Chlorobium

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A Microbial Biorealm page on the genus Chlorobium

Electron micrograph of Chlorobium tepidum. Ovoid chlorosomes can be seen attached to the cytoplasmic membrane. From CyanoBase.

Light micrograph of Chlorobium tepidum. From CyanoBase.

Classification

Higher order taxa:

Bacteria; Bacteroidetes/Chlorobi group; Bacteroidetes; Chlorobi; Chlorobia; Chlorobiales; Chlorobiaceae

Species:

Chlorobium ferrooxidans, Chlorobium limicola, Chlorobium phaeobacteroides, Chlorobium phaeovibraoides, Chlorobium sp.

NCBI: Taxonomy Genome

Description and Significance

Chlorobium tepidum is a thermophilic green sulfer bacteria originally isolated from a New Zealand hot spring. This type of bacterium has special light-harvesting complexes called chlorosomes that contain bacteriochlorophylls and carotenoids. Chlorobium grows in dense mats over hot springs as well as in other warm muds and bodies of water that contain sufficient hydrogen sulfide (used by C. tepidum as an elecron donor). C. tepidum is a valuable model for the green sulfur bacteria because it is easily cultivated and naturally transformable.

Genome Structure

The genome of C. tepidum's single circular chromosome is 2,154,946 bp and was the first sequenced in the phylum Chlorobia. Many genes were found to be highly conserved among photosynthetic species and seemed to have no clear function within C. tepidum; however, these genes are thought to play specific roles in photosynthesis or photobiology. Phylogenomic analysis and comparison showed that C. tepidum contains duplications of genes involved in biosynthetic pathways for photosynthesis and the metabolism of sulfer and nitrogen. These methods also displayed genetic similarities between C. tepidum metabolic processes and many Archaeal species (Eisen et al. 2002) . There are three other species that are currently being sequenced and are in the assembly process. They are Chlorobium phaeobacteroides DSM 266, Chlorobium phaeobacteroides BS1, Chlorobium limicola DSM 245.

Cell Structure and Metabolism

Chlorobium are Gram-negative cells. Chlorobium sp. can from long chains of almost spherical cells. Some strains can form coils of C-shaped cells. Chlorobium tepidum lives by anoxygenic photosynthesis and produces elemental sulfur as a waste product. C. tepidum deposits the elemental sulfer outside its cells unlike Chromatium and Thiothrix, two other sulfer-producing bacteria. In addition, they can photooxidize hydrogen as well as other sulfur compounds like sulfide, polysulfide, and thiosulfate. They are also obligatory autotrophic.

The major light-harvesting antenna organelle, which uses electromagnetic energy to power these processes, in C. tepidum is the chlorosome, which consists of highly aggregated bacteriochlorophyll c and carotenoids surrounded by a lipid-protein envelope. This ovoid structure is different from most other phototrophic organisms. (However, they are similar to a structure contained within the phylogenetically distant family Chloroflexaceae.) The chlorosomes are 70 to 180 nm long and 30 to 60 nm wide. As can be seen on the picture at the top of this page, they are attached to the reactions centers in the cytoplasmic membrane.

Oxidation of sulfide to sulfur has been studied in the past in C. limicola as a possible biocatalyst to remove acid gases made by the hydroprocessing of fossil fuels. The formation of sulfur from sulfide, in which sulfide acts as the reducing substrate, appears as follows (Douglas, et al. 1985):

To see a list of some of the metabolic pathways that occur within C. tepidum, visit The Systems Biology Institute or visit a list of the genes that control known metabolic pathways at Cyanobase.

Ecology

Microbial mats in Yellowstone National park shown to contain Chlorobium tepidum through reflectance spectroscopy. From the Lunar and Planetary Institute.

C. tepidum is a green sulfur bacterium that generally grows in a dense mat over hot springs. They are also found in anoxic and sulfide-rich waters, mud, and sediments. They grow best at tempuratures between 40 and 50 degrees Celsius and at a pH between 6.0 and 4.5.

The phototrophic bacterial composition of a bacterial mat or in any body of water is usuallly dependent on what quality and wavelength of light that the water recieves. Factors that effect this are algae that filter the light and the terrain surrounding the hot spring, mud, or body of water. Bacteria of different pigments capture different wavelengths of light; therefore, Chlorobium and other green sulfur bacteria like Chromotium, as well as other phototrophic bacteria, can be found distributed throughout their environment in a way that allows them to survive and efficiently compete with one another. In addition to the quality of light, C. tepidum and other green sulfur bacteria require sufficient amounts of hydrogen sulfide or other such electron donors for their unique photosynthetic pathways (Montesinos et al.1983).

Because of C. tepidum's ability to grow in low light and oxygen (during the early days of the earth when there were high levels of U.V light) researchers believe that this is where photosythesis may have its origins.