Chlorobium chlorochromatii

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A Microbial Biorealm page on the genus Chlorobium chlorochromatii

Classification

Higher order taxa

Domain: Bacteria; Phylum: Chlorobi; Class: Chlorobia; Order: Chlorobiales; family: Chlorobiacease

Species

Genus: Chlorobium; species: Chlorobium chlorochromatii

Description and significance

C. chlorochromatii is a symbiotic green sulfur bacterium that is usually found in fresh water lakes around the world. It can be found in low-temperature, anaerobic aquatic environments where there is low sulfide and light conditions (1). It is composed of a colorless, rod shaped, central bacterium that is motile surrounded by about twenty green sulfur epibionts which are nonmotile (4). Within the colorless central bacterium, chlorosomes are present which harvest the light into energy. C. chlorochromatii was isolated from an enrichment culture of the phototrophic consortium Chlorochromatium aggregatum, which was changed in 2005 to C. chlorochromatii (8).

Genome structure

C. chlorochromatii consists of a circular chromosome that is 2,572,079 nt in length. The genomes of green sulfur bacteria range between 2 and 3.3 Mb and the size range for genomes of the central rod is 2 to 6 Mb. Thus, the anticipated total genome size for C. aggregatum is less than 10 Mb. This circular chromosome encodes for genes that are involved with numerous processes that are essential to the cell including protein synthesis and also the production of enzymes involved in metabolic pathways such as the Krebs Cycle (4).

Cell structure and metabolism

The phototrophic consortium C. aggregatum consists of a colorless central rod-shaped bacterium which is surrounded by about 20 green-pigmented epibionts, meaning it lives on the body surface of this central rod. The metabolic interactions between the partners in phototrophic consortia are still unknown. However, all green sulfur bacteria studied are obligate anoxygenic photolithotrophs, which tells us two very important factors about the metabolic pathways involved; water is not used as the electron donor (hence no oxygen is produced) and inorganic compounds are used. Inorganic compounds such as hydrogen, sulfide, elemental sulfur or polysulfide, and sometimes thiosulfate can be used as electron donors for carbon dioxide fixation by the reverse TCA cycle.

Most strains show a very limited ability to utilize simple organic compounds such as acetate, pyruvate, and succinate. C. aggregatum direct their movements according to 2-oxoglutarate, a compound which was found to be an essential ingredient of the enrichment medium for this consortium (2). Green sulfur bacteria can be maintained in highly stable co-cultures because of a closed sulfur cycle in which each sulfur atom cycles many times. Due to low sulfide concentrations in the natural environments in which phototrophic consortia are usually found, it has been suggested that a syntrophic sulfur cycle may also exist between the two partner organisms in these consortia. This type of arrangement would ensure a continuous supply of reduced sulfur compounds for energy and reducing power generation by the epibionts. It is also possible that the epibionts may provide reduced carbon or nitrogen to the motile central rod. Sulfide has been shown to stimulate electron transport in the epibionts. Recent molecular analysis shows that the central rod of phototrophic consortia are members of the b-subgroup of the proteobacteria.

Ecology

C. chlorochromatii are found in fresh water lakes in the chemocline, which is the region between the oxygen rich water and the anaerobic water. They usually thrive where there are low sulfide concentrations and low levels of light. These two factors usually are the growth limiting factors in most photosynthetic bacteria. It has also been found that there is an optimal ratio of sulfide to light for growth in phototrophic consortia. Although there are also purple sulfur bacteria, they live in different niches from the green sulfur bacteria due to the wavelength of light that green sulfur bacteria utilize. Purple sulfur require more light because they have less pigment (1). In the environment, C. chlorochromatii are known to represent important components of the sulfur and carbon cycles in freshwater environments and make up nearly two-thirds of the biomass found in the chemocline.

