The rates and concentration dependence of microbial dimethylsulfide (DMS) and dimethylsulfoniopropionate (DMSP) cycling were assessed in Vineyard Sound, Massachusetts, and the Sargasso Sea. Net rates of dissolved DMSP loss, DMS production from DMSP, and DMS consumption were measured at ambient and near-ambient substrate levels. Turnover times of dissolved DMSP, based on loss rates, were short, usually ranging from 1.5 d to several hours. However, the specific process of DMSP cleavage to form DMS accounted for only a portion of total DMSP turnover. Short-term variability obscured possible regional or seasonal trends in these data. Dissolved DMSP removal and cleavage exhibited saturable kinetics in spring in Vineyard Sound and in winter in the Sargasso Sea. In fall and spring in the Sargasso Sea, where dissolved DMSP levels ranged from 2 to 9 nM, DMSP cleavage did not saturate at DMSP levels as high as 1 PM. Kinetic parameters of DMSP uptake appeared to increase with proximity to summer, as did the proportion of DMSP uptake resulting in DMS production.The tertiary sulfonium compound dimethylsulfoniopropionate (DMSP), synthesized by marine algae, is a ubiquitous component of particulate matter in seawater and is the major precursor of marine dimethylsulfide (DMS). As a zwitterion, DMSP is presumably released to the dissolved pool from grazed (Dacey and Wakeham 1986) or senescent (Matrai and Keller 1993) algal cells, and its concentration in seawater often exceeds that of DMS (Turner et al. 1988;Wakeham and Dacey 1989). Due to the potentially significant role of DMS in atmospheric chemistry and climate through formation of sulfate aerosol (Charlson et al. 1987), the microbial processes by which extracellular DMSP is converted to DMS or diverted from this fate have come under increasing scrutiny. Microbial cleavage of DMSP to form DMS has been observed in natural seawater (Wakeham et al. 1987;Kiene 1990; Kiene and Service 199 l), and marine bacteria that can both cleave and demethylate DMSP have been iso-

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