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The most numerous organisms in a compost pile are bacteria. Although bacteria are too small to be seen individually, the effects of their work are easy to detect. Bacteria generate the heat associated with composting, and perform the primary breakdown of organic materials, setting the stage for larger decomposers to continue the job.

Bacteria don't have to be added to the compost pile. They inhabit virtually every surface and enter the pile on every single bit of organic matter. Initially their numbers may be modest, but given the proper conditions (proper moisture and aeration, a favorable balance of carbon and nitrogen, and lots of surface area to work on) bacteria can reproduce at a remarkable rate.

Many species of bacteria are at work in the compost pile. Each type thrives on special conditions and different types of organic materials. Even at temperatures below freezing, some bacteria can be at work on organic matter. These psychrophilic bacteria (a grouping of bacterial species that includes all those working in the lowest temperature range) do their best work at about 55°F, but they are able to carry on right down to 0°F. As these bacteria eat away at organic materials they give off a small amount of heat. If conditions are right for them to grow and reproduce rapidly, this heat will be sufficient to set the stage for the next group, the mesophylic bacteria, or middle temperature range bacteria.

Mesophilic bacteria thrive at temperatures from 70°F to 90°F, and just survive between 40°F to 70°F, and 90°F to 110°F. In many compost piles, these efficient mid-range bacteria do most of the work. However, given optimal conditions, they produce enough heat to kick in the real hot shots, the thermophilic, or heat-loving bacteria.

Thermophilic bacteria work fast, in a temperature range of 104°F to 170°F. In a matter of days they turn green, gold, and tan organic material into a uniform deep brown.

In all of this work, the bacteria are not alone—though at first they are the most active decomposers. Other microbes, fungi, and a host of invertebrates take part in the composting process. Some are active in the heating cycle, but most organisms prefer the cooler temperatures. They proliferate in cold compost piles and along the cooler outer edges of hot piles or when hot piles start to stabilize at lower temperatures.

actinomycetes	fungi

Actinomycetes are a type of primary decomposer common in the early stages of the pile. Actinomycetes produce grayish, cobwebby growths throughout compost that give the pile a pleasing, earthy smell similar to a rotting log. They prefer woody materials and survive a wide range of temperatures and conditions.

Fungi also perform primary decomposition in the compost pile. Fungi send out thin mycelial fibers like roots, far from their spore-forming reproductive structures. The most common of the reproductive structures are mushrooms, which sometimes pop up on a cool pile. Though fungi are major decomposers in the compost pile, fungal decomposition is not as efficient as bacterial decay. The growth of fungi, even more than that of bacteria, is greatly restricted by cold temperatures. Since they have no chlorophyll, fungi must obtain their food from plants and animals. Parasitic fungi exist on living plants or animals. Most fungi are saprophytic, living on decayed vegetable and animal remains.

Go to compost science 5

compost science 1: organics in new york city’s residential waste stream
compost science 2: the decomposition process
compost science 3: how does compost happen?
compost science 4: chemical decomposers—microorganisms
compost science 5: physical decomposers—larger organisms

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