What is Endosymbiosis?
Put simply, endosymbiosis is one organism living within the body of another organism. The relationship between an endosymbiont and its host is usually a mutualistic one, or one that is mutually beneficial.
One example of endosymbiosis is a type of algae called zooxanthellae that lives in the tissue of corals. The zooxanthellae is provided with protection and a place to live, and in return in performs photosynthesis and provides additional energy to the coral host.
Zooxanthellae are what gives many corals their vibrant color. When a coral becomes stressed, such as due to abnormally warm water, it may release its zooxanthellae in a process known as coral bleaching. Bleached corals turn white, and will usually starve to death due to the loss of their energy-producing zooxanthellae.
An even more intimate version of endosymbiosis can be found within the cells of our very own bodies. Mitochondria are found in nearly every cell in the human body, yet they also have their own internal DNA. This is because mitochondria probably used to be free-living prokaryotic cells.
Far back in the evolutionary past, the ancestor of modern mitochondria was engulfed by a primitive eukaryotic cell. Rather than digesting the prokaryotic cell, the eukaryotic host allowed it to remain safely inside as long as it produced energy in return. This created a mutualistic relationship that benefited both parties, and led to the modern mitochondria we rely on today. The endosymbiotic relationship between mitochondria and eukaryotic cells is now so firmly established that both sides would certainly die without the other.
Sea Slug, (Ilyes, 2006)
The photosynthetic chloroplasts found in plant cells likely came about in exactly the same way. Interestingly, the lettuce sea slug is known to “steal” chloroplasts from the algae that it eats and use them to produce energy in its own cells. This is another example of endosymbiosis and one of the only known instances of an animal gleaning energy from photosynthesis.
Ilyes, Laszlo (photographer). (2006). A lettuce sea slug… [photography]. Retrieved from http://en.wikipedia.org/wiki/Elysia_crispata#/media/File: Lettuce_Sea_Slug_11-03-2006.jpg