Purple bacteria

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Purple bacteria grown in Winogradsky column Purp d winogradsky.jpg
Purple bacteria grown in Winogradsky column

Purple bacteria or purple photosynthetic bacteria are Gram-negative 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]

Contents

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
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 metabolic pathways that allow them to adapt to different and even extreme environmental conditions. They are mainly photoautotrophs, but are also known to be chemoautotrophic and photoheterotrophic. Since pigment synthesis does not take place in presence of oxygen, phototrophic growth only occurs in anoxic and light conditions. [9] However purple bacteria can also grow in dark and oxic environments. In fact they can be mixotrophs, capable of anaerobic and aerobic respiration or fermentation [10] basing on the concentration of oxygen and availability of light. [11]

Photosynthesis

Photosynthetic unit

Purple bacteria use bacteriochlorophyll and carotenoids to obtain the light energy for photosynthesis. Electron transfer and photosynthetic reactions occur at the cell membrane in the photosynthetic unit which is composed by the light-harvesting complexes LHI and LHII and the photosynthetic reaction centre where the charge separation reaction occurs. [12] These structures are located in the intracytoplasmic membrane, areas of the cytoplasmic membrane invaginated to form vesicle sacs, tubules, or single-paired or stacked lamellar sheets which have increased surface to maximize light absorption. [13] Light-harvesting complexes are involved in the energy transfer to the reaction centre. These are integral membrane protein complexes consisting of monomers of α- and β-apoproteins, each one binding molecules of bacteriochlorophyll and carotenoids non-covalently. LHI is directly associated with the 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 Bacteria purpura.png
The purple non-sulfur bacterium Rhodospirillum

Mechanism

Purple bacteria use cyclic electron transport driven by a series of redox reactions. [16] Light-harvesting complexes surrounding a reaction centre (RC) harvest photons in the form of resonance energy, exciting chlorophyll pigments P870 or P960 located in the RC. Excited electrons are cycled from P870 to quinones QA and QB, then passed to 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 anoxygenic because they do not use water as electron donor to produce oxygen. Purple sulfur bacteria (PSB), use sulfide, sulfur, thiosulfate or hydrogen as electron donors. [21] In addition, some species use ferrous iron as electron donor and one strain of Thiocapsa can use nitrite. [22] Finally, even if the purple sulfur bacteria are typically photoautotrophic, some of them are photoheterotrophic and use different carbon sources and electron donors such as organic acids. Purple nonsulfur bacteria typically use  hydrogen as an electron donor, but can also use sulfide at lower concentrations compared to PSB and some species can use thiosulfate or 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, purple sulfur bacteria and purple nonsulfur bacteria, coexist in the same habitat, they occupy different niches. Purple sulfur bacteria are strongly photoautotrophs and are not adapted to an efficient metabolism and growth in the dark. A different speech applies to purple nonsulfur bacteria that are strongly photoheterotrophs, even if they are capable of photoautotrophy, and are equipped for living in dark environments. Purple sulfur bacteria can be found in different ecosystems with enough sulfate and light, some examples are shallow lagoons polluted by sewage or deep waters of lakes, in which they could even bloom. Blooms can both involve a single or a mixture of species. They can also be found in 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 photosynthesis in extreme environments, in fact they are quite successful in harsh habitats. In the 1960s the first halophiles and acidophiles of the genus Ectothiorhodospira were discovered. In the 1980s Thermochromatium tepidum , a thermophilic purple bacterium that can be found in 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 (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 green sulfur bacteria, purple sulfur bacteria are also capable of symbiosis and can rapidly create stable associations [36] between other purple sulfur bacteria and sulfur- or sulfate-reducing bacteria. These associations are based on a cycle of sulfur but not carbon compounds. Thus, a simultaneous growth of two bacteria partners takes place, which are fed by the oxidation of 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 mitochondria, 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]

Related Research Articles

<span class="mw-page-title-main">Photosynthesis</span> Biological process to convert light into chemical energy

Photosynthesis is a biological process used by many cellular organisms to convert light energy into chemical energy, which is stored in organic compounds that can later be metabolized through cellular respiration to fuel the organism's activities. The term usually refers to oxygenic photosynthesis, where oxygen is produced as a byproduct and some of the chemical energy produced is stored in carbohydrate molecules such as sugars, starch, glycogen and cellulose, which are synthesized from endergonic reaction of carbon dioxide with water. Most plants, algae and cyanobacteria perform photosynthesis; such organisms are called photoautotrophs. Photosynthesis is largely responsible for producing and maintaining the oxygen content of the Earth's atmosphere, and supplies most of the biological energy necessary for complex life on Earth.

