Important Groups of Procaryotes (page 2)
(This chapter has 10 pages)
© Kenneth Todar, PhD
Current phylogenetic analysis of the Bacteria has demonstrated
of more than 20 distinct groups with taxonomic phylum status (Phylum is the
highest taxon in a Domain). A
phylogenetic tree that displays 13 groups is given shown in Figure
5. Most groups listed consist of a distinct phylum; some are now
more than one phylum. Many groups consist of
that are phenotypically and physiologically unrelated. Furthermore, in
some phylogenetic groups (e.g. Proteobacteria and Gram-positives) there
is considerable phylogenetic diversity, unappreciated in the big Tree
Life. The current edition of Bergey's Manual of Systematic Bacteriology recognizes 24
phyla of Bacteria, but there remains
considerable diversity in phenotype among members of some phyla.
Phyla of the Domain Bacteria (for representative genera see the
Appendix at the end of this page).
discuss the major groups of bacteria herein based on morphology,
ecology, it may be interesting to examine the phylogenetic relatedness
of group members. Sometimes, members of a phylum are quite distinct
spirochetes); other times, our organization involves a confluence of
phototrophs, lithotrophs, pyogenic cocci). This makes for reflection on
how related and unrelated organisms converge on the same strategy to
adapt to specific environments.
5. Phylogenetic tree
Photosynthetic purple and green bacteria
These bacteria conduct anoxygenic photosynthesis, also called bacterial
photosynthesis. Bacterial photosynthesis differs from plant-type
photosynthesis in several ways. Bacterial photosynthesis does not
O2; in fact, it only occurs under anaerobic conditions.
photosynthesis utilizes a type of chlorophyll other than chlorophyll a,
and only one photosystem, photosystem I. The electron donor for
photosynthesis is never H2O but may be H2, H2S
or So, or certain organic compounds. The light-absorbing
of the purple and green bacteria consist of bacterial chlorophylls and
carotenoids. Phycobilins, characteristic of the cyanobacteria, are not
found. Many purple and green sulfur bacteria store elemental sulfur as
a reserve material that can be further oxidized to SO4 as a
photosynthetic electron donor.
The purple and green bacteria
use H2S during photosynthesis in the same manner that
or algae or plants use H2O as an electron donor for
CO2 fixation (the "dark reaction" of photosynthesis). Or
may utilize organic compounds as electron donors for photosynthesis.
example, Rhodobacter can use light as an energy source while
succinate or butyrate in order to obtain electrons for CO2
The bacterium that became an endosymbiont of eucaryotes and evolved
into mitochondria is thought to be a relative of the purple nonsulfur
This conclusion is based on similar metabolic features of mitochondria
and purple nonsulfur bacteria and on comparisons of the base sequences
in their 16S rRNAs.
Figure 6. Photomicrographs
contrast and and ordinary illumination) of various photosynthetic
(Norbert Pfennig). Magnifications are about 1400X. The purple and green
bacteria exhibit a full range of procaryotic morphologies, as these
illustrate. Diversity among their phylogenetic relationships is also
A. Purple sulfur bacteria (L
to R): Chromatium vinosum, Thiospirillum jenense, Thiopedia rosea. The
purple sulfur bacteria are classified among the Gamma Proteobacteria, a
class that also includes Pseudomonas and E. coli.
B. Purple nonsulfur bacteria
(L to R): Rhodospirillum rubrum, Rhodobacter sphaeroides,
vannielii. The purple nonsulfur bacteria are in the
which also includes Rhizobium, Agrobacterium and the
The latter bacteria represent a direct lineage to mitochondria.
C. Green sulfur bacteria (L
to R): Chlorobium limicola, Prosthecochloris aestuarii, Pelodictyon
clathratiforme. The Green sulfur bacteria represent a distinct
lineage and cluster in their own phylum represented by Chlorobium
in Figure 5.
Figure 7. Green nonsulfur
(T.D. Brock). Chloroflexus also represents a phylogenetically
group of green bacteria. Chloroflexus is a thermophilic,
Figure 8. Photosynthetic
growing in a hot spring run-off channel (T.D. Brock). The white area of
the channel is too hot for photosynthetic life, but as the water cools
along a gradient, the colored phototrophic bacteria colonize and
construct the colored microbial mats composed of a consortium of
Heliobacteria The heliobacteria
are a unique group of anoxygenic photosynthetic bacteria. They have a
typical bacterial PSI type reaction center, but the primary pigment
involved is bacteriochlorophyll g, which is unique to the group and has
a unique absorption spectrum. This gives the heliobacteria their own
environmental niche. Photosynthesis takes place at the cell membrane,
which does not form folds or compartments as it does in purple
Interestingly, phylogenetic analysis place the heliobacteria among
the Firmicutes (Gram-positive bacteria), but they do not stain
Gram-positive. They have no outer membrane and like certain other
firmicutes (clostridia) they form heat resistant endospores. Also, are
the only firmicutes known to conduct photosynthesis.
Heliobacteria are photoheterotrophic, requiring organic carbon
sources, and they are exclusively anaerobic. heliobacteria have been
found in soils and are apparently widespread in the waterlogged soils
of paddy fields. They fix nitrogen and are probably important in the
fertility of paddy fields.
Prospecting for genetic data in the microbial mats of Yellowstone Park
and looking for evidence of a green sulfur bacteria, microbiologist
David Ward of Montana State University found a "wildly different kind
of phototrophic bacterium" in the 66oC water. Evidence for
the organism was found using the techniques of metagenomics, and
subsequently by generating an enriched monoculture. The organism
described and grown was named Chloracidobacterium
thermophilum. By demonstrating that the monoculture was tolerant
to O2, but could not be maintained in darkness, it was
concluded that the organism was a new type of photosynthetic microbe.
Despite its affiliation with the bacterial phylum Acidobacterium,
which consists of hyperthermophilic acidophiles, Chloracidobacterium thermophilum
thrives in alkaline environments.
In addition to being able perform photosynthesis in the presence of
thermophilum is also the only oxygen-tolerant bacterium that
providing light-harversting molecular antennae that allow the organisms
to live at vanishingly low light intensities.