Biochemistry, physiology, and ecology of marine microbial dimethyl sulfide production.

摘要

Dimethylsulfoniopropionate (DMSP) is produced by some marine algae and by Spartina alterniflora, the marsh grass. It is cleaved by an enzyme DMSP lyase giving rise to dimethylsulfide (DMS), acrylate and a proton. DMS comprises 90% of biogenic sulfur emissions from sea water, is thought to affect climate, increase cloud reflectivity and the acidity of rainfall. DMSP acted as an energy source for bacteria because it stimulated energy requiring processes like nitrogen fixation, denitrification and CO{dollar}sb2{dollar} production in anoxic salt marsh sediments. The physiology of DMS production by DMSP lyase was studied in two bacteria, Alcaligenes strain M3A and Pseudomonas doudoroffii, and a macroalga, Ulva curvata, and then the enzyme was purified and characterized from these organisms. This enzyme had never been purified previously and N-terminal amino acid sequencing revealed no known homology to any protein sequenced thus far. Kinetic and inhibitor studies led to the postulation of a model of DMS production and the putative role of a DMSP-binding protein. Polyclonal antibodies raised against the Alcaligenes strain M3A DMSP lyase, cross reacted with cell-free extracts of eight other marine bacterial isolates and eight algal species revealing a number of isoforms with diverse molecular masses; this diversity may enable DMSP lyase-producers to successfully compete for the varying concentrations of DMSP found in marine environments. The antibodies were also used to quantitate DMSP lyase in water samples along an estuarine salinity gradient. Enzyme concentrations, DMSP lyase activity, and salinity all decreased with distance along the transect. Using capillary tube assays and computer assisted video motion analysis Alcaligenes strain M3A which possessed a DMSP lyase was shown to respond chemotactically to DMSP at concentrations occurring naturally in ocean water. In contrast, genetically-identical cells that did not possess a DMSP lyase were not chemotactic towards DMSP. Chemotaxis enables bacteria like Alcaligenes M3A to move up gradients of DMSP, degrade this compound, release DMS and utilize acrylate as an energy source. These data provide some explanations of the mechanisms leading to DMS production in marine environments.