<P> Primary producers are organisms in an ecosystem that produce biomass from inorganic compounds (autotrophs). In almost all cases these are photosynthetically active organisms (plants, cyanobacteria, protists and a number of other unicellular organisms; see article on photosynthesis). However, there are examples of archaea and bacteria (unicellular organisms) that produce biomass from the oxidation of inorganic chemical compounds (chemoautotrophs) in hydrothermal vents in the deep ocean . These organisms are considered to exist at the lowest trophic level . </P> <P> Fungi and other organisms that gain their biomass from oxidizing organic materials are called decomposers and are not primary producers . However, lichens located in tundra climates are an exceptional example of a primary producer that, by mutualistic symbiosis, combine photosynthesis by algae (or additionally nitrogen fixation by cyanobacteria) with the protection of a decomposer fungus . </P>

Which groups contain organisms that can be classified as primary producers