Endosymbiosis

Endosymbiosis is a form of symbiotic relationship wherein an organism lives in the body or cell of another. The invasive organisms, which are known as endosymbionts, may either occupy the intracellular space or live extracellularly within a host organism.

The relationship is usually mutually beneficial, and in some cases obligate, meaning that neither are able to live without the other. The acquisition of an endosymbiont can result in the production of unique and important abilities or structures in the host organism. Well known examples include the nodules formed by the nitrogen-fixing bacteria that inhabit legume root cells, or the dinoflagellate that live within stony coral.

The word come from the Greek terms endo = inner, sym = together, and biosis = living.

Coral-dinoflagellate Symbiosis
In tropical marine environments, perhaps the most significant symbiosis is that between calcifying corals (scleractinian corals) and photosynthetic dinoflagellates. The presence of photosynthetic symbionts within the tissues of the host results in high levels of primary productivity as well as rapid deposition of CaCO3 that builds up to form the rock substrate of the coral reef. Thus not only are scleractinian corals key organisms in the formation of the most phylogenetically diverse of marine ecosystems, coral reefs, but they are a major component of the global carbon cycle. Gross primary productivity from coral reefs is among the highest in the tropical seas, between 300 and 5,000 gmC/m2/year (compared to 8-50 gmC/m2/year for non-reefal tropical marine environments).

Legume root nodules


Tiny (2 mm) round organs, called “root nodules,” form on the roots of legumes that are occupied by an endosymbiont. The nodules are the effect of specialized bacteria that capture nitrogen from the air and trade it to the legume plant for sugars the plant produces.

A pinkish color visible in the picture at right comes from a compound similar to the hemoglobin in animal red blood cells.

Endosymbiotic Theory
Evolutionists believe that eukaryotic cells originated through a symbiotic relationship between two or more cells. It is suggested that bacteria entered a proto-eukaryotic cell, and their genetic systems and cellular functions became integrated to form a single functioning cellular unit. Organelles such as the mitochondria and chloroplast are thought to be remnants of these internalized bacteria.

The endosymbiosis theory has many problems, such as the lack of evidence that prokaryotes are capable of ingesting another cell and keeping it alive, and the large differences in genes between mitochondria and prokaryotes.

Mitochondria
Support for an endosymbiont origin of the mitochondria is largely derived from the fact that mitochondria have their own DNA, RNA, and ribosomes. Further support is found by the existence of a eukaryotic organism, called the amoeba, which lacks mitochondria and must always have a symbiotic relationship with an aerobic bacterium. In addition, the energy-conversion process that takes place in the mitochondria is aerobic (with oxygen), while other energy conversion processes in the cell take place anaerobically, or without oxygen. The independent aerobic function of these organelles is thought to have come from bacteria that lived inside of other simple organisms in a mutually beneficial, or symbiotic, relationship, providing them with aerobic capacity.