Photoheterotroph

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Photoheterotrophs (Gk: photo = light, hetero = (an)other, troph = nourishment) are heterotrophic phototrophs—that is, they are organisms that use light for energy, but cannot use carbon dioxide as their sole carbon source. Consequently, they use organic compounds from the environment to satisfy their carbon requirements; these compounds include carbohydrates, fatty acids, and alcohols. Examples of photoheterotrophic organisms include purple non-sulfur bacteria, green non-sulfur bacteria, and heliobacteria.[1] These microorganisms are ubiquitous in aquatic habitats, occupy unique niche-spaces, and contribute to global biogeochemical cycling. Recent research has also indicated that the oriental hornet and some aphids may be able to use light to supplement their energy supply.[2]

Research[edit]

Studies have shown that mammalian mitochondria can also capture light and synthesize ATP when mixed with pheophorbide, a light-capturing metabolite of chlorophyll.[3] Research demonstrated that the same metabolite when fed to the worm Caenorhabditis elegans leads to increase in ATP synthesis upon light exposure, along with an increase in life span.[4]

Furthermore, inoculation experiments suggest that mixotrophic Ochromonas danica (i.e., Golden algae)—and comparable eukaryotes—favor photoheterotrophy in oligotrophic (i.e., nutrient-limited) aquatic habitats.[5] This preference may increase energy-use efficiency and growth by reducing investment in inorganic carbon fixation (e.g., production of autotrophic machineries such as RuBisCo and PSII).

Metabolism[edit]

Photoheterotrophs generate ATP using light, in one of two ways:[6][7] they use a bacteriochlorophyll-based reaction center, or they use a bacteriorhodopsin. The chlorophyll-based mechanism is similar to that used in photosynthesis, where light excites the molecules in a reaction center and causes a flow of electrons through an electron transport chain (ETS). This flow of electrons through the proteins causes hydrogen ions to be pumped across a membrane. The energy stored in this proton gradient is used to drive ATP synthesis. Unlike in photoautotrophs, the electrons flow only in a cyclic pathway: electrons released from the reaction center flow through the ETS and return to the reaction center. They are not utilized to reduce any organic compounds. Purple non-sulfur bacteria, green non-sulfur bacteria, and heliobacteria are examples of bacteria that carry out this scheme of photoheterotrophy.

Other organisms, including halobacteria and flavobacteria[8] and vibrios[9] have purple-rhodopsin-based proton pumps that supplement their energy supply. The archaeal version is called bacteriorhodopsin, while the eubacterial version is called proteorhodopsin. The pump consists of a single protein bound to a Vitamin A derivative, retinal. The pump may have accessory pigments (e.g., carotenoids) associated with the protein. When light is absorbed by the retinal molecule, the molecule isomerises. This drives the protein to change shape and pump a proton across the membrane. The hydrogen ion gradient can then be used to generate ATP, transport solutes across the membrane, or drive a flagellar motor. One particular flavobacterium cannot reduce carbon dioxide using light, but uses the energy from its rhodopsin system to fix carbon dioxide through anaplerotic fixation.[8] The flavobacterium is still a heterotroph as it needs reduced carbon compounds to live and cannot subsist on only light and CO2. It cannot carry out reactions in the form of

n CO2 + 2n H2D + photons → (CH2O)n + 2n D + n H2O,

where H2D may be water, H2S or another compound/compounds providing the reducing electrons and protons; the 2D + H2O pair represents an oxidized form.

However, it can fix carbon in reactions like:

CO2 + pyruvate + ATP (from photons) → malate + ADP +Pi

where malate or other useful molecules are otherwise obtained by breaking down other compounds by

carbohydrate + O2 → malate + CO2 + energy.

This method of carbon fixation is useful when reduced carbon compounds are scarce and cannot be wasted as CO2 during interconversions, but energy is plentiful in the form of sunlight.

Ecology[edit]

Photoheterotrophs are ubiquitous in marine ecosystems. Notably, bacteria and archaea may use proteorhodopsin as a supplementary, light-driven energy source.

