Kinetics of DMSP lyases in whole cell extracts of four Phaeocystis species: Response to temperature and DMSP analogs

Dimethylsulfoniopropionate (DMSP) lyases are present in a wide variety of marine phytoplankton and are responsible for converting the osmolyte DMSP into dimethylsulfide (DMS) and acrylate. The physiological functions of DMSP lyases are not well understood, but they have received considerable attention because of the role of volatile DMS in trophic dynamics and ocean–atmosphere sulfur exchange. Marine phytoplankton of the genus Phaeocystis are important DMSP producers in the ocean and play a pivotal role in global biogeochemical cycles by forming massive blooms and emitting large amounts of DMS to the atmosphere (~ 0.05 Tmol DMS year −1 ). In this study, we used an in vitro, whole-cell extract assay to examine the pH and temperature dependence as well as substrate-specific kinetics of DMSP lyase activity in five different strains of Phaeocystis, including colonial growth forms of Phaeocystis antarctica CCMP 1871, Phaeocystis globosa CCMP 627 and P. globosa CCMP 628, and single cell growth forms of Phaeocystis jahnii CCMP 2496 and Phaeocystis cordata CCMP 3104. All of the tested strains had optimum lyase activity at pH 5. At this pH, the highest Vmax and lowest Km values were recorded at 20 °C for P. antarctica ; 25 °C for P. globosa 627, P. jahnii and P. cordata ; and 30 °C for P. globosa 628. Under optimal conditions, Vmax and Km ranged from 22.2 to 56.4 nmol DMS min −1 μg Chl a −1 and 2.11 to 7.12 mM, respectively, and higher Vmax values were found in colonial Phaeocystis species ( P. antarctica and P. globosa ) as compared with the single-celled species ( P. cordata and P. jahnii ). Substrate specificity tests indicated that the lyases of all five Phaeocystis strains cleaved the DMSP analogs, diethylsulfoniopropionate (DESP) and 2-chloroDMSP with liberation of diethylsulfide (DES) and DMS, respectively. However, the lyases of all five Phaeocystis strains had at least 1.5 to 3.4 times higher affinity towards DMSP compared to its analogs 2-chloroDMSP and DESP, indicating moderate specificity of the enzyme for DMSP. Chlorophyll a -normalized lyase activities in Phaeocystis species are in the upper range of what has been measured in field samples, highlighting the potential importance of Phaeocystis spp. in oceanic DMS production.