Dynamics of Dimethyl Sulfide in a Marine Microbial Mat

Abstract The microbial mat was chosen as a model ecosystem to study dynamics of dimethyl sulfide (DMS) in marine sediments in order to gain insight into key processes and factors which determine emission rates. A practical advantage, compared to open ocean ecosystems, is that microbial mats contain high biomasses of different functional groups of bacteria involved in DMS dynamics, and that DMS concentrations are generally high enough to allow direct measurement of emission rates. Field data showed that, during the seasonal development of microbial mats, concentrations of chlorophyll a corresponded to dimethylsulfoniopropionate (DMSP). DMSP is an important precursor of DMS. It was demonstrated, with laboratory cultures, that various species of benthic diatoms produce substantial amounts of DMSP. The abundances of aerobic and anaerobic DMS- or DMSO-utilizing bacteria were estimated using the most-probable-number technique. Laboratory experiments with relatively undisturbed sediment cores showed that microbial mats act as a sink for DMS under oxic/light (day) conditions, and as a source of DMS under anoxic/dark (night) conditions. Axenic culture studies with Chromatium vinosum M2 and Thiocapsa pfennigii M8 (isolated from a microbial mat) showed that, under anoxic/light conditions, DMS was quantitatively converted to dimethylsulfoxide (DMSO). T. roseopersicina M11 converted DMSP to DMS and acrylate, apparently without use of either substrate. Received: 5 May 1997; Accepted: 21 August 1997     

Effect of UV- radiation on DMSP content and DMS formation of Phaeocystis antarctica

DMSP (dimethyl sulphonium propionate) contents produced by an Antarctic marine phytoplankton species, Phaeocystis antarctica (Prymnesiophyta), which were incubated under light conditions with radiations of different UV wavebands, were measured by gas chromatography after various exposure times. Full light (UV-B + UV-A + PAR) caused the strongest decrease in the production of DMSP in the alga. A marked depression of DMSP content was also observed with short UV-B and UV-A wavebands after 3 h. It was therefore hypothesised that DMSP production in Phaeocystis antarctica was inhibited by UV radiation. There was a negative correlation on change of DMSP contents under UV radiation. There was a negative correlation on change of DMSP contents under UV radiation with exposure times. The conversion rate of DMSP dissolved to DMS (dimethyl sulphide) was significantly increased with UV radiation. The possibility could not be excluded that a high concentration of free chemical radicals in seawater due to UV radiation resulted in an increase of DMSP cleavage in seawater. The oxidation of DMS in seawater due to UV-B radiation could result in a decrease of its flux to the atmosphere. The effect of UV radiation on DMSP production and oxidation of DMS may be an important factor in the variability of DMSP and the global flux of DMS from ocean to atmosphere. Received: 17 June 1996 / Accepted: 17 July 1997     

Structural and molecular basis for the novel catalytic mechanism and evolution of DddP,an abundant peptidase‐like bacterial Dimethylsulfoniopropionate lyase: a new enzyme from an old fold

The microbial cleavage of dimethylsulfoniopropionate (DMSP) generates volatile dimethyl sulfide (DMS) and is an important step in global sulfur and carbon cycles. DddP is a DMSP lyase in marine bacteria, and the deduced dddP gene product is abundant in marine metagenomic data sets. However, DddP belongs to the M24 peptidase family according to sequence alignment. Peptidases hydrolyze C‐N bonds, but DddP is deduced to cleave C‐S bonds. Mechanisms responsible for this striking functional shift are currently unknown. We determined the structures of DMSP lyase RlDddP (the DddP from Ruegeria lacuscaerulensis ITI_1157) bound to inhibitory 2‐(N‐morpholino) ethanesulfonic acid or PO₄^3? and of two mutants of RlDddP bound to acrylate. Based on structural, mutational and biochemical analyses, we characterized a new ion‐shift catalytic mechanism of RlDddP for DMSP cleavage. Furthermore, we suggested the structural mechanism leading to the loss of peptidase activity and the subsequent development of DMSP lyase activity in DddP. This study sheds light on the catalytic mechanism and the divergent evolution of DddP, leading to a better understanding of marine bacterial DMSP catabolism and global DMS production.     

