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=== '''Freshwater''' ===
=== '''Freshwater''' ===
Anoxygenic photosynthetic bacteria typically stratify in anoxic water, such as shallow ponds of stagnant water<ref name=":10" />, hot springs<ref name=":10" />, or stratified sulfuric lakes[6aw], forming thick brown, green, or pink aggregations at the mixing layer, where very sharp gradients in temperature, light, and chemical concentrations (e.g. Oxygen and sulfide) favor their growth [6aw]. The mixing layer depth within lakes may vary seasonally and cause changes in anaerobic primary production due to the intensity of ambient light. Optimal growth conditions may be self-limiting as a reduction of light intensity from self-shading can limit growth even when nutrients are non-limiting.[6aw]
Anoxygenic photosynthetic bacteria typically stratify in anoxic water, such as shallow ponds of stagnant water<ref name=":10" />, hot springs<ref name=":10" />, or stratified sulfuric lakes<ref name=":11">{{Cite journal|last=Pedrós-Alió|first=Carlos|title=Phototrophic sulfur bacteria in two Spanish lakes: Vertical distribution and limiting factors|url=https://www.academia.edu/6396552/Phototrophic_sulfur_bacteria_in_two_Spanish_lakes_Vertical_distribution_and_limiting_factors|journal=Limnology and Oceanography|language=en}}</ref>, forming thick brown, green, or pink aggregations at the mixing layer, where very sharp gradients in temperature, light, and chemical concentrations (e.g. Oxygen and sulfide) favor their growth<ref name=":11" />. The mixing layer depth within lakes may vary seasonally and cause changes in anaerobic primary production due to the intensity of ambient light. Optimal growth conditions may be self-limiting as a reduction of light intensity from self-shading can limit growth even when nutrients are non-limiting.<ref name=":11" />


=== '''Terrestrial''' ===
=== '''Terrestrial''' ===

Revision as of 21:36, 9 November 2020

Example of Anoxygenic Photosynthesis (in Green Sulfur Bacteria)

Anoxygenic photosynthesis is the metabolic process of reducing carbon without producing oxygen. Though such organisms require niche ecological conditions to thrive, the community of anoxygenic phototrophs is diverse. Bacteria that can do anoxygenic photosynthesis employ a variety of strategies with differing photosynthetic complexes and pigments[1]. Studies suggest that anoxygenic photosynthesis was the first anabolic process of harnessing solar energy to evolve[2]. Certain organisms were likely utilizing anoxygenic photosynthesis 3 billion years ago when the planet had extremely low levels of oxygen and organisms thrived under reduced conditions. Anoxygenic phototrophs contain photosynthetic enzymes called Reaction Centers (RC) 1 and 2. RC 1 and 2 are the ancient analogs to Photosystem I and II found in oxygenic phototrophs and likely evolved in sequence as the availability of FeS became more limited and the planet more oxidized[1].



Organisms

File:Green Sulfur Bacteria Image.png
Green Sulfur Bacteria

Bacteria

While oxygenic photosynthesis only exists in one bacteria phylum Cyanobacteria, anoxygenic photosynthesis is widespread in bacteria[3]. Some examples of bacteria phylum that can utilize anoxygenic photosynthesis are Chlorobi, Chloroflexi, Acidobacteria, and Heliobacterium[4][3][5].

Within each phylum there are many bacteria that do not rely on anoxygenic photosynthesis for ATP production, and the ones that do go about its production in unique ways. Various anoxic bacteria use different chemicals and compounds as electron donors. Green Sulfur Bacteria (phyla: Chlorobi) utilize sulfur or sulfide instead of water [4][6]. Other species of Green Bacteria such as Filamentous Anoxygenic Phototrophic Bacteria (Green Non-sulfur Bacteria), use thiosulfate, molecular hydrogen, or reduced iron as their electron donors[6][3]. Most anoxygenic phototrophs use one or multiple of the listed electron donors and the specific electron donor used is dependent on the type of pigment and reaction centers employed by the specific bacteria.