Distribution and Niche Partitioning

Photoheterotrophs—either 1) cyanobacteria (i.e. facultative heterotrophs in nutrient-limited environments like Synechococcus and Prochlorococcus), 2) aerobic anoxygenic photoheterotrophic bacteria (AAP; employing bacteriochlorophyll-based reaction centers), 3) proteorhodopsin (PR)-containing bacteria and archaea, and 4) heliobacteria (i.e., the only phototroph with bacteriochlorophyll g pigments, or Gram-positive membrane) are found in various aquatic habitats including oceans, stratified lakes, rice fields, and environmental extremes.[10][11][12][13]

In oceans' photic zones, up to 10% of bacterial cells are capable of AAP, whereas greater than 50% of net marine microorganisms house PR—reaching up to 90% in coastal biomes.[14] As demonstrated in inoculation experiments, photoheterotrophy may provide these planktonic microbes competitive advantages 1) relative to chemoheterotrophs in oligotrophic (i.e., nutrient-poor) environments via increased nutrient use-efficiency (i.e., organic carbon fuels biosynthesis, excessively, versus energy production) and 2) by eliminating investment in physiologically costly autotrophic enzymes/complexes (RuBisCo and PSII).[15][16] Furthermore, in Arctic oceans, AAP and PR photoheterotrophs are prominent in ice-covered regions during wintertime per light scarcity.[17] Lastly, seasonal turnover has been observed in marine AAPs as ecotypes (i.e., genetically similar taxa with differing functional trait and/or environmental preferences) segregate into temporal niches.[18]

In stratified (i.e., euxinic) lakes, photoheterotrophs—alongside other anoxygenic phototrophs (e.g., purple/green sulfur bacteria fixing carbon dioxide via electron donors such as ferrous iron, sulfide, and hydrogen gas)—often occupy the chemocline in the water column and/or sediments.[19] In this zone, dissolved oxygen is reduced, light is limited to long wavelengths (e.g., red and infrared) left-over by oxygenic phototrophs (e.g., cyanobacteria), and anaerobic metabolisms (i.e., those occurring in the absence of oxygen) begin introducing sulfide and bioavailable nutrients (e.g., organic carbon, phosphate, and ammonia) through upward diffusion.[20]

Heliobacteria are obligate anaerobes primarily located in rice fields, where low sulfide concentrations prevent competitive exclusion of purple/green sulfur bacteria.[21] These waterlogged environments may facilitate symbiotic relationships between heliobacteria and rice plants as fixed nitrogen—from the former—is exchanged for carbon-rich root exudates.

Observation studies have characterized photoheterotrophs (e.g., Green non-sulfur bacteria such as Chloroflexi and AAPs) within photosynthetic mats at environmental extremes (e.g., hot springs and hypersaline lagoons).[12][22] Notably, temperature and pH drive anoxygenic phototroph community composition in Yellowstone National Park's geothermal features.[12] In addition, various, light-dependent niches in the Great Salk Lake's hypersaline mats support phototrophic diversity as microbes optimize energy production and combat osmotic stress.[22]

Biogeochemical Cycling

Photoheterotrophs influence global carbon cycling by assimilating dissolved organic carbon (DOC).[23][20] Therefore, when harvesting light-energy, carbon is maintained in the microbial loop without corresponding respiration (i.e., carbon dioxide release to the atmosphere as DOC is oxidized to fuel energy production). This disconnect, the discovery of facultative photoheterotrophs (e.g., AAPs with flexible energy sources), and previous measurements taken in the dark (i.e., to avoid skewed oxygen consumption values due to photooxidation, UV light, and oxygenic photosynthesis) lead to overestimated aquatic CO2 emissions. For example, a 15.2% decrease in community respiration was observed in Cep Lake, Czechia—alongside preferential glucose and pyruvate uptake—is attributed to facultative photoheterotrophs preferring light-energy during the daytime, given fitness benefits mentioned previously.[23]

Flowchart[edit]

