What are purple bacteria? These microorganisms are pigmented with bacteriochlorophyll a or b together with various carotenoids, which give them colors ranging from purple, red, brown and orange. This is a fairly diverse group. They can be divided into two groups: purple sulfur bacteria and simple purple bacteria (Rhodospirillaceae). A 2018 Frontiers in Energy Research paper suggested using them as bioresources.
Biology
Purple bacteria are mainly photoautotrophic, but chemoautotrophic and photoheterotropic species are also known. They can be mixotrophs capable of aerobic respiration and fermentation.
The photosynthesis of purple bacteria occurs in the reaction centers on the cell membrane, where photosynthetic pigments (i.e. bacteriochlorophyll, carotenoids) and pigment-binding proteins are introduced into the invagination with the formation of special vesicles, tubes, or single-pair or stacked laminated sheets. This is called an intracytoplasmic membrane (ICM), which has an increased surface area to maximize light absorption.
Physics and chemistry
Purple bacteria use cyclic electron transfer caused by a series of redox reactions. The light harvesting complexes surrounding the reaction center (RC) collect photons in the form of resonant energy, capturing P870 or P960 chlorophyll pigments located in RC. The excited electrons cycle from P870 to the quinones QA and QB, then go into cytochrome bc1, cytochrome c2 and back to P870. The reduced quinone QB attracts two cytoplasmic protons and becomes QH2, ultimately oxidizing and releasing protons to be injected into the periplasm by the cytochrome bc1 complex. The resulting charge separation between the cytoplasm and periplasm creates the proton motive force used by ATP synthase to produce ATP energy.
Purple bacteria also transfer electrons from external donors directly to cytochrome bc1 to generate NADH or NADPH used for anabolism. They are single crystals, because they do not use water as an electron donor to produce oxygen. One type of violet bacteria called violet sulfur bacteria (PSB) uses sulfide or sulfur as electron donors. Another type, called purple non-sulphurous bacteria, usually uses hydrogen as an electron donor, but can also use sulfide or organic compounds at lower concentrations compared to PSB.
Violet bacteria lack external electron carriers to spontaneously reduce NAD (P) + to NAD (P) H, so they should use their reduced quinones to enanthetically reduce NAD (P) +. This process is caused by the driving force of the proton and is called the reverse flow of electrons.
Sulfur instead of oxygen
Purple non-sulfur bacteria were the first bacteria in which photosynthesis was detected without a by-product in the form of oxygen. Instead, sulfur is their byproduct. This was proven when bacteria reactions to various oxygen concentrations were first established. It was found that bacteria quickly move away from the slightest trace of oxygen. Then they conducted an experiment where a dish with bacteria was used, and the light was focused on one part of it, and the other was left in the dark. Because bacteria cannot survive without light, they move into a circle of light. If the by-product of their livelihoods were oxygen, the distance between the individual individuals would become greater as the amount of oxygen increased. But due to the behavior of the purple and green bacteria in focused light, it was concluded that the by-product of bacterial photosynthesis cannot be oxygen.
Researchers have suggested that some violet bacteria today are associated with mitochondria, symbiotic bacteria in plant and animal cells that act as organelles. Comparison of their protein structure shows that there is a common ancestor of these structures. Purple green bacteria and heliobacteria also have a similar structure.
Serobacteria (sulfur bacteria)
Violet sulfur bacteria (PSB) are part of the Proteobacteria group, capable of photosynthesis, collectively called purple bacteria. They are anaerobic or microaerophilic and are often found in stratified aquatic environments, including hot springs, stagnant water bodies, and microbial accumulations in tidal zones. Unlike plants, algae and cyanobacteria, purple sulfur bacteria do not use water as a reducing agent and therefore do not produce oxygen. Instead, they can use sulfur in the form of sulfide or thiosulfate (and also some species can use H2, Fe2 + or NO2-) as an electron donor in their photosynthetic pathways. Sulfur is oxidized to produce elemental sulfur granules. It, in turn, can be oxidized to form sulfuric acid.
Classification
The group of purple bacteria is divided into two families: Chromatiaceae and Ectothiorhodospiraceae, which produce internal and external sulfur granules, respectively, and show differences in the structure of their internal membranes. They form part of the Chromatiales order included in the Proteobacteria gamma unit. The genus Halothiobacillus is also included in Chromatiales in its own family, but it is not photosynthetic.
Habitat
Violet sulfur bacteria are usually found in illuminated anoxic zones of lakes and other water areas where hydrogen sulfide is accumulated, as well as in “sulfur springs”, where geochemically or biologically produced hydrogen sulfide can cause the formation of violet sulfur bacteria. Anoxic conditions are needed for photosynthesis; these bacteria cannot thrive in oxygen-containing environments.
The most favorable for the development of violet sulfur bacteria are meromictic (constantly stratified) lakes. They stratify because they have denser (usually physiological) water below and less dense (usually fresh water) closer to the surface. The growth of violet sulfur bacteria is also supported by stratification in holomictic lakes. They are thermally stratified: in spring and summer, water on the surface heats up, making the upper water less dense than the lower, which provides a fairly stable stratification for the growth of purple sulfur bacteria. If enough sulfate is present to support sulfation, the sulfide formed in the sediments diffuses upward into the oxygen-free bottom waters, where the violet sulfur bacteria can form dense cell masses.
Accumulations
Purple sulfur bacteria can also be found and are a prominent component in intermediate microbial clusters. Clusters, such as the Sippewissett microbial rug, have a dynamic environment due to the flow of tides and incoming fresh water leading to similar stratified environments as meromictic lakes. The growth of violet sulfur bacteria is activated as sulfur is supplied due to death and decomposition of the microorganisms located above them. Stratification and a source of sulfur allow PSB to grow in these tidal pools where accumulations occur. PSB can help stabilize microbial sedimentation by secreting extracellular polymeric substances that can bind sedimentation in pools of water bodies.
Ecology
Violet sulfur bacteria are able to influence the environment, promoting the cycling of nutrients, using their metabolism to change the environment. They can play a significant role in primary production, affecting the carbon cycle through carbon fixation. Violet sulfur bacteria also contribute to the production of phosphorus in their habitat. Through the vital activity of these organisms, phosphorus, which limits the nutrient in the oxy layer of lakes, is recycled and provided to heterotrophic bacteria for use. This indicates that although purple sulfur bacteria are in the anoxic layer of their habitat, they are able to stimulate the growth of many heterotrophic organisms by supplying inorganic nutrients to the aforementioned oxide layer.