BIOL/CSES 4684: Sulfur Cycle

BIOGEOCHEMICAL CYCLES

SOIL MICROBIOLOGY

BIOL/CSES 4684

SULFUR (SULFATE) REDUCTION

Sulfate Reduction may be divided into two categories, Assimilatory and Dissimilatory:

Assimilatory:In general, sulfate is converted to a protein containing Sulfur by:
SO₄^2- + PAP (phosphoadenosine-5-phosphate)------> PAPS (phosphoadenosine-5-phosphosulfate------->PAPS + reduction------>PAPS-reductase------> Sulfite---->Thiosulphate---->Sulfide---->a series of biochemical steps---->methionine----> sulfur containing protein.
Sulfate must be 'activated' initially by PAP before the reaction can proceed. Assimilatory sulfate reduction occurs anaerobically as well as aerobically.

Dissimilatory:In soils that become deficient in oxygen, usually the result of flooding, the sulfide level will increase to relatively high concentrations.
The basic reaction: Oxidizable C and 4H₂ + SO₄^2- ---> S^2- + 4HOH
The formation of sulfide by sulfate reduction in nature is enhanced in warm, wet, or water logged soils with a pH of above 6.0. Sulfide accumulation may be particularly pronounced in sulfate-rich saline areas in which plant excretions (release of carbon compounds) serve as the oxidizable carbon source in addition to the hydrogen. Thus, like denitrification, an oxidizable carbon source serves as the electron donor, while the sulfate serves as the electron acceptor.

The metabolic dissimilatory process is similar to the assimiliatory sulfate reduction in that the sulfate must be first activated by a molecule called ADP (adenosine-5-phosphate).

Sulfate + ADP--->ADPS (adenosine-5-phosphosulfate)--->sulfite--->sulfide (H₂S)
OR if tetrathionate is reduced
tetrathionate (S₄O₆)--->thiosulfate (S₂O₂)---> sulfite (SO₃)--->sulfide (H₂S)

Dissimilatory Sulfate- or Sulfur-Reducing Bacteria

The sulfur- and sulfate-reducing bacteria represent a large and diverse group of anaerobic microorganisms. Most are eubacteria, but several archaeabacteria that reduce sulfur have also been described. Cells can be spherical, ovoid, rod-shaped, spiral, or vibrioid-shaped; 0.4-3.0 micrometers in diameter; occurring singly, in pairs, or sometimes in aggregates. Uniseriately multicellular filamentous forms also occur. Most genera stain Gram negative; filamentous and spore-forming types may stain Gram positive. A few species contain gas vacuoles. These bacteria are motile and nonmotile. Their motility is mainly due to flagella; filamentous forms exhibit gliding motility. Flagella are polar or peritrichous. Colonies in agar are usually yellowish brown, pink, or reddish; whitish colonies sometimes occur. Strictly anaerobic. Sulfate-reducing bacteria reduce sulfate and in a few cases also sulfur H₂S; sulfur-reducing bacteria are unable to reduce sulfate or other oxoanions of sulfur. H₂ or organic compounds serve as electron donors; oxidation of organic compounds is either incomplete, leading to acetate as an end product, or complete leading to CO₂. Electron donors utilized by many species are H₂, lactate, fatty acids, ethanol, or dicarboxylic acids. Autotrophic growth on H₂, CO₂, and sulfate may occur. Ammonium salts are generally used as the nitrogen source. All species examined thus far can fix dinitrogen. Typical habitats are anoxic sediments or bottom waters of freshwater, marine, or hypersaline aquatic environments; thermophilic species occur in hot springs and submarine hydrothermal vents.

The sulfur reducing bacteria are divided into four groups, as seen in the following overview table. Below that table are four numbered tables that provide the characteristics of the major genera (and some species) within each subgroup.

Overview Table: Differentiation of Subgroups 1 through 4 of the dissimilatory sulfate- or sulfur-reducing bacteria.

