The 3-dimethylsulphoniopropionate (DMSP) produced by marine algae is the main biogenic precursor of atmospheric dimethylsulphide (DMS)(1-3). This biogenic DMS, formed by bacterial and algal degradation of DMSP4,5, contributes about 1.5 x 10(13) g of sulphur to the atmosphere annually(3), and plays a major part in the global sulphur cycle, in cloud formation and potentially in climate regulation(1,3). Although DMSP biosynthesis has been partially elucidated in a higher plant(6,7), nothing is known about how alg-ae make DMSP except that the whole molecule is derived from methionine(8-12). Here we use in vivo isotope labelling to demonstrate that DMSP synthesis in the green macroalga Enteromorpha intestinalis proceeds by a route entirely distinct from that in higher plants. From methionine, the steps are transamination, reduction and S-methylation to give the novel sulphonium compound 4-dimethylsulphonio-2-hydroxybutyrate (DMSHB), which is oxidatively decarboxylated to DMSP. The key intermediate DMSHB was also identified in three diverse phytoplankton species, indicating that the same pathway operates in other algal classes that are important sources of DMS. The fact that a transamination initiates this pathway could help explain how algal DMSP (and thereby DMS) production is enhanced by nitrogen deficiency(12).