Phototrophs (from
Ancient Greek
φῶς,
φωτός (phôs, phōtós) 'light', and
τροφή (trophḗ) 'nourishment') are
organisms that carry out
photon capture to produce complex
organic compounds (e.g.
carbohydrates) and 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 electrochemical gradient which is utilized by
ATP synthase, to provide the molecular energy currency for the cell. Phototrophs can be either
autotrophs or
heterotrophs. If their electron and hydrogen donors are inorganic compounds (e.g., Na
2S
2O
3, as in some
purple sulfur bacteria, or H
2S, as in some
green sulfur bacteria) they can be also called
lithotrophs, and so, some photoautotrophs are also called photolithoautotrophs. Examples of phototroph organisms are
Rhodobacter capsulatus,
Chromatium, and
Chlorobium.
Originally used with a different meaning, the term took its current definition after Lwoff and collaborators (1946). [1] [2]
Most of the well-recognized phototrophs are autotrophic, 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. Photoautotrophs are capable of synthesizing their own food from inorganic substances using light as an energy source. Green plants and photosynthetic bacteria are photoautotrophs. Photoautotrophic organisms are sometimes referred to as holophytic. [3]
Oxygenic photosynthetic organisms use chlorophyll for light-energy capture and oxidize water, "splitting" it into molecular oxygen.
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).
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 derived from an [4] endosymbiotic cyanobacterium. [5] This bacterium can use water as a source of electrons in order to perform CO2 reduction reactions.
A photolithoautotroph is an autotrophic organism that uses light energy, and an inorganic electron donor (e.g., H2O, H2, H2S), and CO2 as its carbon source.
In contrast to photoautotrophs, photoheterotrophs are organisms that depend solely on light for their energy and principally on organic compounds for their carbon. Photoheterotrophs produce ATP through photophosphorylation but use environmentally obtained organic compounds to build structures and other bio-molecules. [6]
Most phototrophs use chlorophyll or the related bacteriochlorophyll to capture light and are known as chlorophototrophs. Others, however, use retinal and are retinalophototrophs. [7]
Energy source Carbon source |
Chemotroph | Phototroph |
---|---|---|
Autotroph | Chemoautotroph | Photoautotroph |
Heterotroph | Chemoheterotroph | Photoheterotroph |