Temperature ranges of microorganisms

Microorganisms can be grouped into broad (but not very precise) categories, according to their temperature ranges for growth.

• Psychrophiles (cold-loving) can grow at 0^oC, and some even as low as -10^oC; their upper limit is often about 25^oC.
• Mesophiles grow in the moderate temperature range, from about 20^oC (or lower) to 45^oC.
• Thermophiles are heat-loving, with an optimum growth temperature of 50^o or more, a maximum of up to 70^oC or more, and a minimum of about 20^oC.
• Hyperthermophiles have an optimum above 75^oC and thus can grow at the highest temperatures tolerated by any organism. An extreme example is the genus Pyrodictium, found on geothermally heated areas of the seabed. It has a temperature minimum of 82^o, optimum of 105^o and growth maximum of 110^oC.

It must be stressed that the temperature ranges for the groupings above are only approximate. For example, we would use different criteria to classify prokaryotes and eukaryotes. The upper temperature limit for growth of any thermophilic eukaryotic organism is about 62-65^oC. And the upper limit for any photosynthetic eukaryote is about 57^o - for the red alga Cyanidium caldarium, which grows around hot springs and has a temperature optimum of 45^oC. In contrast to this, some unicellular cyanobacteria can grow at up to 75^oC, and some non-photosynthetic prokaryotes can grow at 100^oC or more.

Below, we consider two major types of thermophile - the microbes that grow in geothermal sites, and those that grow in "self-heating" materials such as composts. However, some very recent reports suggest that these different types of environment can share some common organisms.

The study of extreme environments has considerable biotechnological potential. For example, the two thermophilic species Thermus aquaticus and Thermococcus litoralis are used as sources of the enzyme DNA polymerase, for the polymerase chain reaction (PCR) in DNA fingerprinting, etc. The enzymes from these organisms are stable at relatively high temperatures, which is necessary for the PCR process which involves cycles of heating to break the hydrogen bonds in DNA and leave single strands that can be copied repeatedly. Another thermophile, Bacillus stearothermophilus (temperature maximum 75^oC) has been grown commercially to obtain the enzymes used in 'biological' washing powders.

Hot springs and geothermal vents are found in several parts of the world, but the largest single concentration is in Yellowstone National Park, USA. The images below show how some thermophilic prokaryotes (bacteria and archaea) are specially adapted to grow in these environments. In each case we find a zonation of microorganisms according to their temperature optima. Often these organisms are coloured, due to the presence of photosynthetic pigments (blue-green of cyanobacteria, red of red algae or purple bacteria) or carotenoid pigments (yellows and browns of some archaea).

Click for more about thermophiles in Yellowstone National Park

MT Madigan, JM Martinko & J Parker (1997) Brock Biology of Microorganisms. Eighth edition.. Prentice Hall. (see http://www.prenhall.com/brock/)

British Natural History Museum website: (http://nhm.ac.uk/zoology/extreme.html/)

MT Madigan & BL Marrs (1997) Punishing Environments: Extremophiles. Scientific American issue 4 (http://www.sciam.com/0497issue/0497marrs.html)

M.W.W. Adams and R.M. Kelly. (1995) Enzymes Isolated from Microorganisms That Grow in Extreme Environments. Chemical and Engineering News 73, 32-42.

Extremophiles. Special issue of Federation of European Microbiological Societies (FEMS) Microbiology Reviews 18, Nos. 2-3; May 1996.

K. O.Stetter (1996) Hyperthermophiles in the History of Life. in Evolution of Hydrothermal Ecosystems on Earth (and Mars?). Edited by GR Bock & JA Goode. John Wiley & Sons.

For images of extremophiles: http://www-micrbiol.sci.kun.nl/micrbiol/micrgala.html

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A compost consists of any readily degradable organic matter that is kept in a heap with sufficient mineral nutrients (e.g. nitrogen) and sufficient aeration to enable rapid microbial growth. The most familiar example is the garden compost heap, but more important examples are the composting plants used to process municipal wastes, and the composts used for commercial mushroom production.

