Autotroph Definition

Autotrophs are organisms that can produce their own food, using materials from inorganic sources. The word “autotroph” comes from the root words “auto” for “self” and “troph” for “food.” An autotroph is an organism that feeds itself, without the assistance of any other organisms.

Autotrophs are extremely important because without them, no other forms of life can exist. Without plants that create sugars from carbon dioxide gas and sunlight via the process of photosynthesis, for example, no herbivorous animals could exist, and no carnivorous animals that eat herbivores could exist.

For this reason, autotrophs are often called “producers.” They form the base of an ecosystem’s energy pyramid, and provide the fuel that all the heterotrophs (organisms that must get their food from others) need to exist.

The first life forms on Earth would have had to be autotrophs, in order to exist and make energy and biological materials in a previously non-living environment. Heterotrophs most likely evolved as autotrophs became more common, and some life forms discovered that it was easier to simply eat the autotrophs than to make energy and organic materials for themselves.

Types of Autotrophs

Scientists classify autotrophs according to how they obtain their energy. Types of autotrophs include photoautotrophs, and chemoautotrophs.


Photoautotrophs are organisms who get the energy to make organic materials from sunlight. Photoautotrophs include all plants, green algaes, and bacteria which perform photosynthesis.

All photoautotrophs perform photosynthesis – a word that comes from the root words “light” and “to make.” Photoautotrophs capture photons from the Sun and harvest their energy, using it to perform important biochemical processes such as making ATP.

Photoautotrophs make more than just fuel and organic compounds for heterotrophs like ourselves!

Many photoautotrophs take carbon from the atmosphere and use it to make sugars and other molecules that store the Sun’s energy in their molecular bonds. To do this, they take in molecules of CO2, which is created by nonliving geological processes, and release molecules of O2 – also known as the oxygen we need to breathe!

It is thought that free oxygen was not present in Earth’s atmosphere until after photoautotrophs became common in Earth’s seas. Then, they produced so much free oxygen that large amounts of iron that had previously been dissolved in ocean water reacted with the oxygen and turned into rust!

This process created rocks called banded iron formations, which we can still look at today to see this record of our Earth’s history. The release of large amounts of free oxygen into Earth’s atmosphere by photoautotrophs paved the way for large animals, like ourselves, who need the highly efficient process of aerobic respiration to survive.

It is thought that some of the oxygen produced by photoautotrophs also created the Earth’s ozone layer, which allowed life to move onto dry land without fear of DNA damage from the Sun’s UV light.


Chemoautotrophs are organisms that obtain energy from inorganic chemical processes. Today, chemoautotrophs are most commonly found in deep water environments which receive no sunlight. Many need to live around deep sea volcanic vents, which produce enough heat to allow metabolism to occur at a high rate.

Chemoautotrophs use volatile chemicals such as molecular hydrogen, hydrogen sulfide, elemental sulfur, ferrous iron, and ammonia as their energy sources. This makes them well-suited to live in places that would be toxic to many other organisms, as well as places without sunlight. Chemoautotrophs are usually bacteria or archaebacteria, as their metabolisms are usually not efficient enough to support multicellularity.

Scientists have speculated that life might be able to exist in dark, chemically volatile environments such as the seas of Jupiter’s moon Titan by using similar metabolisms to those seen in chemoautotrophs on Earth. No proof of such life has yet been found, but some scientists believe that the range of metabolic options offered by chemosynthesis drastically expands the range of places in the universe where we can expect to find life.

It is actually unknown whether photoautotrophs or chemoautotrophs were the first life forms on Earth. Many favor the idea that the first cells were photosynthetic, since the Sun’s light shines on the entire surface of the Earth. But some scientists believe that volcanic sites in the deep sea or on the surface of the Earth could have supplied more concentrated energy and more volatile chemicals, potentially leading to the creation of the first cells.

These scientists speculate that these cells could then have evolved photosynthesis as an energy source that would work anywhere on the Earth’s surface they spread further from their volcanic points of origin.

Because single cells and their biochemistry do not fossilize well, we may never know whether chemoautotrophs or photoautotrophs were the first ever forms of life on Earth.

Examples of Autotrophs

Example #1: Plants

Plants, with very few exceptions (such as the venus fly trap which can eat insects) are photoautotrophs. They produce sugars and other essential ingredients for life by using their pigments, such as chlorophyll, to capture photons and harness their energy. When plants are consumed by animals, animals are then able to use that energy and those organic materials for themselves.

Example #2: Green Algae

Green algaes, which may be familiar to you as pond scum, are also photoautotrophs. Green algae may in fact bear a great resemblance to the first common life form on Earth – cyanobacteria, a green bacteria that grew in mats and began the process of turning Earth into a world with an oxygen atmosphere.

Example #3: ”Iron Bacteria” – Acidithiobacillus ferrooxidans

The bacterium Acidithiobacillus ferrooxidans obtains energy from ferrous iron. In the process, it converts the iron atoms from a molecular form where they cannot be dissolved in water to a molecular form where they can.

As a result, Acidithiobacillus ferrooxidans has been used to extract iron from ores that could not be extracted through conventional means.

The field of biohydrometallurgy is the study of using living organisms to obtain metals by dissolving them in water, where they can be further processed.

Related Biology Terms

  • Energy pyramid – A structure that shows the flow of energy through an ecosystem.
  • Heterotroph – An organism that relies on other organisms, such as plants or prey animals, for food.
  • Photosynthesis – The process used by phototrophs to extract energy from sunlight.


1. Which of the following statements is true of chemoautotrophs?
A. They harness energy from sunlight to make food.
B. They rely on other organisms, such as plants and prey animals, for food.
C. They harness energy from chemicals such as hydrogen, sulfur, and iron to make food.
D. None of the above.

Answer to Question #1

2. Which of the following is NOT an example of a photoautotroph?
A. Daisies
B. Iron bacteria
C. Cyanobacteria
D. None of the above.

Answer to Question #2

3. The first form of life on Earth was likely…
A. A photoautotroph.
B. A chemoautotroph.
C. Neither of the above.
D. No one knows.

Answer to Question #3

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