DIMETHYLSULFONIOPROPIONATE CLEAVAGE BY MARINE PHYTOPLANKTON IN RESPONSE TO MECHANICAL,CHEMICAL, OR DARK STRESS1

Several bloom‐forming marine algae produce concentrated intracellular dimethylsulfoniopropionate (DMSP) and display high DMSP cleavage activity in vitro and during lysis after grazing or viral attack. Here we show evidence for cleavage of DMSP in response to environmental cues among different strains of the haptophyte Emiliania huxleyi (Lohmann) Hay et Mohler and the dinoflagellate Alexandrium spp. (Halim). Sparging or shaking live cells of either taxon increased dimethyl sulfide (DMS), especially in dinoflagellates, known to be very sensitive to shear stresses. Additions of polyamines, known triggers of exocytosis in some protists, also stimulated DMSP cleavage in a dose‐responsive manner. We observed DMS production by some algae after shifts in light regime. When most exponential‐phase E. huxleyi were transferred to continuous darkness, cells decreased in volume and DMSP content within 24 h; DMSP content per unit cell volume remained relatively steady. DMS accumulated as long as cells remained in the dark, but on returning to a light:dark cycle DMS accumulation ceased within 24 h. However, E. huxleyi strain CCMP 373, containing highly active in vitro DMSP lyase, produced only transient accumulations of DMS in the dark. This was apparently due to production and concomitant oxidation or uptake of DMS, because cells of this strain rapidly removed DMS added to cultures. Three strains of the dinoflagellate Alexandrium tamarense containing high in vitro DMSP lyase activity showed no DMS production in the dark, and all appeared to remove additions of DMS. Alexandrium tamarense strain CCMP 1771 also removed dimethyl disulfide, an inhibitor of bacterial DMS consumption. These data suggest that physical or chemical cues can trigger algal DMSP cleavage, but DMS production may be masked by subsequent oxidation and/or uptake.     

Aerobic turnover of dimethyl sulfide by the anoxygenic phototrophic bacterium Thiocapsa roseopersicina

This is the first report describing the complete oxidation of dimethyl sulfide (DMS) to sulfate by an anoxygenic, phototrophic purple sulfur bacterium. Complete DMS oxidation was observed in cultures of Thiocapsa roseopersicina M11 incubated under oxic/light conditions, resulting in a yield of 30.1 mg protein mmol^–1. No oxidation of DMS occurred under anoxic/light conditions. Chloroform, methyl butyl ether, and 3-amino-1,2,4-triazole, which are specific inhibitors of aerobic DMS oxidation in thiobacilli and hyphomicrobia, did not affect DMS oxidation in strain M11. This could be due to limited transport of the inhibitors through the cell membrane. The growth yield on sulfide as sole electron donor was 22.2 mg protein mmol^–1 under anoxic/light conditions. Since aerobic respiration of sulfide would have resulted in yields lower than 22 mg protein mmol^–1, the higher yield on DMS under oxic/light conditions suggests that the methyl groups of DMS have served as an additional carbon source or as an electron donor in addition to the sulfide moiety. The kinetic parameters V _max and K _m for DMS oxidation under oxic/light conditions were 12.4 ± 1.3 nmol (mg protein)^–1 min^–1 and 2 μM, respectively. T. roseopersicina M11 also produced DMS by cleavage of dimethylsulfoniopropionate (DMSP). Specific DMSP cleavage rates increased with increasing initial substrate concentrations, suggesting that DMSP lyase was only partly induced at lower initial DMSP concentrations. A comparison of T. roseopersicina strains revealed that only strain M11 was able to oxidize DMS and cleave DMSP. Both strain M11 and strain 5811 accumulated DMSP intracellularly during growth, while strain 1711 showed neither of these characteristics. Phylogenetic comparison based on 16S rRNA gene sequence revealed a similarity of 99.0% between strain M11 and strain 5811, and 97.6% between strain M11 and strain 1711. DMS and DMSP utilization thus appear to be strain-specific. Received: 26 March 1999 / Accepted: 18 June 1999     

Towards a systematic understanding of structure–function relationship of dimethylsulfoniopropionate‐catabolizing enzymes

