Autotrophs (self-feeders) are organisms that use an external energy source to assimilate inorganic resources from the environment and synthesize the biological molecules needed to sustain life. In doing so, they prime the biosphere with energy, regenerate atmospheric O2, and provide the primary productivity required for complex food webs and ecosystems. Most of the autotrophic activity in living organisms involves the assimilation of carbon from CO2; however, autotrophs also assimilate inorganic forms of nitrogen, sulfur, phosphorus, and other mineral nutrients.

Photoautotrophs use the energy in light to assimilate inorganic molecules. The ultimate source of this energy is the nuclear fusion of hydrogen in the Sun. Photo-autotrophs include plants, algae, and photosynthetic bacteria, and they account for the vast majority of energy entering the portion of the biosphere that includes humans and the millions of species in the eukaryotic domain. True plants are species within the kingdom Plantae, which includes multicellular land plants (mosses, ferns, gymnosperms, and angiosperms), and related species that have diversified into aquatic habitats. Algae are photosynthetic organisms that are generally (but not always) restricted to aquatic habitats and lack the defining features of true plants. The term 'algae' is not a taxonomic term, as these organisms are prokaryotic or eukaryotic, single- or multicellular, and occur in 12 or more taxo-nomic kingdoms.

Chemoautotrophs utilize energy derived from inorganic or organic molecules to assimilate CO2 and essential mineral elements. Chemoautotrophs that use inorganic molecules such as sulfides, nitrates, hydrogen, and reduced metals are termed lithotrophs (rock eaters). Organotrophs are chemoautotrophs that use organic molecules and thus generally depend upon other organisms for energy, in contrast to photoautotrophs and lithoautotrophs which assimilate energy from abiotic sources. Lithoautotrophs are prokaryotic bacteria and archeobacteria, and include many organisms that live in extreme environments such as hot springs, geothermal vents, and fissures deep within the Earth's crust. Lithoautotrophs are important in the weathering of rock, the regeneration of soil, and the biogeochemical cycling of sulfur and nitrogen. Many lithotrophs derive their energy from geological sources such as reduced compounds in rock, or emissions in volcanic vents and hot springs. Geological energy is ultimately derived from nuclear fission of radioactive elements within the Earth. The use of geological energy demonstrates that energy input into the biosphere is not exclusively derived from solar energy via photosynthesis.

Although once considered to be minor elements of the biosphere, lithoautotrophs are now recognized to form a large, hidden biomass within rock fissures and mineral grains throughout the Earth's upper crust. Ecosystems supported by lithoautotrophs can be complex, such as the tube worms systems near deep-sea vents; however, the flow of energy and carbon into the surface and marine portions of the biosphere where people live and work is almost exclusively photoautotrophic. For this reason, this article will emphasize productivity by photoautotrophs.

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