Purple bacteria or purple photosynthetic bacteria are proteobacteria that are phototrophic, that is, capable of producing their own food via photosynthesis. 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 (Rhodospirillaceae).
Photosynthesis occurs at reaction centers on the cell membrane, where the photosynthetic pigments (i.e. bacteriochlorophyll, carotenoids) and pigment-binding proteins are invaginated to form vesicle sacs, tubules, or single-paired or stacked lamellar sheets. This is called the intracytoplasmic membrane (ICM), which has increased surface area to maximize light absorption.
Purple bacteria use cyclic electron transport driven by a series of redox reactions. 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. The resulting charge separation between the cytoplasm and periplasm generates a proton motive force used by ATP synthase to produce ATP energy.
Electron donors for anabolism
Purple bacteria also transfer electrons from external electron donors directly to cytochrome bc1 to generate NADH or NADPH used for anabolism. They are anoxygenic because they do not use water as an electron donor to produce oxygen. One type of purple bacteria, called purple sulfur bacteria (PSB), use sulfide or sulfur as electron donors. Another type, called purple non-sulfur bacteria, typically use hydrogen as an electron donor, but can also use sulfide or organic compounds at lower concentrations compared to PSB.
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.
Purple bacteria were the first bacteria discovered 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.
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.
- Other families
Purple sulfur bacteria are included among the gamma subgroup and make up the order Chromatiales. 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.
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