Phylogenetic identification and metabolism of marine dimethylsulfide-consuming bacteria

Microbial consumption is one of the main processes, along with photolysis and ventilation, that remove the biogenic trace gas dimethylsulfide (DMS) from the surface ocean. Although a few isolates of marine bacteria have been studied for their ability to utilize DMS, little is known about the characteristics or phylogenetic affiliation of DMS consumers in seawater. We enriched coastal and open-ocean waters with different carbon sources to stimulate different bacterial communities (glucose-consuming bacteria, methyl group-consuming bacteria and DMS consumers) in order to test how this affected DMS consumption and to examine which organisms might be involved. Dimethylsulfide consumption was greatly stimulated in the DMS addition treatments whereas there was no stimulation in the other treatments. Analysis of microbial DNA by two different techniques (sequenced bands from DGGE gels and clone libraries) showed that bacteria grown specifically with the presence of DMS were closely related to the genus Methylophaga. We also followed the fate of consumed DMS in some of the enrichments. Dimethylsulfide was converted mostly to DMSO in glucose or methanol enrichments, whereas it was converted mostly to sulfate in DMS enrichments, the latter suggesting use of DMS as a carbon and energy source. Our results indicate that unlike the biochemical precursor of DMS, dimethylsulfoniopropionate (DMSP), which is consumed by a broad spectrum of marine microorganisms, DMS seems to be utilized as a carbon and electron source by specialists. This is consistent with the usual observation that DMSP turns over at much higher rates than DMS.     

Hydrophobic Components in Delipidated Granule of Egg Yolk

Dimethyl sulfide (DMS) is volatile compound important as one of the characteristic flavor compounds of marine food products. The precursor of DMS in marine products is dimethyl-β- propiothetin (DMPT), which is abundunt in green algae. DMPT was effectively extracted by the use of hydrophilic solvents from dried hitoegusa (Monostroma nitidum), a green alga. At around pH 4 and at over pH 9, the extracted DMPT was rapidly degraded to DMS; at around pH 7.5, this degradation was much slower. The DMS obtained volatilized immediately from aqueous solution. However, when the DMPT was formed into a powder with dextrin and heated to release the DMS, 40 — 60% of the DMS remained in the powder. The amount of DMS remaining was 80 % when cyclodextrin was used to form the powder.     

Isolation of Methylophaga spp. from marine dimethylsulfide-degrading enrichment cultures and identification of polypeptides induced during growth on dimethylsulfide

Dimethylsulfide (DMS)-degrading enrichment cultures were established from samples of coastal seawater, nonaxenic Emiliania huxleyi cultures, and mixed marine methyl halide-degrading enrichment cultures. Bacterial populations from a broad phylogenetic range were identified in the mixed DMS-degrading enrichment cultures by denaturing gradient gel electrophoresis (DGGE). Sequences of dominant DGGE bands were similar to those of members of the genera Methylophaga and Alcanivorax. Several closely related Methylophaga strains were obtained that were able to grow on DMS as the carbon and energy source. Roseobacter-related populations were detected in some of the enrichment cultures; however, none of the Roseobacter group isolates that were tested were able to grow on DMS. Oxidation of DMS by Methylophaga sp. strain DMS010 was not affected by addition of the inhibitor chloroform or methyl tert-butyl ether, suggesting that DMS metabolism may occur by a route different from those described for Thiobacillus species and other unidentified marine isolates. Addition of DMS and methanethiol to whole-cell suspensions of strain DMS010 induced oxygen uptake when strain DMS010 was grown on DMS but not in cells grown on methanol. The apparent K(m)s of strain DMS010 for DMS and for methanethiol were 2.1 and 4.6 microM, respectively, when grown on DMS. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of the biomass of strain DMS010 and analysis of peptide bands by mass spectrometry techniques and N-terminal sequencing provided the first insight into the identity of polypeptides induced during growth on DMS. These included XoxF, a homolog of the large subunit of methanol dehydrogenase for which a biological role has not been identified previously.     

Isolation and characterization of Methanomethylovorans hollandica gen. nov., sp. nov., isolated from freshwater sediment, a methylotrophic methanogen able to grow on dimethyl sulfide and methanethiol.

