Phylum Chlorobi (Green sulfur Bacteria)

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Taxonomy

  • Phylum Chlorobi
    • Class Chlorobia
      • Order Chorobiales
        • Family Chlorobiaceae
          • Genus Chlorobium
          • Genus Anacalochloris
          • Genus Chlorobaculum
          • Genus Chloroherpeton
          • Genus Pelodictyon
          • Genus Prosthecochloris

General characteristics of the Chlorobi

Diversity

This phylum contains only a small number of relatively closely-related species. Most are poorly characterized, and the relationships between them are uncertain. This phylum is related to the phylum Bacteroidetes, and is often grouped with it.

Metabolism

These organisms are strict photoautolithotrophs, using hydrogen or sulfide as the electron donor to generate NADH for carbon fixation via the reverse TCA cycle. Energy is generated by cyclic photophosphorylation. Most are also capable of fixing nitrogen, but cannot assimilate sulfate, and so require sulfide as a sulfur source even when using hydrogen as the electron donor for CO2 fixation. Elemental sulfur is the product of sulfide oxidation, and accumulates as extracellular globules.

Morphology

Cellular morphology is diverse in this group, with short chains of rods being the most common. Some produce gas vacuoles, and they are otherwise non-motile except Chloroherpeton, which is a unicellular gliding filament. Cells contain internal membranous chlorosomes along the inside of the cytoplasmic membrane; these house high concentrations of the accessory photopigment bacteriochlorophyls c, d, or e, and carotenoids. Unlike cyanobacterial antenna complexes, in which the pigments are associated with proteins, these pigments in Chlorobi and Cloroflexi are not bound by protein, existing nearly in a solid state, and transfer light energy in very efficiently. Reaction center chlorophyl is bacteriochlorophyl a, which is located in the cytoplasmic membrane. Sulfur granules are produced externally.

Chlorosomes
Chlorosomes in Chlorobium tepidum, cross-section TEM : unattributed from cyanobase: http://bacteria.kazusa.or.jp/cyanobase/Chlorobium/about.html

Habitat

Chlorobi are commonly found in anaerobic, sulfide-rich freshwater and marine sediments. Because they require less light than other phototrophic organisms (about 1/4th as much as typical green or purple Bacteria), they can deeper in the more anoxic zones of their environments, and so are less conspicuous than the other green Bacteria. They can be conspicuous, however, in meromictic (permanently stratified) lakes, in which they form a brown or green layer in the water column beneath the red or purple layer of purple Bacteria at the chemocline (the interface between the oxygenated surface water and the denser anaerobic sulfide-rich deep water).

Symbiotic consortia

Chlorobium consortium
Symbiotic Chorobium consortium : Croome RL, Tyler PA.. J Gen Microbiol 1984; 130:2717–23.

Some members of this phyla participate in a symbiosis with an uncultivated rod-shaped heterotroph (beta-proteobacteria in known cases). The motile heterotroph is bound by many (about a dozen) non-motile Chlorobi; the heterotroph is provided resources by the green bacteria, which in turn are provided motility by the heterotroph. The two cell types divide synchronously, and the heterotroph swims phototactically, attracted by light of the absorption maximum of bacteriochlorophyl c (740nm) in the phototroph; the cells are in close communication, but the details of this are unknown. The green bacteria can often be cultivated alone (although they are distinct from the free-living species), but the heterotrophs have not. These consortia are common in meromictic lakes, and provide one of the best examples of specific bacterial:bacterial symbiosis.

Example species

Chlorobium limicola

C. limicola
Chlorobium limicola
: Truper & Pfennig pp3588 The Prokaryotes

C. limicola strains are sometimes found in symbiosis with motile beta-proteobacteria, as described above, but most often are free-living sediment dwellers. They grow a short chains of rod-shaped cell, each 0.7-1.1 x 0.9-1.5μm, forming large sulfur globules. This species does not produce gas vaculoes. Cultures are green and the predominant photopigment is bacteriochlorophyl c or d.

Pelodictyon phaeoclathratiforme

Pelodictyon
Pelodictyon phaeoclathratiforme : unattributed from the JGI genome web site :
http://genome.jgi-psf.org/draft_microbes/pelph/pelph.home.html

P. clathratiforme is common in meromictic lakes, and is composed of trapezoidal rod-shaped cells ca. 1 x 2μm that branched chains, circles, and three-dimensional nets. Although non-motile, it produces gas vacuoles that allow it to control its vertical location in the water column. The accessory pigment is primarily bacteriochlorophyl e. Requires very highly reduced conditions for growth, but can use elemental sulfur or thiosulfate as well as sulfide for reducing power for carbon fixation. Unlike most members of this phylum, it is capable of using acetate to grow photoheterotrophically.