These are Gram-negative, non-sporing non-acid fast organisms. There are rod-shaped, coccoid, ovoid and spiral-shaped species. The rods are generally 1 x 2-4mµ but some species may be larger. The cocci are generally 2-3mµ in diameter and the spirals up to 40mµ long. There are a number of genera recognised including Amoebobacter (coccoid), Chromatium (rods), Lamprobacter (rods), Lamprocystis (cocci), Thiocapsa (cocci), Thiocystis (cocci), Thiodictyon (rods), Thiopedia (ovoid) and Thiospirillum (spiral). They may be motile. The organisms of this group comprise the purple sulphur bacteria, characterised by their ability to carry out anoygenic photosynthesis. In light they produce ATP by cyclic photophosphorylation, using bacteriochlorophyll as the primary electron acceptor. CO2 fixation and suphide (or thiosulphate) oxidation are stoichiometrically linked. They fix CO2 autotrophically through the Calvin cycle. Many can also grow on assimilated carbon compounds but some, particularly Chromatium okenii and Thiospirillum jenense, are obligately photolithoautotrophic. Generally anaerobic growth in the dark is poor but some species have a fermentative capability. During dark fermentation they store "suphur" globules, which act as an electron sink. In the dark in the presence of oxygen many species have the usual respiratory pathways. The GC content of the DNA is 45-70 mol%.

Genera

Amoebobacter: These are coccoid purple sulphur bacteria with diameter usually around 2-3µm but A. purpureus may be 3-4.5µm in diameter. Most strains either require vitamin B12 for growth or have their growth stimulated by it. They are not motile and form a slime around themselves. The cells contain gas vacuoles. A cell suspension is generally pink-red. Depending on the species the predominant carotenoid pigment is spirilloxanthin, okenone or rhodopin. The GC content of the DNA is 64-66 mol%.

Chromatium: These are rod-shaped purple sulphur bacteria usually around 1-2 x 2-4µm but some may be as large as 3.5-4 x 5-11µm (C. warmingii). Some strains either require vitamin B12 for growth or have their growth stimulated by it. C. purpuratum requires 5% NaCl. The optimum teperature for C. tepidum is 48-50°C. They are motile and a slime is only formed by some species. The cells do not contain gas vacuoles. A cell suspension is generally brown or purple-red. Depending on the species the predominant carotenoid pigment is spirilloxanthin, okenone, lycopene, rhodopinal or rhodopin. The GC content of the DNA is 55-69 mol%.

Lamprobacter: These are rod-shaped purple sulphur bacteria usually around 2-2.5 x 4-5µm. They do not require vitamin B12 for growth but require 1-2% NaCl. They are motile and a slime is formed. The cells contain gas vacuoles. A cell suspension is generally purple-red. The predominant carotenoid pigment is okenone. The GC content of the DNA is 64 mol%.

Lamprocystis: These are coccoid purple sulphur bacteria usually around 3-3.5µm in diameter. They do not require vitamin B12 for growth. They are motile and a slime is not formed. The cells contain gas vacuoles. A cell suspension is generally purple. The predominant carotenoid pigment is lycopenal and lycopenol. The GC content of the DNA is 63-64 mol%.

Thiocapsa: These are coccoid purple sulphur bacteria usually around 1-3µm in diameter. They do not require vitamin B12 for growth. They may require up to 3% NaCl for growth. They are not motile and a slime is not formed by T. pfennigii but is formed by T. roseopersica. The cells do not contain gas vacuoles. A cell suspension is generally orange-brown (T. pfennigii) or pink-red (T. roseopersica). The predominant carotenoid pigment is tetrahydrospirilloxanthin (T. pfennigii) or spirilloxanthin (T. roseopersica). The GC content of the DNA is 63-70 mol%.

Thiocystis: These are coccoid purple sulphur bacteria usually around 2.5-3µm in diameter. They do not require vitamin B12 for growth. They may require up to 2% NaCl for growth. They are motile and a slime is formed. The cells do not contain gas vacuoles. A cell suspension is generally purple-red (T. gelatinosa) or purple-violet (T. violacea). The predominant carotenoid pigment is okenone (T. gelatinosa) or rhodopinal (T. violacea). The GC content of the DNA is 61-68 mol%.

Thiodictyon: These are rod-shaped purple sulphur bacteria usually around 1.5 -2 x 3-8µm. They do not require vitamin B12 for growth. They are not motile. A slime is not formed. The cells contain gas vacuoles. A cell suspension is generally purple-violet. The predominant carotenoid pigments are rhodopinal and rhodopin. The GC content of the DNA is 64 -65 mol%.

Thiopedia: These are ovoid purple sulphur bacteria usually around 1-2 x 1.2-1.5µm. They require vitamin B12 for growth. They are not motile and a slime is not formed. The cells contain gas vacuoles. A cell suspension is generally purple-red. The predominant carotenoid pigment is okenone. The GC content of the DNA is 62-64 mol%.(ovoid)

Thiospirillum: These are spiral purple sulphur bacteria usually around 2.5-4.5 x 30-40µm. They require vitamin B12 for growth. They are motile but a slime is not formed. The cells do not contain gas vacuoles. A cell suspension is generally orange-brown. The predominant carotenoid pigments are lycopene and rhodopinal. The GC content of the DNA is 45.5 mol%.

Species Profiles

C. okenii

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