Symbiosis

Symbiosis

The term symbiosis, from the Greek words syn (together with) and bios (life), refers to different kinds of organisms living together in ongoing physical association. Although symbiosis is a fundamental biological relationship, it was a disputed concept until the late 1800s, and the term was only first used in 1878. Its role in ecology and evolutionary theory is still developing.

Biologists recognize several variations of symbiotic association. Obligate symbiosis, such as the tropical reef relationship between Zooxanthellae algae and the coral they inhabit, is necessary for the survival of one or more partners. Facultative symbionts are optional; in tidepools, some sea anemones have green flecks of algae growing inside them, while neighboring anemones do not. Endosymbiosis occurs when one species lives inside another, as cellulose-digesting bacteria inhabit the gut of herbivores. Ectosymbiosis, which does not involve internalization, occurs when, for example, birds or fish clean larger species. Finally, there is a range of interactive impacts. In mutualism, both species benefit; all the above and what is perhaps the first-described case, the algae-fungus association that forms lichens, are examples of mutualism. Commensalism involves advantage to one species and neutral impact on another. Parasitic symbiosis benefits one species at a cost to another. Some biologists use the term symbiosis only for mutualistic associations, although scholarly literature and popular textbooks are ambiguous on this point.

Symbiosis was catapulted to prominence in evolutionary theory by the notion that mitochondria and chloroplasts (internal organelles within cells) originated through the endosymbiotic internalization of simpler prokaryotic cells. This theory has been championed by Lynn Margulis, who developed the serial endosymbiosis theory, which attempts to account for the successive development of all eukaryotic cells (cells with nuclei), through a sequence of unions between various prokaryotic bacteria (non-nucleated cells). While some details of serial endosymbiosis theory are still debated, the endosymbiotic origin of eukaryotes is found in virtually all textbooks.

Symbiosis theory has been extended in several profound but controversial ways. The notion of symbiogenesis suggests that symbiosis contributes significantly to the origin of novel traits and new species. Traditional Darwinian theory argues that speciation occurs by natural selection operating on random genetic mutations. Symbiogenesis posits that the symbiotic union of diverse genetic information is a source of creative novelty on which selection acts. Some symbioses, such as lichens, result in an altogether different kind of organism. Moreover, instead of the win-lose scenarios of competitive individual selection, symbiogenesis may more readily create win-win cooperative scenarios that entail new capabilities and resources. Symbiosis as a major evolutionary mechanism has significant though still debated implications, especially for notions of cooperation and complexity in evolutionary history.

Another provocative extension of symbiosis theory entails the scale at which symbiotic associations are conceived to exist. Traditional examples of symbiosis involve individual organisms in physical association with other individuals: for example, a plant and the nitrogen-fixing fungi in its roots. However, one could think of symbioses as involving groups of organisms, such as oxygen-breathing animals and oxygen-generating plants in a pond community. In principle, this could be extended to communities interacting in an ecosystem, or global ecosystems interacting with each other on a planetary scale. James Lovelock's notion of Gaia holds that the entire living world, or biosphere, interacts to regulate water, atmospheric gasses, pH, and temperature. Margulis and others suggest that this reflects the symbiotic integration of life into a global superorganism.

See also Competition; Evolution, Biological

Bibliography

margulis, lynn. symbiotic planet: a new look at evolution. new york: basic books, 1998.

paracer, surindar, and ahmadjian, vernon. symbiosis: an introduction to biological associations. new york: oxford university press, 2000.

sapp, jan. evolution by association: a history of symbiosis. new york: oxford university press, 1994.

seckbach, joseph, ed. symbiosis: mechanisms and model systems. dordrecht, netherlands: kluwer, 2002.

jeffrey p. schloss