An autotroph is an organism that produces complex organic compounds from simple inorganic molecules using energy from light or inorganic chemical reactions.
Autotrophs are the producers in a food chain. Plants and other organisms which carry out photosynthesis are photoautotrophs (or phototrophs). Bacteria which derive energy from oxidizing inorganic compounds (such as hydrogen sulfide, ammonium and ferrous iron) are chemoautotrophs, and include the lithotrophs.
Autotrophs are fundamental to the food chains of all plant ecosystems. They take energy from the environment (in the form of sunlight or inorganic chemicals) and use it to create carbon-based organic molecules. Other organisms, called heterotrophs, take in autotrophs as food to carry out such functions. Thus, heterotrophs — animals, fungi, as well as most bacteria and protozoa — depend on autotrophs for the energy and raw materials they need. This mechanism is called primary production in the sea. Heterotrophs obtain energy by breaking down organic molecules obtained in food. Carnivorous animals ultimately rely on autotrophs because the energy and organic building blocks obtained from their prey comes from autotrophs they preyed upon.
There are some species of organisms that require organic compounds as a source of carbon, but are able to use light or inorganic compounds as a source of energy. 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 called a photoheterotroph, while an organism that obtains carbon from organic compounds but obtains energy from the oxidation of inorganic compounds is termed a chemoheterotroph.
Chemosynthesis is the biological conversion of one or more carbon molecules (usually carbon dioxide or methane) and nutrients into organic matter using the oxidation of inorganic molecules (e.g. hydrogen gas, hydrogen sulfide) or methane as a source of energy, rather than sunlight, as in photosynthesis. Large populations of animals can be supported by chemosynthetic primary production at hydrothermal vents, methane clathrates, cold seeps, and whale falls. Chemoautotrophs, organisms that obtain carbon through chemosynthesis, and are responsible for the primary production in oxygen-deficient environments, generally fall into four groups: methanogens, halophiles, sulfur reducers, and thermoacidophiles.
Many microorganisms in dark regions of the oceans use chemosynthesis to produce biomass from 1-carbon molecules. Two categories can be distinguished. In the rare sites at which hydrogen molecules (H2) are available, the energy available from the reaction between CO2 and H2 (leading to production of methane, CH4) can be large enough to drive the production of biomass. Alternatively, in most oceanic environments, energy for chemosynthesis derives from reactions between O2 and substances such as hydrogen sulfide or ammonia. In this second case, the chemosynthetic microorganisms are dependent on photosynthesis which occurs elsewhere and which produces the O2 that they require. Many chemosynthetic microorganisms are consumed by other organisms in the ocean, and symbiotic associations between chemosynthesizers and respiring heterotrophs are quite common.