[2][3] Most of the species are photolithoautotrophs and conduct an anoxygenic photosynthesis, but there are also representatives capable of growing under dark and/or microaerobic conditions as either chemolithoautotrophs or chemoorganoheterotrophs.

[3][4] Both Ectothiorhodospiraceae and Chromatiaceae bacteria produce elemental sulfur globules, the difference is that in the second case they are stored inside the cells rather than outside.

[2] Although purple sulfur bacteria have been known for some time, the difficulty in cultivating these microorganisms in the laboratory has made that few scientific depositions are available to date, and even less are those that provide comparative studies between the two families of the order: Chromatiaceae and Ectothiorhodospiraceae.

This is evidenced by the fact that most of the publications at disposal of the scientific community are the result of the work of a relatively small number of scientists, first of all Norbert Pfennig, Johannes Imhoff and Jörg Overmann.

[5] The taxonomy of these two families was originally drawn up entirely on characteristics easy to observe, such as the storage of elemental sulfur inside or outside the cells, the presence of gas vesicles and the internal membrane systems.

[2][10] Then, the term Chromatiaceae was used for the first time by Bavendamm in 1924, in particular as referred to all those purple bacteria which follow the pathway of anoxygenic photosynthesis, that is to say without using water as electron donor (and consequently without oxygen production), but rather oxidizing sulfide and storing the resulting sulfur either inside or outside the cells.

[11] Therefore, the current taxonomic system for Chromatiaceae corresponds primarily to the phylogenetic knowledge,[5] but it also takes into account phenotypic characteristics and ecological features, which allow to broadly distinguish the members of this family to the Ectothiorhodospiraceae ones.

From a physiological and morphological point of view, individual members can be distinguished from each other on the basis of cell size and shape (sphere, rod, vibrio, spirillum), motility (polar flagella-dependent motility or nonmotile members), presence of gas vesicles and ability to form cellular aggregates.

The chemical element, as it is inaccessible to other bacteria, could serve as a useful personal reserve:[3][16] Regarding their photosynthetic pigment composition, in the Chromatiaceae can be found bacteriochlorophyll a or b and different types of carotenoids, according to the species.

They confer to these bacteria their characteristic pigmentation, and if the amount of cells in the medium is high enough, the coloring may also be visible to the naked eye.

Most of them have bacteriochlorophyll a, with a maximum absorption wavelength of 800-900 nm, and a number of species uniquely biosynthesize the red-colored aromatic carotenoid okenone.

Bacteriochlorophylls and carotenoids are part of the light-harvesting complex; together with the reaction center, this one makes up the photosynthetic apparatus of Chromatiaceae, which is localized within an intracytoplasmic membrane system arranged as vesicles or tubules, depending on the species.

The specialized species are obligate anaerobic photolithoautotrophs, they use reduced sulfur compounds as electron donors and photoassimilate exclusively acetate and pyruvate (or propionate).

The versatile species photoassimilate a wider range of organic substrates and most of them do not require the presence of reduced sulfur compounds for their growth, since they are also capable of alternative metabolic strategies.

Sulfur is stored as granules inside the cells and can be used as an electron reserve in dark growth conditions during respiration and fermentation.

[3][26] As mentioned above, although Chromatiaceae largely practice anoxygenic photosynthesis, there are also versatile species able to switch their metabolism in the presence of oxygen.

[27] As in the case of the other purple sulfur bacteria, Chromatiaceae are mainly found in all those anaerobic habitats where the presence of light and the geochemical or biological production of sulfide can support their growth.

The natural distribution of Chromatiaceae strongly depends on the selective environmental factors described above (anoxic conditions, availability of light and sulfide) and follows daily and seasonal fluctuations, especially in response to the changes in sunlight intensity and temperature.

[4] This is the reason why these bacteria can find their optimal growth conditions in small areas of overlap between the countercurrent gradients of the above-mentioned factors.

Since the chemocline is usually relatively deep (from a few centimeters up to several meters, depending on the lake), the blooms of these bacteria tend not to be visible at the water surface.

Regarding the holomictic lakes, here occurs a seasonal, thermal stratification maintained by temperature differences (especially during summer), which also provides a stable enough layering for purple sulfur bacteria growth; in this case, they cluster at the level of the anoxic and sulfidic hypolimnion, that is to say the dense, bottom water layer of the lake.

[3][29][31] Ectothiorhodospiraceae and Chlorobiaceae are the only other families of phototrophic bacteria which thrive under similar environmental conditions:[28] despite their dissimilarities from a genetic and evolutionary point of view, both purple and green sulfur bacteria depend on reduced inorganic sulfur compounds for their growth and it allows them to play similar ecological roles.

This molecule allows to carry out paleoenvironmental reconstructions, since its discovery in marine sediments implies the presence of purple sulfur bacteria during the time of burial and thus a past euxinic environment (that is to say, anoxic and sulfidic waters).

[34][35] An anaerobic lagoon is a shallow, man-made, covered basin designed to hold and pretreat industrial, urban or other kinds of wastewaters.