OCCURRENCE AND TRANSFORMATION OF DISSOLVED DIMETHYL SULFUR SPECIES IN STRATIFIED SEAWATER (WESTERN MEDITERRANEAN-SEA)

Dimethyl sulfide (DMS), dissolved dimethylsulfoniopropionate (DMSP(d)) and dimethylsulfoxide (DMSO) were measured in Mediterranean seawater. In the open surface waters of late spring to early summer, DMSO (18 nM) was the main methylated sulfur compound and dominated over DMS (1.7 nM) by 1 order of magnitude. Conversely, DMSP(d) occurred at lower concentrations (0.55 nM) than DMS. The higher abundance of DMS over DMSP(d), the poor correlation between surface DMS and chlorophyll a, and microscopic examination of the algal populations suggest that DMS essentially originated from decomposition of the DMSP(d) generated in a late decay phase of the spring phytoplankton development. On the other hand, the vertical profile of the dissolved dimethyl sulfur species and the depth distribution of both biological DMS production and consumption rates point to the concurrence of the biogenic cycles of DMS and carbon. Thus, highest DMS production occurs near the subsurface chlorophyll maximum in coincidence with the water column layers of highest microbial heterotrophic activity. Finally, the high DMSO concentrations suggest that this species may act as a major nonvolatile dimethyl sulfur pool in these waters.