Green sulfur bacteria
| Chlorobia | ||||||||||||
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Green sulfur bacteria in a Winogradsky column |
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| Systematics | ||||||||||||
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| Scientific name | ||||||||||||
| Chlorobiaceae | ||||||||||||
| Copeland 1956 |
Green sulfur bacteria or chlorobia (from ancient Greek χλωρός, chlorós - the light green, fresh ) are unicellular, phototrophic , obligate anaerobic bacteria (domain Bacteria) with different external forms and without active movement. The family of the Chlorobiaceae forms a coherent and isolated group within the bacteria, which is why it is classified as an independent phylum in the biological system .
physiology
The representatives of the green sulfur bacteria carry out an anoxygenic photosynthesis with reduced sulfur compounds such as hydrogen sulfide (H 2 S) and thiosulfate (S 2 O 3 2− ), or with elemental sulfur as a reductant . Some species are also able to phototrophically oxidize hydrogen or divalent iron (Fe (II)) . In the case of the oxidation of hydrogen sulphide, oxidation only takes place up to the point of elemental sulfur , which is deposited outside the cells . This sulfur is very easily recognizable under the microscope as brightly radiating spheres that adhere to the bacterial cells from the outside. When the hydrogen sulfide deficiency sets in, the deposited sulfur is then oxidized to sulfate . Thiosulfate, which is only used by a few species, is also completely oxidized phototrophically to sulfate. The carbon fixation does not run like most phototrophs over the Calvin cycle , but on the reverse Krebs cycle . Green sulfur bacteria are generally photoautotrophic , but some can also assimilate organic substances with the help of light energy, so they are photoheterotrophic .
The photosystem of the green sulfur bacteria is located next to the adenosine triphosphate- forming system in their cytoplasmic membrane and contains bacteriochlorophyll a . Bacteriochlorophyll c , d or e , which are contained in so-called chlorosomes , serve as antenna pigments ; these are membrane sacs that are in contact with the inside of the cytoplasmic membrane. The cytoplasmic membrane with one part of the photosystem and the chlorosomes with another part form a functional unit.
Green sulfur bacteria occur in anoxic , hydrogen sulfide-containing water areas. Since they can use light very effectively for their photosynthesis, they also occur in areas with little light, including at greater depths than other phototrophic creatures.
Taxonomy and Phylogeny
Traditionally, a taxonomic classification is based on easily recognizable phenotypic characteristics. In addition to the cell morphology, z. B. used the formation of gas vesicles to differentiate between individual genera within the Chlorobiaceae . The brown chlorobia were distinguished from the green by the different composition of their pigments chlorophyll and carotenoids . A differentiation of subspecies was made on the basis of the use of thiosulfate as a photosynthetic electron donor . These classic distinguishing features, even if they are still used, do not match the current molecular biological results. They are therefore of little importance for the phylogenetic taxonomy of the Chlorobiaceae . According to the current state of research, the Chlorobiaceae are divided into the genera Chlorobium , Prosthecochloris and Chlorobaculum .
Chlorobium
The genus Chlorobium includes the green sulfur bacteria with rod-shaped or curved rod-shaped ( vibroid ) cell shape, which occur individually or in cell aggregates . Some species have gas vesicles. Photolithotrophic growth is possible under anoxic conditions and in the presence of light and reduced sulfur compounds (such as sulfide ) or elemental sulfur . Thiosulfate and hydrogen can also be used by some chlorobias. During the phototrophic oxidation of sulfide, elemental sulfur is temporarily deposited outside the cells. Sulphate is the end product of all reduced sulfur compounds. In the presence of sulfide and bicarbonate , simple organic substrates can be photoassimilated. In the rod-shaped species, the cell thickness (diameter) is about 0.6 to 1.2 µm and they do not need any salt to grow. The vibroid species have a cell thickness (diameter) of 0.3 to 0.9 µm and require low salt concentrations for their growth. Vitamin B12 is mandatory for many species within this genus .
Bacteriochlorophyll (BChl) c , d or e are used as photosynthetic pigments . The BChl c always occurs together with carotenoids of the chlorobactene group , while BChl e is always associated with the carotenoids of the Isorenierat group. Species described so far: Chlorobium limicola , Chlorobium ferrooxidans , Chlorobium phaeobacteroides , Chlorobium clathratiforme , Chlorobium phaeovibrioides , Chlorobium luteolum
Prosthecochloris
The bacterial species grouped together in the genus Prosthecochloris are spherical to ovoid ( ovoid ) or rod-shaped. They are single cells that are able to form unbranched prostheces . The cells reach a diameter of 0.5 to 0.7 µm and a cell length of 0.5 to 1.2 µm. Photolithhotrophic growth occurs under anoxic conditions in light and with reduced sulfur compounds (such as sulfide) or elemental sulfur. During the phototrophic oxidation of sulfide, elemental sulfur is temporarily formed outside the cells. The end product is always sulfate. BChl c and d or BChl e serve as photosynthetic pigments . BChl c and d are accompanied by carotenoids of the chlorobactene group. BChl e, however, from the Isorenierat group. Salt and vitamin B12 are an obligatory growth factor for these representatives of the green sulfur bacteria. Species described so far are: Prosthecochloris aestuarii and Prosthecochloris vibrioformis .
Chlorobaculum
The genus Chlorobaculum includes vibroid or rod-shaped representatives, some of which contain gas vesicles. The cell size is 0.3 to 1.1 µm. Vitamin B12 is mandatory for many types. Photolithhotrophic growth occurs under anoxic conditions in light and with reduced sulfur compounds (such as sulfide and thiosulfate) or elemental sulfur. During the phototrophic oxidation of sulfide, elemental sulfur is temporarily formed outside the cells. The end product is always sulfate. In the presence of sulfide and bicarbonate, simple organic substrates can be photoassimilated. Photosynthetic pigments are BChl c and d or BChl e . The green Chlorobaculum TYPES contain carotenoids of Chlorobacten group, the brown Chlorobaculum TYPES contrast, the Isorenierat group. So far the following species have been described: Chlorobaculum tepidum , Chlorobaculum limnaeum , Chlorobaculum thiosulfatiphilum , Chlorobaculum parvum and Chlorobaculum chlorovibrioides .
Consortia
Some green sulfur bacteria form aggregates with chemoheterotrophic bacteria that are beneficial for both partners ( mutualism ), microbial consortia . An example of this is “ Chlorochromatium aggregatum ” (since we are dealing with two different living beings, this name is not a taxon !): Several individuals of green sulfur bacteria are arranged around a long flagellated heterotrophic bacterium. The heterotrophic organism is a sulfate-reducing bacterium that reduces sulfate to sulfide.
Web links
- JP Euzéby: List of Prokaryotic Names with Standing in Nomenclature - Family Chlorobiaceae