A newly isolated methanogen, strain DMS1(T), is the first obligately anaerobic archaeon which was directly enriched and isolated from a freshwater sediment in defined minimal medium containing dimethyl sulfide (DMS) as the sole carbon and energy source. The use of a chemostat with a continuous DMS-containing gas stream as a method of enrichment, followed by cultivation in deep agar tubes, resulted in a pure culture. Since the only substrates utilized by strain DMS1(T) are methanol, methylamines, methanethiol (MT), and DMS, this organism is considered an obligately methylotrophic methanogen like most other DMS-degrading methanogens. Strain DMS1(T) differs from all other DMS-degrading methanogens, since it was isolated from a freshwater pond and requires NaCl concentrations (0 to 0.04 M) typical of the NaCl concentrations required by freshwater microorganisms for growth. DMS was degraded effectively only in a chemostat culture in the presence of low hydrogen sulfide and MT concentrations. Addition of MT or sulfide to the chemostat significantly decreased degradation of DMS. Transient accumulation of DMS in MT-amended cultures indicated that transfer of the first methyl group during DMS degradation is a reversible process. On the basis of its low level of homology with the most closely related methanogen, Methanococcoides burtonii (94.5%), its position on the phylogenetic tree, its morphology (which is different from that of members of the genera Methanolobus, Methanococcoides, and Methanohalophilus), and its salt tolerance and optimum (which are characteristic of freshwater bacteria), we propose that strain DMS1(T) is a representative of a novel genus. This isolate was named Methanomethylovorans hollandica. Analysis of DMS-amended sediment slurries with a fluorescence microscope revealed the presence of methanogens which were morphologically identical to M. hollandica, as described in this study. Considering its physiological properties, M. hollandica DMS1(T) is probably responsible for degradation of MT and DMS in freshwater sediments in situ. Due to the reversibility of the DMS conversion, methanogens like strain DMS1(T) can also be involved in the formation of DMS through methylation of MT. This phenomenon, which previously has been shown to occur in sediment slurries of freshwater origin, might affect the steady-state concentrations and, consequently, the total flux of DMS and MT in these systems.     

The effects of methanol on the biofiltration of dimethyl sulfide in inorganic biofilters

Air emissions from the pulp and paper industry frequently contain reduced sulfur compounds (RSC), such as dimethyl sulfide (DMS) mixed with volatile organic compounds (VOC) (e.g., methanol, MeOH) and it is desirable to treat either one or both of these groups of compounds. The objective of this study was to assess the effects of VOC (MeOH) on the biofiltration of DMS. Results obtained from continuous experiments using three bench-scale biofilters packed with inorganic material clearly show that MeOH has a positive effect (11-fold increase) on the biofiltration of DMS. Further experiments indicate that MeOH addition enhances biomass concentration and viability (threefold) in the biofilters. However, a suspension of MeOH addition causes a rapid significant increase (twofold) in the removal rate of DMS, suggesting that the presence of MeOH also has a competitive effect on DMS biodegradation. This negative effect was also confirmed in batch experiments. The decrease of biofilter performance with time for a long-term suspension of MeOH addition indicates that MeOH addition is necessary to sustain a high removal rate of DMS in inorganic biofilters. Results on metabolic products of DMS biodegradation demonstrate that DMS is almost completely converted to sulfate in the absence of MeOH, while it is partially oxidized to elemental sulfur in the presence of MeOH. This study suggests that there exists an optimum mix of DMS and MeOH for the treatment of DMS emissions in inorganic biofilters.     

Modeling the biofiltration of dimethyl sulfide in the presence of methanol in inorganic biofilters at steady state

The presence of methanol (MeOH) improves DMS removal (up to 11-fold) by enhancing biomass growth in inorganic biofilters. Although the overall effect is positive, prolonged growth on methanol also negatively affects DMS degradation as a result of competition with DMS. The objectives of this study were to explore the potential to optimize DMS removal with methanol addition and to develop and experimentally validate a mathematical model describing the biofiltration of DMS in the presence of MeOH. Continuous experiments using three bench-scale biofilters packed with inorganic material were performed to examine the removal of DMS under different MeOH addition rates ranging from 0 to 140 g/m3/h. For a constant DMS loading of 3.5 g/m3/h, a maximum DMS removal rate of 1.8 g/m3/h was achieved at a MeOH addition rate of 20 g/m3/h in the inorganic biofilters. A steady-state model incorporating the competitive and activation effects of MeOH on DMS biodegradation was developed, and the modeled results on DMS and MeOH removal were in close agreement with experimental data. Both the experimental data and model simulation suggest that there is an optimum MeOH addition rate for a given DMS loading. A step-feeding strategy for MeOH addition was proposed and tested by the model to optimize DMS removal. The model-predicted results demonstrate that six-step feeding of MeOH enhances DMS treatment by 46% in the biofilters when compared to conventional feeding (one-step) of MeOH at the same total mass loading.     

