Heliobacteria

Marie Asao, The Ohio State University, Columbus, Ohio, USA
Michael T Madigan, The Ohio State University, Columbus, Ohio, USA

Published online: September 2010

DOI: 10.1002/9780470015902.a0021935

Heliobacteria are the most recently discovered of the anoxygenic phototrophic bacteria. Heliobacteria contain bacteriochlorophyll g, a pigment unique to species of this group, and synthesise the simplest photosynthetic complexes of all known phototrophs. Also, unlike all other phototrophs, heliobacteria produce endospores but cannot grow autotrophically. Four genera of heliobacteria are known. Species of Heliobacterium, Heliobacillus and Heliophilum grow best at neutral pH, whereas species of Heliorestis are alkaliphilic. Heliobacterium, Heliobacillus and Heliophilum species form one phylogenetic clade whereas Heliorestis species form a second within the phylum Firmicutes of the domain Bacteria. Heliobacteria have a unique ecology, being primarily terrestrial rather than aquatic, and some species may have evolved a mutualistic relationship with rice plants. The genome sequence of the thermophile Heliobacterium modesticaldum supports the hypothesis that heliobacteria are minimalist phototrophs, and because of this, heliobacteria may have played a pivotal role in the evolution of phototrophic bacteria.

Key Concepts:

  • Heliobacteria are unlike all known anoxygenic phototrophic bacteria because of their unique bacteriochlorophyll, Gram-positive phylogeny, and primarily soil habitat.
  • Heliobacteria employ the simplest of all photosynthetic energy-generating systems and thus their study may yield important clues to when and how photosynthesis evolved.
  • In addition to being phototrophic, heliobacteria are strong nitrogen-fixing bacteria and possibly associate with plants such as rice in a type of symbiotic association.

Keywords: anoxygenic phototrophic bacteria; heliobacteria; bacteriochlorophyll g

Figure 1. Enrichment cultures for anoxygenic phototrophic bacteria using a paddy soil sample. (a) If the enrichment was not pasteurised (heat-treated at 80°C for 15 min) (tube 1), the sample yielded purple nonsulfur bacteria (purple-red turbidity), in this case, Rhodopseudomonas palustris, a common purple bacterium. However, if a duplicate enrichment was pasteurised (tube 2), the purple bacteria in the sample are killed and heliobacteria (green turbidity) develop, presumably from the germination of heliobacterial endospores. (b) Phase-contrast photomicrograph of a pasteurised enrichment culture. Note spores and sporulating cells (arrows).
Figure 2. The thermophilic heliobacterium, Heliobacterium modesticaldum. (a) Thin section transmission electron micrograph of cells of Heliobacterium modesticaldum. (b) Scanning electron micrograph of cells. Note flagella scattered in the preparation. Note the absence of an outer membrane and any type of internal photosynthetic membranes or chlorosomes. Cells of Heliobacterium modesticaldum are approximately 1 m wide.
Figure 3. Heliorestis convoluta. (a) Phase-contrast micrograph of long cell coils. (b) Scanning electron micrograph of a short coil of cells. Coils form from ring-shaped cells that remain connected after cell division. Cells of Heliorestis convoluta are approximately 0.6 m wide.
Figure 4. Phylogenetic tree of the family Heliobacteriaceae based on 16S rRNA gene sequences of 1261 nucleotide positions. The tree was computed using PHYLIP version 3.66 (Felsenstein, 1989). Alignment was converted to a distance matrix using F84 algorithm (transition/transversion ratio=2.0, empirical base frequencies) in the program DNADIST. The tree was based on neighbour-joining method using the program NEIGHBOR. The tree was rooted using Escherichia coli (Gammaproteobacteria) as an outgroup. All sequences have been deposited in GenBank as follows: E. coli (), Bacillus subtilis (), Desulfitobacterium dehalogenans (), Clostridium pasteurianum (), Heliorestis baculata (), Heliorestis convoluta (), Heliophilum fasciatum (), Heliobacterium modesticaldum (), Heliobacterium chlorum () and Heliobacillus mobilis ().
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Related Sub-Topics:

Bacteria | Environmental Microbiology | Microbial Photosynthesis
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Article Title: Heliobacteria
 
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Asao, Marie, and Madigan, Michael T(Sep 2010) Heliobacteria. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0021935]