Rhodospirillum rubrum
Amanda Withers

Rhodospirillum is a genus of photosynthetic bacteria of the family Rhodospirillaceae. Their cells are generally spiral-shaped, polarly flagellated and contain vesicular, lamellar of stacked photosynthetic membranes (Singleton and Sainbury). They range from three to ten micrometers in length and one-half to one and one-half micrometers in width. Cells divide by binary fission. One of the type species of this genus is Rhodospirillum rubrum, a purple nonsulfur bacteria. This bacteria falls under the Alpha subdivision of the kingdom Proteobacteria (Brock pg.456).

The morphology of Rhodospirillum rubrum is shown clearly on the cover, a figure from The Prokaryotes (Balows pg.2146). This purple-red microbe contains vesicular photosynthetic membranes and a cell width of 0.8 to one micrometer. R. rubrum is a gram negative bacteria containing both saturated and unsaturated fatty acids. Its major carotenoids (pigments) are rhodovibrin and spirilloxanthin. Biotin is a required growth factor for R. rubrum and its GC content is 63.8 to 65.8 percent (Balows pg.2148).

Unique in many ways, it grows both aerobically with oxygen or anaerobically using light for its energy metabolism. As a phototroph, R. rubrum can grow autotrophically or heterotrophically. It does not produce oxygen as a by-product of photosynthesis, thus it is an anoxygenic phototroph (Michigan State, Brock). Extracellular elemental sulfur is the final oxidation product in R. rubrum, (Balows pg.2151). This bacterium has been used in many studies, for example, to study radiation resistance of pigmented bacteria and nitrogen fixation.

This organism contains chlorophyll b, which is different than chlorophyll a found in plants. Chlorophyll b distinguished by a lower absorption spectra, absorbs maximally at 660 nm rather than at 680 nm, (Brock pg.577). Anoxygenic phototrophs such as R. rubrum can contain several bacteriochlorophylls, and most purple bacteria have bacteriochlorophyll a, which absorbs maximally between 800 and 925 nm. Organisms with many different types of chlorophylls are at an advantage, because they can use more of the energy of the electromagnetic spectrum, (Brock pg.577).

In prokaryotes, photosynthetic pigments are integrated into internal membrane systems that arise from invagination of the cytoplasmic membrane (purple bacteria), (Brock pg.579). The carotenoids found in R. rubrum not only give the microbe its purple-red color, they also help gather light energy for photosynthesis (Michigan State). Carotenoids function as accessory pigments, play a photoprotective role against bright light and transfer energy to the reaction center to be used in photophosphorylation (Brock pg.581).

Rhodospirillum rubrum and other purple nonsulfur bacteria can be found in natural setting such as pond water, mud or a sewage sample (Brock pg.459). Phototrophic purple nonsulfur bacteria are used in sewage treatment processes, for biomass production as a source of animal food or agricultural fertilizer, and production of molecular hydrogen by evolution from nitrogenase. They may be used also as a source of cell-free systems performing photosynthesis and ATP formation, and for the production of vitamins and other organic molecules (Balows pg.2151).

Characteristics

(1) gram-negative bacteria (all Proteobacteria are)

(2) purple color - from cartenoid pigments rhodovibrin and spirilloxanthin

(3) nonsulfur bacteria - can use sulfide as an electron donor for the reduction of carbon dioxide, but not at high concentrations like sulfur bacteria, See figure 13.7 pg. 460 Brock.

(4) cell morphology - spiral shaped cells

- polarly flagellated, See figure 3.47 p. 80 Brock.
- length 3-10 micrometers
- width 0.8-1 micrometer
- vesticular photosynthetic membranes
See figure 15.12 pg. 583 Brock.
(5) carbon and energy metabolism
- grows aerobically with oxygen (chemotroph)
- also grows anaerobically using light (phototroph)
- can grow auto- and heterotrophically
(6) locations and uses
- studies in radiation resistance and nitrogen fixation
- found in mud, pond water, and sewage samples
- used for:
- sewage treatment processes
- biomass production
- animal food (fish & chicken), agricultural fertilizer
- molecular hydrogen production (evolution from nitrogenase)
- perform photosynthesis and ATP formation
- produce vitamins and other organic molecules

References

Balows, A., Truper, H.G., Dworkin, M., Harder, W., and Schleifer, K.H. (1992);The Prokaryotes, second edition. Springer-Verlag Inc., New York.

Brock, T.D., Madigan, M.T., Martinko, J.M. and Parker, J. (2000) Biology of Microorganisms, ninth edition. Prentice Hall, New Jersey.

Michigan State University. (web page) Microbe Zoo, Water World.

O'Leary, William. (1989) Practical Handbook of Microbiology. CRC Press Inc., Boca Raton, Florida.

Singleton, P. and Sainsbury, D. (1987) Dictionary of Microbiology and Molecular Biology, second edition. John Wiley & Sons Inc., New York.

 

*Disclaimer - This report was written by a student participaring in a microbiology course at the Missouri University of Science and Technology. The accuracy of the contents of this report is not guaranteed and it is recommended that you seek additional sources of information to verify the contents.

 

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