File:Purple Bacteria Image.png
Purple Bacteria

Green Sulfur Bacteria use bacteriochlorophyll (Bchl) c, d, or e which is what gives them their green color[4]. On the other side, (anoxygenic) Purple Bacteria use bacteriochlorophyll a or b which gives them their red-purplish color[7]. There are also the Green Non-sulfur Bacteria which use similar bacteriochlorophyll as Green Sulfur Bacteria but use RC2 like Purple Bacteria[7][3]. Interestingly, some cyanobacteria can do both anoxygenic and oxygenic photosynthesis. When these organisms are in an environment with abundant sulfur, they will preferentially oxidize sulfide rather than water. This is because sulfide is more thermodynamically favorable. Cyanobacteria will temporarily turn off PSII and utilize the reaction centers.[1]

A similar phenomenon was observed in a study by Shiba et al 1979. They isolated a subclass of the Proteobacteria from oxic conditions and documented their ability to synthesize Bchl-a in the presence of oxygen and carry out photosynthesis under oxic conditions, but not produce oxygen in the process. This bacteria are part of a different category of aerobic anoxygenic photosynthetic bacteria. (Harashima et al. 1989; Yurkov and Beatty1998). [2aw]

Archaea

Some archaea (e.g. Halobacterium) capture light energy for metabolic function and are thus phototrophic but none are known to "fix" carbon (i.e. be photosynthetic). Instead of a chlorophyll-type receptor and electron transport chain, proteins such as halorhodopsin capture light energy with the aid of diterpenes to move ions against the gradient and produce ATP via chemiosmosis in the manner of mitochondria. Heliobacteria are also nitrogen fixers. They are similar to Green Sulfur Bacteria in that they conduct a type 1 photosynthesis reaction (RC1), except they use bacteriochlorophyll g unlike any other bacterium[4][8].  

Function

Pigments

The pigments used to carry out anaerobic photosynthesis are similar to chlorophyll but differ in molecular detail and peak wavelength of light absorbed. Bacteriochlorophylls a through g absorb electromagnetic radiation maximally in the near-infrared within their natural membrane milieu. This differs from chlorophyll a, the predominant plant and cyanobacteria pigment, which has peak absorption wavelength approximately 100 nanometers shorter (in the red portion of the visible spectrum).

Reaction Centers

There are two main types of anaerobic photosynthetic electron transport chains in bacteria. The type I reaction centers (RC1) found in GSB, Chloracidobacterium, and Heliobacteria and the type II reaction centers (RC2) found in FAPs and purple bacteria. RC1 uses low-potential FeS clusters as electron acceptors to reduce NADP+ to NADPH. RC2 uses quinones as the ultimate electron acceptor and is similar to PSII, but without the oxygen-evolving complex.

Type I reaction centers

The electron transport chain of green sulfur bacteria — such as is present in model organism Chlorobaculum tepidum — uses the reaction center bacteriochlorophyll pair, P840. When light is absorbed by the reaction center, P840 enters an excited state with a large negative reduction potential, and so readily donates the electron to bacteriochlorophyll 663 which passes it on down the electron chain. The electron is transferred through a series of electron carriers and complexes until it is used to reduce NAD+. P840 regeneration is accomplished with the oxidation of sulfide ion from hydrogen sulfide (or hydrogen or ferrous iron) by cytochrome c555[citation needed].

Type II reaction centers

Although the type II reaction centers are structurally and sequentially analogous to Photosystem II (PSII) in plant chloroplasts and cyanobacteria, known organisms that exhibit anoxygenic photosynthesis do not have a region analogous to the oxygen-evolving complex of PSII.

The electron transport chain of purple non-sulfur bacteria begins when the reaction center bacteriochlorophyll pair, P870, becomes excited from the absorption of light. Excited P870 will then donate an electron to bacteriopheophytin, which then passes it on to a series of electron carriers down the electron chain. In the process, it will generate an electrochemical gradient which can then be used to synthesize ATP by chemiosmosis. P870 has to be regenerated (reduced) to be available again for a photon reaching the reaction-center to start the process anew. Molecular hydrogen in the bacterial environment is the usual electron donor.

