ABSTRACT: We characterised and compared dimethylsulfoniopropionate (DMSP) lyase isozymes in crude extracts of 6 axenic Emiliania huxleyi cultures (CCMP 370, 373, 374, 379, 1516, and strain L). This enzyme cleaves DMSP to form dimethyl sulfide(DMS), acrylate and a proton, but the function of this reaction in algae is still poorly understood. Most of the cultures produced high concentrations of intracellular DMSP, which was constant over the growth cycle and ranged from 157 to 242 mM, exceptfor 1516 which had 50 mM DMSP cell^-1. Extracts of all strains produced DMS from exogenous DMSP in vitro. DMSP lyases appeared constitutive, but enzyme activity and behaviour varied greatly among strains, and did not correlate withintracellular DMSP concentration. Strains 373 and 379 showed high DMSP lyase activities (12.5 and 6.1 fmol DMS cell^-1 min^-1, respectively), whereas DMS production was more than 100-fold lower in 370, 374, 1516 and L. This differencewas intrinsic and the general pattern of high- and low-activity strains remained true over more than a 1 yr cultivation period. The cleavage reaction was optimal at pH 6 in the strains with high lyase activity and pH 5 was optimal for 374, 1516 and L.Strain 370 showed increasing activity with increasing pH. Experiments with additions of 0.125 to 2 M NaCl indicated halotolerant DMSP lyases in 373, 379 and 374. However, the halophilic DMSP lyases in 370 and L required 1 M NaCl addition for optimal DMSPcleavage, and 1516 showed optimal activity at 2 M NaCl. These results suggest that there are several structurally different DMSP lyase isozymes within E. huxleyi. However, it cannot be ruled out that varying concentrations of DMSP lyase per cellmay have contributed to the differences in enzyme activity per cell. Comparison with other algal taxa indicates several families of DMSP lyases, hinting at possibly different cellular locations and functions, and varying DMS production under naturalconditions.