Investigating the inter-relationships between water attenuated irradiance, primary production and DMS(P)

Both solar irradiance and primary production have been proposed as independent controls on seawater dimethyl sulphide (DMS) and dimethylsulphoniopropionate (DMSP) concentrations. However, irradiance also drives photosynthesis, and thus influences a complex set of inter-related processes that modulate marine DMS. We investigate the potential inter-relationships between the rate of primary production (carbon assimilation), water-attenuated irradiance and DMS/DMSP dynamics by applying correlation analysis to a high resolution, concurrently sampled in situ data set from a range of latitudes covering multiple biogeochemical provinces from 3 of the 4 Longhurst biogeochemical domains. The combination of primary production (PP) and underwater irradiance (Iz) within a multivariate regression model is able to explain 55% of the variance in DMS concentrations from all depths within the euphotic zone and 66% of the variance in surface DMS concentrations. Contrary to some previous studies we find a variable representing biological processes is necessary to better account for the variance in DMS. We find that the inclusion of Iz accounts for variance in DMS that is independent from the variance explained by PP. This suggests an important role for solar irradiance (beyond the influence of irradiance upon primary production) in mediating the relationship between the productivity of the ecosystem, DMS/DMSP production and ambient seawater DMS concentrations.     

Optimisation of a fast DMS sensor (FDS) for real time quantification of dimethyl sulfide production by algae

Production of dimethyl sulfide (DMS) from marine samples is often quantified using gas chromatography techniques. Typically, these are labour intensive and have a slow sample turnover rate. Here we demonstrate the use of a portable fast DMS sensor (FDS) that utilises the chemiluminescent reaction of DMS and ozone to measure DMS production in aqueous samples, with a maximum frequency of 10?Hz. We have developed a protocol for quantifying DMS production that removes potential signal interference from other biogenic trace gases such as isoprene (2-methyl-1,3-butadiene) and hydrogen sulfide. The detection limit was 0.89?pM (0.02?ppbv) when using a DMS standard gas mixture. The lowest DMS production rates quantified with the FDS and verified using conventional gas chromatography with flame photometric detection (GC-FPD) were around 0.01?nmol?min^?1. There was a strong correlation in DMS production when comparing the FDS and GC-FPD techniques with a range of marine samples (e.g., r ² ?=?0.94 for Emiliania huxleyi). However, the combined dataset showed the FDS measured 22% higher DMS production than the GC-FPD, with the differences in rates likely due to interfering gases, for example hydrogen sulfide and isoprene. This possible overestimation of DMS production is smaller than the two-fold difference in DMS production between day and night samples from a culture of E. huxleyi. The response time of the instrument to changes in DMS production is method dependent (e.g., geometry of incubation vessel, bubble size) and was approximately 4?min under our conditions when using a culture of E. huxleyi (800?ml) with aeration at 100?ml?min^?1. We suggest the FDS can reduce sample handling, is suitable for short- and long-term measurements of DMS production in algal cultures, and will widen the range of DMS research in marine environments.     

