Dear Carolyn,
I have been led to believe that sulfate assimilation is associated with
bacteria and plants and that higher animals are not able to convert sulfate
into S-amino acids, etc (although they interconvert organic S-compunds).
Our S-isotope food chain investigations tend to support this concept.. The
organic-S in the consumer has an isotope composition very close to that of
organic-S in the diet.. Sulfate in  consumers  is very interesting. Looking
at trace sulfate in teeth and kidney stones  and  the copious sulfate in
urine attests to sulfate dominantly originating from the oxidation of
organic-S. Oxygen isotope data for sulfate also support this. For example,
one finds that the delta 34S value for trace sulfate in marine fish teeth
is not +21 permil (ocean sulfate) but rather + 17 permil, the same as
organic-S.
Roy Krouse.