All organisms metabolize sulfur, and it is incorporated into the structure of proteins, polysaccharides, steroids, and many coenzymes. The biological pathway by which an organism takes up and/or removes sulfur can have significant impacts on the sulfur isotope composition of the organism and its environment.
Microorganisms that consume and reduce sulfate in relatively large quantities perform a different pathway of sulfur uptake called dissimilatory sulfate reduction. These organisms use sulfate reduction as an energy source as opposed to a way to synthesize new cell components, and remove the resulting sulfide as a waste product. Microbial sulfate reduction has been demonstrated to fractionate sulfur isotopes in bacteria, with some studies showing a dependence upon sulfate concentration and/or temperature. Studies examining dozens of species of dissimilatory sulfate reducing microbes have observed sulfur isotope fractionations ranging from −65.6‰ to 0‰.
Some organisms take in relatively small amounts of sulfate in a process called assimilatory sulfate reduction, for the purpose of synthesizing compounds that contain sulfur, such as the amino acids methionine and cysteine that can then be used to make proteins. In phytoplankton, most of the sulfur taken up through assimilatory sulfate reduction is incorporated into biomass as proteins (~35%), sulfate esters (~20%), and low-weight sulfur-containing compounds (~40%). Literature on the isotopic fractionation effects of the assimilatory sulfate reduction pathway is much more limited than that discussing dissimilatory sulfate reduction, but some sources report slight isotopic variations (δS = −4.4‰ to +0.5‰) in the resulting organic sulfur relative to the surrounding sulfate.
While dissimilatory sulfate reduction and assimilatory sulfate reduction are two of the most common pathways by which organisms take up and utilize sulfate, there are many other pathways by which living things take up sulfur. For example, sulfur oxidation of compounds like hydrogen sulfide and elemental sulfur is performed by lithotrophic bacteria and chemosynthetic archaea. Most animals obtain sulfur directly from the methionine and cysteine in the protein they consume.