You\u2019ve probably heard of symbiosis (Greek for \u2018living together\u2019), a close and long-term association between two different organisms. Endosymbiosis occurs when one organism, known as the endosymbiont, lives inside another, the host.<\/p>\n

Generally, endosymbionts are small enough to reside within host cells but they aren\u2019t parasites. The interaction between any two species can range from parasitism \u2013 where one organism exploits a (usually larger) one to the point of being harmful \u2013 to mutualism, in which the association is beneficial to both partners. Unlike parasitic associations, endosymbiotic relationships typically aren\u2019t one-sided.<\/p>\n

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How do the hosts benefit from a endosymbiont?<\/h2>\n
Endosymbionts will often do something their hosts can\u2019t. One such function is photosynthesis<\/a> <\/strong>\u2013 harvesting energy from sunlight to make food. It\u2019s performed by green plants, algae and certain bacteria today, but the process first emerged in a group called cyanobacteria over 2 billion years ago and generated a byproduct that\u2019s since become vital to most life on Earth: oxygen in the atmosphere. Some animal hosts, mainly invertebrates such as marine molluscs and cnidarians (including corals and anemones) are solar-powered thanks to their endosymbionts.<\/p>\n
And what do endosymbionts get?<\/h2>\n
Room and board. Mutualistic relationships are quid pro quo. So in exchange for food, the host provides a home that protects an endosymbiont from predators and other environmental threats, plus nutrients that might have otherwise been hard to obtain. But endosymbionts can also become prey: Elysia chlorotica, a sea slug that resembles a leaf, eats algae to steal chloroplasts (the structures where photosynthesis occurs) \u2013 a phenomenon called kleptoplasty.<\/p>\n
Can relationship between the endosymbiont and host\u00a0break down?<\/h2>\n
Unfortunately, yes. Most famously, coral cells lose their photosynthetic algae, known as zooxanthellae, under stressful conditions such as an abnormal rise in water temperature due to global warming. Then, after a coral colony ejects its colourful endosymbiotic algae, its tissues become transparent to expose the white skeleton beneath, leading to coral bleaching<\/a><\/strong>.<\/p>\n
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