Association in which two species involved benefit, however, each species can only live in the presence of the other. Among examples we will highlight.
- Lichens - Lichens are associations between unicellular algae and fungal cereals. Algae synthesizes organic matter and provides fungi with part of the food produced. These, in turn, remove water and mineral salts from the substrate, supplying them to algae. In addition, the fungi surround the algae group with their hyphae, protecting them against dehydration.
- Termites and protozoa - By eating wood, termites obtain large amounts of cellulose but cannot produce cellulase, an enzyme capable of digesting cellulose. In its gut there are flagellated protozoa that can perform this digestion. Thus, protozoa rely in part on the insect's food, which in turn benefits from the action of protozoa. None of them, however, could live in isolation.
- Ruminates and microorganisms - In the ruminant's paunch or rumen are also bacteria that promote the digestion of the cellulose ingested with the foliage. It is a case identical to the previous one.
- Legume Bacteria and Roots - In the nitrogen cycle, bacteria of the genus Rhizobium produce nitrogenous compounds that are assimilated by legumes, in turn provide these bacteria with the organic matter needed to perform their vital functions.
- Mycorrhizae - They are associations between fungi and roots of certain plants, such as orchids, strawberries, tomatoes, pines, etc. The fungus, which is a decomposer, provides the vegetable with nitrogen and other mineral nutrients; in return, it receives photosynthesized organic matter.