Photosynthetic purple and green bacteria These bacteria conduct anoxygenic photosynthesis, also called bacterial photosynthesis. Bacterial photosynthesis differs from plant-type (oxygenic) photosynthesis in several ways. Bacterial photosynthesis does not produce O₂; in fact, it only occurs under anaerobic conditions. Bacterial photosynthesis utilizes a type of chlorophyll other than chlorophyll a, and only one photosystem, photosystem I. The electron donor for bacterial photosynthesis is never H₂O but may be H₂, H₂S or S^o, or certain organic compounds. The light-absorbing pigments 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 may use H₂S during photosynthesis in the same manner that cyanobacteria or algae or plants use H₂O as an electron donor for autotrophic CO₂ fixation (the "dark reaction" of photosynthesis). Or they may utilize organic compounds as electron donors for photosynthesis. For example, Rhodobacter can use light as an energy source while oxidizing succinate or butyrate in order to obtain electrons for CO₂ fixation.

The bacterium that became an endosymbiont of eucaryotes and evolved into mitochondria is thought to be a relative of the purple nonsulfur bacteria. 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 (phase contrast and and ordinary illumination) of various photosynthetic bacteria (Norbert Pfennig). Magnifications are about 1400X. The purple and green bacteria exhibit a full range of procaryotic morphologies, as these photomicrographs illustrate. Diversity among their phylogenetic relationships is also noted.

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, Rhodomicrobium vannielii. The purple nonsulfur bacteria are in the Alpha Proteobacteria, which also includes Rhizobium, Agrobacterium and the Rickettsias. 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 phylogenetic lineage and cluster in their own phylum represented by Chlorobium in Figure 5.

Figure 7. Green nonsulfur bacterium, Chloroflexus (T.D. Brock). Chloroflexus also represents a phylogenetically distinct group of green bacteria. Chloroflexus is a thermophilic, filamentous gliding bacterium.

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 phototrophic bacteria.

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.

Chloracidobacterium thermophilum 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 66^oC 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 oxygen, Chloroacidobacterium thermophilum is also the only oxygen-tolerant bacterium that constructs chlorosomes, providing light-harversting molecular antennae that allow the organisms to live at vanishingly low light intensities.