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A wide array of microorganisms live in a compost pile. Bacteria are especially abundant and are usually divided into several classes based upon the temperatures at which they grow best. The low temperature bacteria are the psychrophiles, which can grow at temperatures down to -10°C, but whose optimum temperature is 15°C (59°F) or lower. The mesophiles live at medium temperatures, 20-45°C (68-113°F), and include human pathogens. Thermophiles thrive above 45°C (113°F), and some live at or even above the boiling point of water.

Strains of thermophilic bacteria have been identified with optimum temperatures ranging from 55°C to an incredible 105°C (above the boiling point of water), and many temperatures in between.20 The strains that survive at extremely high temperatures are called, appropriately enough, extreme thermophiles, or hyperthermophiles, and have a temperature optimum of 80°C (176°F) or higher. Thermophilic bacteria occur naturally in hot springs, tropical soils, compost heaps, in your excrement, in hot water heaters (both domestic and industrial), and in your garbage, to name a few places.21

Thermophilic bacteria were first isolated in 1879 by Miquel, who found bacteria capable of developing at 72°C (162°F). He found these bacteria in soil, dust, excrement, sewage, and river mud. It wasn't long afterward that a variety of thermophilic bacteria were discovered in soil - bacteria that readily thrived at high temperatures, but not at room temperature. These bacteria are said to be found in the sands of the Sahara Desert, but not in the soil of cool forests. Composted or manured garden soils may contain 1-10 percent thermophilic types of bacteria, while field soils may have only 0.25% or less. Uncultivated soils may be entirely free of thermophilic bacteria.22

[table camparing different types of manures]

Thermophiles are responsible for the spontaneous heating of hay stacks which can cause them to burst into flame. Compost itself can sometimes spontaneously combust. This occurs in larger piles (usually over 12 feet high) that become too dry (between 25% and 45% moisture) and overheat.23 Spontaneous fires have started at two American composting plants (Schenectady and Cape May) due to excessively dry compost. According to the EPA, fires can start at surprisingly low temperatures (194°F) in too-dry compost, although this is not a problem for the back yard composter. When growing on bread, thermophiles can raise the temperature of the bread to 74°C (165°F). Heat from bacteria also warms germinating seeds, as seeds in a sterile environment are found to remain cool while germinating.24

Both mesophilic and thermophilic microorganisms are found widely distributed in nature, and are commonly resident on food material, garbage, and manures. This is not so surprising when considering mesophiles, because the temperatures they find to be optimum for their reproduction are commonly found in nature. These temperatures include those of warm-blooded animals, which excrete mesophiles in their stools in huge numbers.

A mystery presents itself, on the other hand, when we consider thermophilic microorganisms, since they prefer living at temperatures not commonly found in nature, but in hot springs, water heaters, and compost piles. Their preferences for hot temperatures has given rise to some speculation about their evolution. One theory suggests that the thermophiles were among the first living things on this planet, developing and evolving during the primordial birthing days of Earth, when surface temperatures were quite hot. They have thus been called the "Universal Ancestor." Estimated at 3.6 billion years old, they are said to be so abundant as to "comprise as much as half of all living things on the planet." 25 This is a rather startling concept, as it would mean that thermophilic organisms are perhaps more ancient than anything else alive. Their age would make dinosaurs look like new born babes, still wet behind the ears (however extinct). Of course, we humans, in comparison, have just shown up on the Earth. Thermophiles could, therefore, be the common ancestral organism of all life forms on our planet.

Just as startling is the concept that thermophiles, despite their need for a hot environment, are found everywhere. They're lingering in your garbage, and in your stool, and have been since we humans first began to crawl on this planet. They have quietly waited since the beginning of time, and we haven't been aware of them until recently. Researchers insist that thermophiles do not grow at ambient or room temperatures.26 Yet, like a miracle, when we collect our organic refuse in a tidy pile, the thermophiles seem to be sparked out of their dormant slumber to work furiously toward creating the primordial heat they so long for. And they succeed - if we help them by creating compost piles. They reward us for our help by converting our garbage and other organic discards into life-sustaining earth.

The knowledge of living creatures incomprehensibly ancient, so small as to be entirely invisible, thriving at temperatures hotter than those normally found in nature, and yet found alive everywhere, is remarkable enough. The fact that they are so willing to work for our benefit, however, is rather humbling.

By some estimates, humanure contains up to 1,000,000,000,000 (a trillion) bacteria per gram.27 These are, of course, mixed species, and not by any means all thermophiles. A trillion bacteria is equivalent to the entire human population of the Earth multiplied by 166, and all squeezed into a gram of organic material. These microbiological concepts of size and number are difficult for us humans to grasp. Ten people crammed into an elevator we can understand. A trillion living organisms in a teaspoonful of crap is a bit mind-boggling.

Has anyone identified the species of microorganism that heats up compost? Actually, a large variety of species, a biodiversity, is critical to the success of compost. However, the thermophilic stage of the process is dominated by thermophilic bacteria. One examination of compost microorganisms at two compost plants showed that most of the bacteria (87%) were of the genus Bacillus, which are bacteria that form spores,28 while another researcher found that above 65°C, the organisms in the compost were almost purely Bacillus stearothermophilus.29

Source: The Humanure Handbook. Jenkins Publishing, PO Box 607, Grove City, PA 16127. To order, phone: 1-800-639-4099.

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weblife.org | library | Humanure Handbook | Chapter 3: Thermophilic Microorganisms