Heliobacteria

1. Structure
   1. Gram positive cell wall
   2. Neither intercytoplasmic membrane systems nor chlorosomes are present
   3. Most species can form spores
   4. Flagellar motility in most, Heliobacterium chlorum has gliding mobility
   5. Most are rods, some are curved rods or spirilla
2. Photosynthesis
   1. Bacteriochlorophylls g in the reaction center (P798)
   2. Carotenoids - neurosporene
   3. Major absorbtion peak at 790nm -- unique to heliobacteria
   4. Photosystem-I like photosystem (Fe-S type)
   5. Homodimeric reaction center
   6. Electron acceptor is FeS, with a redox potential of -0.5 V
      1. This is adequate to reduce ferredoxin, and thus can reduce NAD+ to NADH directly (unlike proteobacteria).
   7. Photoheterotrophs
      1. No carbon fixation
      2. Lack rubisco; No Calvin cycle
      3. Do not fix carbon by reverse TCA
3. Classification
   1. Gram positive bacteria (Firmicutes)
   2. Small group, only known photrophy in gram positives
   3. Three genera, Heliobacterium (3 spp.), Heliobacillus (1 sp.), and Heliophilum (1 sp.)
4. Representative Organisms
   1. Heliobacterium chlorum
5. Ecology
   1. Obligate anaerobe, very sensitive to oxygen
   2. Photoheterotrophic
   3. Cannot tolerate sulfide
   4. All known species can fix nitrogen
   5. Isolated from soil samples, apparently not aquatic
      1. Dry paddy soils
      2. Same soils, if moist, yield proteobacteria
      3. Rarely, if ever, isolated from aquatic habitats
   6. Typical habitat is rice paddy soil
   7. Absorbtion spectrum suggests heliobacteria can grow under a cyanobacterial mat
   8. Natural history is almost completely unknown

Required Reading:

Supplementary Reading:

The Prokaryotes; chapter:

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