Photoheterotrophy

Photoheterotrophy is a mode of metabolism in which organisms use light as an energy source while obtaining their carbon from organic compounds rather than fixing carbon dioxide. In photoheterotrophs, photosynthesis is typically anoxygenic, meaning no oxygen is produced, and light energy drives ATP formation through photophosphorylation. Organic substrates such as acetate, succinate, malate, propionate, or fatty acids provide carbon and often additional electrons for metabolism.

Key photosynthetic pigments in photoheterotrophs include bacteriochlorophylls and carotenoids, organized in antenna complexes that capture light energy. The absorbed light energy powers electron transport and generates a proton motive force, which drives ATP synthesis. In many photoheterotrophs, carbon from organic substrates is assimilated, while the organism simultaneously uses light to supplement its energy needs. Some groups can also redirect reducing equivalents to other biosynthetic pathways, reflecting metabolic flexibility.

Organisms known for photoheterotrophy include purple non-sulfur bacteria (for example, Rhodospirillum and Rhodobacter species) and green non-sulfur bacteria (such as Chloroflexus). These organisms are commonly found in anaerobic or microaerophilic environments, including stratified freshwater and marine sediments, muds, and hot springs, where light and organic matter are available but oxygen is limited.

Photoheterotrophy is one of several nutritional modes in photosynthesis, distinguished from photoautotrophy (CO2 fixation with light energy) and chemoheterotrophy (organic carbon as energy and carbon sources without photosynthesis). It plays a role in carbon cycling and the ecological dynamics of microbial communities in illuminated, a‑norobic habitats.