Producers and microorganisms are able to use sulfur easiest if it is in the form of sulfide (S²⁻). This is because sulfide is a reduced form of sulfur and can be readily incorporated into organic molecules through processes such as assimilatory sulfate reduction.
Sulfhydryl groups (-SH) and hydrogen sulfide (H₂S) also contain sulfur, but they are not as readily used by organisms as sulfide. Sulfate (SO₄²⁻), on the other hand, is an oxidized form of sulfur and must be reduced before it can be used by most organisms. Therefore, sulfide is the form of sulfur that is most easily used by producers and microorganisms.