Bacteria perform the primary breakdown of organic materials and generate the heat associated with composting. Bateria don’t have to be added to the compost pile. They are present virtually everywhere and enter the pile on every single bit of organic matter. Many types of bacteria participate in the composting process, thriving at different temperatures and on different materials. Other microscopic creatures, such as actinomycetes, protozoa, and fungi, also play important roles. Together, these microscopic decomposers change the chemistry of the organic wastes; they carry the name of chemical decomposers.
- Psychrophilic - First wave of microbial activity. They do their best work at 55ºF, but can carry on right down to 0ºF. As they eat away at organic materials, they give off a small amount of heat.
- Mesophilic - Second wave of microbial activity. Most of the decomposition in a compost pile is mesophilic. They do their work at temperatures between 70ºF and 90ºF.
- Thermophilic - Third and final wave of microbial activity. At 100ºF, the thermophiles take over and raise the temperature to about 160ºF. However, the highest range temperatures last only 3 to 5 days as the microbes use up the air and moisture in the compost pile.
Bacteria. The bacteria likely to be found in a compost pile are those that specialize in breaking down organic compounds, those that thrive in temperatures ranging up to 170ºF (77ºC) in the thermophilic range, and those that are aerobic, needing air to survive. Bacterial populations differ from pile to pile, depending upon the raw materials of the compost, degree of heat, amount of air present, moisture level and other considerations.
Bacteria are single-celled and can be shaped like spheres, rods or spiral twists. They are so small that it would take 25,000 bacteria laid end to end to take up one inch on a rule and an amount of garden soil the size of a pea may contain up to a billion bacteria. Most bacteria are colorless and cannot make carbohydrates from sunshine, water and carbon dioxide the way green plants can. Some bacteria produce colonies; others are free-living. All reproduce by means of binary fission.
Bacteria are the most nutrionally diverse of all organisms, which is to say, as a group, they can eat nearly anything. Most compost bacteria, similar to fungi and animals, can use living or dead organic materials. Some are so adaptable they can use more than 100 different organic compounds as their source of carbon because of their ability to produce a variety of enzymes. Usually, they can produce the appropriate enzyme to digest whatever material they find themselves on. In addition, respiratory enzymes in the cell membrane make aerobic respiration possible.
Since bacteria are smaller, less mobile, and less complexly organized than most organisms, they are less able to escape an environment that becomes unfavorable. A decrease in the temperature of the pile or a sharp change in its acidity can render bacteria inactive or kill them. When the environment of a pile begins to change, bacteria that formerly dominated may be decimated by another species.
At the beginning of the composting process, mesophilic bacteria and fungi predominate. They gradually give way to thermophilic bacteria as the pile becomes hotter; the more thermophilic bateria that are present, breaking down compounds and releasing heat as a by-product, the hotter the pile becomes. As stability approaches, actinomycetes and fungi that have so far been contained to the cooler edges of the pile begin to dominate the compost and hasten it toward further stability.
Actinomycetes. The characteristically earthy smell of newly plowed soil in the spring is caused by actinomycetes, higher-form bacteria similar to fungi and mold. Actinomycetes are especially important in the formation of humus. While most bacteria are found in the top foot or so of topsoil, actinomycetes may work many feet below the surface. Deep under the roots they convert dead plant matter to a peatlike substance.
While they are decomposing animal and vegetable matter, actinomycetes liberate carbon, nitrogen and ammonia, making nutrients available for higher plants. They are found on every natural substrate, and the majority are aerobic and mesophilic. Five percent or more of the soil’s bacterial population is comprised of actinomycetes.
The reason that other bacteria tend to die rapidly as actinomycete populations grow in the compost pile is that actinomycetes have the ability to produce antibiotics - chemical substances that inhibit bacterial growth.
Protozoa. Protozoa are the simplest form of animal organism. Even though they are single celled and microscopic in size, they are larger and more complex in their activities that most bacteria. A gram of soil can contain as many as a million protozoa, but compost has far fewer, especially during the thermophilic stage. Protozoa obtain their food from organic matter in the same way bacteria do. In fact, they are so much like bacteria ans so much less important to composting that they need only brief mention in the compost biological census.
Fungi. Fungi are primitive plants that are single celled or are many celled and filamentous. Unlike more complex green plants, they lack chlorophyll and therefore lack the ability to make their own carbohydrates. Most of them are classified as saprophytes because they live on dead or dying material and obtain energy by breaking down organic matter in dead plants and animals.
Like the actinomycetes, fungi take over during the final stages of the pile when the compost has been changed to a more easily digested form. The best temperature for active fungi in the compost heap is around 70º to 75ºF (21º to 24ºC), though some thermophilic forms prefer much greater heat and survive to 120ºF (49ºC).