There are five main groups of sulfur-oxidizing bacteria:

Sulfide Oxidation

Hydrogen sulfide is normally toxic to aerobic plant and animal tissue; the exception is ruminant animals. Whenever sulfide is generated, a specialized microflora usually develops that is capable of oxidizing the sulfide to elemental sulfur.

Sulfide may be oxidized to elemental sulfur aerobically by species of Thiothrix and Beggiatoa (morphologically conspicuous sulfur oxidizers), and anaerobically by the purple sulfur bacteria. Both of these groups are primarily aquatic microbes. In soil, the predominant microbes involved in the oxidation of sulfide to elemental sulfur belong to the genus Thiobacillus. However, many thiobacilli are aquatic as well, especially in acidic mine waters. For the following two "generic" reactions, the first is typical of oxidation of sulfide to sulfur, and the second of oxidation of sulfide to sulfate. As shown in the diagram to the left, the rate-limiting step is mediated by the enzyme sulfite oxidase (cofactors for the enzyme are Mo and Fe).

CO2 + 2H2S -----> CH2O + 2S + H2O
2CO2 + H2S + 2H2O -------> 2(CH2O) + H2SO4

Oxidation of Sulfur and Related Compounds

2S + 3O2 + 2HOH ------> 2H2SO4 (Thiobacillus thiooxidans)
12FeSO4 + 3O2 + 6HOH ------> 4Fe2(SO4)3 + 4Fe(OH)3 (T.ferroxidans)

Certain thiobacilli (T.denitrificans) can also carry out anaerobic respiration, using nitrate as the terminal electron acceptor, while oxidizing the elemental sulfur to sulfate. However, they lack the ability to synthesize assimilatory nitrate and nitrite reductases, therefore a source of ammonia/nitrogen must be present for cell/protein synthesis to take place.

Table 1: Optimum pH for Thiobacilli.
T.intermedius pH 1.9 to 7.0
T.thiooxidans pH 2.0 to 3.5
T.ferroxidans pH 2.5 to 4.0
T.thioparus and T.denitrificans pH 7.0 to 8.5

Sulfur oxidation will take place from a pH of 8.5 to 1.9, the species involved will vary, yet each in turn oxidizing the sulfur to sulfate. Therefore, a "succesion" of species takes place as the pH of the soil is lowered by the production of sulfate.

The thiobacilli can also oxidize thiosulfate---
Na2S2O2 + O2 + HOH ------> Na2SO4 + H2SO4

and tetrathionate---
2Na2S4O6 + 7O2 + 6HOH -----> 2Na2SO4 + 6H2SO4
During the oxidation process, energy is liberated as follows:
S -15kcal ------> S2O2 -5kcal -----> S4O6 - 100kcal ---->SO4
Sulfur - 0 ------>Thiosulfate - -2 ------>Tetrathionate - -2 ----->Sulfate - -2

The Colorless Sulfur-Oxidizing Bacteria

The colorless sulfur bacteria comprise a very heterogeneous group of organisms which share the ability to oxidize reduced or partially oxidized inorganic sulfur compounds. The use of this ability as a taxonomic criterion has linked many genera that have no taxonomic relationship. The position has been further complicated by the results of 5S and 16S RNA analysis carried out in the last few years, which have indicated that species currently classified within some of the larger genera (e.g., Thiobacillus) may be only distantly related to each other, although they are (eco)physiologically similar.

Virtually all morphological forms and types of motility occur among the colorless sulfur bacteria, and representatives can be found growing over most of the pH range(pH 1.0-10.5). The main factors linking the genera known as the "colorless sulfur bacteria" are that they are Gram-negative aerobes (some may denitrify) and chemoautotrophic. They do not contain bacteriochlorophyll.

Both obligate and facultative chemoautotrophs occur, as well as chemohoheterotrophs. A few organisms classified as colorless sulfur bacteria (e.g., some Beggiatoa spp.) may not be true chemoheterotrophs, as they do not gain energy from the oxidation of sulfur compounds. They do, however, profit from the oxidation in other ways. Colorless sulfur bacteria can be found wherever reduced sulfur compounds are available (e.g., in sediments, soil, at aerobic/anaerobic interfaces in water, and at volcanic sources such as the hydrothermal vents). Natural enrichments with sometimes 108 cells/mL or more may be found at sulfide/oxygen interfaces and where sulfur compounds and oxygen mix. The presence of colorless sulfur bacteria may often be evident to the naked eye by the appearance of copious white deposits of sulfur, streamers, veils, rosettes, or films. Samples taken from sites exposed to light may also contain (at least below 55oC) photoautotrophic (purple) sulfur-oxidizing bacteria, many of which can also be grown as chemoautotrophs. Pellets or concentrated suspensions of the bacteria should be examined for color (red/purple, brown), and the presence of a phototroph can be confirmed by scanning the in vivo light absorption of the suspension in a spectrophotometer and checking for the characteristic (bacterio)chlorophyll peaks of purple bacteria between 850 and 1040 nm. This is especially important with pinkish or brownish pellets that may owe their color, instead, to cytochromes.

