What is purple bacteria? These microorganisms are pigmented with bacteriochlorophyll a or b along with various carotenoids that 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). The 2018 Frontiers in Energy Research paper proposed using them as bio-resources.
Biology
Purple bacteria are mostly photoautotrophic, but chemoautotrophic and photoheterotropic species are also known. They may be mixotrophs capable of aerobic respiration and fermentation.
Photosynthesis of purple bacteria occurs in reaction centers on the cell membrane where photosynthetic pigments (i.e. bacteriochlorophyll, carotenoids) and pigment-binding proteins are introduced into the invagination to form specific vesicles, tubules, or single-pair or stacked lamellar sheets. This is called the intracytoplasmic membrane (ICM), which has an enlargedsurface area to maximize light absorption.
Physics and chemistry
Purple bacteria use cyclic electron transfer caused by a series of redox reactions. Light harvesting complexes surrounding the reaction center (RC) collect photons in the form of resonant energy, capturing the P870 or P960 chlorophyll pigments located in the RC. Excited electrons cycle from P870 to quinones QA and QB, then go 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 protons to be pumped into the periplasm by the cytochrome bc1 complex. The resulting charge sharing between the cytoplasm and periplasm creates a proton driving force used by ATP synthase to generate 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 purple bacteria, called purple sulfur bacteria (PSB), uses sulfide or sulfur as electron donors. Another type, called purple non-sulfur bacteria, usually uses hydrogen as an electron donor, but can also use sulfide or organic compounds at lower concentrations compared to PSB.
Violet bacteriathere are not enough external electron carriers to spontaneously reduce NAD(P)+ to NAD(P)H, so they must use their reduced quinones to reduce NAD(P)+ enanghorously. This process is driven 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 to be found to have photosynthesis without oxygen as a by-product. Instead, their by-product is sulfur. This was proven when the reactions of bacteria to different concentrations of oxygen were first established. Bacteria have been found to quickly move away from the slightest trace of oxygen. Then they did an experiment where a dish of bacteria was used, and the light was focused on one part of it, while the other was left in the dark. Because bacteria cannot survive without light, they move into the circle of light. If the by-product of their life were oxygen, the distances between individuals would become larger as the amount of oxygen increased. But due to the behavior of purple and green bacteria in focused light, it was concluded that the by-product of bacterial photosynthesis could not be oxygen.
Researchers have suggested that some purple 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.
Sulfur bacteria (sulfur bacteria)
Purple sulfur bacteria (PSB) are part of the Proteobacteria group capable of photosynthesis, collectively referred to as purple bacteria. They are anaerobic or microaerophilic and are often found in stratified aquatic environments, including hot springs, stagnant pools, and microbial aggregations in high water areas. Unlike plants, algae, and cyanobacteria, purple sulfur bacteria do not use water as a reducing agent and therefore do not produce oxygen. Instead, they may use sulfur in the form of sulfide or thiosulfate (and some species may also use H2, Fe2+ or NO2-) as an electron donor in their photosynthesis pathways. Sulfur is oxidized to produce elemental sulfur granules. This, 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 order Chromatiales, included in the gamma division Proteobacteria. The genus Halothiobacillus is also included in Chromatiales in its own family, but it is not photosynthetic.
Habitats
Purple sulfur bacteria are typically found in the illuminated anoxic zones of lakes and other aquatic habitats where hydrogen sulfide accumulates,as well as in "sulfur springs" where geochemically or biologically produced hydrogen sulfide can cause purple sulfur bacteria to bloom. Photosynthesis requires anoxic conditions; these bacteria cannot thrive in oxygenated environments.
Meromictic (permanently stratified) lakes are the most favorable for the development of purple sulfur bacteria. They stratify because they have denser (usually physiological) water at the bottom and less dense (usually fresh water) closer to the surface. The growth of purple sulfur bacteria is also supported by layering in holomictic lakes. They are thermally stratified: during spring and summer, the surface water 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 sediment diffuses upward into anoxic bottom waters where purple sulfur bacteria can form dense cell masses.
Clusters
Purple sulfur bacteria can also be found and are a prominent component in intermediate microbial aggregations. Clusters such as the Sippewissett microbial rug have a dynamic environment due to the flow of tides and incoming fresh water, resulting in similarly stratified environments as meromictic lakes. Growth of purple sulfur bacteriais activated as sulfur is supplied due to the death and decomposition of microorganisms located above them. The stratification and source of sulfur allows PSB to grow in these tidal basins where aggregations occur. PSB can help stabilize microbial sediment through the secretion of extracellular polymeric substances that can bind sediment in watersheds.
Ecology
Purple sulfur bacteria are able to influence the environment by promoting nutrient cycling, using their metabolism to change the environment. They may play a significant role in primary production by influencing the carbon cycle through carbon fixation. Purple 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 oxic layer of lakes, is recycled and provided to heterotrophic bacteria for use. This indicates that although purple sulfur bacteria are found 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.
