Symbiosis is a mutually beneficial relationship between two organisms that are completely unrelated. There are numerous examples of symbiosis in nature and many of them involve unicellular plants, called algae, living in cooperation with a wide variety of animals and fungi.
Most organisms that rely on algal symbiosis could not survive without their partners. And most algae involved in such a relationship are in a similar situation throughout at least some of the stages of their lives.
Corals and Zooxanthelles
Zooxanthelles are golden brown algae that live in symbiosis with a wide variety of aquatic animals. Through photosynthesis, algae convert carbon dioxide and nutrients bearing nitrogen and phosphorus into oxygen and sugars. Animals, such as the coral in which zooxanthelles live, produce no nutrients directly but consume oxygen and sugars to survive.
As in the defining characteristic of the mutually beneficial relationship the algae are provided with a place to live and survive by the coral. The corals waste excretions are high in the nitrogen and phosphorus compounds required for the algae to grow. And, like all animals, the cellular processes of the coral release carbon dioxide that is used by the algae in photosynthesis. And, along with the chemicals used by the algae, the coral provides a relatively safe place for the algae to live.
Other ocean organisms also form symbiotic relationships with zooxanthelles. These include species of jellyfish, sea slugs, anemones and clams.
Lichens are formed through a symbiosis between a fungus and an algae. The symbiosis of lichens is similar to that of corals but there is an added component that enables lichens to survive in very dry and harsh environments.
In lichens the fungus wraps itself largely around the algae. Since algae are typically water plants and require a moist environment for survival, the protection of the fungus enables the algae to live in areas with unreliable or limited water. The fungus also breaks down the surfaces on which the lichens live and provides nutrients to the algae from the rocks, trees and other objects that the algae can use.
Formanifera and Algae
Several species of unicellular organisms known as formanifera form symbiotic relationships with a variety of algae. Green algae, red algae, diatoms and many others are symbiotic with different formanifera species. The relationships between these two, very different, unicellular organisms are very similar to the other algal symbioses.
Some formanifera have even taken this relationship a step further. Formanifera can't grow the photosynthetic organelles, called chloroplasts, that are used by plants to convert carbon dioxide and water into sugars. Instead of partnering with algae, some formanifera consume algae and incorporate the chloroplasts into their own cells.
Some evolutionary biologists believe that symbiosis as exhibited between algae and many other organisms hints at how much of life on Earth may have evolved. Most forms of life on the planet are more complex than can easily be explained from basic evolutionary concepts. The existence of organelles in most cells such as mitochondria and chloroplasts could have come from symbiotic relationships that became so intertwined that the symbiotes merged into a single organism. And, just maybe, some of the organs and other parts of multicellular organisms joined through a similar method many eons ago.