Characteristic	Subgroup 1. Spore-forming sulfate-reducing bacteria	Subgroup 2. Non-spore- forming sulfate- reducing bacteria; incomplete substrate oxidation	Subgroup 3. Non-spore- forming sulfate- reducing bacteria; complete substrate oxidation	Subgroup 4. Sulfur-reducing bacteria
Sulfate reduced to sulfide	+	+	+	-
Sulfur reduced to sulfide	-	+ or -	-	+
Organic substrates completely oxidized to CO₂	+ or -	-	+	+

Table 1: Differential characteristics of the genus Desulfotomaculum/Subgroup 1.

Characteristic	D.acetoxidans	D.antarcticum	D.geothermicum	D.guttoideum	D.kuznetsovii
Organic substrates completely oxidized to CO₂	+	-	-	-	+
Utilized with sulfate:
H₂+CO₂, + or - acetate as carbon source	-	-	+	-	+
Lactate	-	+	+	+	-
Acetate	+	-	-	-	+
Butyrate	+	-	+	-	+
Fatty Acids(C6-C16)	-	-	+	-	+
Optimum temperature range:
25-40^oC	+	+	-	+	-
50-60^oC	-	-	+	-	+
60-65^oC	-	-	-	-	+
Gas vacuoles formed during sporulation	+	-	+	-	-

Table 1. (cont).

Characteristic	D.nigrificans	D.orientis	D.ruminis	D.sapomandens	D.thermoacetoxidans
Organic substrates completely oxidized to CO₂	-	-	-	+	+
Utilized with sulfate:
H₂+CO₂, + or - acetate as carbon source	+	+	+	-	+
Lactate	+	+	+	-	+
Acetate	-	-	-	+	+
Butyrate	-	-	-	+	+
Fatty Acids(C6-C16)	-	-	-	+	-
Optimum temperature range:
25-40^oC	-	+	+	+	-
50-60^oC	+	-	-	-	+
60-65^oC	-	-	-	-	+
Gas vacuoles formed during sporulation	-	-	-	+	-

Table 2: Differential characteristics of the genus Desulfomonas/Subgroup 2.

Characteristic	Desulfobulbus	Desulfomicrobium	Desulfomonas	Desulfovibrio	Thermodesulfobacterium
Cells spiral or vibrioid-shaped	-	-	-	+	-
Cells ovoid to rod-shaped	+	+	+	-	+
Motile by polar flagella	+ or -	+	-	+ or -	+ or -
Optimum temp. range
25-40^oC	+	+	+	+	-
65-70^oC	-	-	-	-	+
Utilized with sulfate:
H₂+CO₂+acetate as carbon source	+	+	+	+	+
Lactate	+	+	+	+	+
Propionate	+	-	-	-	-
Desulfoviridin present	-	-	+	+	-

Table 3: Differential characteristics of the geneus Desulfobacter/Subgroup 3.

Characteristic	Desulfobacter	Desulfobacterium	Desulfococcus	Desulfomonile	Desulfonema	Desulfosarcina
Desulfoviridin present	-	-	+	+	+ or -	-
Cells spiral or vibrioid-shaped	+ or -	-	-	-	-	-
Cells ovoid to rod-shaped	+ or -	+	-	+	-	+
Cells spherical	-	+ or -	+	-	-	+ or -
Multicellular filaments	-	-	-	-	+	-
Motility:
Flagella	+ or -	+ or -	-	-	-	+ or -
Gliding	-	-	-	-	+	-
Utilized with sulfate:
Acetate:
Doubling time 15-30h	+	-	-	-	-	-
Doubling time greater than 30h	-	+	+	+	+	+
Fatty acids (C4-C8)	-	+ or -	+	-	+	+
Benzoate	-	+ or -	+ or -	+	+ or -	+

Table 4: Differential characteristics of the genera of Subgroup 4 (poorly described).

Characteristic	Desulfurella	Desulfuromonas
Electron acceptor reduce to sulfide:
Sulfate	-	-
Sulfur	+	+
Optimum temp. range:
25-40^oC	-	+
50-57^oC	+	-
Color of colonies in agar:
Pink to reddish brown	-	+
Whitish	+	-
Growth in the absence of sulfur on fumarate or malate with or without acetate as additional substrate	-	+ or -