Involved in the assimilation of dimethylsulphoniopropionate (DMSP), an important compound in the fixation of carbon in marine phytoplankton, by mediating demethylation of dimethylsulfonioproprionate (DMSP) to methyl-mercaptopropionate (MMPA). The intracellular concentration of DMSP is estimated to be 70 mM. (6S)-5,6,7,8-tetrahydrofolate + S,S-dimethyl-beta-propiothetin = (6S)-5-methyl-5,6,7,8-tetrahydrofolate + 3-(methylsulfanyl)propanoate + H(+) In extracts of strain DSS-3. Optimum pH is 7.0-8.0. By dimethylsulfonioproprionate (DMSP) and acrylate. Loss of DMSP demethylation to MMPA. 25-fold increased sensitivity to acrylate, growth is strongly inhibited by 20 mM dimethylsulfonioproprionate (DMSP). Disruption of this gene has a polar effect on acuI, the downstream gene, which accounts for the increased sensitivity. Dimethyl sulfide (DMS) production is enhanced. DMSP is used as an intracellular osmolyte, predator deterrent and antioxidant. Belongs to the GcvT family. DmdA subfamily.