to be driven by a proton gradient across the plasma membrane. The possession of a high affinity sugar transport system (Km_mm = 290 µM) would allow harnessing of substrate when present in the environment in low concentrations. Protein Breakdown An early hypothesis proposed that thermophilic microorganisms can grow and survive at high temperatures because of the ability to rapidly replace heat labile proteins. The rate of protein breakdown in thermophilic fungi was comparable to that in mesophilic fungi (3.5% per hour). The hypothesis of rapid protein turnover was therefore discounted as a general mechanism for thermophily. Ecology Microbiological examination of soil samples collected from different places in India confirmed the widespread distribution of thermophilic fungi in soil. As soil temperatures are normally far below the optima for thermophilic fungi, experiments were designed to understand their common occurrence in soil. An immunofluorescent approach was taken to study the autecology of T. lanuginosus using the buried slide technique. The ability of the fungus to grow in soil in the open could not be proved. Experiments were also designed to study the competitive ability of thermophilic fungi in soil plates exposed to diurnally varying thermal regimes. Development of thermophilic fungal colonies in soil plates occurred only when the incubation temperature was varied between 40 °C and 48 °C. The experimental results do not favour the view that thermophilic fungi normally grow and reproduce in soil. Their widespread presence in soil is considered to be a result of dissemination of propagules from growth in self heating masses of organic material (compost) where temperatures up to 55 °C are recorded.