In the marine environment, production of dimethylsulfide (DMS) from dissolved dimethylsulfoniopropionate (DMSP(d)) - an algal osmolyte - is thought to occur mainly through bacterial activity. We have investigated the possibility that phytoplankton cells convert DMSP(d) into DMS, using axenic batch cultures of Phaeocystis sp. at different growth stages. DMSP(d) added to the medium was converted enzymatically to DMS by Phaeocystis sp, A culture in the exponential growth phase displayed Michaelis-Menten type kinetics for DMSP(d) conversion, yielding an apparent K(m) value for DMSP(d) Of 11.7 +/- 3.1 muM and a V(max) value of 3.05 +/- 0.48 nmol DMS produced min-1 (10(6) cells)-1. DMSP(d) conversion rates declined during the transition from exponential to stationary growth phase, at least partly due to a diminished overall affinity of the enzyme system(s) involved in DMSP conversion. No evidence was obtained for accumulation of inhibiting substances in the medium. Intracellular DMSP concentrations in Phaeocystis sp. batch cultures increased from 71 mM in exponential-phase cells to ca 150 mM in stationary-phase cells. DMS and DMSP(d) concentrations in the culture remained very low during the exponential growth phase. DMS production started in early stationary phase. In a senescent culture DMSP(d) appeared when cell numbers started to decline. DMSP production in this culture continued even when cell numbers declined. In completely lysed batch cultures some 25% of total DMSP remained as DMSP(d). The results indicate that Phaeocystis sp. may contribute significantly to DMS production from DMSP(d) during bloom situations in the field.