A Microbial Biorealm page on the genus Rhodobacter sphaeroides
Higher order taxa
Bacteria; Proteobacteria; Alphaproteobacteria; Rhodobaterales; Rhodobacteraceae; Rhodobacter; sphaeroides
Description and significance
Rhodobacter sphaeroides is a rod-shaped bacterium that has an unusual single flagellum. Unlike other bacteria, the flagellum of R. sphaeroides is composed of a straight hook and hook-associated body (HBB) complexes. Due to the shape of the flagellum, it can only rotate in a clockwise direction with a fast, slow or stop mechanism. Rhodobacter sphaeroides also has a complex genome and a versatile metabolism that cannot be found in most other microorganisms. R. sphaeroides is a gram-negative purple nonsulfur phototrophic bacterium belonging to α-3 Proteobacteria. Like other species of Rhodobacter, it is a metabolically diverse organism able to grow in a wide range of lifestyles including aerobic, anaerobic, photosynthetic, and diazotrophic growth modes. It responds to environmental changes by undergoing both physiological and morphological adaptations. R. sphaeroides is also the first organism that was found to possess multiple chromosomes. This discovery was made by Suwanto and Kaplan. Having two chromosomes gives the R. sphaeroides an advantage in adapting to various conditions.
Rhodobacter sphaeroides contains two distinct circular chromosomes, CⅠ(3,046kb) and CⅡ(914kb), and five endogenous plasmids (450kb). Thus, the total genome size is about 4,400kb and G+C content of its genome is 67.3 mol% and 65.7 mol% for CⅠ and CⅡ, respectively. It is revealed that a number of essential duplicate copies of R. sphaeroides are distributed between the two chromosomes. For example, one ribosomal RNA (rRNA) operon (rrnA) is found on CⅠ, while two rRNA operons (rrnB and rrnC) are on CⅡ. The difference between two chromosomes makes R. sphaeroides unique in its metabolic flexibility. Recent study found that CⅠ of R. sphaeroides has more coding abilities and conserved sequences than CⅡ. Since CⅡ is a rapidly evolving copy, it makes R. sphaeroides possible to grow in various conditions. In addition, genes on CⅡ encodes a various set of functions that are unusual for this photosynthetic organism—genes that are involved in proteins synthesis, amino acid biosynthesis, fatty acid metabolism, transcriptional regulation, energy metabolism, and structural components.
Cell Structure and Metabolism
As mentioned above, Rhodobacter sphaeroides is highly adaptive to various environmental conditions. In oxygenic conditions, it uses aerobic respiration for energy generation and the organism is similar to a normal gram-negative cell envelope structure. Under anoxygenic conditions, in the light or dark, R. sphaeroides respires anaerobically but, in the dark, it uses dimethyl sulfoxide (DMSO) or trimethylamine N-oxide (TMAO) as the terminal electron acceptor. Under aerobic-to-anaerobic shift conditions, R. sphaeroides changes morphologically by synthesizing the intracytoplasmic membrane (ICM) through an invagination process. The ICM possesses the photosynthetic apparatus and the structural components required for light energy capture, electron transport, and energy transduction.
R. sphaeroides is well-known for its diverse biochemical processes including metal reduction, nitrogen and carbon fixation, and also production of hydrogen as a source of energy. These processes are coupled to photosynthetic apparatus of the organism.
Rhodobacter sphaeroides is found in various conditions, especially in organic-rich habitat.
This organism is not pathogenic.
Application to Biotechnology
1) Production of indole Under anoxygenic conditions, R. sphaeroides OU5 is used to mediate production of indole and its derivatives from anthranilate. Indole is an aromatic compound that can be useful for growth and production of valuable compounds. It is used as the main commercial source of the material that benefits production of paddy crop and plant hormone.
2) Production of ZnS nanoparticles Because R. sphaeroides grow diverse conditions and is resistant to heavy metals, it is used to prepare ZnS nanoparticles. ZnS nanoparticle is highly industrial material in IR optical devices.
3) Production of Rhodethrin Rhodethrin can be isolated when R. sphaeroides OU5 is grown on a L-tryptophan as sole source of nitrogen in the absence of oxygen. The metabolite has phytohormonal activity and phytotoxicity against cancer cell lines and also inhibitory activity of cyclooxxygenase-2.
4) Extraction of carotenoids Carotenoids are naturally occurring compounds found in photosynthetic bacteria. Studies found that carotenoids having antioxidant activity and provitamin A function are able to inhibit various types of cancer and protect from cardiovascular disease and age-related macular degeneration.
1) Energy trapping in photosynthesis Since R. sphaeroides is a photosynthetic bacterium in diverse environmental conditions, how energy flows through complexes has been an interest to researchers. The reaction center (RC) from Rhodobacter sphaeroides contains bacteriopheophytin (BPhy), bacteriochlorophyll (BChl), and bacteriochlorophyll dimer (P). The energy in the RC is transferred from BPhy via BChl to P, taking about 100 to 200 femtoseconds (fs). Researchers suggest that there might have correlated effects between proteins that result in long-lived electronic coherence in RC, allowing the energy to transfer rapidly into space. And then, energy can be trapped efficiently.
2) Studies on optimal conditions of H2 production under photoheterotrophic condition. Since hydrogen is expected to be an energy source in the future, researchers have made efforts to find efficient ways ofproducing hydrogen. Iron is used because it functions as a cofactor for proteins responsible for energy metabolism. Studies found that concentration of Fe2+ have a greater effect on production of hydrogen by R. sphaeroides. With increases in concentration of Fe2+, hydrogen production increases linearly, indicating they have direct relationship.
[Sample reference] Takai, K., Sugai, A., Itoh, T., and Horikoshi, K. "Palaeococcus ferrophilus gen. nov., sp. nov., a barophilic, hyperthermophilic archaeon from a deep-sea hydrothermal vent chimney". International Journal of Systematic and Evolutionary Microbiology. 2000. Volume 50. p. 489-500.
Edited by student of Rachel Larsen