Көкірт бактерияларының Chromatiaceae тұқымдасы

Family of purple sulfur bacteria

The Chromatiaceae are one of the two families of purple sulfur bacteria, together with the Ectothiorhodospiraceae. They belong to the order Chromatiales of the class Gammaproteobacteria, which is composed by unicellular Gram negative organisms. 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. 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. Sulfur is an intermediate in the oxidization of sulfide, which is ultimately converted into sulfate, and may serve as a reserve. 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. The first taxonomic system (1907) was based on the Molisch's pigment and the storage of elemental sulfur. According to these traits, Molisch made a first distinction of the purple sulfur bacteria into two groups: the Thiorhodaceae, including all the members that store globules of elemental sulfur inside their cells, and the Athiorhodaceae, without this feature. These two groups appeared as families of the order Pseudomonadales in the Bergey's Manual of Determinative Bacteriology, 7th ed. In the next edition, they were split in the Rhodospirillaceae (Pfennig and Triiper, 1971) and the Chromatiaceae (Bavendamm, 1924), respectively. 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. As a consequence of this definition, both the bacteria that accumulate sulfur inside (current Chromatiaceae) and outside the cells (current Ectothiorhodospiraceae) were initially grouped together. With the development of chemotaxonomy and DNA sequencing, thanks to analyses on 16S rRNA and pufLM genes as well as on some peculiar phenotypic features (such as lipid and fatty acid composition, quinone structure and amino acid sequence of cytochrome c551), it was possible to demonstrate the phylogenetic distance between the two families. This emerging data led Imhoff (1984) to redefine them: he assigned the genus Ectothiorhodospira (Pelsh, 1936) to a new family called Ectothiorhodospiraceae, and the genus Chromatium (Perty, 1852) to the family Chromatiaceae. Referring to the latest definition, Chromatiaceae family includes only those purple sulfur bacteria that perform anoxygenic photosynthesis and store elemental sulfur inside their cells. This is in line with the first Molisch definition of the Thiorhodaceae group and reflects the distance, but phylogenetic correlation between the two families. Allochromatium spp., Thermochromatium spp., Thiocystis spp. and Chromatium okenii. They share some cytological properties, such as rod shaped or spherical cells, flagellar movements and the absence of vesicles. This group includes the most versatile species of the family Chromatiaceae: although they are considered primarily freshwater bacteria as they do not have a specific need for salt, some members can grow under low salinity conditions and thus also in brackish and marine habitats. Thiocapsa spp., Thiolamprovum spp., Thiobaca spp., Lamprocystis spp. and Thiodictyon spp. As well as in the previous case, the members of this group are primarily freshwater bacteria, even if some of them can be salt tolerant enough to grow in coastal habitats. Thiocapsa roseopersicina and Lamprocystis roseopersicina are among the well known species of this group; the first one lacks gas vesicles, while in the second one gas vesicles and flagella dependent movements occur. Marichromatium spp. and Thiorhodococcus spp. They are typical marine bacteria and capable of flagellar movements, but they differ from each other for their cell shape, which is rod and spherical, respectively. The species Thiophaeococcus mangrovi is associated to this group. The fourth group is phylogenetically more distant to the others and is composed by halophilic (Halochromatium spp. and Thiohalocapsa spp. ), marine (Thiorhodovibrio spp., Rhabdochromatium spp. and Isochromatium spp.) and bacteriochlorophyll b containing genera (Thiococcus spp., Thioflavicoccus spp. and Thioalkalicoccus spp.). Some species belonging to the last three genera are characterized by the presence of tubular internal membranes, a feature which distinguishes them from all the other genera of the family Chromatiaceae, which instead have internal membrane systems arranged as vesicles.

The majority of Chromatiaceae species utilize CO2 as the sole carbon source for autotrophic growth. They employ the Calvin cycle with the key enzymes ribulose bisphosphate carboxylase (RubisCO) and phosphoribulokinase. Versatile species are able to metabolize a larger number of organic compounds than specialized species, which instead mainly assimilate acetate and pyruvate.

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

The diagenetic end product of okenone, okenane, is considered a valid biomarker. When bacteria die, they sink to the seafloor together with some of their pigment molecules, including okenone. Under conditions of preservation, the environment is often anoxic and reducing, leading to the chemical loss of functional groups and the partial saturation of okenone to okenane, which in this form can evade microbial and chemical degradation and therefore is buried in marine sediments. 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).

An anaerobic lagoon is a shallow, man made, covered basin designed to hold and pretreat industrial, urban or other kinds of wastewaters. Here Chromatiaceae (and purple sulfur bacteria in general) are the predominant phototrophic bacteria, since they play a key role in metabolizing and thus lowering the concentration of undesirable compounds present in the influent wastewater stream. This is an example of in situ bioremediation, during which the bacteria inhabiting the lagoon exploit these compounds to support their photoheterotrophic growth. For example, they can use the toxic and bad smelling hydrogen sulfide as the electron source and the greenhouse gas methane as the carbon source for anoxygenic photosynthesis; in this way, the contaminants are removed from the lagoon thus reducing odour and toxicity as well as atmospheric pollution, respectively.