Substrate kinetics of DMSP-lyases in axenic cultures and mesocosm populations of Emiliania huxleyi

Most oceanic dimethyl sulphide (DMS) is produced through the enzymatic cleavage of dimethylsulphoniopropionate (DMSP), a process governed by the activity of DMSP-lyases in algae and bacteria. Laboratory and field experiments with the DMS-producing coccolithophorid Emiliania huxleyi have advanced our understanding of algal DMSP lyases. However, despite their central role in the global biogeochemical cycle of sulphur, little is known about the physiological and biochemical properties of this group of isozymes. Here we investigate the apparent substrate kinetic properties of two axenic E. huxleyi cultures (CCMP 373 and CCMP 379) under two light conditions and a mesocosm assemblage dominated by this species. Assays of in vivo and in vitro DMSP-lyase activity (DLA) were used to estimate the kinetic parameters Km and Vmax. Whereas in vivo DLA was not detected in CCMP 379, both laboratory cultures showed high DLA during in vitro tests. The mesocosm population was initially characterised by low substrate affinity (low Vmax and high Km), but shifted to a high affinity community when more dissolved DMSP became available after the collapse of the E. huxleyi bloom. At natural DMSP concentrations, DMS production is probably more affected by changes in Km than Vmax and future studies should include further investigations of the kinetic properties of DMSP-lyases.