Each year, several million tons of dimethylsulfoniopropionate (DMSP) are produced by marine phytoplankton and bacteria as an important osmolyte to regulate their cellular osmosis. Microbial breakdown of DMSP to the volatile gas dimethylsulfide (DMS) plays an important role in global biogeochemical cycles of the sulphur element between land and the sea. Understanding the enzymes involved in the transformation of DMSP and DMS holds the key to a better understanding of oceanic DMSP cycles. Recent work by Shao et al. (2019) has resolved the crystal structure of two important enzymes, DmdB and DmdC, involved in DMSP transformation through the demethylation pathway. Their work represents an important step towards a systematic understanding of the structure–function relationships of DMSP‐catabolizing enzymes in marine microbes.     

Abundance and Distribution of Dimethylsulfoniopropionate Degradation Genes and the Corresponding Bacterial Community Structure at Dimethyl Sulfide Hot Spots in the Tropical and Subtropical Pacific Ocean

Dimethylsulfoniopropionate (DMSP) is mainly produced by marine phytoplankton but is released into the microbial food web and degraded by marine bacteria to dimethyl sulfide (DMS) and other products. To reveal the abundance and distribution of bacterial DMSP degradation genes and the corresponding bacterial communities in relation to DMS and DMSP concentrations in seawater, we collected surface seawater samples from DMS hot spot sites during a cruise across the Pacific Ocean. We analyzed the genes encoding DMSP lyase (dddP) and DMSP demethylase (dmdA), which are responsible for the transformation of DMSP to DMS and DMSP assimilation, respectively. The averaged abundance (±standard deviation) of these DMSP degradation genes relative to that of the 16S rRNA genes was 33% ± 12%. The abundances of these genes showed large spatial variations. dddP genes showed more variation in abundances than dmdA genes. Multidimensional analysis based on the abundances of DMSP degradation genes and environmental factors revealed that the distribution pattern of these genes was influenced by chlorophyll a concentrations and temperatures. dddP genes, dmdA subclade C/2 genes, and dmdA subclade D genes exhibited significant correlations with the marine Roseobacter clade, SAR11 subgroup Ib, and SAR11 subgroup Ia, respectively. SAR11 subgroups Ia and Ib, which possessed dmdA genes, were suggested to be the main potential DMSP consumers. The Roseobacter clade members possessing dddP genes in oligotrophic subtropical regions were possible DMS producers. These results suggest that DMSP degradation genes are abundant and widely distributed in the surface seawater and that the marine bacteria possessing these genes influence the degradation of DMSP and regulate the emissions of DMS in subtropical gyres of the Pacific Ocean.     

Interacting effects of ocean acidification and warming on growth and DMS‐production in the haptophyte coccolithophore Emiliania huxleyi

The production of the marine trace gas dimethyl sulfide (DMS) provides 90% of the marine biogenic sulfur in the atmosphere where it affects cloud formation and climate. The effects of increasing anthropogenic CO₂ and the resulting warming and ocean acidification on trace gas production in the oceans are poorly understood. Here we report the first measurements of DMS‐production and data on growth, DMSP and DMS concentrations in pH‐stated cultures of the phytoplankton haptophyte Emiliania huxleyi. Four different environmental conditions were tested: ambient, elevated CO₂ (+CO₂), elevated temperature (+T) and elevated temperature and CO₂ (+TCO₂). In comparison to the ambient treatment, average DMS production was about 50% lower in the +CO₂ treatment. Importantly, temperature had a strong effect on DMS production and the impacts outweighed the effects of a decrease in pH. As a result, the +T and +TCO₂ treatments showed significantly higher DMS production of 36.2 ± 2.58 and 31.5 ± 4.66 μmol L^?1 cell volume (CV) h^?1 in comparison with the +CO₂ treatment (14.9 ± 4.20 μmol L^?1 CV h^?1). As the cultures were aerated with an air/CO₂ mixture, DMS was effectively removed from the incubation bottles so that concentration remained relatively low (3.6–6.1 mmol L^?1 CV). Intracellular DMSP has been shown to increase in E. huxleyi as a result of elevated temperature and/or elevated CO₂ and our results are in agreement with this finding: the ambient and +CO₂ treatments showed 125 ± 20.4 and 162 ± 27.7 mmol L^?1 CV, whereas +T and +TCO₂ showed significantly increased intracellular DMSP concentrations of 195 ± 15.8 and 211 ± 28.2 mmol L^?1 CV respectively. Growth was unaffected by the treatments, but cell diameter decreased significantly under elevated temperature. These results indicate that DMS production is sensitive to CO₂ and temperature in E. huxleyi. Hence, global environmental change that manifests in ocean acidification and warming may not result in decreased DMS as suggested by earlier studies investigating the effect of elevated CO₂ in isolation.     

