Green nonsulfur (gliding) bacteria

1. Structure
   1. Form flexible filaments; also called the green flexibacteria
   2. Gliding mobility
   3. Most do not have gas vesicles
   4. Form chlorosomes
   5. May have an Intramembrane system reminiscent of that in proteobacteria (e.g., Oscillochloris)
2. Photosynthesis
   1. Anoxygenic phototrophs
   2. Bacteriochlorophylls c, or d, with small amounts of chlorophyll a
      1. Antenna pigments are Bchl c arranged in chlorosomes
      2. Heliothrix has Bchl a only
   3. Carotenoids are gamma- and beta- carotene (Isorenieratene and chlorobactene series)
   4. Reaction center P840, with Bchl a
      1. Carotenoids are not present in reaction center
      2. Reaction center composed of 2 subunits (homodimer, lacks H subunit)
   5. Electron acceptor is a quinone, with a redox potential of -0.15 V
      1. PS-II like, (similar to that in proteobacteria)
   1. Menaquinone rather than ubiquinone as secondary electron acceptor
   6. Like proteobacteria, reduction of NAD+ to NADH requires reverse electron flow
   7. Lack rubisco
      1. Carbon fixation by reverse TCA (Tricarboxylic acid cycle)
3. Classification
   1. Closely related to nonphotosynthetic gliding bacteria (e.g., Herpetosiphon), and perhaps to the thermophile Thermomicrobium
   2. Thermus thermophilus may also be related, but there are conflicting data on the relationships among these groups.
   3. Despite similarities in pigmentation, only distantly related to green sulfur bacteria according to SSU rRNA data
4. Representative Organisms
   1. Chloroflexus auranticus is the best studied
   2. Oscillochloris
      1. Large, filamentous, and has invaginations of the cell membrane
      2. Can be mistaken for a cyanobacterium
      3. Distinguish in field samples by fluorescence; Oscillochloris fluoresces in the infrared, cyanobacteria in both infrared and visible
5. Ecology
1. Most strains are facultatively aerobic; when living aerobically they are not photosynthetic, and live heterotrophically.
2. Chloroflexus was originally isolated from hot springs
   1. Temperature optimum at ~55°C (maximum ~70°C)
3. In hot springs with H2S, will form mat without cyanobacteria, and may be green in color
   1. Photoautotrophic, with sulfide as the electron donor
   2. These strains are obligately anaerobic, and although capable of photoautotrophy, grow best as photoheterotrophs
4. Chloroflexus auranticus is found in alcaline hot springs (pH 5.5-10), where it occurs in an orange mat below a layer of cyanobacteria
   1. Under these conditions, Chloroflexus is probably living photoheterotrophically, dependent upon the cyanobacteria for fixed carbon
   2. These strains are facultatively aerobic
5. Temperature limit is higher than that for cyanobacteria, and this may govern distribution
6. Chloroflexus-like organisms have also been found in marine and hypersaline environments, i.e., in intertidal sand flats
   1. Gliding mobility is advantageous in this type of environment
7. Do not fix nitrogen

Required Reading: M&K Chapters 2 & 3

Supplementary Reading:

The Prokaryotes; chapters 13 (for all phototrophic prokaryotes), and 206

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