Purple bacteria
Purple bacteria or purple photosynthetic bacteria are
Taxonomy
All purple bacteria belong in the phylum of Pseudomonadota. This phylum was established as Proteobacteria by Carl Woese in 1987 calling it "purple bacteria and their relatives".[3] Purple bacteria are distributed between 3 classes: Alphaproteobacteria, Betaproteobacteria, Gammaproteobacteria[4] each characterized by a photosynthetic phenotype. All these classes also contain numerous non-photosynthetic numbers, such as the nitrogen-fixing Rhizobium and the human gut bacterium Escherichia coli.
Purple non-sulfur bacteria are found in Alphaproteobacteria and Betaproteobacteria. The families are:[5]
- Class Alphaproteobacteria (17 purple genera)
- Order Rhodospirillales
- Family Rhodospirillaceae, e.g. Rhodospirillum rubrum
- Family Acetobacteraceae, e.g. Rhodopila globiformis
- Order Hyphomicrobiales
- Family Nitrobacteraceae, e.g. Rhodopseudomonas palustris
- Family Hyphomicrobiaceae, e.g. Rhodomicrobium
- Family Rhodobiaceae, e.g. Rhodobium(1 purple genus)
- Order Rhodobacterales, family Rhodobacteraceae (3 purple genera)
- Order Rhodospirillales
- Class Betaproteobacteria (3 purple genera)
- Family Rhodocyclaceae, e.g. Rhodocyclus (1 purple genus)
- Family Comamonadaceae, e.g. Rhodoferax (2 purple genera)
Purple sulfur bacteria are named for the ability to produce elemental sulfur. They are included in the class Gammaproteobacteria, in the two families Chromatiaceae and Ectothiorhodospiraceae. While the former family stores the produced sulfur inside the cell, the latter sends the sulfur outside the cell.[5] According to a 1985 phylogeny, Gammaproteobacteria is divided into three sub-lineages, with both families falling into the first along with non-photosynthetic species such as Nitrosococcus oceani.[6]
The similarity between the photosynthetic machinery in these different lines indicates that it had a common origin, either from some common ancestor or passed by lateral transfer. Purple sulfur bacteria and purple nonsulfur bacteria were distinguished on the basis of physiological factors of their tolerance and utilization of sulfide: was considered that purple sulfur bacteria tolerate millimolar levels of sulfide and oxidized sulfide to sulfur globules stored intracellulary while purple nonsulfur bacteria species did neither.[7] This kind of classification was not absoluted. It was refuted with classic chemostat experiments by Hansen and Van Gemerden (1972) that demonstrate the growing of many purple nonsulfur bacteria species at low levels of sulfide (0.5 mM) and in so doing, oxidize sulfide to S0, S
4O2−
6, or SO2−
4. The important distinction that remains from these two different metabolisms is that: any S0 formed by purple nonsulfur bacteria is not stored intracellularly but is deposited outside the cell[8] (even if there are exception for this as Ectothiorhodospiraceae). So if grown on sulfide it is easy to differentiate purple sulfur bacteria from purple non-sulfur bacteria because the microscopically globules of S0 are formed.[5]
Metabolism
Purple bacteria are able to perform different
Photosynthesis
Photosynthetic unit
Purple bacteria use
Mechanism
Purple bacteria use cyclic
Electron donors for anabolism
Purple bacteria are
Purple bacteria lack external electron carriers to spontaneously reduce NAD(P)+ to NAD(P)H, so they must use their reduced quinones to
Ecology
Distribution
Purple bacteria inhabit illuminated anoxic aquatic and terrestrial environments. Even if sometimes the two major groups of purple bacteria,
Purple bacteria have evolved effective strategies for
Biogeochemical cycles
Purple bacteria are involved in the biogeochemical cycles of different nutrients. In fact they are able to photoautotrophically fix carbon, or to consume it photoheterotrophically; in both cases in anoxic conditions. However the most important role is played by consuming hydrogen sulfide: a highly toxic substance for plants, animals and other bacteria. In fact, the oxidation of hydrogen sulfide by purple bacteria produces non-toxic forms of sulfur, such as elemental sulfur and sulfate.[5]