Variability in abundance and fluxes of dimethyl sulphide in the Indian Ocean

Dimethyl sulphide (DMS) is a biogenic gas of climatic significance on which limited information is available from the Indian Ocean. To fill this gap, we collected data on DMS and total dimethylsulphoniopropionate (DMSP_t) by participating in a dozen cruises. Here, we discuss the variability in DMS and DMSP_t in the north and central Indian Ocean in terms of their spatial and temporal variation. DMS and DMSP_t exhibited significant spatial and temporal variability. Apart from the concentration gradients in DMS within the Arabian Sea, Bay of Bengal and Central Indian Ocean basins, differences in average abundances were conspicuous between these basins. The Arabian Sea contained more DMS (mixed layer average was 7.8 nM) followed by the Bay of Bengal (2.8 nM) and the Central Indian Ocean (2.7 nM). The highest concentrations of DMS and DMSP_t (525 nM and 916 nM, respectively) were found in upwelling regimes along the west coast of India during the Southwest monsoon and fall intermonsoon seasons. Average surface DMS was the highest in the Arabian Sea. On the other hand observed sea-to-air fluxes of DMS were higher in the Bay of Bengal due to the prevalence of turbulent conditions. In the Arabian Sea wind speeds were low and hence the sea-to-air fluxes. The total diffusive flux of DMS from the study area to atmosphere is estimated to be about 1.02 × 10¹² g S y⁻¹, which contributes to 4.1–6.3% of the global DMS emission     

Dimethyl sulfide metabolism in salt marsh sediments

Abstract Anoxic sediment slurries prepared from Spartina salt marsh soils contained dimethyl sulfide (DMS) at concentrations ranging from 1 to 10 μM. DMS was produced in slurries over the initial 1–24 h incubation. After the initial period of production, DMS decreased to undetectable levels and methane thiol (MSH) was produced. Inhibition of methanogenesis caused a 20% decrease in the rate of DMS consumption, while inhibition of sulfate reduction caused a 80% decrease in DMS consumption. When sulfate reduction and methanogenesis were simultaneously inhibited, DMS did not decrease. DMS contributed about 28% to the methane production rate, while DMS probably contributed only 1% or less to the sulfate reduction rate. Incubation of the sediment slurries under an atmosphere of air resulted in similar DMS consumption compared to anaerobic incubations, but MSH and CH₄ were not evolved.
Sediments from the marsh released significant quantities of DMS when treated with cold alkali, indicating that potentially significant sources of DMS existed in the sediments. Values of base-hydrolyzable DMS as high as 190 μmol per liter of sediment were observed near the sediment surface, and values always decreased with depth in the sediment. Simple flux experiments with small intact sediment cores, showed that DMS was emitted from the marsh surface when cores were injected with glutaraldehyde or molybdate and 2-bromoethanesulfonate (BES), but nit when cores were left uninhibited. These results showed that DMS was readily metabolized by microbes in marsh sediments and that this metabolism may be responsible for reducing the emission of DMS from the marsh surface.     

Dimethyl sulphide and methanethiol formation in microbial mats: potential pathways for biogenic signatures

Mechanisms of dimethyl sulphide (DMS) and methanethiol (MT) production and consumption were determined in moderately hypersaline mats, Guerrero Negro, Mexico. Biological pathways regulated the net flux of DMS and MT as revealed by increases in flux resulting from decreased salinity, increased temperature and the removal of oxygen. Dimethylsulphoniopropionate (DMSP) was not present in these microbial mats and DMS and MT are probably formed by the reaction of photosynthetically produced low-molecular weight organic carbon and biogenic hydrogen sulphide derived from sulphate reduction. These observations provide an alternative to the notion that DMSP or S-containing amino acids are the dominant precursors of DMS in intertidal sediment systems. The major sink for DMS in the microbial mats was biological consumption, whereas photochemical oxidation to dimethylsulphoxide was the major sink for DMS in the overlying water column. Diel flux measurements demonstrated that significantly more DMS is released from the system during the night than during the day. The major consumers of DMS in the presence of oxygen were monooxygenase-utilizing bacteria, whereas under anoxic conditions, DMS was predominantly consumed by sulphate-reducing bacteria and methanethiol was consumed by methanogenic bacteria. Aerobic and anaerobic consumption rates of DMS were nearly identical. Mass balance estimates suggest that the consumption in the water column is likely to be smaller than net the flux from the mats. Volatile organic sulphur compounds are thus indicators of high rates of carbon fixation and sulphate reduction in these laminated sediment ecosystems, and atmospheric sulphur can be generated as a biogenic signature of the microbial mat community.     

