Autotrophs, which include plants and some kinds of protists and bacteria, manufacture their own food. Because autotrophs capture energy and use it to make organic molecules, they are called producers. Recall that organic molecules are molecules that contain carbon.

Most producers are photosynthetic, so they use solar energy to power the production of food. However, some autotrophic bacteria do not use sunlight as an energy source. These bacteria carry out chemosynthesis (KEE-moh-SIN-thuh-sis), in which they use energy stored in inorganic molecules to produce carbohydrates. In terrestrial ecosystems, plants are usually the major producers. In aquatic ecosystems, photosynthetic protists and bacteria are usually the major producers.

Measuring Productivity

Gross primary productivity is the rate at which producers in an ecosystem capture the energy of sunlight by producing organic compounds. Photosynthetic producers use energy and carbon dioxide to make sugar, an energy-rich organic molecule. Some of the sugar is used for cellular respiration, some is used for maintenance and repair, and some is used for making new organic material through either growth or reproduction. Ecologists refer to the organic material that has been produced in an ecosystem as biomass. Producers add biomass to an ecosystem by making organic molecules.

Only energy stored as biomass is available to other organisms in the ecosystem. Ecologists often measure the rate at which biomass accumulates, called the net primary productivity. Net primary productivity is typically expressed in units of energy per unit area per year (kcal/m2/y) or in units of dry organic mass per unit area per year (g/m2/y). Net primary productivity equals gross primary productivity minus the rate of respiration in producers.

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