The following tables contain information on the facultative autotrophic, obligately autotrophic, and morphologically conspicuous sulfur oxidizung bacteria.

Table 2. Basic Characteristics of the facultatively autotrophic colorless sulfur bacteria.
Species % G+C Motility Carboxysomes pH NO3- reduction to NO2- NO3- reduction to N2 Opt. temp.
Thiobacillus novellus 66-68 - - 6-8 - - 25-30
Thiobacillus versutus 65-68 + - 6-8 + + 30-35
Thiobacillus intermedius 65-67 + + 5-7 - - 30-35
Thiobacillus perometabolis 65-68 + - 5-7 - - 30-35
Thiobacillus delicatus 66-67 - Not determined 5-7 + - 30-35
Thiobacillus aquaesulis 65-66 + Not determined 7-9 + - 40-50
Thiobacillus thyasiris 52 - + 7-8 + + 35-40
Thiobacillus acidophilus 61-64 + + 2-4 - - 25-30
Thiobacillus cuprinus 66-69 + Not determined 3-4 Not determined Not determined 30-36
Thiosphaera pantotropha 66 - - 7-9 + + 30-40
Paracoccus denitrificans 64-67 - - 7-9 + + 25-35

Table 3. Physical Characteristics of the morphologically conspicuous colorless sulfur bacteria. If filaments are observed, isolates may belong to genera of gliding bacteria such as Beggiatoa, Thiothrix, Thioploca, and Thiospirillopsis.
Species Size- micro-m Shape Motility CaCO3
Achromatium oxaliferum >5 x <100 Round to cylindrical Peritrichous or gliding +
Achromatium volutans 5 x <40 Spherical to ovoid Slow, jerky, gliding -
Macromonas mobilis 6-14 x 10-30 Cylindrical to oval Polar Tuft +
Macromonas bipunctata 2-4 x 3-7 Cylindrical to pear shaped Polar Tuft +
Thiobacterium bovista 0.4- >1 x <3-9 Rods Nonmotile -
Thiospira winogradskyi 2-2.5 x <50 Spirilla, pointed ends Polar or polar tuft -
Thiospira bipunctata 1.7 x >2 x 6-14 Spirilla, pointed ends Polar or polar tuft -
Thiovulum majus 5-25 Round to ovoid Peritrichous -

Table 4. Basic characteristics of the obligately autotrophic colorless sulfur bacteria. Carboxysomes indicated the possession of carboxysomes under some, if not all growth conditions; pH and temp. indicate the most favorable ranges for growth. ND: not determined.
Species %G+C Motility Carboxysomes pH NO3- reduction to NO2- NO3- reduction to N2 Opt. temp. Ubiquinone
Thiobacillus thioparus 61-66 + + 6-8 + - 25-30 Q-8
Thiobacillus neapolitanus 52-56 + + 6-8 - - 25-30 Q-8
Thiobacillus capsulatus 54.5 + + 5-7 - - 25-30 ND
Thiobacillus tepidarius 66.6 + ND 6-8 + - 40-45 Q-8
Thiobacillus denitrificans 63-68 + - 6-8 + + 25-30 Q-8
Thiobacillus ferroxidans 55-65 + + 2-4 - - 30-35 Q-8
Thiobacillus thiooxidans 51-53 + + 2-4 - - 25-30 Q-8
Thiobacillus albertis 61.5 + + 2-4 - - 25-30 ND
Thiobacillus prosperus 61-64 + + 1-4 ND ND 30-35 Q-8
Thiomicrospira denitrificans 36 - - 7 + + 20-25 ND
Thiomicrospira pelophila 44 + - 6-8 - - 25-30 ND
Thiomicrospira crunogena 42-43 + - 7-8 - - 28-32 ND