Phototroph

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.

This article is about phototrophism, obtaining energy from light. For the tropism that governs growth toward or away from a light source, see Phototropism.

Terrestrial and aquatic phototrophs: plants grow on a fallen log floating in algae-rich water

History

Originally used with a different meaning, the term took its current definition after Lwoff and collaborators (1946).[1][2]

Photoautotroph
Main article: Photoautotrophism

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.

Ecology

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.

Photoheterotroph
Main article: Photoheterotroph

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]

Classification by light-capturing molecule

Most phototrophs use chlorophyll or the related bacteriochlorophyll to capture light and are known as chlorophototrophs. Others, however, use retinal and are retinalophototrophs.[7]

Flowchart
Flowchart to determine if a species is autotroph, heterotroph, or a subtype
Energy source
Carbon source
ChemotrophPhototroph
Autotroph ChemoautotrophPhotoautotroph
Heterotroph ChemoheterotrophPhotoheterotroph

See also

References
  1. Lwoff, A., C.B. van Niel, P.J. Ryan, and E.L. Tatum (1946). Nomenclature of nutritional types of microorganisms. Cold Spring Harbor Symposia on Quantitative Biology (5th edn.), Vol. XI, The Biological Laboratory, Cold Spring Harbor, NY, pp. 302–303, .
  2. Schneider, С. K. 1917. Illustriertes Handwörterbuch der Botanik. 2. Aufl., herausgeg. von K. Linsbauer. Leipzig: Engelmann, .
  3. Hine, Robert (2005). The Facts on File dictionary of biology. Infobase Publishing. p. 175. ISBN 978-0-8160-5648-4.
  4. Hill, Malcolm S. "Production Possibility Frontiers in Phototroph:heterotroph Symbioses: Trade-Offs in Allocating Fixed Carbon Pools and the Challenges These Alternatives Present for Understanding the Acquisition of Intracellular Habitats." Frontiers in Microbiology 5 (2014): 357. PMC. Web. 11 March 2016.
  5. 3. Johnson, Lewis, Morgan, Raff, Roberts, and Walter. "Energy Conversion: Mitochondria and Chloroplast." Molecular Biology of the Cell, Sixth Edition By Alberts. 6th ed. New York: Garland Science, Taylor & Francis Group, 2015. 774+. Print.
  6. 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.
  7. Gómez-Consarnau, Laura; Raven, John A.; Levine, Naomi M.; Cutter, Lynda S.; Wang, Deli; Seegers, Brian; Arístegui, Javier; Fuhrman, Jed A.; Gasol, Josep M.; Sañudo-Wilhelmy, Sergio A. (August 2019). "Microbial rhodopsins are major contributors to the solar energy captured in the sea". Science Advances. 5 (8): eaaw8855. Bibcode:2019SciA....5.8855G. doi:10.1126/sciadv.aaw8855. ISSN 2375-2548. PMC 6685716. PMID 31457093.
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