Seasonal changes in fluxes of methane and volatile sulfur compounds from rice paddies and their concentrations in soil water

Rice paddies emit not only methane but also several volatile sulfur compounds such as dimethyl sulfide (DMS: CH₃SCH₃). However, little is known about DMS emission from rice paddies. Fluxes of methane and DMS, and the concentrations of methane and several volatile sulfur compounds including hydrogen sulfide (H₂S), carbonyl disulfide (CS₂), methyl mercaptan (CH₃SH) and DMS in soil water and flood water were measured in four lysimeter rice paddies (2.5 × 4 m, depth 2.0 m) once per week throughout the entire cultivation period in 1995 in Tsukuba, Japan. The addition of exogenous organic matter (rice straw) was also examined for its influence on methane or DMS emissions. Methane fluxes greatly differed between treatments in which rice straw had been incorporated into the paddy soil (rice straw plot) and plots without rice straw (mineral fertilizer plot). The annual methane emission from the rice straw plots (37.7 g m^-2) was approximately 8 times higher than that from the mineral fertilizer plots (4.8 g m^-2). Application of rice straw had little influence on DMS fluxes. Significant diurnal and seasonal changes in DMS fluxes were observed. Peak DMS fluxes were found around noon. DMS was emitted from the flood water in the early growth stage of rice and began to be emitted from rice plants during the middle stage. DMS fluxes increased with the growth of rice plants and the highest flux, 15.1 µg m^-2 h^-1, was recorded before heading. DMS in the soil water was negligible during the entire cultivation period. These facts indicate that the DMS emitted from rice paddies is produced by metabolic processes in rice plants. The total amount of DMS emitted from rice paddies over the cultivated period was estimated to be approximately 5–6 mg m^-2. CH₃SH was emitted only from flood water during the first month after flooding.     

A mechanistic explanation of the Sargasso Sea DMS “summer paradox”

In the Sargasso Sea, maximum dimethylsulfide (DMS) accumulation occurs in summer, concomitant with the minimum of chlorophyll and 2?months later than its precursor, dimethylsulfoniopropionate (DMSP). This phenomenon is often referred to as the DMS “summer paradox”. It has been previously suggested that the main agent triggering this pattern is increasing irradiance leading to light stress-induced DMS release from phytoplankton cells. We have developed a new model describing DMS(P) dynamics in the water column and used it to investigate how and to what extent processes other than light induced DMS exudation from phytoplankton, may contribute to the DMS summer paradox. To do this, we have conceptually divided the DMS “summer paradox” into two components: (1) the temporal decoupling between chlorophyll and DMSP and (2) the temporal decoupling between DMSP and DMS. Our results suggest that it is possible to explain the above cited patterns by means of two different dynamics, respectively: (1) a succession of phytoplankton types in the surface water and (2) the bacterially mediated DMSP(d) to DMS conversion, seasonally varying as a function of nutrient limitation. This work differs from previous modelling studies in that the presented model suggests that phytoplankton light-stress induced processes may only partially explain the summer paradox, not being able to explain the decoupling between DMSP and DMS, which is possibly the more challenging aspect of this phenomenon. Our study, therefore, provides an “alternative” explanation to the summer paradox further underlining the major role that bacteria potentially play in DMS production and fate.     

Isolation of Methylophaga spp. from Marine Dimethylsulfide-Degrading Enrichment Cultures and Identification of Polypeptides Induced during Growth on Dimethylsulfide

Dimethylsulfide (DMS)-degrading enrichment cultures were established from samples of coastal seawater, nonaxenic Emiliania huxleyi cultures, and mixed marine methyl halide-degrading enrichment cultures. Bacterial populations from a broad phylogenetic range were identified in the mixed DMS-degrading enrichment cultures by denaturing gradient gel electrophoresis (DGGE). Sequences of dominant DGGE bands were similar to those of members of the genera Methylophaga and Alcanivorax. Several closely related Methylophaga strains were obtained that were able to grow on DMS as the carbon and energy source. Roseobacter-related populations were detected in some of the enrichment cultures; however, none of the Roseobacter group isolates that were tested were able to grow on DMS. Oxidation of DMS by Methylophaga sp. strain DMS010 was not affected by addition of the inhibitor chloroform or methyl tert-butyl ether, suggesting that DMS metabolism may occur by a route different from those described for Thiobacillus species and other unidentified marine isolates. Addition of DMS and methanethiol to whole-cell suspensions of strain DMS010 induced oxygen uptake when strain DMS010 was grown on DMS but not in cells grown on methanol. The apparent K_ms of strain DMS010 for DMS and for methanethiol were 2.1 and 4.6 μM, respectively, when grown on DMS. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of the biomass of strain DMS010 and analysis of peptide bands by mass spectrometry techniques and N-terminal sequencing provided the first insight into the identity of polypeptides induced during growth on DMS. These included XoxF, a homolog of the large subunit of methanol dehydrogenase for which a biological role has not been identified previously.     