The typical composting process is illustrated in Figure E below. There is an initial phase of rapid microbial growth (a) on the most readily available sugars and amino acids. This phase is initiated by mesophilic organisms, which generate heat by their metabolism and raise the temperature to a point where their own activities are suppressed. Then a few thermophilic fungi (e.g. Rhizomucor pusillus, Figure F) and several thermophilic bacteria (e.g. Bacillus stearothermophilus) continue the process, raising the temperature of the material to 70-80^oC within a few days. This peak heating phase (b) has a profound effect on the microbial population, because it destroys or inactivates all the mesophilic organisms (and the initial thermophilic fungi such as Rhizomucor pusillus) and leads to a prolonged high-temperature phase that favours other thermophilic species.

Figure E. Changes in temperature (solid line) and populations of mesophilic fungi (broken line) and thermophilic fungi (dotted line) in a wheat straw compost. Based on data in Chang & Hudson, 1967. The left axis shows fungal populations (logarithm of colony forming units per gram of compost plated onto agar); the right axis shows temperature in the centre of the compost. Stages a-d are referred to in the text.

Eventually the temperature declines and mesophilic organisms then recolonise the compost and displace the thermophiles (d in Fig. E). However, some heat-tolerant species such as Aspergillus fumigatus can continue to grow. This fungus can grow at temperatures ranging from 12^o to about 52-55^o. Strictly speaking, it is not a thermophile because its temperature optimum is below 50^o, but it is a very common and important member of the high-temperature compost community.

All the thermophilic fungi shown below were obtained by plating small particles of garden compost on potato-dextose agar containing antibacterial agents (streptomycin plus chlortetracycline) at 45^oC.

Figure F. Rhizomucor pusillus. Typical grey coloured colony on a plate of potato-dextrose agar at 45^oC (left-hand image). This fungus produces abundant "fluffy" aerial hyphae and spore-bearing stalks (sporangiophores) which are branched (centre and right images) and have sporangia at the tips of the branches. The delicate sporangial walls break to release numerous spores, leaving only a central bulbous region (the columella, c) and remains of the sporangial wall (arrowhead in right-hand image).

With a temperature range of 20-55^oC, this fungus is a typical early coloniser of composts, exploiting simple sugars, amino acids etc. that are present initially in the plant material. It is inactivated during peak-heating, and it does not recolonise afterwards.

Figure G. Humicola (or Thermomyces) lanuginosus. Colonies growing on potato-dextrose agar (top left) and malt extract agar (top right) at 45^oC. This fungus produces single spores by a balloon-like swelling process at the tips of short hyphal branches (bottom, left). At maturity (bottom right) the spores have brown, ornamented walls.

H. lanuginosus grows from 30 to 52-55^oC. It is extremely common in all types of self-heating material and also in birds' nests and sun-heated soils. It colonises composts after peak-heating and persists throughout the high-temperature phase. However, it cannot degrade cellulose and it seems to live as a commensal with cellulose-decomposing species, sharing some of the sugars released from the plant cell walls by their cellulolytic activities.

Figure H. Thermoascus aurantiacus colony (left) growing on malt-extract agar at 45^oC. The orange-brown colour is caused by the presence of many small (about 1 millimetre) fruiting bodies (ascocarps), which are seen at higher magnification (top right). These ascocarps are closed bodies, termed cleistothecia, containing many asci, each with 8 ascospores. The cleistothecia and the ascus walls break down at maturity to release the ascospores. Four asci containing ascospores are shown in the composite image at bottom right. They were released when a cleistothecium was crushed on a slide, and they show ascospores in various stages of maturity - the brown spores are nearly mature.

This fungus grows from about 25 to 55^oC and is a vigorous cellulose degrader.

Figure I. Paecilomyces species growing on malt extract agar at 45^oC (left image). The yellow-buff colour of the colony is caused by the presence of asexual sporing structures on the aerial hyphae. The sporing stages of the genus Paecilomyces (right-hand image) superficially resemble those of Penicillium, because the spores (conidia, c) are formed from flask-shaped cells (phialides, p) borne at the tips of short, brush-like branching structures. But the branching pattern of these "brushes" is less regular than in Penicillium.

