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. 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. 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. 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 sulphide: a highly toxic substance for plants, animals and other bacteria. In fact, the oxidation of hydrogen sulphide by purple bacteria produces non-toxic forms of sulfur, such as elemental sulfur and sulphate. In addition, almost all non-sulfur purple bacteria are able to fix nitrogen (), and Rba Sphaeroides, an alpha proteobacter, is capable of reducing nitrate to molecular nitrogen by denitrification.<ref>{{cite journal | vauthors = Satoh T, Hoshino Y, Kitamura H | title = Rhodopseudomonas sphaeroides''' forma sp. denitrificans, a denitrifying strain as a subspecies of Rhodopseudomonas sphaeroides | journal = Archives of Microbiology | volume = 108 | issue = 3 | pages = 265–269 | date = July 1976 | pmid = 1085137 | doi = 10.1007/BF00454851 | s2cid = 20375188 }}</ref> 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 deep (in some cases even 30 m) of pelagic environments. 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% Furthermore, Chromatiaceae have been found in chemocline environments over 20 m 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