Dimethylsulfide is an energy source for the heterotrophic marine bacterium Sagittula stellata

Dimethylsulfide (DMS) is a volatile organosulfur compound, ubiquitous in the oceans, that has been credited with various roles in biogeochemical cycling and in climate control. Various oceanic sinks of DMS are known - both chemical and biological - although they are poorly understood. In addition to the utilization of DMS as a carbon or a sulfur source, some Bacteria are known to oxidize it to dimethylsulfoxide (DMSO). Sagittula stellata is a heterotrophic member of the Alphaproteobacteria found in marine environments. It has been shown to oxidize DMS during heterotrophic growth on sugars, but the reasons for and the mechanisms of this oxidation have not been investigated. Here, we show that the oxidation of DMS to DMSO is coupled to ATP synthesis in S. stellata and that DMS acts as an energy source during chemoorganoheterotrophic growth of the organism on fructose and on succinate. DMS dehydrogenase (which is responsible for the oxidation of DMS to DMSO in other marine Bacteria) and DMSO reductase activities were absent from cells grown in the presence of DMS, indicating an alternative route of DMS oxidation in this organism.     

Isolation and Characterization of Methanomethylovorans hollandica gen. nov., sp. nov., Isolated from Freshwater Sediment, a Methylotrophic Methanogen Able To Grow on Dimethyl Sulfide and Methanethiol

A newly isolated methanogen, strain DMS1^T, is the first obligately anaerobic archaeon which was directly enriched and isolated from a freshwater sediment in defined minimal medium containing dimethyl sulfide (DMS) as the sole carbon and energy source. The use of a chemostat with a continuous DMS-containing gas stream as a method of enrichment, followed by cultivation in deep agar tubes, resulted in a pure culture. Since the only substrates utilized by strain DMS1^T are methanol, methylamines, methanethiol (MT), and DMS, this organism is considered an obligately methylotrophic methanogen like most other DMS-degrading methanogens. Strain DMS1^T differs from all other DMS-degrading methanogens, since it was isolated from a freshwater pond and requires NaCl concentrations (0 to 0.04 M) typical of the NaCl concentrations required by freshwater microorganisms for growth. DMS was degraded effectively only in a chemostat culture in the presence of low hydrogen sulfide and MT concentrations. Addition of MT or sulfide to the chemostat significantly decreased degradation of DMS. Transient accumulation of DMS in MT-amended cultures indicated that transfer of the first methyl group during DMS degradation is a reversible process. On the basis of its low level of homology with the most closely related methanogen, Methanococcoides burtonii (94.5%), its position on the phylogenetic tree, its morphology (which is different from that of members of the genera Methanolobus, Methanococcoides, and Methanohalophilus), and its salt tolerance and optimum (which are characteristic of freshwater bacteria), we propose that strain DMS1^T is a representative of a novel genus. This isolate was named Methanomethylovorans hollandica. Analysis of DMS-amended sediment slurries with a fluorescence microscope revealed the presence of methanogens which were morphologically identical to M. hollandica, as described in this study. Considering its physiological properties, M. hollandica DMS1^T is probably responsible for degradation of MT and DMS in freshwater sediments in situ. Due to the reversibility of the DMS conversion, methanogens like strain DMS1^T can also be involved in the formation of DMS through methylation of MT. This phenomenon, which previously has been shown to occur in sediment slurries of freshwater origin, might affect the steady-state concentrations and, consequently, the total flux of DMS and MT in these systems.     

Coupling of dimethylsulfide oxidation to biomass production by a marine flavobacterium

Dimethylsulfide (DMS) is an important climatically active gas. In the sea, DMS is produced primarily by microbial metabolism of the compatible solute dimethylsulfoniopropionate. Laboratory growth of Bacteroidetes with DMS resulted in its oxidation to dimethyl sulfoxide but only in the presence of glucose. We hypothesized that electrons liberated from sulfur oxidation were used to augment biomass production.     

