• Cells are rod shaped.
• 0.6-2.5 um x 0.6-5.0 um in size.
• Cells stain gram positive.
• Only species with flagella are motile.
• Colonies may appear red, green, or yellowish brown.
• Internal photosynthetic membranes, known as lamellae, may be vesicular, tubular, or membranous (Figure 1).
• Reproduction is characterized by "budding" (Figure 2).
• Bacteriochlorophyll necessary for photoautotrophic  growth consists of Bchl a and Bchl b.
• Species that reduce sulfide produce sulfate, sulfur, and thiosulfate. 
• Restriction endonuclease activity of the RsrII enzyme yields a staggered 5' overhang upon cleavage of the 7 bp recognition site.

Figure 1:  Internal photosynthetic membranes of 
species within the genus Rhodopseudomonas. 

 

2. TAXONOMIC DESCRIPTION         Rhodopseudomonas is a genus within the family Athiorhodaceae.  It consists of 8 described species for which 7 strains have been identified (Str. 37, 6, x, 18, Morita, 2.8.2, and 2.10.1).  These purple nonsulfur bacteria may grow chemotrophically under microaerobic or aerobic conditions, although most species display photoheterotrophic growth under anaerobic conditions.  Hydrogen, thiosulfate, or sulfide generally serve as the electron donor in these instances.  Growth is optimal at 25-35oC and 6.5-7.0 pH.  In addition, the photosystems associated with photoautotrophic growth operate at redox potentials around 450 mV.

Phase contrast image of  R. acidophila.

     Although chemotrophy may be less favorable for bacterial growth, facultative chemotrophic growth is an adaptation that enables Rhodopseudomonas to survive in the absence of light.²  However, the degree of development of the intercytoplasmic membrane (ICM) primarily depends on light intensity and oxygen availability (Figures 3 and 4).  Despite observation of assimilatory nitrate and nitrite reduction by R. sphaeroides ³,  research has shown that the growth of Rhodopseudomonas spp. is not significantly affected by the replacement of oxygen with nitrate in either light or dark conditions.⁴ 
 

(A)       (B)             Figure 3:  (A)  Section of R. sphaeroides after semi-aerobic culture in the dark.  Note the absence  of distinguishable chromatophores and presence of numerous poly-beta-hydroxybutyrate granules.  (B) Section of R. sphaeroides grown aerobically at moderate light intensity.  The proliferating vesicles  within and along the cytoplasmic membrane indicate areas of chromatophore continuity.4

     Isolation of rhodopseudomonads is usually accomplished by the agar shake method.   Presence and numbers of Rhodopseudomonas have been determined using a serial dilution technique following the preparation of the agar shake cultures.⁶  Difficulty with colony separation has been documented, so second stage enrichment may be necessary to promote the dominance of one species, even among as few as 2 or 3 species in a sample.  Streaking on the surface of agar plates and exposing them to light will separate the more oxygen-tolerant varieties (e.g. R. palustris).  In most instances, basal medium and yeast extract together provide sufficient conditions for enrichment and subsequent isolation, given appropriate ionic concentrations and pH.  Some strains of Rhodopseudomonas behave like true anaerobes when first isolated, and can be grown in strictly anaerobic conditions.  Through repeated transfers, these organisms physiologically evolve a tolerance for aerobic conditions.
     The distribution of several Rhodopseudomonas spp. is restricted by light and oxygen levels.  Since light intensity is the primary limiting factor, these organisms can generally be found in the higher in the water column of several aquatic habitats.  Very little information concerning the ecology of these organisms is available.  Much of the current research involving Rhodopseudomonas has focused on light-harvesting complexes and reaction center complexes at the biochemical and molecular levels. ⁷

4. ADDITIONAL SOURCES OF INFORMATION

¹ Holt, J.G., N.R. Krieg, P.H.A. Sneath, J.T. Staley, and S.T. Williams. 1994. Bergey's Manual of Determinative Bacteriology.  Ninth Edition.  Baltimore, Maryland:  Williams and Wilkins.  pp. 359.

² Butow, B. and Bergstein-Ben Dan, T.  1991.  Effects of growth conditions on acetate utilization by Rhodopseudomonas palustris isolated from a freshwater lake.  Microb. Ecol.  22(3):317-329.

³ Yoch, D. C.  1978.  "Ch. 34:  Nitrogen Fixation and Hydrogen Metabolism by Photosynthetic Bacteria."  In:  R. K. Clayton and W. R. Sistrom, Eds.  The Photosynthetic Bacteria.  New York:  Plenum Press.  pp.657-676.

⁴ Remsen, C. C.  1978.  "Ch. 3:  Comparative Subcellular Architecture of Photosynthetic Bacteria."  In:  R. K. Clayton and W. R. Sistrom Eds.  The Photosynthetic Bacteria.  New York:  Plenum Press.  pp.31-60.

⁵ Whittenbury, R.  1971.  "Enrichment and Isolation of Photosynthetic Bacteria."  In:  D. A. Shapton and R. G. Board, Eds.  Isolation of Anaerobes.  London, England:  Academic Press.  pp. 241-249.

⁶ Butow, B. and Bergstein-Ben Dan, T.  1992.  Occurrence of Rhodopseudomonas palustris and Chlorobium phaeobacteriodes blooms in Lake Kinneret.  Hydrobiologia.  232(3):193-200.
 
⁷ Jones, C. W.  1982.  Bacterial Respiration and Photosynthesis.  Surry, Kentucky:  Thomas Nelson and Sons, Ltd.  P.68

Protein coding in R. blastica -- Studies of the protein encoded by URF 6 of the atp operon in Rhodopseudomonas
     blastica: structure determination, physiological function and occurence in other organisms.

ASU Photosynthesis Center