Flowchart to determine if a species is autotroph, heterotroph, or a subtype
Energy source
Carbon source
 Chemotroph Phototroph
Autotroph Chemoautotroph Photoautotroph
Heterotroph Chemoheterotroph Photoheterotroph

See also[edit]

References[edit]

  1. ^ Bryant, D.A.; Frigaard, N.-U. (November 2006). "Prokaryotic photosynthesis and phototrophy illuminated". Trends Microbiol. 14 (11): 488–496. doi:10.1016/j.tim.2006.09.001. PMID 16997562.
  2. ^ Valmalette, J.C.; Dombrovsky, A.; Brat, P.; Mertz, C.; Capovilla, M.; Robichon, A. (2012). "Light-induced electron transfer and ATP synthesis in a carotene synthesizing insect". Scientific Reports. 2: 579. Bibcode:2012NatSR...2E.579V. doi:10.1038/srep00579. PMC 3420219. PMID 22900140.
  3. ^ Xu, Chen; Zhang, Junhua; Mihai, Doina M.; Washington, Ilyas (2014-01-15). "Light-harvesting chlorophyll pigments enable mammalian mitochondria to capture photonic energy and produce ATP". Journal of Cell Science. 127 (2): 388–399. doi:10.1242/jcs.134262. ISSN 0021-9533. PMC 6518289. PMID 24198392.
  4. ^ Zhang, Dan; Robinson, Kiera; Mihai, Doina M.; Washington, Ilyas (2016-10-12). "Sequestration of ubiquitous dietary derived pigments enables mitochondrial light sensing". Scientific Reports. 6 (1): 34320. Bibcode:2016NatSR...634320Z. doi:10.1038/srep34320. ISSN 2045-2322. PMC 5059631. PMID 27731322.
  5. ^ Wilken, Susanne; Schuurmans, J. Merijn; Matthijs, Hans C. P. (December 2014). "Do mixotrophs grow as photoheterotrophs? Photophysiological acclimation of the chrysophyte Ochromonas danica after feeding". New Phytologist. 204 (4): 882–889. doi:10.1111/nph.12975. ISSN 0028-646X. PMID 25138174.
  6. ^ Bryant, Donald A.; Frigaard, Niels-Ulrik (November 2006). "Prokaryotic photosynthesis and phototrophy illuminated". Trends in Microbiology. 14 (11): 488–496. doi:10.1016/j.tim.2006.09.001. ISSN 0966-842X. PMID 16997562.
  7. ^ Zubkov, Mikhail V. (1 September 2009). "Photoheterotrophy in Marine Prokaryotes". Journal of Plankton Research. 31 (9): 933–938. doi:10.1093/plankt/fbp043. ISSN 0142-7873.
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  12. ^ a b c Hamilton, Trinity L.; Bennett, Annastacia C.; Murugapiran, Senthil K.; Havig, Jeff R. (2019-12-17). Flynn, Theodore M. (ed.). "Anoxygenic Phototrophs Span Geochemical Gradients and Diverse Morphologies in Terrestrial Geothermal Springs". mSystems. 4 (6): e00498–19. doi:10.1128/mSystems.00498-19. ISSN 2379-5077. PMC 6832021. PMID 31690593.
  13. ^ Boeuf, D.; Humily, F.; Jeanthon, C. (2014-06-20). "Diversity of Arctic pelagic <i>Bacteria</i> with an emphasis on photoheterotrophs: a review". Biogeosciences. 11 (12): 3309–3322. Bibcode:2014BGeo...11.3309B. doi:10.5194/bg-11-3309-2014. ISSN 1726-4189.
  14. ^ Arandia‐Gorostidi, Nestor; González, José M.; Huete‐Stauffer, Tamara M.; Ansari, Mohd I.; Morán, Xosé Anxelu G.; Alonso‐Sáez, Laura (2020-08-02). "Light supports cell‐integrity and growth rates of taxonomically diverse coastal photoheterotrophs". Environmental Microbiology. 22 (9): 3823–3837. doi:10.1111/1462-2920.15158. hdl:10754/664249. ISSN 1462-2912. PMID 32643243. S2CID 220435352.
