Symbiosis

Symbiosis is a word used to refer to intimate relationships among species . Symbioses can involve interactions of individuals of different species, or associations of populations of one or more species. Symbiosis can involve obligate relationships, in which the symbionts cannot live apart in nature, but usually the association is more flexible than this.

Various types of symbiosis

Mutualism is a symbiosis between species in which both partners benefit. Mutualism is considered by some biologists to be the archetypal form of symbiosis. The examples of symbiosis that are discussed in the next section are all mutualisms.

Parasitism is another type of symbiotic association, in which one organism obtains nourishment from a host, usually without killing it. In most parasitisms, the parasite has a close and sometimes obligate relationship with the host. However, to be healthy, the host by no means needs the parasite, and in fact usually suffers a detriment from the symbiosis. Commensalism is a relationship in which one symbiont benefits from the interaction, while the host species is neither positively or negatively affected. For example, small epiphytic plants derive a substantial ecological benefit from living on larger plants, but the latter are not usually affected to a meaningful degree.

Examples of natural symbioses

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 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).

Symbioses between humans and other species

Humans live in symbioses of various intensities with a number of domesticated animals and plants. To varying degrees, these cultural symbioses are mutualistic, with both humans and the other species benefitting.

For example, all important agricultural plants exist in tight mutualisms with humans. Agricultural varieties of corn or maize (Zea mays), for example, are no longer capable of reproducing independently of human management. This is because over time the fruiting structure of maize has been selected to be enclosed in a leafy sheath that does not open, and to have seeds that do not easily separate (or shatter) from the supporting tissue (the cob). If humans did not plant the seeds of maize, the species would rapidly become extinct, because it no longer occurs as wild populations. The same is substantially true for most agricultural plants that have become extensively modified through cultural selection by humans. Humans, of course, benefit greatly from their mutualisms with agricultural plants, through the provision of crops of food, fiber, and other products.

Similarly, agricultural animals live in a symbiotic mutualism with humans. Cows (Bos taurus), for example, benefit from their human-managed access to fodder, veterinary services, and protection from predators, while humans benefit from access to milk and meat.

Even the keeping of animals as pets represents a type of mutualism. Pet dogs (Canis familiaris) and cats (Felis catus) are fed and kept safe in domestication, while humans benefit from the companionship of these animals, and sometimes from other services, as when cats kill pest rodents .

Symbiosis and evolution

Ideas about symbiosis have made some important contributions to theories that help explain the evolution of complex life forms on Earth . The first organisms on Earth were prokaryotic viruses and blue-green bacteria, which do not have an organized nucleus. Eukaryotic cells are more complex, having their nuclear material bounded within a nucleus, as well as other cellular organelles such as ribosomes , chloroplasts, cilia, flagellae, and other important structures. An exciting theory of the origin of eukaryotic cellular organization postulates the occurrence of a series of symbioses, in which prokaryotic cells became intimately associated with each other, with certain cellular functions being mutualistically divided amongst the symbionts. For example, certain tiny symbionts might have become responsible for the most of the respiratory function of the mutualism, and could then have evolved into mitochondria. Other symbionts, such as blue-green bacteria, could have been responsible for photosynthesis by the cell , and may have evolved into chloroplasts. To a degree these ideas are supported by the observation that both ribosomes and chloroplasts contain small amounts of genetic material (DNA, or deoxyribonucleic acid), which may be relict from an earlier, independent existence.

Another recent and highly controversial theory, called the Gaia hypothesis , suggests that Earth may represent an enormous, quasi-organismic entity, in which all species comprise a global, symbiotic, physiological system that maintains environmental conditions within a range that life can tolerate. Supporting evidence for this hypothesis includes the suggestion that the oxygen in Earth's atmosphere is ultimately of biological origin, having been emitted by photosynthetic organisms. Without oxygen, of course, most species could not survive. In addition, some ecologists suggest that the concentration of carbon dioxide in Earth's atmosphere is to a large degree regulated by a complex of integrated biological and physical processes by which CO2 is emitted and absorbed. This gas is well known to be important in the planet's greenhouse effect , which is critical in maintaining the average surface temperature within a range that organisms can tolerate.

See also Parasites.

Resources

books

Begon, M., J.L. Harper, and C.R. Townsend. Ecology: Individuals, Populations and Communities. 2nd ed. London: Blackwell Sci. Pub., 1990.

Brewer, R. The Science of Ecology. 2nd ed. Fort Worth: Saunders, 1994.

Margulis, L., and L. Olendzenski, eds. Environmental Evolution: Effects of the Origin and Evolution of Life on Planet Earth. Cambridge: MIT Press, 1992.

Bill Freedman

KEY TERMS

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Mutualism

—A mutually beneficial relationship between species.

Symbiosis

—A biological relationship between two or more organisms that is mutually beneficial. The relationship is obligate, meaning that the partners cannot successfully live apart in nature.