History

Geologic Timeline showing the Boring Billion

Anoxygenic photosynthesis evolved in bacteria approximately 3.5 billion years ago during the Archaen. One billion years later, oxygenic photosynthesis evolved and the Great Oxidation Event occurred. While this produced substantial levels of atmospheric oxygen for the first time in Earth's history, it took approximately 900 million years before the atmosphere contained oxygen concentrations conducive for the life we see today (often called the Boring Billion). The Canfield Ocean is a theorized model that explains how the feedback mechanisms associated with anoxygenic photosynthesis made it difficult for the planet to fully oxidize for nearly 1 billion years following the onset of oxygenic photosynthesis.[9]

Canfield Ocean

Biogeochemical processes in the Boring Billion

The Canfield Ocean is one theory on why oxygen levels were so low in the ocean during the boring billion. Though the Great Oxidation event that occurred approximately 2.7 billion years ago produced substantial atmospheric oxygen concentrations, the ocean did not experience substantial oxidation and rather stayed reduced. The Canfield Ocean model explains how the oxygen levels in the ocean were intertwined with the marine sulfide budget. The weathering of pyrite and sulfate brought high amounts of iron and sulfur to the ocean. The healthy input of sulfur to the marine environment provided anoxygenic phototrophs with the ingredients they needed to continue to thrive. This healthy input lasted several thousands of years.[1]

During the proterozoic, anoxygenic cyanobacteria maintained highly euxinic conditions in the world’s oceans. In the anoxic OMZ, nitrogen falls below the necessary Redfield Ratio of 16:1, N:P, respectively, needed for oxygenic photosynthesis. Anoxygenic photoautotrophs would have taken up all the free nitrogen in the middle ocean before oxygenic photoautotrophs would have consumed all the nitrogen which led to nitrogen being limited in the euphotic zone[1]. This would have limited oxygenic photosynthesis in the surface waters and produced a severe chemocline. Molecular fossils from the Proterozoic oceans show pigments derived from anoxygenic photosynthesizers which is direct evidence for photic zone euxinia as Canfield theorizes.  Anoxygenic photosynthesizers were the dominant photosynthesizers in the ocean at that time.[1]

Geologic Timescale

The Canfield Ocean model provides an explanation for the Boring Billion through feedback mechanisms associated with euxinia. Euxinic conditions are maintained by a high prevalence of anoxygenic phototrophs. When these organisms die, they sink to the ocean floor, and are buried as organic, carbon-rich material. When euxinia was rampant in the Boring Billion, there was a large burial of this organic material, and the Canfield Ocean theorized that a significant amount of sulfide was released. Sulfide would have reacted with dissolved iron in the water to form pyrite (pyritization)[10]. As more carbon was buried due to the increased production from anoxygenic phototrophs, more pyritization occurred. As more organic material decays, sulfide (S2-) was released, which reacted with dissolved iron in the water and the outer layer of the organism was  replaced with Iron pyrite (FeS2)[10]. A positive feedback loop occurred because as anoxygenic photosynthesizers increased because of the abundant sulfur in the oceans meant more sinking biomass which increased  carbon burial in the sediment and the more carbon burial led to more pyritization which in turn lowered the sulfur budget in the ocean[9]. A strong negative feedback loop was present because as anoxia increased, the organic matter decaying increased the fraction of organic matter that escaped decay by burial in the sediments[9].




Present-day examples

Marine

Marine Euxinia

Anoxygenic photosynthesis by marine algae (cyanobacteria/dinoflagellates) and bacteria is potentially of great ecological importance to the world's oceans where they are abundant[11] [12]. Anoxygenic photosynthetic bacteria facilitate the process in photic, anoxic marine environments where hydrogen sulfide is available and light intensity is low [13]. Optimal habitats include stratified seas [xxxx], sediment layers[14], salt marshes[15] , intertidal microbial mats[14], anoxic microzones associated with particulate matter[12], and even within the guts of zooplankton (though not their fecal pellets)[13].

Some aerobic photoheterotrophic alpha-bacteria are also capable of anoxygenic photosynthesis and account for as much as 10% of the bacterial biomass off the coast of Washington and Oregon [kolber 2001]. Their ability for anoxygenic photosynthesis may be more important in oligotrophic waters where photoheterotrophic bacteria are more likely to be limited by the availability of reduced carbon[12].

One of the best known examples of sustained anoxygenic photosynthesis is the Black Sea.[9] The feedback mechanisms described above maintain a shallow oxic surface layer overlying a steep chemocline and a euxinic deep sea.