Figure J. Aspergillus fumigatus. This common fungus of composts and mouldy grain has a grey-green colour on agar plates (left image), in contrast to the brighter green colour of several other Aspergillus species. The typical asexual sporing stage of Aspergillus consists of a spore-bearing hypha (conidiophore, centre image) which swells into a vesicle at the tip, and the vesicle bears flask-shaped cells (phialides) that produce the spores (conidia). In A. fumigatus the vesicle typically is club-shaped, the phialides arise only from the upper part of the vesicle, and the phialides all point upwards. Together with the grey-green colour and temperature range of about 12-52^oC, these features distinguish A. fumigatus from all other Aspergillus species.

A. fumigatus is an extremely common, interesting and dangerous fungus because of its nutritional opportunism. It is strongly cellulolytic, but it also can grow on hydrocarbons in aviation kerosene, and it can enter the lungs as inhaled spores, causing allergies or growing in the lung cavities, causing aspergillomas (see Airborne Microorganisms). Its ability to grow readily at 37^oC has made this fungus a significant problem in operating theatres, where it can establish infections of the internal organs via surgical wounds, especially during transplant surgery when the patient's immune system is suppressed.

Although the thermophilic fungi play a major role in degrading cellulose and other major polymers in composts, the activities of bacteria also are important. Two recent discoveries highlight this point.

• In 1996 it was reported that composts of many different types (garden and kitchen wastes, sewage sludge, industrial composting systems) contain high numbers of bacteria of the genus Thermus which grow on organic substrates at temperatures from 40-80^oC, with optimum growth between 65 and 75^oC. The numbers were as high as 10⁷ to 10¹⁰ per gram dry weight of compost. Spore-forming Bacillus species were also found, but they were unable to grow above 70^oC. Thus, it seems that Thermus species, previously known only from geothermal sites, have probably adapted to the hot-compost system and play a major role in the peak-heating phase. [T. Beffa et al., 1996. Applied & Environmental Microbiology 62, 1723-1727].
• Also in 1996, a number of autotrophic (self-feeding) bacteria were isolated from composts. These non-sporing bacteria grew at 60-80^oC, with optima of 70-75^oC, and closely resembled Hydrogenobacter strains that previously were know only from geothermal sites. They obtain their energy by oxidising sulphur or hydrogen, and synthesise their organic matter from CO₂. [T. Beffa et al., 1996. Archives of Microbiology 165, 34-40]

There is much to be learned about the interactions of microorganisms in composting systems. Work in this field is driven largely by commercial needs to produce composts for high mushroom yields (Agaricus species) and for rapid, efficient processing of municipal (domestic) and industrial wastes.

In contrast to the typical "natural" composting sequence (Figure E), commercial mushroom composts are produced by a truncated, two-phase process, designed to minimise the loss of cellulosic materials that Agaricus can use for growth. Phase I involves peak-heating of straw compost to 70-80^oC for several days. Then the compost is pasteurised at 70^oC and held at about 45^oC for a further few days (Phase II). Finally, the temperature is lowered and the compost is inoculated with Agaricus. Both of the preliminary phases are essential for high mushroom yields. Recent work suggests that the thermophilic fungus Scytalidium thermophilum becomes dominant in phase II and its presence can almost double the mushroom yield [G. Straatsma et al., 1995. Canadian Journal of Botany 73, S1019-1024]. The reason for this is still unknown.

Other work has shown that Agaricus bisporus (the commercial mushroom) can use either living or heat-killed bacteria as its sole source of nitrogen for growth. Like many other members of the fungal group basidiomycota, this fungus does not thrive on inorganic nitrogen sources such as ammonium or nitrate, but readily utilises organic nitrogen, which it can degrade by releasing protease enzymes. Thus, an initial high bacterial activity in the compost might provide the fungus with its favoured nitrogen source.

Websites:

Compost Resource Page. A "general interest" site, good fun, but perhaps a bit evangelical. http://www.oldgrowth.org/compost/

Glacier Gold Compost Inc (a commercial composting company): http://www.speakeasy.org/ggc/

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