Particle size-fractionated kinetics of DMS production: where does DMSP cleavage occur at the microscale?

Dissolved dimethylsulfoniopropionate (DMSPd) is normally present in seawater at low concentrations (typically a few nM). However, marine bacteria have been shown to exhibit low-affinity kinetics for the enzymatic consumption of DMSPd, with apparent half-saturation constants (Km(app)) orders of magnitude higher than normal DMSPd concentrations. It has been speculated that microzones with high concentrations of dissolved organic compounds (including DMSPd) exist around phytoplankton cells and detrital particles. This study uses filter fractionation of natural seawater samples to demonstrate different kinetic responses of DMSPd consumption and DMS production between free-living and particle-associated fractions of the microbial community. Seawater samples were passed through 2 μm GMF filters to remove the majority of phytoplankton cells and detrital particles but to allow the passage of free-living bacteria and free enzymes. Kinetic assays for net DMSPd consumption and net DMS production in filtered and unfiltered samples show a 10-fold difference between the substrate affinities in free-living versus particle-associated organisms. In the free-living fraction, Km(app)=29.4 nM for DMSPd consumption, 119 nM for DMS production. In the particle-associated fraction, Km(app)=223 nM for DMSPd consumption, 935 nM for DMS production. This is consistent with the interpretation that particle-associated DMSP-lyases (bacterial or phytoplanktonic) are acclimated to high DMSPd concentrations while free-living bacteria are acclimated to the (much lower) bulk phase DMSPd. In addition, the total enzyme activity and the relative importance of DMS production (as a percentage of net DMSPd consumption) were much higher in the particle-associated fraction. Further understanding of these relationships could aid in modelling the ocean–atmosphere flux of DMS.