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Heterotrophs Rely on Autotrophs
When autotrophs undergo photosynthesis, they convert the raw energy of light from the sun into chemical bonds. For instance, they make energy storage molecules, such as fructose (a type of sugar), and energy transfer molecules, such as ATP, which acts as an energy source for many biochemical reactions. Since heterotrophs, such as humans, cannot carry out photosynthesis, they obtain energy by consuming autotrophs and their byproducts.
One major source of energy is oxygen. Autotrophs release oxygen as a byproduct of photosynthesis. Heterotrophic cells burn oxygen in a carefully-controlled combustion reaction to power the formation of high-energy biochemical bonds. They also ingest energy molecules, such as sugars and fats, by consuming plants or animals that ate plants. In this manner, heterotrophs are thoroughly reliant on photosynthesis to live. However, other groups of organisms are equally reliant upon autotrophs.
Autotrophs Benefit From Photosynthesis
For the past several billion years, autotrophs have been the basis for the bounty and diversity of life on Earth. As the primary executors of photosynthesis, autotrophs are the greatest beneficiaries of this process. With a few notable exceptions, autotrophs obtain all of their energy from sunlight, and are, therefore, able to take in low-energy molecules, like carbon dioxide, and turn them into large, useful biomolecules.
Biologists currently believe that plants are organisms that used to be heterotrophs in their distant evolutionary past, but became autotrophs when they formed a symbiotic relationship with autotrophic organisms called cyanobacteria in a process called endosymbiosis. Internalized cyanobacteria became cellular structures called chloroplasts, which are the biochemical engines of photosynthesis in plants and many nonbacterial microorganisms.
Saprotrophs Power Decomposition
Saprotrophs obtain their energy by decomposing dead organic matter, such as the bodies of dead plants and animals. They are also known as detritivores. Many types of fungi are saprotrophs. As decomposers, these organisms occupy a shady middle-ground between heterotrophs and chemotrophs (organisms that obtain their energy by breaking down nonorganic chemical bonds).
Opportunistic saprotrophs are especially plentiful on newly-dead substrates, such as the carcasses of dead animals or recently deceased plants. They, too, are taking advantage of the solar energy originally harnessed by photosynthesizing autotrophs. This is an essential part of the carbon cycle where the structural components of dead organisms are recycled into new forms.
Chemotrophs Use Oxygen
The first organisms on Earth were likely chemotrophs. These are organisms that obtain their energy by breaking down chemical bonds formed by nonsolar energy. Chemotrophs include bacteria that live in deep-sea vents and the organisms, such as the giant tube worm, which utilize energy from these bacteria in a symbiotic relationship. These animals are still dependent upon photosynthesis, though; the free oxygen that they use to harness this chemical energy was produced as a byproduct of photosynthesis.
Mixotrophs Can Perform Photosynthesis
Some organisms alternate between autotrophic and heterotrophic lifestyles. These creatures are called mixotrophs since they have a mix of different energy sources. One notable example of these is the sea slug, Elysia chlorotica. This animal "steals" the DNA needed to produce chloroplasts and photosynthesize from the algae it eats. These chloroplasts can remain viable for months within the slug, thus allowing the animal to alternate between autotrophic and heterotrophic lifestyles.