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

Dissolved dimethylsulfoniopropionate (DMSPd) is normally present in seawate r 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 (K-m(app)) orders of magn itude higher than normal DMSPd concentrations. It has been speculated that microzones with high concentrations of dissolved organic compounds (includi ng DMSPd) exist around phytoplankton cells and detrital particles. This stu dy uses filter fractionation of natural seawater samples to demonstrate dif ferent kinetic responses of DMSPd consumption and DMS production between fr ee-living and particle-associated fractions of the microbial community. Sea water samples were passed through 2 mu m 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 consump tion and net DMS production in filtered and unfiltered samples show a 10-fo ld difference between the substrate affinities in free-living versus partic le-associated organisms. In the free-living fraction, K-m(app) = 29.4 nM fo r DMSPd consumption, 119 nM for DMS production. In the particle-associated fraction, K-m(app) = 223 nM for DMSPd consumption, 935 nM for DMS productio n. This is consistent with the interpretation that particle-associated DMSP -lyases (bacterial or phytoplanktonic) are acclimated to high DMSPd concent rations while free-living bacteria are acclimated to the (much lower) bulk phase DMSPd. In addition, the total enzyme activity and the relative import ance 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. Cro wn Copyright (C) 2000 Published by Elsevier Science B.V. All rights reserve d.