Primary producer

A primary producer is an autotroph, an organism that builds sugars and other organic molecules from carbon dioxide and water instead of eating other living things. Most capture sunlight through photosynthesis: plants on land, and algae, seaweeds and photosynthetic bacteria such as cyanobacteria in water. A smaller group, the chemoautotrophs, draw their energy from chemical reactions instead, for example by oxidizing hydrogen sulfide that seeps from hydrothermal vents on the deep seafloor, where no light reaches.

Primary producers make up the first trophic level, and every consumer, from a grazing deer to a top predator, ultimately depends on the energy they capture. Ecologists measure this input as primary productivity. Gross primary productivity is the total rate at which producers take in energy, while net primary productivity is what remains after they use some for their own respiration. Only the net amount is available to herbivores and the rest of the food web, and because energy is lost at each later step, the producer level always holds the most energy in an ecosystem.

In the oceans the main primary producers are microscopic phytoplankton, which together carry out roughly half of all the photosynthesis on Earth.