Phototrophs (Gr: φῶς, φωτός = light, τροϕή = nourishment) are the organisms that carry out photon capture to acquire energy. They use the energy from light to carry out various cellular metabolic processes. It is a common misconception that phototrophs are obligatorily photosynthetic. Many, but not all, phototrophs often photosynthesize: they anabolically convert carbon dioxide into organic material to be utilized structurally, functionally, or as a source for later catabolic processes (e.g. in the form of starches, sugars and fats). All phototrophs either use electron transport chains or direct proton pumping to establish an electro-chemical gradient which is utilized by ATP synthase, to provide the molecular energy currency for the cell.
Most of the well-recognized phototrophs are autotrophs, also known as photoautotrophs, and can fix carbon. They can be contrasted with chemotrophs that obtain their energy by the oxidation of electron donors in their environments. Photoheterotrophs produce ATP through photophosphorylation but use environmentally obtained organic compounds to build structures and other bio-molecules. Photoautotrophic organisms are sometimes referred to as holophytic.
In an ecological context, phototrophs are often the food source for neighboring heterotrophic life. In terrestrial environments, plants are the predominant variety, while aquatic environments include a range of phototrophic organisms such as algae (e.g., kelp), other protists (such as euglena), phytoplankton, and bacteria (such as cyanobacteria).
Oxygenic photosynthetic organisms use chlorophyll for light-energy capture and oxidize water, "splitting" it into molecular oxygen. In contrast, anoxygenic photosynthetic bacteria have a substance called bacteriochlorophyll - which absorbs predominantly at non-optical wavelengths - for light-energy capture, live in aquatic environments, and will, using light, oxidize chemical substances such as hydrogen sulfide rather than water.
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 actually derived from an endosymbiosis cyanobacteria.
The depth to which sunlight or artificial light can penetrate into water, so that photosynthesis may occur, is known as the photic zone.
- Campbell, Neil A.; Reece, Jane B.; Urry, Lisa A.; Cain, Michael L.; Wasserman, Steven A.; Minorsky, Peter V.; Jackson, Robert B. (2008). Biology (8th ed.). p. 564. ISBN 978-0-8053-6844-4.
- Hine, Robert (2005). The Facts on File dictionary of biology. Infobase Publishing. p. 175. ISBN 978-0-8160-5648-4.