Cyanobacteria, which are prokaryotic organisms which carry out oxygenic photosynthesis, occupy many environmental conditions, including fresh water, seas, soil, and lichen. Cyanobacteria carry out plant-like photosynthesis because the organelle in plants that carries out photosynthesis is derived from an endosymbiotic cyanobacterium. This bacterium can use water as a source of electrons in order to perform CO2 reduction reactions. Evolutionarily, cyanobacteria's ability to survive in oxygenic conditions, which are considered toxic to most anaerobic bacteria, might have given the bacteria an adaptive advantage which could have allowed the cyanobacteria to populate more efficiently. A photolithoautotroph is an autotrophic organism that uses light energy, and an inorganic electron donor (e.g., H2O, H2, H2S), and CO2 as its carbon source. Examples include plants. Photoheterotroph In contrast to photoautotrophs, photoheterotrophs are organisms that depend solely on light for their energy and principally on organic compounds for their carbon. Photoheterotrophs produce ATP through photophosphorylation but use environmentally obtained organic compounds to build structures and other bio-molecules. Classification by light-capturing molecule Most phototrophs use chlorophyll or the related bacteriochlorophyll to capture light and are known as chlorophototrophs. Others, however, use retinal and are retinalophototrophs. Flowchart See also Primary nutritional groups Prototroph References Photosynthesis Trophic ecology Microbial growth and nutrition Biology terminology