CONCENTRATIONS OF DIMETHYLSULFONIOPROPIONATE AND DIMETHYL SULFIDE ARE STRAIN‐SPECIFIC IN SYMBIOTIC DINOFLAGELLATES (SYMBIODINIUM SP., DINOPHYCEAE)1

Dimethyl sulfide (DMS) and dimethylsulfoniopropionate (DMSP) are sulfur compounds that may function as antioxidants in algae. Symbiotic dinoflagellates of the genus Symbiodinium show strain‐specific differences in their susceptibility to temperature‐induced oxidative stress and have been shown to contain high concentrations of DMSP. We investigated continuous cultures of four strains from distinct phylotypes (A1, A13, A2, and B1) that can be characterized by differential thermal tolerances. We hypothesized that strains with high thermal tolerance have higher concentrations of DMSP and DMS in comparison to strains with low thermal tolerance. DMSP concentrations were strain‐specific with highest concentrations occurring in A1 (225 ± 3.5 mmol · L^?1 cell volume [CV]) and lowest in A2 (158 ± 3.8 mmol · L^?1 CV). Both strains have high thermal tolerance. Strains with low thermal tolerance (A13 and B1) showed DMSP concentrations in between these extremes (194 ± 19.0 and 160 ± 6.1 mmol · L^?1 CV, respectively). DMS data further confirmed this general pattern with high DMS concentrations in A1 and A13 (4.1 ± 1.22 and 2.1 ± 0.37 mmol · L^?1 CV, respectively) and low DMS concentrations in A2 and B1 (0.3 ± 0.06 and 0.5 ± 0.22 mmol · L^?1 CV, respectively). Hence, the strain‐specific differences in DMSP and DMS concentrations did not match the different abilities of the four phylotypes to withstand thermal stress. Future work should quantify the possible dynamics in DMSP and DMS concentrations during periods of high oxidative stress in Symbiodinium sp. and address the role of these antioxidants in zooxanthellate cnidarians.     

Recent insights into oceanic dimethylsulfoniopropionate biosynthesis and catabolism

Dimethylsulfoniopropionate (DMSP), a globally important organosulfur compound is produced in prodigious amounts (2.0 Pg sulfur) annually in the marine environment by phytoplankton, macroalgae, heterotrophic bacteria, some corals and certain higher plants. It is an important marine osmolyte and a major precursor molecule for the production of climate-active volatile gas dimethyl sulfide (DMS). DMSP synthesis take place via three pathways: a transamination ‘pathway-’ in some marine bacteria and algae, a Met-methylation ‘pathway-’ in angiosperms and bacteria and a decarboxylation ‘pathway-’ in the dinoflagellate, Crypthecodinium. The enzymes DSYB and TpMMT are involved in the DMSP biosynthesis in eukaryotes while marine heterotrophic bacteria engage key enzymes such as DsyB and MmtN. Several marine bacterial communities import DMSP and degrade it via cleavage or demethylation pathways or oxidation pathway, thereby generating DMS, methanethiol, and dimethylsulfoxonium propionate, respectively. DMSP is cleaved through diverse DMSP lyase enzymes in bacteria and via Alma1 enzyme in phytoplankton. The demethylation pathway involves four different enzymes, namely DmdA, DmdB, DmdC and DmdD/AcuH. However, enzymes involved in the oxidation pathway have not been yet identified. We reviewed the recent advances on the synthesis and catabolism of DMSP and enzymes that are involved in these processes.     

