Gammaproteobacteria is a class of bacteria in the phylum Pseudomonadota (synonym Proteobacteria). It contains about 250 genera, which makes it the most genus-rich taxon of the
Prokaryotes.[1] Several medically, ecologically, and scientifically important groups of bacteria belong to this class. All members of this class are
Gram-negative. It is the most phylogenetically and physiologically diverse class of the Pseudomonadota.[2]
Members of Gammaproteobacteria live in several terrestrial and marine environments, in which they play various important roles, including in extreme environments such as
hydrothermal vents. They can have different shapes, rods, curved rods, cocci, spirilla, and filaments,[3] and include free living bacteria,
biofilm formers, commensals and symbionts;[4] some also have the distinctive trait of being bioluminescent.[5] Diverse metabolisms are found in Gammaproteobacteria; there are both aerobic and anaerobic (obligate or facultative) species,
chemolithoautotrophics,
chemoorganotrophics,
photoautotrophs and
heterotrophs.[6]
Etymology
The element "
gamma" (third letter of the Greek alphabet) indicates that this is Class III in Bergey's Manual of Systematic Bacteriology (Vol. II, page 1). Proteus refers to the Greek sea god who could change his shape. Bacteria (Greek βακτήριον; "rod" "little stick"), in terms of etymological history, refers to
Bacillus (rod-shaped bacteria), but in this case is "useful in the interim while the phylogenetic data are being integrated into formal bacterial taxonomy."[7]
Phylogeny of Gammaproteobacteria after[10] Not all orders are monophyletic, consequently families or genera are shown for the Pseudomonadales, Oceanospirillales, and Alteromonadales. In the case of singleton orders, the genus is shown. (In
bacterial taxonomy, orders have the suffix -ales, while families have -aceae.)
Gammaproteobacteria, especially the orders Alteromonadales and
Vibrionales, are fundamental in marine and coastal ecosystems because they are the major groups involved in nutrient cycling.[12] Despite their fame as pathogens, they find application in a huge number of fields, such as bioremediation and biosynthesis.
Gammaproteobacteria can be used as a
microbial fuel cell (MFC)[13] element that applies their ability to dissimilate various metals.[14] The produced energy could be collected as one of the most environmentally friendly and sustainable energy production systems.[15] They are also used as biological methane filters.[16]
Gammaproteobacteria are widely distributed and abundant in various ecosystems such as soil, freshwater lakes and rivers, oceans and salt lakes. For example, they constitute about 6–20% (average of 14%) of
bacterioplankton in different oceans,[23] and they are distributed world-wide in both deep-sea and coastal sediments.[24] In seawater, bacterial community composition could be shaped by environmental parameters such as phosphorus availability, total organic carbon, salinity, and pH.[25] In soil, higher pH is correlated with higher relative abundance of Alphaproteobacteria, Betaproteobacteria and Gammaproteobacteria.[26] The relative abundance of Betaproteobacteria and Gammaproteobacteria is also positively correlated to the
dissolved organic carbon (DOC) concentration, which is a key environmental parameter shaping bacterial community composition.[27]Gammaproteobacteria are also key players in the dark carbon fixation in coastal sediments, which are the largest
carbon sink on Earth, and the majority of these bacteria have not been cultured yet.[28] The
deep-sea hydrothermal system is one of the most extreme environments on Earth. Almost all vent-endemic animals are strongly associated with the primary production of the endo- and/or episymbiotic
chemoautotrophic microorganisms.[29] Analyses of both the symbiotic and free-living microbial communities in the various deep-sea hydrothermal environments have revealed a predominance in biomass of members of the Gammaproteobacteria.[30]
Gammaproteobacteria have a wide diversity, metabolic versatility, and functional redundancy in the hydrothermal sediments, and they are responsible for the important organic carbon turnover and nitrogen and sulfur cycling processes.[31] Anoxic hydrothermal fluids contain several reduced compounds such as H2, CH4, and reduced metal ions in addition to H2S. Chemoautotrophs that oxidize hydrogen sulfide and reduce oxygen potentially sustain the primary production in these unique ecosystems.[32] In the last decades, it has been found that orders belonging to Gammaproteobacteria, like Pseudomonas, Moraxella, are able to degrade different types of plastics and these microbes might have a key role in plastic biodegradation.[33]
Metabolism
Gammaproteobacteria are metabolically diverse, employing a variety of electron donors for
respiration and biosynthesis.
