Microorganisms can be physically associated with other organisms in a variety of ways. One organism can be located on the surface of another, as an ectosymbiont. In this case, the ectosymbiont usually is a smaller organism located on the surface of a larger organism. Often, dissimilar organisms of similar size are in physical contact. The term consortium can be used to describe this physical relationship. Consortia in aquatic environments are frequently complex, involving multiple layers of similar-looking microorganisms that often have complementary physiological properties. In contrast, one organism can be located within another organism as an endosymbiont. There also are many cases in which microorganisms live on both the inside and the outside of another organism, a phenomenon called ecto/ endosymbiosis. Interesting examples of ecto/endosymbioses include a Thiothrix species, a sulfur-using bacterium, which is attached to the surface of a mayfly larva and which itself contains a parasitic bacterium. Fungi associated with plant roots (mycorrhizal fungi) often contain endosymbiotic bacteria, as well as having bacteria living on their surfaces (see pp. 679-82).
Everything is everywhere, the environment selects.
M. W Beijerinck
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