Intertidal

Anoxygenic photosynthesis has the potential to be substantial in intertidal mudflats and salt marsh systems where ideal conditions for anoxygenic photosynthesis exist, with large sulfur and iron redox gradients in near surface sediments and there is ample light availability[16].

Freshwater

Anoxygenic photosynthetic bacteria typically stratify in anoxic water, such as shallow ponds of stagnant water[15], hot springs[15], or stratified sulfuric lakes[17], forming thick brown, green, or pink aggregations at the mixing layer, where very sharp gradients in temperature, light, and chemical concentrations (e.g. Oxygen and sulfide) favor their growth[17]. The mixing layer depth within lakes may vary seasonally and cause changes in anaerobic primary production due to the intensity of ambient light. Optimal growth conditions may be self-limiting as a reduction of light intensity from self-shading can limit growth even when nutrients are non-limiting.[17]

Terrestrial

Aerobic anoxygenic phototrophic bacteria have been reported to exist in biological soils, comprising 0.1–5.9% of the cultivable bacterial community in moss, lichen and cyanobacteria‐dominated crust from sand dunes and sandy soils, and could accelerate organic carbon cycling in nutrient-poor arid soils. Their effects will be especially important as global climate change enhances soil erosion and consequent nutrient loss.[18] [19]

See also

References

  1. ^ a b c d e f Johnston, D. T.; Wolfe-Simon, F.; Pearson, A.; Knoll, A. H. (2009-09-28). "Anoxygenic photosynthesis modulated Proterozoic oxygen and sustained Earth's middle age". Proceedings of the National Academy of Sciences. 106 (40): 16925–16929. doi:10.1073/pnas.0909248106. ISSN 0027-8424.
  2. ^ Mulkidjanian, Armen Y.; Koonin, Eugene V.; Makarova, Kira S.; Mekhedov, Sergey L.; Sorokin, Alexander; Wolf, Yuri I.; Dufresne, Alexis; Partensky, Frédéric; Burd, Henry; Kaznadzey, Denis; Haselkorn, Robert (2006). "The Cyanobacterial Genome Core and the Origin of Photosynthesis". Proceedings of the National Academy of Sciences of the United States of America. 103 (35): 13126–13131. ISSN 0027-8424.
  3. ^ a b c d Hanada, Satoshi (2016-3). "Anoxygenic Photosynthesis —A Photochemical Reaction That Does Not Contribute to Oxygen Reproduction—". Microbes and Environments. 31 (1): 1–3. doi:10.1264/jsme2.ME3101rh. ISSN 1342-6311. PMC 4791109. PMID 27021204. {{cite journal}}: Check date values in: |date= (help)
  4. ^ a b c d "Green Sulfur Bacteria - an overview | ScienceDirect Topics". www.sciencedirect.com. Retrieved 2020-11-08.
  5. ^ "Photoheterotrophs - an overview | ScienceDirect Topics". www.sciencedirect.com. Retrieved 2020-11-08.
  6. ^ a b "Green Sulfur Bacteria - an overview | ScienceDirect Topics". www.sciencedirect.com. Retrieved 2020-11-08.
  7. ^ a b Imhoff, Johannes F.; Hiraishi, Akira; Süling, Jörg (2005), Brenner, Don J.; Krieg, Noel R.; Staley, James T.; Garrity, George M. (eds.), "Anoxygenic Phototrophic Purple Bacteria", Bergey’s Manual® of Systematic Bacteriology: Volume Two: The Proteobacteria, Part A Introductory Essays, Boston, MA: Springer US, pp. 119–132, doi:10.1007/0-387-28021-9_15, ISBN 978-0-387-28021-9, retrieved 2020-11-09
  8. ^ Gest, Howard; Favinger, Jeffrey L. (1983-10-01). "Heliobacterium chlorum, an anoxygenic brownish-green photosynthetic bacterium containing a "new" form of bacteriochlorophyll". Archives of Microbiology. 136 (1): 11–16. doi:10.1007/BF00415602. ISSN 1432-072X.
