Demethylation of dimethylsulfoniopropionate and production of thiols in anoxic marine sediments.

Dimethylsulfoniopropionate (DMSP) is a natural product of algae and aquatic plants, particularly those from saline environments. We investigated whether DMSP could serve as a precursor of thiols in anoxic coastal marine sediments. The addition of 10 or 60 muM DMSP to anoxic sediment slurries caused the concentrations of 3-mercaptopropionate (3-MPA) and methanethiol (MSH) to increase. Antibiotics prevented the appearance of these thiols, indicating biological formation. Dimethyl sulfide (DMS) and acrylate also accumulated after the addition of DMSP, but these compounds were rapidly metabolized by microbes and did not reach high levels. Acrylate and DMS were probably generated by the enzymatic cleavage of DMSP. MSH arose from the microbial metabolism of DMS, since the direct addition of DMS greatly increased MSH production. Additions of 3-methiolpropionate gave rise to 3-MPA at rates similar to those with DMSP, suggesting that sequential demethylation of DMSP leads to 3-MPA formation. Only small amounts of MSH were liberated from 3-methiolpropionate, indicating that demethiolation was not a major transformation for 3-methiolpropionate. We conclude that DMSP was degraded in anoxic sediments by two different pathways. One involved the well-known enzymatic cleavage to acrylate and DMS, with DMS subsequently serving as a precursor of MSH. In the other pathway, successive demethylations of the sulfur atom proceeded via 3-methiolpropionate to 3-MPA.