Cyanobacteria
These photoautotrophs are the only prokaryotes with plantlike, oxygen-generating photosynthesis. Both solitary and colonial cyanobacteria are abundant wherever there is water, providing an enormous amount of food for freshwater and marine ecosystems. Some filamentous colonies have cells specialized for nitrogen fixation, the process that incorporates atmospheric nitrogen into inorgnanic compounds that can be used in the synthesis of amino acids and other organic molecules.