Involved in the dimethyl sulfide degradation pathway. Catalyzes the oxidation of dimethylsulfone (DMSO2) to yield methanesulfinate, which is oxidized spontaneously to methanesulfonate in the presence of dioxygen and FMNH(2). dimethyl sulfone + FMNH2 + O2 = FMN + formaldehyde + 2 H(+) + H2O + methanesulfinate Transcriptionally activated by SfnR. Cells lacking this gene are unable to grow on dimethyl sulfide (DMS), dimethylsulfone (DMSO2) and diethyl sulphone (DESO2). Belongs to the SsuD family.