Primary nutritional groups are groups of organisms, divided in relation to the nutrition mode according to the sources of energy and carbon, needed for living, growth and reproduction. The sources of energy can be light or chemical compounds; the sources of carbon can be of organic or inorganic origin.

<span class="mw-page-title-main">Green sulfur bacteria</span> Family of bacteria

The green sulfur bacteria are a phylum, Chlorobiota, of obligately anaerobic photoautotrophic bacteria that metabolize sulfur.

<i>Chloroflexus aurantiacus</i> Species of bacterium

Chloroflexus aurantiacus is a photosynthetic bacterium isolated from hot springs, belonging to the green non-sulfur bacteria. This organism is thermophilic and can grow at temperatures from 35 °C to 70 °C. Chloroflexus aurantiacus can survive in the dark if oxygen is available. When grown in the dark, Chloroflexus aurantiacus has a dark orange color. When grown in sunlight it is dark green. The individual bacteria tend to form filamentous colonies enclosed in sheaths, which are known as trichomes.

The purple sulfur bacteria (PSB) are part of a group of Pseudomonadota capable of photosynthesis, collectively referred to as purple bacteria. They are anaerobic or microaerophilic, and are often found in stratified water environments including hot springs, stagnant water bodies, as well as microbial mats in intertidal zones. Unlike plants, algae, and cyanobacteria, purple sulfur bacteria do not use water as their reducing agent, and therefore do not produce oxygen. Instead, they can use sulfur in the form of sulfide, or thiosulfate (as well, some species can use H2, Fe2+, or NO2−) as the electron donor in their photosynthetic pathways. The sulfur is oxidized to produce granules of elemental sulfur. This, in turn, may be oxidized to form sulfuric acid.

<span class="mw-page-title-main">Bacteriochlorophyll</span> Chemical compound

Bacteriochlorophylls (BChl) are photosynthetic pigments that occur in various phototrophic bacteria. They were discovered by C. B. van Niel in 1932. They are related to chlorophylls, which are the primary pigments in plants, algae, and cyanobacteria. Organisms that contain bacteriochlorophyll conduct photosynthesis to sustain their energy requirements, but the process is anoxygenic and does not produce oxygen as a byproduct. They use wavelengths of light not absorbed by plants or cyanobacteria. Replacement of Mg2+ with protons gives bacteriophaeophytin (BPh), the phaeophytin form.

Heliobacteria are a unique subset of prokaryotic bacteria that process light for energy. Distinguishable from other phototrophic bacteria, they utilize a unique photosynthetic pigment, bacteriochlorophyll g and are the only known Gram-positive phototroph. They are a key player in symbiotic nitrogen fixation alongside plants, and use a type I reaction center like green-sulfur bacteria.

<span class="mw-page-title-main">Chromatiaceae</span> Family of purple sulfur bacteria

The Chromatiaceae are one of the two families of purple sulfur bacteria, together with the Ectothiorhodospiraceae. They belong to the order Chromatiales of the class Gammaproteobacteria, which is composed by unicellular Gram-negative organisms. Most of the species are photolithoautotrophs and conduct an anoxygenic photosynthesis, but there are also representatives capable of growing under dark and/or microaerobic conditions as either chemolithoautotrophs or chemoorganoheterotrophs.

Photoheterotrophs 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. 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.

γ-Carotene (gamma-carotene) is a carotenoid, and is a biosynthetic intermediate for cyclized carotenoid synthesis in plants. It is formed from cyclization of lycopene by lycopene cyclase epsilon. Along with several other carotenoids, γ-carotene is a vitamer of vitamin A in herbivores and omnivores. Carotenoids with a cyclized, beta-ionone ring can be converted to vitamin A, also known as retinol, by the enzyme beta-carotene 15,15'-dioxygenase; however, the bioconversion of γ-carotene to retinol has not been well-characterized. γ-Carotene has tentatively been identified as a biomarker for green and purple sulfur bacteria in a sample from the 1.640 ± 0.003-Gyr-old Barney Creek Formation in Northern Australia which comprises marine sediments. Tentative discovery of γ-carotene in marine sediments implies a past euxinic environment, where water columns were anoxic and sulfidic. This is significant for reconstructing past oceanic conditions, but so far γ-carotene has only been potentially identified in the one measured sample.

