The massive quantities of phytoplankton in the North Atlantic and Antarctic oceans producing dimethylsulfoniopropionate (DMSP) as an osmoprotectant, much of which is degraded by marine bacteria to dimethylsulfide (DMS), ensures an important role for both compounds in the global sulfur cycle. The closest to a comprehensive review on this topic is a book of symposium proceedings edited by Kiene et al. (75); the more recent developments related specifically to DMSP degradation by microbial communities are found elsewhere (68). This article is more comprehensive, as it includes some of the earlier literature in describing the sources of DMSP, its release and linkage to the marine (primarily microbial) food web and subsequent degradation via cleavage to DMS and acrylic acid or demethylation and demethiolation to methanethiol.DMS production from DMSP has long been associated with marine algae according to the following reaction (20, 22):DMSP is a tertiary sulfonium compound produced in high concentration by certain species of marine algae and plant halophytes for the regulation of their internal osmotic environment (1,41,47,120), although its role in plants remains unclear. This alga-associated, i.e., particulate DMSP (DMSPp), when released into the marine environment as dissolved DMSP (DMSPd), can serve as a link between primary production and the microbial population, as it is readily degraded by chemoheterotrophic bacteria (59). DMSP turnover usually exceeds DMS production in natural waters (60) because DMSP is also demethylated to 3-methiolpropionate, which can be further demethylated to 3-mercaptopropionate or demethiolated, releasing methanethiol (72,118). These reactions will be discussed in more detail below.The biogeochemical significance of DMSP cleavage was first suggested in 1972, when DMS was found to be universally present in seawater and emitted at a significant rate to the atmosphere (87). It was proposed that DMS, rather than H 2 S from coastal waters and mud flats, was the missing gaseous sulfur compound needed to enable the steady-state flow of sulfur between marine and terrestrial environments, making DMS emissions a key step in the global sulfur cycle (87). Atmospheric H 2 S, which arises primarily from dissimilatory sulfate reduction in organic matter-rich environments, could never be measured in sufficient quantity to be the vehicle for transferring large quantities of sulfur from sea to air to land.The total annual flux of biogenic DMS released to the atmosphere ranges from 28 to 45 Tg of S year Ϫ1 , at least 10-fold higher than from all other sources (Table 1). Recent, more comprehensive calculations of global annual DMS flux from the oceans gave values that ranged from 13 to 37 Tg of S year Ϫ1 (57). This sea-to-air flux represents about 50% of the global biogenic sulfur flux to the atmosphere (3). However, anthropogenic sulfur emissions dominate the sulfur flux, representing 80 to 90% of the input to the global sulfur cycle (12,23,88).The magnitude of the marine DMS emissions is all the more ...

Dimethyl sulfide (DMS) is quantitatively the most important biogenic sulfur compound emitted from oceans and salt marshes. It is formed primarily by the action of dimethylsulfoniopropionate (DMSP) lyase which cleaves DMSP, an algal osmolyte, to equimolar amounts of DMS and acrylate. This report is the first to describe the isolation and purification of DMSP lyase. The soluble enzyme was purified to electrophoretic homogeneity from a facultatively anaerobic gram-negative rod-shaped marine bacterium identified as an Alcaligenes species by the Vitek gram-negative identification method. The key to successful purification of the enzyme was its binding to, and hydrophobic chromatography on, a phenyl-Sepharose CL-4B column. DMSP lyase biosynthesis was induced by its substrate, DMSP; its product, acrylate; and also by acrylamide. The relative effectivenesses of the inducers were 100, 90, and 204%, respectively. DMSP lyase is a 48-kDa monomer with a Michaelis-Menten constant (K m) for DMSP of 1.4 mM and a V max of 408 mol/min/mg of protein. It converted DMSP to DMS and acrylate stoichiometrically. The similar K m values measured for pure DMSP lyase and the axenic culture, seawater, and surface marsh sediment suggest that the microbes in these ecosystems must have enzymes similar to the one purified from our marine isolate. Anoxic sediment populations, however, have a 40-fold-lower K m for this enzyme (30 M), possibly giving them the capability to metabolize much lower levels of DMSP than the aerobes.

