Endosymbiosis

Endosymbiosis

Endosymbiosis is a mutually beneficial relationship between a host organism and an internal associate organism. The term is derived from the prefix "endo," meaning within, and the word symbiosis, which refers to a mutually beneficial relationship between two closely associated organisms. Another term for symbiosis is mutualism, which highlights the fact that both organisms are benefiting from the relationship.

Examples of Endosymbiosis

A well-known example of endosymbiosis is the relationship between a termite and the microorganisms in its gut. The termite consumes wood, but it cannot digest it without the help of protozoans in the termite's gut that break down the cellulose to a form that the termite can metabolize. Thus, the termite supplies food for the protozoan, and the protozoan provides food for the termite. In this example, the protozoan is the endosymbiont, or the internal organism in the endosymbiotic relationship.

There are a variety of levels of dependency between the two associates, including at one extreme an entirely voluntary relationship in which each partner can survive alone, and at the other extreme a situation where both are entirely dependent on the other. Also, the endosymbiont can be at different places within the host organism, from within a body cavity such as the gut to within individual cells. Endosymbiosis also plays a role in evolution, affecting the structure, behavior, and life history of the associated organisms.

Although there are various levels of dependency between the two organisms in an endosymbiotic relationship, it is nearly always advantageous for the two to stay together. An example that demonstrates this is the mutualism between corals and their endosymbiotic algae. The type of algae involved here are called dinoflagellates, and they are specialized to photosynthesize or use organic foods as their energy source. However, certain nutrients are not readily available in the ocean, so it is beneficial for the dinoflagelletes to live within the corals, where the nutrients are available. Similarly, corals can gather some dissolved organic carbon from the water or from prey items, but it is much easier and faster to gather them from the photosynthetic activity of dinoflagellate endosymbionts. A side effect of photosynthesis is that calcium carbonate is precipitated from the water that forms the coral structures of coral reefs.

Both of these organisms have been cultured independently in the laboratory to show the extent of their interdependence. Under these circumstances, both have significantly reduced growth rates. Sometimes they even stop growing and rely on energy reserves. When they are allowed to circulate in the same water, but not make contact, their growth nearly doubles. When put into contact, growth is even greater, indicating that actual contact can spur a higher than normal release and uptake of chemicals they exchange. Clearly, then, it is to the advantage of both to remain together.

Some sea anemones with these dinoflagellate endosymbionts have adapted their behavior to the needs of their algae. For example, free-swimming jellyfish will make vertical migrations to layers of water that are rich in ammonium for the dinoflagellates. During the day, sessile sea anemones expose those parts of their bodies where the dinoflagellates are located to allow for photosynthesis. At night they retract those parts and expose their stinging tentacles to catch prey in order to sequester food and provide nitrogen to their endosymbionts. These examples of behavior modifications by the host associate organism show how the two organisms have evolved to benefit one another, and, in turn, themselves.

Locations of Endosymbionts

Endosymbionts can live within their associate organism at a variety of places. They can be within a cavity of the organism, within cavities and within cells, or entirely within cells. Intracellularly, the location can be in cells that have special vacuoles for the isolation of the endosymbiont from the interior of the cell, or in cells that maintain the endosymbiont directly within the cell fluid.

Termites and their protozoan gut inhabitants are one example of the endosymbiont living within a cavity of the associate organism. Another common example is the fauna in the stomach of ruminating animals, or animals that regurgitate and rechew food particles, such as deer, cattle, and antelope. Stomachs of ruminants have chambers, the first of which is called the rumen and is specially designed to maintain populations of bacteria and protozoa that break down the food of their host using fermentation. The rumen is supplied with food and kept within a certain range of pH by specialized salivary glands. This affords the microbial community with a substrate to feed off of and a favorable environment to do so. There are a diverse number of microorganisms living there, including bacteria that digest cellulose, protozoa that digest cellulose with the help of their own endosymbionts, and others still that are predators on these protozoa. An entire community of different species with different lifestyles lives there.

