Iron(II) Sulfide - Biology and Biochemistry

Biology and Biochemistry

As organic matter decays under low-oxygen (or hypoxic) conditions such as in swamps or dead zones of lakes and oceans, sulfate-reducing bacteria will use the sulfates present in the water to oxidize the organic matter, producing hydrogen sulfide as waste. Some of the hydrogen sulfide will react with metal ions in the water to produce metal sulfides, which are not water soluble. These metal sulfides, such as iron(II) sulfide, are often black or brown, leading to the color of sludge.

Pyrrotite is a waste product of the Desulfovibrio bacteria, a sulfate reducing bacteria.

When eggs are cooked for a long time, the yolk's surface may turn green. This is due to iron(II) sulfide which forms as iron from the yolk meets hydrogen sulfide released from the egg white by the heat. This reaction occurs more rapidly in older eggs as the whites are more alkaline.

The presence of ferrous sulfide as a visible black precipitate in the growth medium peptone iron agar can be used to distinguish between microorganisms that produce the cysteine metabolizing enzyme cysteine desulfhydrase and those that do not. Peptone iron agar contains the amino acid cysteine and a chemical indicator, ferric citrate. The degradation of cysteine releases hydrogen sulfide gas that reacts with the ferric citrate to produce ferrous sulfide.