at low levels of sulfide (0.5 mM) and in so doing, oxidize sulfide to S0, , or . 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 (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. Metabolism Purple Bacteria are able to perform different metabolisms 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 pigments synthesis does not take place in presence of oxygen, phototrophic growth only occurs in anoxic and light conditions. 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 basing on the concentration of oxygen and availability of light. 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. 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. 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. 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. Mechanism Purple bacteria use cyclic electron transport driven by a series of redox reactions.