What organisms are considered primary producers? What important function do they perform in the biosphere?
The biosphere maintains itself in part through a food pyramid. At the base of the pyramid are large numbers of plentiful and small organisms which get their food directly from the sun. (There are a few deep sea exceptions which get their food energy directly from chemical sources on the sea bed as well.) Each level of the pyrimid gets its food energy from the previous level. Generaly, the size and complexity of the organisms at each level gets larger and more complex.
The organisms at the bottom of the food pyramid are plants. A vast majority of the plants that make up this level are single celled plants found in the oceans. Because these organisms create the food from which all other subsequent levels of the food chain get their food, and because they generate the food by the direct conversions of solar energy they are considered the primary food producers for a biosystem. Therefore, plants are considered the primary producers.