<P> In ecology, primary production is the synthesis of organic compounds from atmospheric or aqueous carbon dioxide . It principally occurs through the process of photosynthesis, which uses light as its source of energy, but it also occurs through chemosynthesis, which uses the oxidation or reduction of inorganic chemical compounds as its source of energy . Almost all life on Earth relies directly or indirectly on primary production . The organisms responsible for primary production are known as primary producers or autotrophs, and form the base of the food chain . In terrestrial ecoregions, these are mainly plants, while in aquatic ecoregions algae predominate in this role . Ecologists distinguish primary production as either net or gross, the former accounting for losses to processes such as cellular respiration, the latter not . </P> <P> Primary production is the production of chemical energy in organic compounds by living organisms . The main source of this energy is sunlight but a minute fraction of primary production is driven by lithotrophic organisms using the chemical energy of inorganic molecules . </P>

Define and compare gross primary production and net primary production
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