Symbiosis

Most biologists, when confronted by the need to illustrate the concept of symbiotic mutualism, describe the case of lichens. Lichens are an obligate association between a fungus (the mycobiont) and an alga or blue-green bacterium (the phycobiont). Lichen mutualisms are very distinctive, and they can be identified on the basis of the size, shape, color, and biochemistry of their biomass. Lichenologists have developed systematic and taxonomic treatments of lichens, even though these mutualisms are not true "organisms" in the conventional A cape buffalo with an oxpecker on its back in Kenya. The relationship between the oxpecker and the buffalo is a type of symbiosis called mutualism; the oxpecker feeds from the supply of ticks on the buffalo, which in turn benefits from tick removal. JLM Visuals. Reproduced by permission. meaning of the word. The fungus benefits from the lichen mutualism through access to photosynthetic products of the alga or blue-green bacterium, while the phycobiont benefits from provision of a relatively moist habitat and enhanced access to inorganic nutrients.

Certain species of fungi also occur in intimate associations with the roots of vascular plants, in a mutualism referred to as mycorrhizae. The plant benefits from the mycorrhiza through increased access to inorganic nutrients, especially phosphate, while the fungus gains an advantage through access to nutritious exudates from the roots of the plant. This is a very widespread mutualism—most vascular plants have mycorrhizae.

Some vascular plants live in a mutualism with particular microorganisms that have the ability to fix atmospheric dinitrogen into ammonia, a form of inorganic nitrogen that the plant can utilize in its nutrition (see entry on nitrogen cycle). The best known examples of this mutualism involve various species of plants in the legume family (Fabaceae) and a bacterium known as Rhizobium japonicum. In this mutualism, the plant benefits from increased access to an important nutrient, while the bacterium gains an advantage through the provision of an appropriate habitat in the form of root nodules, as well as fixed energy provided by the host plant.

Another common mutualism occurs in the guts of animals that eat plant matter. Many animals consume plant biomass, but most are not very effective at digesting polymeric biochemicals such as cellulose and lignin. Often, these animals live in a symbiosis with microorganisms, which inhabit part of the gut and secrete specialized enzymes, such as cellulases, which digest cellulose. The herbivorous animal benefits from access to a large source of fixed energy, while the microorganisms benefit from access to a safe and appropriate habitat, and to nutritious chemicals available in the animal gut. This sort of mutualism occurs, for example, between termites and symbiotic bacteria and protozoans. In the case of the termite Eutermes, protozoans in the gut may account for 60% of the insect's weight. Many herbivorous mammals also live in a cellulose-digesting symbiosis with bacteria and protozoans, as is the case of ruminants such as the domestic sheep (Ovis aries) and cow (Bos taurus).