Primary producers (or autotrophs) are organisms that synthesize their own biochemical constituents using simple inorganic compounds and an external energy source to drive the process. The amount of energy fixed by autotrophs is known as primary production, and the rate of fixation is primary productivity. Both primary productivity and primary production may be measured in units of carbon dioxide (CO₂) fixed, energy fixed (as calories or joules), or biomass produced. Productivity in terrestrial ecosystems is often expressed in such units as kilograms of dry biomass (or its energy equivalent) per hectare per year (e.g., kg/ha.yr or kJ/ha.yr), while aquatic productivity is often measured on a volumetric basis (e.g., kg/m³.yr). Many studies have been made of the primary productivity of various kinds of ecosystems.

Most primary producers are photoautotrophs that use sunlight as...