  15. ^ Wilken, Susanne; Schuurmans, J. Merijn; Matthijs, Hans C. P. (2014-08-19). "Do mixotrophs grow as photoheterotrophs? Photophysiological acclimation of the chrysophyte Ochromonas danica after feeding". New Phytologist. 204 (4): 882–889. doi:10.1111/nph.12975. ISSN 0028-646X. PMID 25138174.
  16. ^ Fenchel, T.; King, G.M.; Blackburn, T.H. (2012), "Biogeochemical Cycling in Soils", Bacterial Biogeochemistry, Elsevier, pp. 89–120, doi:10.1016/b978-0-12-415836-8.00006-2, ISBN 9780124158368, retrieved 2023-04-15
  17. ^ Cottrell, Matthew T.; Kirchman, David L. (2009). "Photoheterotrophic Microbes in the Arctic Ocean in Summer and Winter". Applied and Environmental Microbiology. 75 (15): 4958–4966. Bibcode:2009ApEnM..75.4958C. doi:10.1128/aem.00117-09. ISSN 0099-2240. PMC 2725502. PMID 19502441.
  18. ^ Auladell, Adrià; Sánchez, Pablo; Sánchez, Olga; Gasol, Josep M.; Ferrera, Isabel (2019). "Long-term seasonal and interannual variability of marine aerobic anoxygenic photoheterotrophic bacteria". The ISME Journal. 13 (8): 1975–1987. doi:10.1038/s41396-019-0401-4. ISSN 1751-7370. PMC 6776013. PMID 30914777. S2CID 256555570.
  19. ^ Posth, N. R.; Bristow, L. A.; Cox, R. P.; Habicht, K. S.; Danza, F.; Tonolla, M.; Frigaard, N.-U.; Canfield, D. E. (2017). "Carbon isotope fractionation by anoxygenic phototrophic bacteria in euxinic Lake Cadagno". Geobiology. 15 (6): 798–816. Bibcode:2017Gbio...15..798P. doi:10.1111/gbi.12254. ISSN 1472-4669. PMID 28866873. S2CID 11132075.
  20. ^ a b Fedonkin, M. A. (December 2009). "Eukaryotization of the early biosphere: A biogeochemical aspect". Geochemistry International. 47 (13): 1265–1333. doi:10.1134/s0016702909130011. ISSN 0016-7029. S2CID 98430905.
  21. ^ Madigan, Michael T.; Ormerod, John G. (2004), Taxonomy, Physiology and Ecology of Heliobacteria, Advances in Photosynthesis and Respiration, vol. 2, Dordrecht: Kluwer Academic Publishers, pp. 17–30, doi:10.1007/0-306-47954-0_2, ISBN 0-7923-3681-X, retrieved 2023-04-16
  22. ^ a b Kanik, Mert; Munro-Ehrlich, Mason; Fernandes-Martins, Maria Clara; Payne, Devon; Gianoulias, Kathryn; Keller, Lisa; Kubacki, Alexander; Lindsay, Melody R.; Baxter, Bonnie K.; Vanden Berg, Michael D.; Colman, Daniel R.; Boyd, Eric S. (2020-05-05). "Unexpected Abundance and Diversity of Phototrophs in Mats from Morphologically Variable Microbialites in Great Salt Lake, Utah". Applied and Environmental Microbiology. 86 (10): e00165–20. Bibcode:2020ApEnM..86E.165K. doi:10.1128/AEM.00165-20. ISSN 1098-5336. PMC 7205499. PMID 32198176.
  23. ^ a b Piwosz, Kasia; Villena-Alemany, Cristian; Mujakić, Izabela (2022). "Photoheterotrophy by aerobic anoxygenic bacteria modulates carbon fluxes in a freshwater lake". The ISME Journal. 16 (4): 1046–1054. doi:10.1038/s41396-021-01142-2. ISSN 1751-7370. PMC 8941148. PMID 34802055.

Sources[edit]

"Microbiology Online" (textbook). University of Wisconsin, Madison.

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