Current Research

One of current research project was to be able to detect green sulfur bacteria by using an oligodeoxynucleotide probe (GSB-532). The development of this probe allowed for the highly specific detection of green sulfur bacteria in their natural environment as well as rapid screening of natural bacterial communities. The probe GSB-532 allowed for the “phylogenetic affiliation of the epibionts in Chlorochromatium aggregatum and Pelochromatium roseum for the first time.”(7)

Another interesting study done on C. chlorochromatii is the diversity of epibionts involved in the phototrophic consortia. To date, there are seven different morphological types of such consortia and two immotile associations involving green sulfur bacteria are known. By using a culture-independent method, different types of phototrophic consortia were isolated from fourteen freshwater environments, and partial 16S rRNA gene sequences of the green sulfur epibionts were determined. Nineteen different types of epibionts were detected in the different lakes whereas the epibionts within one geographic region were very similar. None of the epibiont 16S rRNA gene sequences have been detected so far in the green sulfur bacteria, suggesting that the interaction between epibionts and central bacteria is an obligate interaction. The present study thus demonstrates that there is great diversity and nonrandom geographical distribution of phototrophic consortia in the natural environment (3).

Another interesting research that involved green sulfur bacteria was the population analysis obtained from the chemocline in Lake Cadagno, Switzerland. Purple sulfur bacteria were the numerically most prominent phototrophic sulfur bacteria in samples obtained from 1994 to 2001 and represented between 70 and 95% of the phototrophic sulfur bacteria. All populations of purple sulfur bacteria showed large fluctuations in time with populations belonging to the green sulfur bacteria. During the last 2 years of the analysis, there was a shift in dominance from purple sulfur bacteria to green sulfur bacteria in the chemocline. At this time, numbers of purple sulfur bacteria had decreased and those of green sulfur bacteria increased and C. clathratiforme represented about 95% of the phototrophic sulfur bacteria. This major change in community structure in the chemocline was accompanied by changes in the cloudiness of water, sulfide concentrations, and light intensity. Overall, these findings suggest that a disruption of the chemocline in 2000 due to very cold winters may have altered environmental niches and populations in subsequent years (5).

References

1. Chapin, B., Denoyelles F., Gaham, D.W., Smith, V.H. “A deep maximum of green sulphur bacteria (‘Chlorochromatium aggregatum’) in a strongly stratified reservoir.” Freshwater Biology (2004) 49, 1337–1354.

2. Fröstl, J. M. and Overmann, J. 1998. Physiology and tactic response of the phototrophic consortium “Chlorochromatium aggregatum.” Arch. Microbiol. 169: 129-135.

3. Glaeser, J., Overmann, J. “Biogeography, Evolution, and Diversity of Epibionts in Phototrophic Consortia.” Applied and Environmental Microbiology, Vol. 70, No. 8. American Society for Microbiology. 4821-4830.

4. Kanzler BE, Pfannes KR, Vogl K, Overmann J. Molecular characterization of the nonphotosynthetic partner bacterium in the consortium "Chlorochromatium aggregatum.” Applied and Environmental Microbiology, Vol. 71, No. 11. American Society for Microbiology. 7434-7441

5. Tonolla, M., Peduzzi, R., and Hahn, D. “Long-Term Population Dynamics of Phototrophic Sulfur Bacteria in the Chemocline of Lake Cadagno, Switzerland.” Applied and Environmental Microbiology, Vol. 71, No. 7. American Society for Microbiology. 3554-3550.

6. Overmann, J. and van Gemerden, H. 2000. Microbial interactions involving sulfur bacteria: implications for the ecology and evolution of bacterial communities. FEMS Microbiol. Lett. 24: 591-599.

7. Tuschak, C., Glaeser, J. and Overmann, J. Specific detection of green sulfur bacteria by in situ hybridization with a fluorescently labeled oligonucleotide probe. Arch. Microbiol. 171: 265-272.

8. Vogl, K., Glaeser, J., Pfanees, K.R., Wanner, G., and Overmann, J. “Chlorobium chlorochromatii sp. nov., a symbiotic green sulfur bacterium isolated from the phototrophic consortium "Chlorochromatium aggregatum".

Edited by student of Rachel Larsen and Kit Pogliano