Bacteria:

Bacteria are very effective decomposers. These microorganisms play a vital role in the transformation of organic matter of the  compost. They use a wide range of enzymes to chemically break down plant molecules. Using these enzymes, they will first break down the cell walls of soft plant tissue. After, they will feed from the contents of the interior of cells. The first bacteria to colonize the compost are mesophiles who like to live at temperatures between 30 º and 45 º Celsius. With the increasing temperature, the thermophilic bacteria will settle. They prefer a temperature around 60 ° C. Bacteria are always present in the compost.

Cyanophytes:

The cyanobacteria are oxygenic photosynthetic bacteria. They can fix atmospheric nitrogen through specialized cells called heterocysts. They possess chlorophyll "a". Their photosynthetic system is very close to chlorophyllous plants. They are often called blue-green algae. They live in the first centimeter of the composts exposed to light.

Actinomycetes:

Actinomycetes are a group of filamentous bacteria of the soil. They look like fungus. These are bacteria that develop very fine filaments 0.5 microns around the spore. It was thought that this bacterium was an organisim between a fungus and bacteria. However, molecular phylogeny has established that they belonged to the group of Eubacteria. These bacteria have enzymes capable of degrading complex organic materials such as cellulose, lignin and chitin in simple sugar. Actinomycetes become active when bacterial activity decreases. Actinomycetes are the source of several antibiotics that we use. They play a very important role in recycling the dead organic matter of soil.

Fungi:

Fungi play a complementary role to the work of bacteria in the decomposition of the organic matter. They can very quickly degrade cellulose and lignin of plants. Although they are present at all stages of the composting process, they do not tolerate well temperatures above 50 C. After the thermophilic phase and the hard work of bacteria, they will expand their network of mycelium throughout the compost to feed on cellulose and lignin and produce simple sugar. This is the saccharification.

Viruses:

Pathogenic viruses  to humans generally can not withstand the high heat generated by compost. Exposure to a temperature of 60 °C for one week ensures good disinfection of organic matter. Viruses are ubiquitous in the environment. Each virus infects a particular host as a plant, insect, bacteria, etc.. Viruses present in the compost probably infects a bacterium, an insect or Collembola, but they are not pathogenic to humans.

Nematodes (or Nemathelminthes):

Nematodes are microscopic round and translucent worms that devour  fungus, bacteria and insect larvae. They will release a large amount of nitrogen in the soil. If the compost becomes too dry, they will form cysts to protect themselves. After re-hydration, they will resume their activities.

Acari:

Acari are tiny arachnids representative. There are several species of Acari. In the compost, we might find there two species. Saprophagous Oribatida will contribute to the fragmentation of organic compost. The Gamasina carnivores feed on small worms, eggs of Collembola.

Collembola:

Collembola look like small insects. However, they are wingless and they do not pass through a larval phase. Today, they are classified in hexapods. They are more closely related to crustaceans. They feed on microorganisms in the compost.

Wood lice:

Wood lice are small crustaceans land. They need a moist environment to live. They feed on dead organic matter of the compost.