autotroph (ôtˈətrōfˌ) [key], in biology, an organism capable of synthesizing its own organic substances from inorganic compounds. Autotrophs produce their own sugars, lipids, and amino acids using carbon dioxide as a source of carbon, and ammonia or nitrates as a source of nitrogen. Organisms that use light for the energy to synthesize organic compounds are called photosynthetic autotrophs; organisms that oxidize such compounds as hydrogen sulfide (H2S) to obtain energy are called chemosynthetic autotrophs, or chemotrophs. Photosynthetic autotrophs include the green plants, certain algae, and the pigmented sulfur bacteria (see photosynthesis). Chemotrophs include the iron bacteria, the nitrifying bacteria, and the nonpigmented sulfur bacteria (see chemosynthesis). Heterotrophs are organisms that must obtain their energy from organic compounds.