Sulfur is metabolized by all organisms, from bacteria and archaea to plants and animals. Sulfur can have an oxidation state from -2 to +6 and is reduced or oxidized by a diverse range of organisms. The element is present in proteins, sulfate esters of polysaccharides, steroids, phenols, and sulfur-containing coenzymes.

<span class="mw-page-title-main">Anoxygenic photosynthesis</span> Process used by obligate anaerobes

Anoxygenic photosynthesis is a special form of photosynthesis used by some bacteria and archaea, which differs from the better known oxygenic photosynthesis in plants in the reductant used and the byproduct generated.

Aerobic anoxygenic phototrophic bacteria (AAPBs) are Alphaproteobacteria and Gammaproteobacteria that are obligate aerobes that capture energy from light by anoxygenic photosynthesis. Anoxygenic photosynthesis is the phototrophic process where light energy is captured and stored as ATP. The production of oxygen is non-existent and, therefore, water is not used as an electron donor. They are widely distributed marine bacteria that may constitute over 10% of the open ocean microbial community. They can be particularly abundant in oligotrophic conditions where they were found to be 24% of the community. Aerobic anoxygenic phototrophic bacteria are photoheterotrophic (phototroph) microbes that exist in a variety of aquatic environments. Most are obligately aerobic, meaning they require oxygen to grow. One aspect of these bacteria is that they, unlike other similar bacteria, are unable to utilize BChl (bacteriochlorophyll) for anaerobic growth. The only photosynthetic pigment that exists in AAPB is BChl-a. Anaerobic phototrophic bacteria, on the contrary, can contain numerous species of photosynthetic pigments like bacteriochlorophyll-a. These bacteria can be isolated using carotenoid presence and medias containing organic compounds. Predation, as well as the availability of phosphorus and light, have been shown to be important factors that influence AAPB growth in their natural environments. AAPBs are thought to play an important role in carbon cycling by relying on organic matter substrates and acting as sinks for dissolved organic carbon. There is still a knowledge gap in research areas regarding the abundance and genetic diversity of AAPB, as well as the environmental variables that regulate these two properties.

Rhodovulum sulfidophilum is a gram-negative purple nonsulfur bacteria. The cells are rod-shaped, and range in size from 0.6 to 0.9 μm wide and 0.9 to 2.0 μm long, and have a polar flagella. These cells reproduce asexually by binary fission. This bacterium can grow anaerobically when light is present, or aerobically (chemoheterotrophic) under dark conditions. It contains the photosynthetic pigments bacteriochlorophyll a and of carotenoids.

Chlorobium chlorochromatii, originally known as Chlorobium aggregatum, is a symbiotic green sulfur bacteria that performs anoxygenic photosynthesis and functions as an obligate photoautotroph using reduced sulfur species as electron donors. Chlorobium chlorochromatii can be found in stratified freshwater lakes.

In some forms of photosynthetic bacteria, a chromatophore is a pigmented(coloured), membrane-associated vesicle used to perform photosynthesis. They contain different coloured pigments.

Okenane, the diagenetic end product of okenone, is a biomarker for Chromatiaceae, the purple sulfur bacteria. These anoxygenic phototrophs use light for energy and sulfide as their electron donor and sulfur source. Discovery of okenane in marine sediments implies a past euxinic environment, where water columns were anoxic and sulfidic. This is potentially tremendously important for reconstructing past oceanic conditions, but so far okenane has only been identified in one Paleoproterozoic rock sample from Northern Australia.

<span class="mw-page-title-main">Microbial oxidation of sulfur</span>

Microbial oxidation of sulfur is the oxidation of sulfur by microorganisms to build their structural components. The oxidation of inorganic compounds is the strategy primarily used by chemolithotrophic microorganisms to obtain energy to survive, grow and reproduce. Some inorganic forms of reduced sulfur, mainly sulfide (H2S/HS−) and elemental sulfur (S0), can be oxidized by chemolithotrophic sulfur-oxidizing prokaryotes, usually coupled to the reduction of oxygen (O2) or nitrate (NO3−). Anaerobic sulfur oxidizers include photolithoautotrophs that obtain their energy from sunlight, hydrogen from sulfide, and carbon from carbon dioxide (CO2).

Thioflavicoccus is a Gram-negative, obligately phototrophic, strictly anaerobic and motile genus of bacteria from the family of Chromatiaceae with one known species.

<i>Pseudoblepharisma</i>

Pseudoblepharisma is a genus of heterotrich ciliates inhabiting oxygen depleted freshwater habitats. Most sources report that it contains one species, Pseudoblepharisma tenue, but at least four have been seen in literature.

References

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