Skip to main content

Microorganisms in your Compost: The Famous Five

As I mentioned a previous article about bin composting, there are many different types of microorganisms that are responsible for breaking down the organic matter in your compost pile into nutrient rich humus. They can be broken down into five main categories:


Bacteria are the smallest and most commonly found organism found in compost; they make up about 80-90% of the microorganisms. They come in three different shapes and are all single celled organisms. They are structured as either rod shaped bacilli, spiral shaped spirilla or sphere shaped cocci. Bacteria are the most nutritionally diverse compost organisms and use many different enzymes to aid in the decomposing.

Most of the bacteria are mobile or move under their own power. They move by slipping and sliding around in the water that is in the compost so it is important to keep the pile moist. If not, then the bacteria will not be cable to move around and the pile will be unevenly composted. As the pile heats up, different types of bacteria are present at different temperature levels. There are even bacteria that are present when the pile heats up to 60C and above! At its highest levels, the bacteria Thermus have been isolated. This is a bacterium that was first discovered living in the geysers at Yellowstone National Park and are believed to have evolved there.

Different types of bacteria you could find in your compost

Different types of bacteria you could find in your compost


Acintomycetes are an interesting category of microorganism. They greatly resemble fungi in the way that they grow but they are actually a type of bacteria. They are primarily responsible for breaking down newspaper, and tough woody debris such as woodchips and roots. They also break down more complex organics such as cellulose, lignin, chitin, and proteins.

They are lacking of a nucleus like all bacteria but, like fungi, they grow multicultural filaments. If you notice long, grayish spider web looking things stretching on the top 10-20cm of your compost pile, do not fear, it is only the actinomycetes. They are most commonly seen in the ending stages of the compost process when only the most resistant compounds remain.


Why was the mushroom invited to the party…

Because he was such fungi!!! And also because he played a huge role in the composting pile!!! However, the fungi’s that appear in your compost pile will be mostly in the form of molds, not recognizable mushrooms. Fungi break down all the large organisms that are too large for bacteria alone. They attack organisms that are either too dry, acidic or low in nitrogen for the bacteria to handle. That’s another important reason to keep the carbon-nitrogen balance...balanced. If it’s out of whack, the bacteria will not be at their most effective.

While fungi are found at all stages of composting, they are found mostly on the outer layer of the compost when it’s in its thermophilic phase. Just to clarify, thermophilic is when the compost is at a higher than normal temperature level. Accordingly, a thermophile is a bacterium that grows in hotter than normal climates. Mesophilic would be a moderate or medium temperatur

Ciliated Protozoa tentatively identified as Dileptus anser using Covich's Ecology and Classification of North American Freshwater Invertebrates

Ciliated Protozoa tentatively identified as Dileptus anser using Covich's Ecology and Classification of North American Freshwater Invertebrates

Scroll to Continue


Protozoa are microscopic, one celled organisms similar to bacteria. They play a relatively minor role in the composting process but they are present. They are found in the water that is in the compost. They also act as secondary consumers eating all of the dead bacteria and fungi to keep the pile clean and healthy.


There’s not much to say about rotifers without getting more into biology than we need here. They are small planktonic organisms found in most freshwater worldwide. They are commonly used in fish tanks to keep the water clean because they eat algae and bacteria so they will help to keep the compost free of dead organisms.

As a person with no science background my self, its always nice to have a look at what I’m dealing with. I realize that you probably don’t have the things you need for this experiment at home but a simple internet search can get you all you need and its worth it!

As with all living things, these organisms require the correct quantities of water, oxygen, carbon and nitrogen to thrive. This is one reason that it is very important to maintain the carbon nitrogen balance in your pile. Because these organisms are aerobic, the compost pile will be giving off heat.

The composting process can be broken down into three basic phases. The first is the mesophilic phase. This phase is characterized by a moderate temperature and usually lasts only for a couple of days. The mesophilic bacteria begin the composting process and the heat energy that they release will help the pile to reach the next step. The next is the thermophilic phase, which is characterized by high temperatures and can last from several days to several months.

A compost pile being flipped and giving off heat in the form of steam.

A compost pile being flipped and giving off heat in the form of steam.

As the temperatures rise above 40 degree C, the mesophilic organisms start to die off and are replaced by the more heat resistant bacteria and Actinomycetes. When the pile reaches 50 degree C, many microorganisms that are human or plant pathogens will begin to die off.

Temperatures that are above 65 degree C are non productive. It will slow the decomposition process and kill many forms of microorganism so you will want to make sure to turn or aerate your pile fairly often to keep this from happening.

As the thermophilic organisms use up all the high-energy proteins, fats and complex carbohydrates the pile will begin to cool down again. The mesophilic organisms will again take over for the final stage of composting, the maturing phase.

Related Articles