Anoxygenic Photosynthetic Bacteria: Proteobacteria

  1. Proteobacteria ("purple bacteria")
    1. Probably the largest and most physiologically diverse group of bacteria
  2. Structure
    1. Membrane system infolded from cytoplasmic membrane
    2. Sulfur granules deposited inside (Chromatium) or outside of cells (Ectothiorhodospira)
      1. This is evolutionarily, geologically, and environmentally important
    3. May have gas vesicles, flagella, or gliding
    4. May have stalks, or appendates.
  3. Photosynthesis in proteobacteria
    1. anoxygenic phototrophs
    2. Bacteriochlorophylls a and b
    3. Reaction center P870, with Bchl a
      1. Three subunits, L, M, and H
      2. Four Bchl a (special pair+ two more), 2 bacteriopheophytin, 2 quinone, 2 cartotenoid
    4. Carotenoids in the spirilloxanthin, okenone, or rhodopinal groups
    5. + Quinone-type photosystem (PSII)
      1. Primary electron acceptor is quinone, with a reduction potential of about -0.15
      2. This is more positive than required to form NADH
      3. NADH production requires reverse electron flow
    6. Electron donor H2S, H2, or sometimes other compounds
    7. + rubisco (either or both form I and II)
    8. Carbon fixation via Calvin cycle
      1. Fix carbon with rubisco (Ribulose-1,5-bisphosphate carboxylase/oxygenase)
        1. Substrates are ribulose-5-bisphosphate (5 carbon) and CO2 (or oxygen)
        2. Product is 2 ea 3-Phosphoglycerate (3 carbon)
      2. Must regenerate substrate, but without 1-carbon chemistry
      3. Can also fix carbon by reverse TCA
  4. Classification
    1. Defined on the basis of rRNA sequence
    2. Classically, the photosynthetic proteobacteria were placed in two families, the Chromatiaceae and the Ectothiorhodospiraceae.
    3. In addition to anoxygenic photoheterotrophs, proteobacteria include chemolithotrophs and chemoorganotrophs
      1. lithotrophs are able to fix carbon, organotrophs are not, and require fixed carbon as a nutrient
    4. Five subgroups (alpha-, beta-, etc.)
      1. Photosynthesis (anoxygenic photoautotrophy) is found in alpha, beta, and gamma proteobacteria
        1. Alpha proteobacteria
            Rhodospirillum, Rhodobacter
        2. Gamma proteobacteria are paraphyletic with respect to beta proteobacteria
        3. Beta and gamma proteobacteria
          1. Ectothiorhodospiraceae are gamma-proteobacteria, but are distinctive because they deposit sulfur on the outside of their cells. Thus in classical taxonomy, they were given their own family.
            1. Ectothiorhodospira
          2. Chromatiaceae deposit sulfur inside their cells
            1. Chromatium
      2. The purple sulfur bacteria are generally beta or gamma proteobacteria, utilize sulfide as an electron donor and can tolerate high concentrations of sulfide. However, extremely high levels of sulfide are toxic even to sulfur bacteria.
      3. The purple nonsulfur bacteria are primarily alpha proteobacteria
        1. Purple nonsulfur bacteria are able to utilize sulfide as an electron donor, but cannot tolerate it at high concentration
      4. Alpha, beta, and gamma gropus also include chemolithotrophs and chemoorganotrophs
      5. Delta- and epsilon- proteobacteria are not photosynthetic; all are chemoorganotrophs
      6. Significant horizontal gene transfer may have occurred among proteobacterial groups
    5. Some proteobacteria (e.g., Methylobacterium) produce bacteriochlorophyll, but are not photosynthetic
  5. Ecology
    1. Photosynthesis is generally facultative
    2. The implications of using H2S as an electron donor
    3. Habitats where H2S is available

Required Reading: C&M, Chapter 2; finish Chapter 3

Supplementary Reading:

The Prokaryotes; chapters 170 and 171

Blankenship, R., M.T. Madigan, and C. Bauer (eds.) 1996. Anoxygenic Photosynthetic Bacteria. Kluwer, Dordrecht & Boston.