organisms that synthesize the organic substances necessary for life from inorganic materials. The role of autotrophic organisms in nature is enormous, since they create all organic matter, which cannot be synthesized by man and almost all animals. Higher plants (excluding parasites and saphrophytes), algae, and some bacteria are autotrophic organisms. Higher plants and algae contain chlorophyll and are photosynthetic; they synthesize organic substances from simple compounds—carbon dioxide gas and water—using solar energy.
Autotrophic bacteria are chemosynthetic; they synthesize organic matter from mineral compounds, utilizing the energy of certain chemical reactions. For example, the soil bacteria Nitrosomonas and Nitrobacter oxidize ammonia to nitrates and nitrites and use the resulting energy in body building; ferrobacteria use the oxidation energy of ferrous forms of iron; sulfur bacteria oxidize hydrogen sulfide to the sulfuric acid salt (some species of sulfur bacteria are colorless and are typically chemosynthetic; others—for example, the purple sulfur bacteria—are colored and are capable of photoreduction, photosynthesis in which the hydrogen source for reduction of carbon dioxide gas is not water but hydrogen sulfide). Autotrophic organisms play an extremely large role in the circulation of matter in nature.