Metabolism of reduced methylated sulfur compounds in anaerobic sediments and by a pure culture of an estuarine methanogen

Addition of dimethylsulfide (DMS), dimethyldisulfide (DMDS), or methane thiol (MSH) to a diversity of anoxic aquatic sediments (e.g., fresh water, estuarine, alkaline/hypersaline) stimulated methane production. The yield of methane recovered from DMS was often 52 to 63%, although high concentrations of DMS (as well as MSH and DMDS) inhibited methanogenesis in some types of sediments. Production of methane from these reduced methylated sulfur compounds was blocked by 2-bromoethanesulfonic acid. Sulfate did not influence the metabolism of millimolar levels of DMS, DMDS, or MSH added to sediments. However, when DMS was added at approximately 2-muM levels as [C]DMS, metabolism by sediments resulted in a CH(4)/CO(2) ratio of only 0.06. Addition of molybdate increased the ratio to 1.8, while 2-bromoethanesulfonic acid decreased it to 0, but did not block CO(2) production. These results indicate the methanogens and sulfate reducers compete for DMS when it is present at low concentrations; however, at high concentrations, DMS is a "noncompetitive" substrate for methanogens. Metabolism of DMS by sediments resulted in the appearance of MSH as a transient intermediate. A pure culture of an obligately methylotrophic estuarine methanogen was isolated which was capable of growth on DMS. Metabolism of DMS by the culture also resulted in the transient appearance of MSH, but the organism could grow on neither MSH nor DMDS. The culture metabolized [C]-DMS to yield a CH(4)/CO(2) ratio of approximately 2.8. Reduced methylated sulfur compounds represent a new class of substrates for methanogens and may be potential precursors of methane in a variety of aquatic habitats.     

l-(−)-Desmethylselegiline, a Metabolite of Selegiline [l-(−)-Deprenyl], Protects Mesencephalic Dopamine Neurons from Excitotoxicity In Vitro

Abstract: Selegiline [ l -(−)-deprenyl], a monoamine oxidase B inhibitor, has been used in the treatment of Parkinson's disease as a putative neuroprotective agent. Selegiline is metabolized rapidly in the gastrointestinal tract and liver to desmethylselegiline (DMS) and methamphetamine. We have previously shown that selegiline protects dopamine neurons in mesencephalic cultures from toxicity resulting from activation of glutamate receptors. In the present study we examined whether DMS has similar neuroprotective effects. Our data show that DMS protects dopamine neurons from N -methyl- d -aspartate receptor-mediated excitotoxic damage. The efficacy of DMS is greater than that of selegiline, as it can cause protection at lower concentrations and provide significantly greater levels of protection at the same concentrations. Our results suggest that DMS might be the active compound responsible for the neuroprotective properties of selegiline.     

Microbial populations involved in cycling of dimethyl sulfide and methanethiol in freshwater sediments

Although several microorganisms that produce and degrade methanethiol (MT) and dimethyl sulfide (DMS) have been isolated from various habitats, little is known about the numbers of these microorganisms in situ. This study reports on the identification and quantification of microorganisms involved in the cycling of MT and DMS in freshwater sediments. Sediment incubation studies revealed that the formation of MT and DMS is well balanced with their degradation. MT formation depends on the concentrations of both sulfide and methyl group-donating compounds. A most-probable number (MPN) dilution series with syringate as the growth substrate showed that methylation of sulfide with methyl groups derived from syringate is a commonly occurring process in situ. MT appeared to be primarily degraded by obligately methylotrophic methanogens, which were found in the highest positive dilutions on DMS and mixed substrates (methanol, trimethylamine [TMA], and DMS). Amplified ribosomal DNA restriction analysis (ARDRA) and 16S rRNA gene sequence analysis of the total DNA isolated from the sediments and of the DNA isolated from the highest positive dilutions of the MPN series (mixed substrates) revealed that the methanogens that are responsible for the degradation of MT, DMS, methanol, and TMA in situ are all phylogenetically closely related to Methanomethylovorans hollandica. This was confirmed by sequence analysis of the product obtained from a nested PCR developed for the selective amplification of the 16S rRNA gene from M. hollandica. The data from sediment incubation experiments, MPN series, and molecular-genetics detection correlated well and provide convincing evidence for the suggested mechanisms for MT and DMS cycling and the common presence of the DMS-degrading methanogen M. hollandica in freshwater sediments.