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Dimethylsulfoniopropionate (DMSP), an abundant osmoprotectant found in marine algae and salt marsh cordgrass, can be metabolized to dimethyl sulfide (DMS) and acrylate by microbes having the enzyme DMSP lyase. A suite of DMS-producing bacteria isolated from a salt marsh and adjacent estuarine water on DMSP agar plates differed markedly from the pelagic strains currently in culture. While many of the salt marsh and estuarine isolates produced DMS and methanethiol from methionine and dimethyl sulfoxide, none appeared to be capable of producing both methanethiol and DMS from DMSP. DMSP, and its degradation products acrylate and ␤-hydroxypropionate but not methyl-3-mecaptopropionate or 3-mercaptopropionate, served as a carbon source for the growth of all the ␣-and ␤-but only some of the ␥-proteobacterium isolates. Phylogenetic analysis of 16S rRNA gene sequences showed that all of the isolates were in the group Proteobacteria, with most of them belonging to the ␣ and ␥ subclasses. Only one isolate was identified as a ␤-proteobacterium, and it had >98% 16S rRNA sequence homology with a terrestrial species of Alcaligenes faecalis. Although bacterial population analysis based on culturability has its limitations, bacteria from the ␣ and ␥ subclasses of the Proteobacteria were the dominant DMS producers isolated from salt marsh sediments and estuaries, with the ␥ subclass representing 80% of the isolates. The ␣-proteobacterium isolates were all in the Roseobacter subgroup, while many of the ␥-proteobacteria were closely related to the pseudomonads; others were phylogenetically related to Marinomonas, Psychrobacter, or Vibrio species. These data suggest that DMSP cleavage to DMS and acrylate is a characteristic widely distributed among different phylotypes in the salt marsh-estuarine ecosystem.Dimethyl sulfide (DMS)-producing bacteria play an important but as yet unquantified role in the biogenic transfer of sulfur from the ocean to the atmosphere (2, 25, 30). DMS production results from the enzymatic degradation of dimethylsulfoniopropionate (DMSP) (9,28,43), an osmoprotectant (13, 14, 22, 47) produced and stored by marine phytoplankton, macroalgae, cyanobacteria, and coastal vascular plants (e.g., Spartina alterniflora) (7,8,23,38,50). When these organisms senesce and decay or phytoplankton are grazed upon by zooplankton, the intracellular DMSP is released into the water column or sediment (9,35,53), where it can be used as a carbon and energy source for the bacterial community (25, 49). The enzymatic degradation of DMSP to DMS and acrylate has been observed in marine and estuarine bacteria (11, 25, 31), fungi (4), and algae (6, 22, 41). The enzyme responsible, DMSP lyase, has been purified from several marine bacteria (11,12,48).Understanding the factors controlling DMS production in the marine environment relies on knowing the abundance, phenotypic diversity, and physiology of the microbes involved in this process. As there is not a functional probe available to quantitate and identify DMS-producing microbes in envir...

Phytoremediation of toxic inorganic selenium compounds by accumulation, assimilation, and volatilization is an ideal way to rid contaminated soils and sediments of these molecules. In this context, salt marsh cordgrass (Spartina alterniflora) was investigated for its potential to produce dimethylselenoniopropionate (DMSeP), which as we have shown can serve as a precursor for the enzymatic volatilization of the relatively nontoxic gas, dimethylselenide (DMSe). Plants grown in sand culture, under varying saline conditions amended with the environmentally toxic form of selenium (selenate) were analyzed for organoselenonium compounds. DMSeP was positively identified in plant tissue and partially purified plant extracts by alkaline degradation to DMSe, 1H and 77Se NMR, and by enzymatic cleavage by DMSP lyase to DMSe (and acrylate). DMSeP levels were highest in plants grown in high salt (full-strength seawater) and high selenium. Preliminary evidence suggests that cordgrass may also produce Se-methyl selenomethionine, the putative precursor of DMSeP. This appears to be the first report for the biological assimilation of selenate into DMSeP by a plant species. These findings suggest a possible mechanism for the volatilization of selenium, as DMSe, analogous to that of dimethylsulfide (DMS) production by the salt tolerant cordgrass, Spartina alterniflora.