Thermophilic fungi are a small assemblage in mycota that have a minimum temperature of growth at or above 20°C and a maximum temperature of growth extending up to 60 to 62°C.
As the only representatives of eukaryotic organisms that can grow at temperatures above 45°C, the thermophilic fungi are valuable experimental systems for investigations of mechanisms that allow growth at moderately high temperature yet limit their growth beyond 60 to 62°C.
Although widespread in terrestrial habitats, they have remained underexplored compared to thermophilic species of eubacteria and archaea. However, thermophilic fungi are potential sources of enzymes with scientific and commercial interests.
This review, for the first time, compiles information on the physiology and enzymes of thermophilic fungi. Thermophilic fungi can be grown in minimal media with metabolic rates and growth yields comparable to those of mesophilic fungi.
Studies of their growth kinetics, respiration, mixed-substrate utilization, nutrient uptake, and protein breakdown rate have provided some basic information not only on thermophilic fungi but also on filamentous fungi in general. Some species have the ability to grow at ambient temperatures if cultures are initiated with germinated spores or mycelial inoculum or if a nutritionally rich medium is used.
Thermophilic fungi have a powerful ability to degrade polysaccharide constituents of biomass. The properties of their enzymes show differences not only among species but also among strains of the same species. Their extracellular enzymes display temperature optima for activity that are close to or above the optimum temperature for the growth of organism and, in general, are more heat stable than those of the mesophilic fungi.
Some extracellular enzymes from thermophilic fungi are being produced commercially, and a few others have commercial prospects. Genes of thermophilic fungi encoding lipase, protease, xylanase, and cellulase have been cloned and overexpressed in heterologous fungi, and pure crystalline proteins have been obtained for elucidation of the mechanisms of their intrinsic thermostability and catalysis.
By contrast, the thermal stability of the few intracellular enzymes that have been purified is comparable to or, in some cases, lower than that of enzymes from the mesophilic fungi. Although rigorous data are lacking, it appears that eukaryotic thermophily involves several mechanisms of stabilization of enzymes or optimization of their activity, with different mechanisms operating for different enzymes.
Among the eukaryotic organisms, only a few species of fungi have the ability to thrive at temperatures between 45 and 55°C. Such fungi comprise thermophilic and thermotolerant forms, which are arbitrarily distinguished on the basis of their minimum and maximum temperature of growth (63): the thermophilic fungi have a growth temperature minimum at or above 20°C and a growth temperature maximum at or above 50°C, and the thermotolerant forms have a temperature range of growth from below 20 to ~55°C.
Thermophily in fungi is not as extreme as in eubacteria or archaea, some species of which are able to grow near or above 100°C in thermal springs, solfatara fields, or hydrothermal vents (36, 45). Perhaps because of their moderate degree of thermophily and because their habitats are not exotic, thermophilic fungi have not received much publicity and attention.
However, considering that the vast majority of eukaryotes cannot survive prolonged exposure to temperatures above 40 to 45°C (8), the ability of some 30 species, out of approximately 50,000 recorded fungal species, to breach the upper temperature limit of eukaryotes is a phenomenon that deserves elucidation.
Moreover, this group of fungi provides scientists with valuable experimental material for investigations of the mechanisms which, although allowing their growth at moderately high temperatures, limit it beyond 60 to 62°C (243).
Thermophilic fungi are the chief components of the microflora that develops in heaped masses of plant material, piles of agricultural and forestry products, and other accumulations of organic matter wherein the warm, humid, and aerobic environment provides the basic conditions for their development (10, 172). They constitute a heterogeneous physiological group of various genera in the Phycomycetes, Ascomycetes, Fungi Imperfecti, and Mycelia Sterilia (182).
Conclusions and Prospects
Modern studies on thermophilic fungi were stimulated by the prospect of finding fungi capable of secreting high levels of enzymes and of finding novel enzyme variants with high temperature optima and a long “shelf-life,” which are desirable characteristics for commercial applications of enzymes. Unfortunately, even before we had a chance to understand or appreciate thermophilic fungi, the questions of their adaptation to high temperatures were brushed aside in favor of practical ends. Consequently, our knowledge of their physiology is very fragmentary.
A long-held belief that thermophilic fungi are unable to grow, or grow poorly, at ambient temperatures needs to be reexamined, since some thermophilic fungi exhibit near optimal growth even at room temperature (20 to 30°C). This has led to uncertainty about their minimal temperature of growth. Because of this, thermophilic fungi may be redefined as fungi whose optimum growth temperature is 45°C or above.
Although it is unlikely that the temperature range of growth of thermophilic fungi will exceed 32 to 36°C, their latent mesophilic capability may be an advantage, since this would obviate the need for a strict temperature regimen for their industrial cultivation, resulting in savings in operational costs.
From the basic point of view, the potential of the same fungus for growth under thermophilic as well as mesophilic conditions poses questions about the biochemical composition of the fungus when grown over a wide temperature range, the rapidity with which metabolism is reorganized, and the mechanisms involved in metabolic adjustments. The question why thermophilic fungi have not been able to breach the upper temperature limit of growth of 60 to 62°C is still not resolved.
