Sulfur compounds undergo frequent metabolic transformations in bacteria, resulting in the sulfur cycle as shown in this animation. Sulfur commonly occurs as sulfate (SO₄^2-) in soils and waters. Most organisms can reduce sulfate to sulfide, an assimilatory process, to produce -SH groups used in certain organic molecules. Release of organic sulfur, by excretion or decomposition, frequently produces reduced hydrogen sulfide (H₂S). In aerobic environments, chemolithotrophic bacteria oxidize hydrogen sulfide to free sulfur (S₀) and often back to sulfate. In anaerobic environments, hydrogen sulfide is used as an electron donor by many anoxygenic sulfuric photosynthetic bacteria. When sulfate enters anaerobic habitats, it can be used as an electron acceptor by certain anaerobic bacteria, producing hydrogen sulfide.

Suggested use: Demonstrate the animation several times in class, explaining the different processes involved, and allow students to access the animation for review.