New Routes for Aerobic Biodegradation of Dimethylsulfoniopropionate

Dimethylsulfoniopropionate (DMSP), an osmolyte in marine plants, is biodegraded by cleavage of dimethyl sulfide (DMS) or by demethylation to 3-methiolpropionate (MMPA) and 3-mercaptopropionate (MPA). Sequential demethylation has been observed only with anoxic slurries of coastal sediments. Bacteria that grew aerobically on MMPA and DMSP were isolated from marine environments and phytoplankton cultures. Enrichments with DMSP selected for bacteria that generated DMS, whereas MMPA enrichments selected organisms that produced methanethiol (CH₃SH) from either DMSP or MMPA. A bacterium isolated on MMPA grew on MMPA and DMSP, but rapid production of CH₃SH from DMSP occurred only with DMSP-grown cells. Low levels of MPA accumulated during growth on MMPA, indicating demethylation as well as demethiolation of MMPA. The alternative routes for DMSP biodegradation via MMPA probably impact on net DMS fluxes to the marine atmosphere.     

Dimethylsulfoniopropionate Turnover Is Linked to the Composition and Dynamics of the Bacterioplankton Assemblage during a Microcosm Phytoplankton Bloom

Processing of the phytoplankton-derived organic sulfur compound dimethylsulfoniopropionate (DMSP) by bacteria was studied in seawater microcosms in the coastal Gulf of Mexico (Alabama). Modest phytoplankton blooms (peak chlorophyll a [Chl a] concentrations of ~2.5 μg liter⁻¹) were induced in nutrient-enriched microcosms, while phytoplankton biomass remained low in unamended controls (Chl a concentrations of ~0.34 μg liter⁻¹). Particulate DMSP concentrations reached 96 nM in the enriched microcosms but remained approximately 14 nM in the controls. Bacterial biomass production increased in parallel with the increase in particulate DMSP, and nutrient limitation bioassays in the initial water showed that enrichment with DMSP or glucose caused a similar stimulation of bacterial growth. Concomitantly, increased bacterial consumption rate constants of dissolved DMSP (up to 20 day⁻¹) and dimethylsulfide (DMS) (up to 6.5 day⁻¹) were observed. Nevertheless, higher DMSP S assimilation efficiencies and higher contribution of DMSP to bacterial S demand were found in the controls compared to the enriched microcosms. This indicated that marine bacterioplankton may rely more on DMSP as a source of S under oligotrophic conditions than under the senescence phase of phytoplankton blooms. Phylogenetic analysis of the bacterial assemblages in all microcosms showed that the DMSP-rich algal bloom favored the occurrence of various Roseobacter members, flavobacteria (Bacteroidetes phylum), and oligotrophic marine Gammaproteobacteria. Our observations suggest that the composition of the bacterial assemblage and the relative contribution of DMSP to the overall dissolved organic sulfur/organic matter pool control how efficiently bacteria assimilate DMSP S and thereby potentially divert it from DMS production.     

Effect of Salinity on DMSP Production in Gambierdiscus belizeanus (Dinophyceae)

Dimethylsulfoniopropionate (DMSP) is produced by many species of marine phytoplankton and has been reported to provide a variety of beneficial functions including osmoregulation. Dinoflagellates are recognized as major DMSP producers; however, accumulation has been shown to be highly variable in this group. We explored the effect of hyposaline transfer in Gambierdiscus belizeanus between ecologically relevant salinities (36 and 31) on DMSP accumulation, Chl a, cell growth, and cell volume, over 12 d. Our results showed that G. belizeanus maintained an intracellular DMSP content of 16.3 pmol cell^?1 and concentration of 139 mM in both salinities. Although this intracellular concentration was near the median reported for other dinoflagellates, the cellular content achieved by G. belizeanus was the highest reported of any dinoflagellate thus far, owing mainly to its large size. DMSP levels were not significantly affected by salinity treatment but did change over time during the experiment. Salinity, however, did have a significant effect on the ratio of DMSP:Chl a, suggesting that salinity transfer of G. belizeanus induced a physiological response other than DMSP adjustment. A survey of DMSP content in a variety of Gambierdiscus species and strains revealed relatively high DMSP concentrations (1.0–16.4 pmol cell^?1) as well as high intrageneric and intraspecific variation. We conclude that, although DMSP may not be involved in long‐term (3–12 d) osmoregulation in this species, G. belizeanus and other Gambierdiscus species may be important contributors to DMSP production in tropical benthic microalgal communities due to their large size and high cellular content.     