Some groups are nitrite-oxidizers[34] and ammonia oxidizers like the members of Nitrosococcus (with the exception of Nitrosococcus mobilis) and they are also obligate
halophilic bacteria.[35]
Others are chemoautotrophic sulfur-oxidizers, like Thiotrichales, which are found in communities such as filamentous microbial biofilms in the Tor Caldara shallow-water gas vent in the
Tyrrhenian Sea .[36] Moreover, thanks to 16S rRNA gene analysis, different sulfide oxidizers in the Gammaporteobacteria class have been detected, and the most important among them are Beggiatoa, Thioploca and Thiomargarita; besides, large amounts of hydrogen sulfide are produced by sulfate-reducing bacteria in organic-rich coastal sediments.[37]
The most widespread pathway for
carbon fixation among Gammaproteobacteria is the
Calvin–Benson–Bassham (CBB) cycle, although a minority may use the
rTCA cycle.[44]Thioflavicoccus mobilis (a free living species) and "Candidatus Endoriftia persephone" (symbiont of the giant tubeworm Riftia pachyptila) may use the
rTCA cycle in addition to the
CBB cycle, and may express these two different pathways simultaneously.[45]
Symbiosis
Symbiosis is a close and a long-term biological interaction between two different biological organisms. A large number of Gammaproteobacteria are able to join in a close endosymbiosis with various species. Evidence for this can be found in a wide variety of ecological niches: on the ground,[46][47] within plants,[48] or deep on the ocean floor.[49] On the land, it has been reported that Gammaproteobacteria species have been isolated from Robinia pseudoacacia[50] and other plants,[51][52] while in the deep sea a sulfur-oxidizing gammaproteobacteria was found in a hydrothermal vent chimney;[53] by entering into symbiotic relationships in deep sea areas, sulfur-oxidizing chemolithotrophic microbes receive additional organic hydrocarbons in hydrothermal ecosystems. Some Gammaproteobacteria are symbiotic with geothermic ocean vent-downwelling animals,[54] and in addition, Gammaproteobacteria can have complex relationships with other species that live around thermal springs,[55] for example, with the shrimp Rimicaris exoculata living from
hydrothermal vents on the
Mid-Atlantic Ridge.
Regarding the endosymbionts, most of them lack many of their family characteristics due to significant genome reduction.[56][57]
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^T. Gutierrez – 2019 - Institute of Mechanical, Process and Energy Engineering, School of Engineering and Physical Sciences, Heriot-Watt University, Edinburgh, UK
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^Broszat M, Nacke H, Blasi R, Siebe C, Huebner J, Daniel R, Grohmann E. 2014. Wastewater irrigation increases the abundance of potentially harmful Gammaproteobacteria in soils in Mezquital Valley, Mexico. Appl Environ Microbiol.
^Jiang H, Dong H, Ji S, Ye Y, Wu N (2007-09-26). "Microbial Diversity in the Deep Marine Sediments from the Qiongdongnan Basin in South China Sea". Geomicrobiology Journal. 24 (6): 505–517.
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^Sabine Lenk, Julia Arnds, Katrice Zerjatke, Niculina Musat, Rudolf Amann and Marc Mußmann* Max Planck Institute for Marine Microbiology, Celsiusstraße 1, 28359 Bremen, Germany. Novel groups of Gammaproteobacteria catalyse sulfur oxidation and carbon fixation in a coastal, intertidal sediment. (2011)
^Sabrina Hedrich,Michael Schlomann and D. Barrie Johnson. The iron-oxidizing proteobacteria. School of Biological Sciences, College of Natural Sciences, Bangor University, Deiniol Road, Bangor LL57 2UW, UK 2 Interdisciplinary Ecological Center, TU Bergakademie Freiberg, Leipziger Strasse 29, 09599 Freiberg, Germany. (2011)
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^Shiraishi A, Matsushita N, Hougetsu T (August 2010). "Nodulation in black locust by the Gammaproteobacteria Pseudomonas sp. and the Betaproteobacteria Burkholderia sp". Systematic and Applied Microbiology. 33 (5): 269–74.
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