Photoheterotroph

Photoheterotroph is a type of microorganism that obtains energy from light (phototrophy) but relies on organic compounds as a source of carbon (heterotrophy). Unlike photoautotrophs, they do not fix carbon dioxide to build biomass. Photosynthesis in these organisms is anoxygenic and usually uses bacteriochlorophyll pigments, enabling light capture without releasing oxygen.

Most well-known photoheterotrophs are purple nonsulfur bacteria in the phylum Proteobacteria, including Rhodopseudomonas, Rhodobacter, and Rhodospirillum. Also, members of the green non-sulfur bacteria (Chloroflexi) can function as photoheterotrophs in certain conditions. These organisms typically inhabit illuminated, often anaerobic or microaerophilic environments such as stagnant freshwater habitats, mudflats, and microbial mats, and can be found in wastewater. They use organic compounds such as acetate, lactate, propionate, or succinate as both carbon sources and electron donors, with light supplying energy to drive ATP production and biosynthesis.

Photosynthetic apparatus in these bacteria consists of chlorophyll or bacteriochlorophyll and accessory pigments in intracytoplasmic membranes. Their metabolism is primarily anoxygenic; oxygen is not produced and may even be detrimental. Photoheterotrophs play a role in carbon recycling in illuminated sediments and are studied for insights into alternative metabolic strategies and potential biotechnological applications.