They used to be composers, but by the beginning of the 20 TH century, they had all become de composers. I can, however name three rather large groups of decomposer organisms and direct you to a pretty cool web site . Bacterial decomposers are grouped by the temperature ranges in which they work the best. Fungi Fungi is the name for a family of simple organisms which includes molds and yeasts. Next to bacteria, fungi are the most efficient decomposer organisms.
Molds can grow both as unseen filaments and as gray or white fuzzy colonies on the surface.
Fungi are important because they break down tough debris, enabling bacteria to continue the decomposition process once most of the cellulose has been exhausted. They spread and grow vigorously by producing many cells and filaments, and they can attack organic residues that are too dry or acidic for bacterial decomposition. Actinomycetes are a form of fungi-like bacteria that form long filaments resembling spider webs stretching through the humus. Actinomycetes are the primary decomposers of tough plant tissues like bark, paper and stems. They are especially effective at softening up tough materials, such as cellulose, chitin, and lignin.