Bound thiols are probably significant microbial substrates, since over 95% of the 3-MPA and methanethiol fractions recovered from the sediments are bound by disulfide linkages to sediment particles. In previous research, 3-Mercaptopropionate (3-MPA) was detected as major thiol in the anoxic, coastal, marine sediments; present at lower concentrations were mercaptopyruvate, mercaptoethanol and mercaptoacetate. 3-MPA occurred as its oxidized, dithio-form, in oxic regions of the sediments. Dimethylsulfoniopropionate (DMSP), an osmolyte in marine plants, was transformed in sediment slurries to 3-MPA by sequential demethylation with 3-methiolpropionate as an intermediate. Homocysteine and methionine were also converted to 3-MPA. These transformations need establishing by the isolation of the micro- organisms that are involved. Bacteria that demethylate DMSP or 3-methiolpropionate will be isolated, as will those which dissimilate and assimilate 3-MPA or its methylated precursors. Sulfate-reducers that incompletely oxidize organic compounds will be tested for conversions of sulfur-containing amino acids to 3- MPA, mercaptopyruvate, mercaptoethanol and mercaptoacetate. The ability of sulfate-reducers to oxidize mercaptopyruvate, mercaptoacetate and mercaptoethanol will also be studied. The research will reveal biochemical conversions of organic thiols that are characteristic of, and perhaps unique to, marine sediments.
