Chlorobium (also known as Chlorochromatium) is a genus of green sulfur bacteria. They are photolithotrophic oxidizers of sulfur and most notably utilise a noncyclic electron transport chain to reduce NAD+. Photosynthesis is achieved using a Type 1 Reaction Centre using bacteriochlorophyll (BChl) a. Two photosynthetic antenna complexes aid in light absorption: the Fenna-Matthews-Olson complex ("FMO", also containing BChl a), and the chlorosomes which employ mostly BChl c, d, or e. Hydrogen sulfide is used as an electron source and carbon dioxide its carbon source.
Chlorobium species exhibit a dark green color; in a Winogradsky column, the green layer often observed is composed of Chlorobium. This genus lives in strictly anaerobic conditions below the surface of a body of water, commonly the anaerobic zone of a eutrophic lake.
Chlorobium aggregatum is a species which exists in a symbiotic relationship with a colorless, nonphotosynthetic bacteria. This species looks like a bundle of green bacteria, attached to a central rod-like cell which can move around with a flagellum. The green, outer bacteria use light to oxidize sulfide into sulfate. The inner cell, which is not able to perform photosynthesis, reduces the sulfate into sulfide. These bacteria divide in unison, giving the structure a multicellular appearance which is highly unusual in bacteria.
Chlorobium species are thought to have played an important part in mass extinction events on Earth. If the oceans turn anoxic (due to the shutdown of ocean circulation) then Chlorobium would be able to out compete other photosynthetic life. They would produce huge quantities of methane and hydrogen sulfide which would cause global warming and acid rain. This would have huge consequences for other oceanic organisms and also for terrestrial organisms. Evidence for abundant Chlorobium populations is provided by chemical fossils found in sediments deposited at the Cretaceous mass extinction.
The complete C. tepidum genome, which consists of 2.15 megabases (Mb), was sequenced and published in 2002. It synthesizes chlorophyll a and bacteriochlorophylls (BChls) a and c, of which the model organism has been used to elucidate the biosynthesis of BChl c. Several of its carotenoid metabolic pathways (including a novel lycopene cyclase) have similar counterparts in cyanobacteria.
Molecular signatures for Chlorobi
Comparative genomic analysis has led to the identification of 2 conserved signature indels which are uniquely found in members of the phylum Chlorobi and are thus characteristic of the phylum. The first indel is a 28-amino-acid insertion in DNA polymerase III and the second is a 12 to 14 amino acid insertion in alanyl-tRNA synthetase. These indels are not found in any other bacteria and thus serve as molecular markers for the phylum. In addition to the conserved signature indels, 51 proteins which are uniquely found in members of the phylum Chlorobi. 65 other proteins have been identified which are unique to the Chlorobi phylum, however these proteins are missing in several Chlorobi species and are not distributed throughout the phylum with any clear pattern. This means that significant gene loss may have occurred, or the presence of these proteins may be a result of horizontal gene transfer. Of these 65 proteins, 8 are found only in Chlorobium luteolum and Chlorobium phaeovibrioides. These two species form a strongly supported clade in phylogenetic trees and a close relationship between these species is further supported by the unique sharing of these 8 proteins.
Relatedness of Chlorobi to Bacteroidetes and Fibrobacteres phyla
Species from the Bacteroidetes and Chlorobi phyla branch very closely together in phylogenetic trees, indicating a close relationship. Through the use of comparative genomic analysis, 3 proteins have been identified which are uniquely shared by virtually all members of the Bacteroidetes and Chlorobi phyla. The sharing of these 3 proteins is significant because other than these 3 proteins, no proteins from either the Bacteroidetes or Chlorobi phyla are shared by any other groups of bacteria. Several conserved signature indels have also been identified which are uniquely shared by members of the Bacteroidetes and Chlorobi phyla. The presence of these molecular signatures supports the close relationship of the Bacteroidetes and Chlorobi phyla. Additionally, the phylum Fibrobacteres is indicated to be specifically related to these two phyla. A clade consisting of these three phyla is strongly supported by phylogenetic analyses based upon a number of different proteins These phyla also branch in the same position based upon conserved signature indels in a number of important proteins. Lastly and most importantly, two conserved signature indels (in the RpoC protein and in serine hydroxymethyltransferase) and one signature protein PG00081 have been identified that are uniquely shared by all of the species from these three phyla. All of these results provide compelling evidence that the species from these three phyla shared a common ancestor exclusive of all other bacteria and it has been proposed that they should all recognized as part of a single “FCB”superphylum.
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