Because thermophilic fungi occur in terrestrial habitats which are heterogeneous in terms of temperature and the types and concentrations of nutrients, chemicals, gases, water activity, competing species, and other variables, they may be able to adapt to several factors besides just high temperature.
From this perspective, research should extend to their nutrient uptake systems, their ability to utilize mixed substrates, the nature and concentrations of their intracellular ions and osmolytes, and their effects on enzyme function. In addition, the composition of membrane lipids and of membrane-bound enzymes, the composition of their respiratory chain, and their energy production when grown under thermophilic and mesophilic conditions are other aspects that should be investigated.
Only when several species are studied and the results are compared will we be able to determine which of the observed responses is an adaptive strategy for thermophilic growth. In general, our knowledge of the above areas of fungal physiology is very limited. Perhaps as thermophilic fungi become better known, a fascination for these organisms will lead to basic studies and their physiology and biochemistry will be better understood.
The ease of their isolation and maintenance of cultures, the reduced risk of contamination of thermophilic cultures, their simple nutrition and rapid growth, and the formation of homogeneous mycelia in suspension cultures are desirable features for many types of experimental studies.
Although thermophilic fungi have long been known to be involved in composting and humification, the mechanisms involved in the accelerated decomposition of biomass are not well understood. However, their role in decomposition of vegetable matter suggests that thermophilic fungi may be good sources of a battery of purified enzymes that are capable of disassembling plant cell walls, which are required as tools by plant biologists for elucidation of cell wall architecture.
The extracellular enzymes of thermophilic fungi are appreciably thermostable, but less so than those of hyperthermophilic archaea (2). Currently, enzymes from hyperthermophiles are being favored, with prospects in biotechnology. A realistic assessment of the degree of thermostability needed for industrial applications of enzymes is required.
Since flexibility of proteins is essential for catalysis, the enzymes from hyperthermophiles will have optimal conformational flexibility at the temperatures at which they are adapted to grow (80°C or above) but may become too rigid and have low catalytic rates at the operational temperatures which in most situations range from 50 to 65°C.
Therefore, in most situations, enzymes having moderately high temperature optima and thermostability, as has been observed for extracellular proteins of thermophilic fungi, may be better suited than enzymes from hyperthermophiles. In basic studies, enzymes of thermophilic fungi (eukaryotes) will provide additional material for comparative analysis of the extent of modification in kinetic properties of enzymes vis-à-vis their thermal stability.
Some thermophilic fungi have yielded crystalline preparations of thermostable proteins that are contributing to structural information on the rules that determine thermostability of proteins and the critical groups involved in catalysis. This information will be essential for designing alteration in the genes encoding enzymes for specific applications.
The available information suggests that the thermostability of intracellular enzymes of thermophilic fungi is not appreciably different from that of mesophilic fungi. In fact, instances of unstable enzymes in thermophilic fungi have come to light.
Although the studies so far have been preliminary, there are indications that eukaryotic thermophily involves a repertoire of mechanisms of stabilization of enzymes, with different mechanisms operating for different proteins: intrinsic thermostability and stabilization by ions, other cellular proteins including chaperonin molecules, self-aggregation, and possibly covalent or noncovalent associations with the polymeric constituents of the cell wall.
The activity of certain sensitive enzymes may be optimized by their induced synthesis only when needed, by their placement in the most strategic location of the hypha, and by their rapid resynthesis. In this regard, the finding in a thermophilic fungus of a novel invertasean enzyme that has figured prominently in the development of biochemistrysuggests that investigations may reveal other examples of enzymes that are regulated differently in thermophilic fungi.
Such examples pose challenging questions about the significance of the observed difference, including whether the observed differences are adaptive responses to thermophily or to conditions that normally prevail in the habitat.
Cloning of some genes of thermophilic fungi and their heterologous expression have provided answers to questions concerning their biosynthesis, structure, and role of carbohydrate moieties in proteins.
Heterologous expression will undoubtedly be extended to other enzymes, which are difficult to obtain in sufficient amounts from the native strain. Some thermophilic fungi produce high levels of extracellular enzymes that are of interest in biotechnology. Knowledge of the structure and function of transcription control regions of the genes encoding these proteins may be used to construct recombinant genes for their overexpression.
Because thermophilic fungi are found in a variety of habitats from which they are easily isolated, many investigators have been prompted to use their own strains in enzyme studies. This is of course welcome, since studies with different isolates are required to obtain information on intraspecies differences in the properties of the same enzyme.
However, in some cases the marked differences observed in properties of enzymes in “strains” from different geographical backgrounds have raised doubts about whether the taxon was, in fact, correctly identified. A comparison of the properties of an enzyme from two or more strains by the same method will be necessary to determine if high variability is a characteristic feature of thermophilic fungi.
If so, investigations of the mechanisms generating variation may provide opportunities for fundamental discoveries. Finally, as has been said repeatedly, progress in science depends on prior investigations. It is therefore important that scientists share cultures of thermophilic fungi and cloned genes.