Even though a few organisms gain their Sulphur requirements in such organic forms as amino acids, inorganic sulfate is the major source of biologically significant sulfur, being absorbed through plant roots.
Most of the biologically incorporated sulphydryl Sulphur (-SH) is mineralized by bacteria and fungi in ordinary decomposition.
Under anaerobic conditions, however some may be reduced directly to sulphide, including hydrogen sulphide (H2S) by bacteria (thiobacills) ( SO4- + 2H+ -- H2S + 2O2 ). Sulphate is also reduced under anaerobic conditions to elemented sulfur or to sulfides including hydrogen sulfide.
Some organic sulfur gains entry into the atmosphere as sulfur dioxide (SO2) through incomplete combustion of fossil fuels.
Atmospheric sulfur in the form of elemental sulfur, hydrogen sulfide, or sulfur dioxide is oxidized to sulfur trioxide (SO3). This combines with water to form sulfuric acid (H2SO4) the form in which it is precipitated out as acid rain.