Comparison of selenium and sulfur volatilization by dimethylsulfoniopropionate lyase (DMSP) in two marine bacteria and estuarine sediments

The volatile forms of selenium emitted from soils and plants have been identified by others as dimethylselenide and dimethyldiselenide. Dimethylselenoniopropionate, a possible precursor of dimethylselenide, is the selenium analogue of the common marine osmoprotectant, dimethylsulfoniopropionate. Dimethylselenoniopropionate was organically synthesized from dimethylselenide and acrylate and used in a comparative study with dimethylsulfoniopropionate as a growth substrate, an inducer and substrate for dimethylsulfoniopropionate lyase in two marine bacterial isolates, Alcaligenes sp. strain M3A and Pseudomonas doudoroffii, and in salt marsh sediments and estuarine creek water samples. In P. doudoroffii, the rate of dimethylselenoniopropionate transport into cells was about half that observed with dimethylsulfoniopropionate; Alcaligenes does not take up these molecules. The rate and extent of induction of dimethylsulfoniopropionate lyase in P. doudoroffii at low concentrations of dimethylselenoniopropionate were similar to that of dimethylsulfoniopropionate. In Alcaligenes the low constitutive level of dimethylsulfoniopropionate lyase cleaved dimethylselenoniopropionate into dimethylselenide and acrylate, but induction of dimethylsulfoniopropionate lyase by dimethylselenoniopropionate did not follow. Dimethylsulfoniopropionate lyases from both strains, when induced by dimethylsulfoniopropionate, utilized dimethylselenoniopropionate as a substrate with emission of dimethylselenide. Rates of partially purified dimethylsulfoniopropionate lyase activity with dimethylselenoniopropionate were half those with dimethylsulfoniopropionate. Anoxic salt marsh sediments catalyzed dimethylsulfoniopropionate and dimethylselenoniopropionate degradation at equal rates. In sediments containing high organic matter, both dimethylsulfide and dimethylselenide served as substrates for methanogenesis. These data support the hypothesis that organic selenium molecules can be volatilized by the same biochemical pathways as organic sulfur molecules.