2. Any organism capable of self-nourishment by using inorganic materials as a source of nutrients and using photosynthesis or chemosynthesis as a source of energy, as most plants and certain bacteria and protists.
An autotroph is an organism that produces organic compounds from carbon dioxide as a carbon source, using either light or reactions of inorganic chemical compounds as a source of energy.
Plants and other organisms using photosynthesis are photolithoautotrophs; bacteria that utilize the oxidation of inorganic compounds; such as, hydrogen sulfide or ferrous iron as an energy source so they are considered to be chemolithoautotrophs.
There are some organisms that require organic compounds as a source of carbon, but which are able to use light or inorganic compounds as sources of energy; and so, such organisms are not defined as autotrophic, but rather as heterotrophic.
An organism that obtains carbon from organic compounds but obtains energy from light is known as a photoheterotroph; while an organism which obtains carbon from organic compunds but obtains energy from the oxidation of inorganic compounds, is referred to as a chemoheterotroph.
Autotrophs are a vital part of the food chain because they take energy from the sun or from inorganic sources and convert them into a form (organic molecules) that they use to carry out biological functions including cell growth, and which other organisms (called heterotrophs) utilize as food.
So it is that heterotrophs; such as, animals, fungi, as well as most bacteria and protozoa, all depend on autotrophs for energy and for the raw materials to make complex organic molecules. Heterotrophs obtain energy by breaking down organic molecules obtained in their food.
Carnivorous animals ultimately rely on autotrophs because the energy and organic building blocks obtained from their prey comes from autotrophs which were eaten by the prey.