Metabolism of DMSP, DMS and DMSO by the cultivable bacterial community associated with the DMSP-producing dinoflagellate Scrippsiella trochoidea

Bacterial species associated with the dimethylsulfoniopropionate (DMSP)-producing phytoplankton Scrippsiella trochoidea were cultured and identified, with the aim of establishing their ability to metabolise DMSP, dimethylsulfide (DMS) and dimethylsulfoxide (DMSO). Results demonstrate that of the cultivable bacteria only α-Proteobacteria were capable of producing DMS from DMSP. The concentration of DMSP was shown to affect the amount of DMS produced. Lower DMSP concentrations (1.5?μmol?dm^?3) were completely assimilated, whereas higher concentrations (10?μmol?dm^?3) resulted in increasing amounts of DMS being produced. By contrast to the restricted set of bacteria that metabolised DMSP,?~?70% of the bacterial isolates were able to ‘consume’ DMS. However, 98-100% of the DMS removed was accounted for as DMSO. Notably, a number of these bacteria would only oxidise DMS in the presence of glucose, including members of the γ-Proteobacteria and Bacteroidetes. The observations from this study, coupled with published field data, identify DMS oxidation to DMSO as a major transformation pathway for DMS, and we speculate that the fate of DMS and DMSP in the field are tightly coupled to the available carbon produced by phytoplankton.     

Effect of PAR irradiance intensity on Phaeocystis antarctica (Prymnesiophyceae) growth and DMSP,DMSO, and acrylate concentrations

Phaeocystis antarctica forms extensive spring blooms in the Southern Ocean that coincide with high concentrations of dimethylsulfoniopropionate (DMSP), dimethylsulfoxide (DMSO), dimethylsulfide (DMS), and acrylate. We determined how concentrations of these compounds changed during the growth of axenic P. antarctica cultures exposed to light-limiting, sub-saturating, and saturating PAR irradiances. Cellular DMSP concentrations per liter cell volume (CV) ranged between 199 and 403 mmol · L_CV⁻¹, with the highest concentrations observed under light-limiting PAR. Cellular acrylate concentrations did not change appreciably with a change in irradiance level or growth, ranging between 18 and 45 mmol · L_CV⁻¹, constituting an estimated 0.2%–2.8% of cellular carbon. Both dissolved acrylate and DMSO increased substantially with irradiance during exponential growth on a per-cell basis, ranging from 0.91 to 3.15 and 0.24 to 1.39 fmol · cell⁻¹, respectively, indicating substantial export of these compounds into the dissolved phase. Average cellular DMSO:DMSP ratios increased 7.6-fold between exponential and stationary phases of batch growth, with a 3- to 13-fold increase in cellular DMSO likely formed from abiotic reactions of DMSP and DMS with reactive oxygen species (ROS). At mM levels, cellular DMSP and acrylate are proposed to serve as de facto antioxidants in P. antarctica not regulated by oxidative stress or changes in ROS. Instead, cellular DMSP concentrations are likely controlled by other physiological processes including an overflow mechanism to remove excess carbon via acrylate, DMS, and DMSO during times of unbalanced growth brought on by physical stress or nutrient limitation. Together, these compounds should aid P. antarctica in adapting to a range of PAR irradiances by maintaining cellular functions and reducing oxidative stress.     