In addition, almost all non-sulfur purple bacteria are able to fix nitrogen (N2 + 8 H+ → 2 NH3 + H2),[27] and Rba Sphaeroides, an alpha proteobacter, is capable of reducing nitrate to molecular nitrogen by denitrification.[28]
Ecological niches
Quantity and quality of light
Several studies have shown that a strong accumulation of phototrophic sulfur bacteria has been observed between 2 and 20 meters (6 ft 7 in and 65 ft 7 in) deep, in some cases even 30 m (98 ft), of pelagic environments.[29] This is due to the fact that in some environments the light transmission for various populations of phototrophic sulfur bacteria varies with a density from 0.015 to 10%[30] Furthermore, Chromatiaceae have been found in chemocline environments over 20 m (66 ft) depths. The correlation between anoxygenic photosynthesis and the availability of solar radiation suggests that light is the main factor controlling all the activities of phototrophic sulfur bacteria. The density of pelagic communities of phototrophic sulfur bacteria extends beyond a depth range of 10 cm (3.9 in),[30] while the less dense population (found in the Black Sea (0.068–0.94 μg BChle/dm3), scattered over an interval of 30 m (98 ft).[31] Communities of phototrophic sulfur bacteria located in the coastal sediments of sandy, saline or muddy beaches live in an environment with a higher light gradient, limiting growth to the highest value between 1.5–5 mm (1⁄16–3⁄16 in) of the sediments.[32] At the same time, biomass densities of 900 mg bacteriochlorophyll/dm−3 can be attained in these latter systems.[33]
Temperature and salinity
Purple sulfur bacteria (like green sulfur bacteria) typically form blooms in non-thermal aquatic ecosystems, some members have been found in hot springs.[34] For example Chlorobaculum tepidum can only be found in some hot springs in New Zealand at a pH value between 4.3 and 6.2 and at a temperature above 56 °C (133 °F). Another example, Thermochromatium tepidum, has been found in several hot springs in western North America at temperatures above 58 °C (136 °F) and may represent the most thermophilic extant Pseudomonadota.[30] Of the purple sulfur bacteria, many members of the Chromatiaceae family are often found in fresh water and marine environments. About 10 species of Chromatiaceae are halophilic.[35]
Syntrophy and symbioses
Like
History
Purple bacteria were the first bacteria discovered[when?] to photosynthesize without having an oxygen byproduct. Instead, their byproduct is sulfur. This was demonstrated by first establishing the bacteria's reactions to different concentrations of oxygen. It was found that the bacteria moved quickly away from even the slightest trace of oxygen. Then a dish of the bacteria was taken, and a light was focused on one part of the dish, leaving the rest dark. As the bacteria cannot survive without light, all the bacteria moved into the circle of light, becoming very crowded. If the bacteria's byproduct was oxygen, the distances between individuals would become larger and larger as more oxygen was produced. But because of the bacteria's behavior in the focused light, it was concluded that the bacteria's photosynthetic byproduct could not be oxygen.[citation needed]
In a 2018
article, it has been suggested that purple bacteria can be used as aEvolution
Researchers have theorized that some purple bacteria are related to the
References
- PMID 16997562.
- PMID 13416343.
- ISSN 1466-5026.
- ISBN 978-1-4020-8814-8.
- ^ ISBN 978-1-4020-8815-5.
- ISSN 0723-2020.
- S2CID 19597530.
- S2CID 7410927.
- PMID 25509391.
- S2CID 14240332.
- PMID 21943387.
- PMID 12503169.
- ISBN 978-3-540-32524-6.
- S2CID 23254767.
- .
- PMID 18197174.
- S2CID 2409162.
- S2CID 46208080.
- OCLC 49273347.
- PMID 9600895.
- S2CID 84224465.
- PMID 7811087.
- ISBN 978-0-306-47954-0.
- ^ "The architecture and function of the light-harvesting apparatus of purple bacteria: from single molecules to in vivo membranes". ProQuest.
- PMID 623470.
- ISBN 978-3-319-46259-2.
- ISBN 978-0-306-47954-0.
- 'S2CID 20375188.
- PMID 16011763.
- ^ ISBN 978-1-4020-6862-1.
- PMID 16332785.
- ISBN 978-0-7923-3681-5.
- ISSN 0168-6496.
- ISSN 0378-1097.
- ISBN 0-387-24145-0.
- S2CID 25411079.
- PMID 10653757.
- Science Daily. November 13, 2018. Retrieved November 14, 2018.
- .
- PMID 8790385.