  9. ^ a b c d Lyons, T. W.; Reinhard, C. T. (2009-10-21). "An early productive ocean unfit for aerobics". Proceedings of the National Academy of Sciences. 106 (43): 18045–18046. doi:10.1073/pnas.0910345106. ISSN 0027-8424. PMC 2775325. PMID 19846788.{{cite journal}}: CS1 maint: PMC format (link)
  10. ^ a b Mukherjee, Indrani; Large, Ross R.; Corkrey, Ross; Danyushevsky, Leonid V. (2018-03-13). "The Boring Billion, a slingshot for Complex Life on Earth". Scientific Reports. 8 (1): 4432. doi:10.1038/s41598-018-22695-x. ISSN 2045-2322. PMC 5849639. PMID 29535324.{{cite journal}}: CS1 maint: PMC format (link)
  11. ^ Allgaier, Martin; Uphoff, Heike; Felske, Andreas; Wagner-Döbler, Irene (2003-09). "Aerobic Anoxygenic Photosynthesis in Roseobacter Clade Bacteria from Diverse Marine Habitats". Applied and Environmental Microbiology. 69 (9): 5051–5059. doi:10.1128/AEM.69.9.5051-5059.2003. ISSN 0099-2240. PMC 194994. PMID 12957886. {{cite journal}}: Check date values in: |date= (help)CS1 maint: PMC format (link)
  12. ^ a b c Goericke, Ralf (2002-01). "Bacteriochlorophyll a in the ocean: Is anoxygenic bacterial photosynthesis important?". Limnology and Oceanography. 47 (1): 290–295. doi:10.4319/lo.2002.47.1.0290. ISSN 0024-3590. {{cite journal}}: Check date values in: |date= (help); line feed character in |title= at position 22 (help)
  13. ^ a b Proctor, LM (1997). "Nitrogen-fixing, photosynthetic, anaerobic bacteria associated with pelagic copepods". Aquatic Microbial Ecology. 12: 105–113. doi:10.3354/ame012105. ISSN 0948-3055.
  14. ^ a b Hunter, C. Neil; Daldal, Fevzi; Thurnauer, Marion C.; Beatty, J. Thomas, eds. (2009). "The Purple Phototrophic Bacteria". Advances in Photosynthesis and Respiration. doi:10.1007/978-1-4020-8815-5. ISSN 1572-0233.
  15. ^ a b c Paterek, J. Robert; Paynter, M. J. B. (1988). "Populations of Anaerobic Phototrophic Bacteria in a Spartina alterniflora Salt Marsh". Applied and Environmental Microbiology. 54 (6): 1360–1364. doi:10.1128/AEM.54.6.1360-1364.1988. ISSN 0099-2240.
  16. ^ Wang, Shiyu Rachel; Iorio, Daniela Di; Cai, Wei-Jun; Hopkinson, Charles S. (2018). "Inorganic carbon and oxygen dynamics in a marsh-dominated estuary". Limnology and Oceanography. 63 (1): 47–71. doi:10.1002/lno.10614. ISSN 1939-5590. PMC 5812098. PMID 29456267.{{cite journal}}: CS1 maint: PMC format (link)
  17. ^ a b c Pedrós-Alió, Carlos. "Phototrophic sulfur bacteria in two Spanish lakes: Vertical distribution and limiting factors". Limnology and Oceanography.
  18. ^ Csotonyi, Julius T.; Swiderski, Jolantha; Stackebrandt, Erko; Yurkov, Vladimir (2010-10). "A new environment for aerobic anoxygenic phototrophic bacteria: biological soil crusts: Anoxygenic phototrophs in biological soil crusts". Environmental Microbiology Reports. 2 (5): 651–656. doi:10.1111/j.1758-2229.2010.00151.x. {{cite journal}}: Check date values in: |date= (help)
  19. ^ Lehours, Anne-Catherine; Enault, François; Boeuf, Dominique; Jeanthon, Christian (2018-12). "Biogeographic patterns of aerobic anoxygenic phototrophic bacteria reveal an ecological consistency of phylogenetic clades in different oceanic biomes". Scientific Reports. 8 (1): 4105. doi:10.1038/s41598-018-22413-7. ISSN 2045-2322. PMC 5841314. PMID 29515205. {{cite journal}}: Check date values in: |date= (help)CS1 maint: PMC format (link)
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