Involved in the assimilation of dimethylsulphoniopropionate (DMSP), an important compound in the fixation of carbon in marine phytoplankton, by mediating the conversion of 3-(methylthio)propanoyl-CoA (MMPA-CoA) to 3-(methylthio)acryloyl-CoA (MTA-CoA). 3-(methylsulfanyl)propanoyl-CoA + H(+) + oxidized [electron-transfer flavoprotein] = 3-(methylsulfanyl)acryloyl-CoA + reduced [electron-transfer flavoprotein] Cells lacking this gene do not grow on methylmercaptopropionate (MMPA) as the sole source of carbon. Belongs to the acyl-CoA dehydrogenase family.