Photoautotrophs are organisms that can utilize light energy from sunlight and elements (such as carbon) from inorganic compounds to produce organic materials needed to sustain their own metabolism (i.e. autotrophy). This biological activity is known as photosynthesis, and examples of such photosynthetic organisms include plants, algae and cyanobacteria.
Eukaryotic photoautotrophs absorb photon energy through the photopigment chlorophyll (a porphyrin derivative) in their endosymbiont chloroplasts, which splits water and carbon dioxide to synthesize carbohydrates that can be metabolized later to produce adenosine triphosphate (ATP). Prokaryotic photoautotrophs use both chlorophylls and bacteriochlorophylls (which split hydrogen sulfide instead of water) present in free-floating cytoplasmic thylakoids to produce carbohydrates, or, in rare cases, use membrane-bound retinal derivatives such as bacteriorhodopsin proton pumps which captures light to directly produce ATP. The vast majority of known photoautotrophs perform photosynthesis that split water molecules to produce oxygen as a byproduct, while a small minority (such as haloarchaea and sulfur-reducing bacteria) perform anoxygenic photosynthesis.