Reduction of Dimethylsulfoxide to Dimethylsulfide by Marine Phytoplankton

Dimethylsulfoxide (DMSO) is an abundant but poorly understood methylated sulfur compound in the marine environment. One potentially significant loss pathway for DMSO is through its biological reduction to dimethylsulfide (DMS), which has been documented in a number of organisms, most notably bacteria. Here we present the first detailed study of DMSO reduction by several marine phytoplankton in axenic cultures. Reduction of DMSO was observed in four algal classes, with in vivo reduction rates ranging from 0.006 to 1.5/ µmol [L cell volume]⁻¹ s⁻¹ at 1.0 mmol L⁻¹ DMSO. Corresponding turnover times for measured intracellular DMSO pools varied from hours to days. Michaelis-Menton kinetic parameters were estimated for Isochrysis galbana, Thalassiosira pseudonana, and Amphidinium carterae. The half-saturation constant $(K_m )$ and maximal rate $(V_{\max } )$ for DMSO reduction ranged between 0.96 and 2.7 mmol [L cell volume]⁻¹ and 17–118 nmol [L cell volume]⁻¹ s⁻¹ , respectively. Our results suggest that DMSO reduction is a universal activity in marine phytoplankton, even in algae with no detectable dimethylsulfoniopropionate (DMSP). Although reduction of DMSO by marine eukaryotes may not contribute significantly to removal of DMSO from the dissolved phase, this reduction is likely to be a major source of DMS in species lacking detectable DMSP lyase activity. The ability of marine phytoplankton to reduce DMSO to DMS should allow algae to cycle these compounds as part of an antioxidant system.