Sulfur Cycle

BIOGEOCHEMICAL CYCLES

SOIL MICROBIOLOGY

BIOL/CSES 4684

THE SULFUR CYCLE

Compound	Chemical Form	Valence
Sulfide	H₂S	-2
Sulfur	S	0
Hyposulfite	S₂O₄	-2
Sulfite	SO₃	-2
Thiosulfite	S₂O₃	-2
Dithionate	S₂O₆	-2
Trithionate	S₃O₆	-2
Tetrathionate	S₄O₆	-2
Petathionate	S₅O₆	-2
Sulfate	SO₄	-2

The Sulfur Cycle: Steps in the Cycle

Mineralization of organic sulfur to the inorganic form, sulfide (H₂S) (see below).

OXIDATION OF SULFIDE AND ELEMENTAL SULFUR (S) and related compounds to sulfate (SO₄).

REDUCTION OF SULFATE TO SULFIDE.

Microbial immobilization of the sulfur compounds and subsequent incorporation into the organic form of sulfur (see below).

Human impact on the sulfur cycle is primarily in the production of sulfur dioxide (SO₂) from industry (e.g. burning coal) and the internal combustion engine (see above diagram). Sulfur dioxide can precipitate onto surfaces where it can be oxidized to sulfate in the soil (it is also toxic to some plants), reduced to sulfide in the atmosphere, or oxidized to sulfate in the atmosphere as sulfuric acid, a principal component of acid rain.

The above table lists the forms of sulfur that can occur in soil. Notice that the net valence of each chemical form is minus 2, except for elemental sulfur which has a valence of zero. Sulfate, sulfide, and sulfur are the major and most important forms, all others are minor.

Mineralization of Organic Sulfur

Most organic sulfur in living tissue is in S-containing amino acids and sulfate esters. As an example, consider the degradation of cysteine:

R-HS-CH₂-CHNH₂-CO₂H (cysteine) -------> NH₃ + R-HS-CH₂-CO-CO₂H (Beta-mercaptopyruvic acid)------> H₂S + CH₃-CO-C₂H (pyruvic acid) ------> utilized in a number of ways for metabolism

The H₂S and NH₃ are released into the atmosphere, contributing to the characteristc smell associated with putrefaction. In soil, 90% of the sulfur present is organic: 50% is C-O-S linkages (sulfate esters), 20% is S-amino acids, and 20% as a wide range of different sulfur compounds.

Sulfur Immobilization

Also called Assimilatory Sulfate Reduction
In general, sulfate is converted to a protein containing sulfur by:
SO₄² + 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, and can be performed by microbes, plants, and animals.

Materials on the this webpage were created by Katie Corbin.