Phototrophs (from Ancient Greek φῶς,φωτός (phôs, phōtós) 'light' and τροφή (trophḗ) 'nourishment') areorganisms that carry outphoton capture to acquire energy. They use theenergy fromlight to carry out various cellular metabolic processes. It is acommon misconception that phototrophs are obligatorilyphotosynthetic. Many, but not all, phototrophs photosynthesize: theyanabolically convertcarbon dioxide intobiomolecules to be utilized structurally (e.g.cellulose andmembrane lipids), functionally (e.g.vitamins,nucleotides, andamino acids), or as a source for latercatabolic processes (e.g.starches,sugars andfats). All phototrophs either useelectron transport chains or directproton pumping to establish an electrochemical gradient, which is utilized byATP synthase to provideadenosine triphosphate (ATP) for the cell. Phototrophs can be eitherautotrophs orheterotrophs. If their electron and hydrogen donors are inorganic compounds (e.g.,Na
2S
2O
3, as in somepurple sulfur bacteria, orH
2S, as in somegreen sulfur bacteria) they can be also calledlithotrophs, and so, some photoautotrophs are also called photolithoautotrophs. Examples of phototroph organisms areRhodobacter capsulatus,Chromatium, andChlorobium.
Originally used with a different meaning, the term took its current definition afterLwoff and collaborators (1946).[1][2]
Most well-known phototrophs arephotoautotrophs, which means they synthesize their own food from inorganic substances (i.e. carbon dioxide) in a process calledcarbon fixation, using light as an energy source. Green plants and most photosynthetic bacteria are photoautotrophs. Photoautotrophic organisms are sometimes referred to asholophytic.[3]
Oxygenic photosynthetic organisms usephotosystem II to capture light-energy and oxidize water (H2O), splitting it into molecular oxygen (O2) and 4 protons (H+) in the process calledphotolysis.
In anecological context, photoautotrophs 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 asalgae (e.g.,kelp), otherprotists (such aseuglena),phytoplankton, andbacteria (such ascyanobacteria).
Cyanobacteria, which are prokaryotic organisms which carry out oxygenic photosynthesis, occupy many environmental conditions, including fresh water, seas,soil, andlichen. Cyanobacteria carry out plant-like photosynthesis because theorganelle in plants that carries out photosynthesis is derived from an endosymbiotic cyanobacterium.[4][5] This bacterium can use water as a source ofelectrons in order to perform CO2reduction reactions.
Aphotolithoautotroph is anautotrophic organism that uses light energy, and aninorganicelectron donor (e.g., H2O, H2, H2S), andCO2 as itscarbon source.
In contrast to photoautotrophs,photoheterotrophs are organisms that depend solely on light for their energy, and consumption of organic compounds for biomolecules. Photoheterotrophs produceATP throughphotophosphorylation but use environmentally obtainedorganic compounds to build structures and other biomolecules.[6]
Most phototrophs usechlorophyll or the relatedbacteriochlorophyll to capture light and are known aschlorophototrophs. Others, however, useretinal and areretinalophototrophs.[7]
Energy source Carbon source | From chemical energy (Chemotroph) | From light energy (Phototroph) |
|---|---|---|
| From CO2 (Autotroph) | Chemoautotroph | Photoautotroph |
| From organics (Heterotroph) | Chemoheterotroph | Photoheterotroph |