A common example of the endosymbiont living within the cells of the host is that of bacteria in the cells of insects. The cells of cockroaches contain bacteria, and cockroaches exhibit slowed development if the bacteria are killed with antibiotics. The growth of the cockroach can be restored, however, with certain additions to its diet that the bacteria presumably were providing.

The transmission of these bacteria from one cockroach to an offspring is hereditary, although not genetically based, because the bacteria invade the cytoplasm of the egg. Then, when the egg is fertilized and develops, it already has the endosymbiont that the mother had.

Another example of maternal transmission can be found in ruminating animals. In these animals, the mother passes the rumen microorganisms to her baby after it is born through her saliva and ruminated food, which contain all the microbial species the baby will need in life. If a baby ruminating animal is not allowed to be in contact with its mother, the baby may never get the microbes necessary for it to be able to digest plant material and will die.

Endosymbiotic Evolution

From behaviors such as the migration of jellyfish to different water layers, and special structures such as the rumen of the stomach, it is clear that endosymbiosis involves complex interactions and that these organisms have evolved together for many generations in order to develop such interactions.

Perhaps the oldest and most widespread example of this endosymbiotic co-evolution is in the origin of eukaryotic cells. They evolved from prokaryotic cells, with the primary differences being that eukaryotic cells are larger and more complex, containing a separate nucleus and numerous organelles (such as mitochondria), whereas prokaryotic cells are smaller with a few organelles floating freely in the cellular fluid. Examples of prokaryotes are simple unicellular organisms such as bacteria. Most multicellular complex organisms, however, from protozoans to fungus to animals, are eukaryotes.

How did eukaryotic cells arise? Athough there is no direct evidence, the most plausible theory is that an early prokaryotic cell, the ancestor to the mitochondrion, entered another prokaryotic cell, either as a food item or a parasite. Over time, the relationship between the two became endosymbiotic, with the mitochondrion supplying energy to the host associate and the host providing the proper environment and nutrients to the mitochondrion. Thus, a cell with a distinct organelle, or a eukaryotic cell, emerged. This means that every single cell in all prokaryotic organisms has endosymbiotic organelles.

Several characteristics of mitochondria support this widely accepted theory of an endosymbiotic evolution giving rise to eukaryotic cells:

• The mutually beneficial relationship between the cell, which provides nutrients and an environment for the organelle, and the mitochondrion, which provides energy for the cell, is seen in many other endosymbiotic systems, including those mentioned above.
• The modern role of the mitochondrion is to provide energy in a usable form for the cell.
• The mitochondrion has a genome within it that lets it reproduce itself and be largely independent from the cell and the cell's genome, which resides in the nucleus. Finally, the mitochondrion does not divide and reproduce in the same manner as the host cell. In sexually reproducing animals, for example, the mitochondria of the off-spring are not a mix of both parents' mitochondria. Instead, they are all inherited from the mother. Thus, the mitochondria do not recombine as does the rest of the cell during sexual reproduction. Rather, they act more as independent organisms, maintaining their identity from host to host.

see also Interspecies Interactions.

Jean K. Krejca

Bibliography

Ahmadjian, Vernon, and Surindar Paracer. Symbiosis: An Introduction to Biological Associations. Hanover, NH: University Press of New England, 1986.

Begon, Michael, John L. Harper, and Colin R. Townsend. Ecology, 2nd ed. Cambridge, MA: Blackwell Scientific Publications, 1990.

Douglas, Angela E. Symbiotic Interactions. Oxford: Oxford University Press, 1994.

Marguilis, Lynn. Symbiosis in Cell Evolution. San Francisco, CA: W. H. Freeman, 1981.

Ridley, Mark. Evolution, 2nd ed. Cambridge, MA: Blackwell Scientific Publications, 1996.

Valiela, Ivan. Marine Ecological Processes, 2nd ed. New York: Springer-Verlag, 1995.