Microbial controls on DMSP degradation and DMS formation in the Sargasso Sea

Bacterial degradation of dimethylsulfoniopropionate (DMSP) represents one of the main sources of the climatically–active trace gas dimethylsulfide (DMS) in the upper ocean. Short-term enrichment studies to stimulate specific pathways of DMSP degradation in oligotrophic waters from the Sargasso Sea were used to explore regulatory connections between the different bacterial DMSP degradation steps and determine potential biological controls on DMS formation in the open ocean. Experiments were conducted with surface water at the BATS station in the western North Atlantic Ocean. We added selected organic substrates (25 nmol L^?1 final concentration) to induce different steps of DMSP degradation in the microbial community, and then measured DMSP dynamics (assimilation and turnover rates), DMS yields (using ³⁵sulfur-DMSP tracer), and bacterial production rates. In most treatments, the main fate of consumed S-DMSP was excretion as a non-volatile S product. ³⁵S-DMSP tracer turnover rates (accumulation + assimilation + excretion of transformed products as DMS or others) increased upon addition of DMSP and glucose, but not acrylate, methymercaptopropionate (MMPA), methanethiol, DMS or glycine betaine. DMS yields from ³⁵S-DMSP never exceeded 16 % except in a short term DMSP enrichment, for which the yield reached 45 % (±17 %). Results show that availability of non-sulfur containing labile C sources (glucose, acrylate) decreased bacterial DMS production while stimulating bacterial heterotrophic production, and suggest an influence of bacterial sulfur demand in controlling DMS-yielding pathways. However, regulatory effects on ³⁵S-DMSP fate were not consistent across all reduced sulfur compounds (i.e., methanethiol or MMPA), and may reflect alternate roles of DMSP as a bacterial energy source and osmolyte.     

Characterization of a DMSP-degrading bacterial isolate from the Sargasso Sea

A bacterium which cleaves dimethylsulfoniopropionate (DMSP) to form dimethylsulfide (DMS) was isolated from surface Sargasso Sea water by a DMSP enrichment technique. The isolate, here designated LFR, is a Gram-negative, obligately aerobic, rod-shaped, carotenoid-containing bacterium with a DNA G+C content of 70%. Sequencing and comparison of its 16S ribosomal ribonucleic acid (rRNA) with that of known eubacteria revealed highest similarity (91% unrestricted sequence similarity) to Roseobacter denitrificans (formerly Erythrobacter species strain OCh114), an aerobic, bacteriochlorophyll-containing marine representative of the -Proteobacteria. However, physiological differences between the two bacteria, and the current lack of other characterized close relatives, preclude assignment of strain LFR to the Roseobacter genus. Screening of fifteen characterized marine bacteria revealed only one, Pseudomonas doudoroffii, capable of degrading DMSP to DMS. Strain LFR is deposited with the American Type Culture Collection (ATCC 51258) and 16S rRNA sequence data are available under GenBank accession number 15345.Contribution no. 8337 of the Woods Hole Oceanographic Institution     

Dimethylsulfoniopropionate lyase from the marine macroalga Ulva curvata: purification and characterization of the enzyme

This is a report on the purification and characterization of an algal dimethylsulfoniopropionate (DMSP) lyase. This enzyme, also found in bacteria, is responsible for producing most of the dimethylsulfide (DMS) in marine environments. It was purified from the green macroalga, Ulva curvata (Kützing) De Toni. Initial in-vivo experiments showed that DMSP lyase activity from endogenous DMSP in Ulva increased for 24 h and then decreased as the culture aged and endogenous DMSP levels were depleted. When amended with exogenous DMSP, rates of DMSP lyase activity remained high even when the culture was 5 d old. Following disruption of the DMSP-depleted U. curvata cells by grinding, a soluble DMSP lyase was purified. This enzyme is a monomer of 78 kDa which has a K _m for DMSP of 0.52 mM. Soluble DMSP lyase had an optimum pH of 8 and an optimum osmotic strength of 75 mM NaCl. Following disruption of the algae by either grinding with sand or blending, and washing out the soluble enzyme, the green tissue, when treated with the non-ionic detergent, Triton X-100, solubilized additional DMSP lyase activity. Three hydrophobic variant forms of Ulva DMSP lyase were isolated and partially characterized from the detergent-solubilized activity. While the molecular and kinetic properties of the algal enzyme are different from the bacterial enzymes we purified earlier, both the soluble and membrane-bound forms did, nevertheless, cross-react with antibodies raised against the bacterial (Alcaligenes strain M3A) DMSP lyase.Abbreviations DMS dimethylsulfide - DMSP dimethylsulfoniopropionate This paper is dedicated to D.I. Arnon (1910–1995). We thank Dr. Richard Zingmark for helpful discussions on the speciation of the natural algal samples used in these experiments, and Robin Krest for collecting samples for us on numerous occasions. This work was supported, in part, by a grant from the University of South Carolina Venture Fund.