Purple bacteria

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Purple bacteria grown in Winogradsky column

Purple bacteria or purple photosynthetic bacteria are

proteobacteria that are phototrophic, capable of producing their own food via photosynthesis.[1] They are pigmented with bacteriochlorophyll a or b, together with various carotenoids, which give them colours ranging between purple, red, brown, and orange. They may be divided into two groups – purple sulfur bacteria (Chromatiales, in part) and purple non-sulfur bacteria. Purple bacteria are anoxygenic phototrophs widely spread in nature, but especially in aquatic environments, where there are anoxic conditions that favor the synthesis of their pigments.[2]

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]

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
4
O2−
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

aerobic respiration or fermentation[10] basing on the concentration of oxygen and availability of light.[11]

Photosynthesis

Photosynthetic unit

Purple bacteria use

reaction centre forming a polymeric ring-like structure around it. LHI has an absorption maximum at 870 nm and it contains most of the bacteriochlorophyll of the photosynthetic unit. LHII contains less bacteriochlorophylls, has lower absorption maximum (850 nm) and is not present in all purple bacteria.[14] Moreover, the photosynthetic unit in Purple Bacteria shows great plasticity, being able to adapt to the constantly changing light conditions. In fact these microorganisms are able to rearrange the composition and the concentration of the pigments, and consequently the absorption spectrum, in response to light variation.[15]

The purple non-sulfur bacterium Rhodospirillum

Mechanism

Purple bacteria use cyclic

cytochrome bc1, cytochrome c2, and back to P870. The reduced quinone QB attracts two cytoplasmic protons and becomes QH2, eventually being oxidized and releasing the protons to be pumped into the periplasm by the cytochrome bc1 complex.[17][18] The resulting charge separation between the cytoplasm and periplasm generates a proton motive force used by ATP synthase to produce ATP energy.[19][20]

Electron donors for anabolism

Purple bacteria are

ferrous iron as electron donor.[23]
In contrast to the purple sulfur bacteria, the purple nonsulfur bacteria are mostly photoheterotrophic and can use a variety of organic compounds as both electron donor and carbon source, such as sugars, amino acids, organic acids, and aromatic compounds like toluene or benzoate.

Purple bacteria lack external electron carriers to spontaneously reduce NAD(P)+ to NAD(P)H, so they must use their reduced quinones to

endergonically reduce NAD(P)+. This process is driven by the proton motive force and is called reverse electron flow.[24]

Ecology

Distribution

Purple bacteria inhabit illuminated anoxic aquatic and terrestrial environments. Even if sometimes the two major groups of purple bacteria,

microbial mats where the lower layer decomposes and sulfate-reduction occurs.[5]

Purple non sulfur bacteria can be found in both illuminated and dark environments with lack of sulfide. However, they hardly form blooms with sufficiently high concentration to be visible without enrichment techniques.[25]

Purple bacteria have evolved effective strategies for

North American hot springs, was isolated for the first time.[26]

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 (116316 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

organic carbon and light substrates. Experiments with Chromatiaceae have pointed out that cell aggregates consisting of sulfate-reducing proteobacterium Desulfocapsa thiozymogenes and small cells of Chromatiaceae have been observed in the chemocline of an alpine meromictic lake.[37]

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

Frontiers in Energy Research [de] article, it has been suggested that purple bacteria can be used as a biorefinery.[38][39]

Evolution

Researchers have theorized that some purple bacteria are related to the

symbiotic bacteria in plant and animal cells today that act as organelles. Comparisons of their protein structure suggests that there is a common ancestor.[40]

References