Dimethylsulfoniopropionate Lyase Research Articles

Dimethylsulfoniopropionate (DMSP) lyase activity in different strains of the symbiotic alga Symbiodinium microadriaticum

Dimethylsulfoniopropionate (DMSP) lyases are responsible for the enzymatic conver- sion of the algal metabolite DMSP into dimethylsulfide (DMS) and other products. DMSP lyase poten- tial activity (DLA) was assayed in 5 cultured strains of Symbiodinium microadriaticum to investigate whether DLA was present and whether variations in DLA occurred among strains. S. microadri- aticum are important primary producers symbiotic with corals and other cnidarians. Four of the 5 tested S. microadriaticum strains were capable of performing enzymatic lysis of DMSP, and the lev- els of DLA varied significantly among the strains. Average DLA normalized to chlorophyll a (chl a) was significantly higher in strain CCMP 829 (5.3 ± 0.9 nmol DMS · min -1 · µg -1 ) compared with all other strains investigated. DLA was not detected in strain CCMP 830, suggesting that DMSP lyase may not be a universal enzyme in S. microadriaticum. DMSP levels also varied between strains, but there was no correlation between cellular DMSP and DLA. Additionally, our data indicate that in non-axenic S. microadriaticum strains, the bacterial contribution to DLA was not substantial in any of the algal cultures investigated. These results are supportive of the idea that S. microadriaticum contain DLA, that DMSP producers are not necessarily capable of DLA, and that, even within a spe- cies, DMSP and DLA levels vary significantly. These findings suggest important differences in the regulation of DMSP and DLA in S. microadriaticum, but the reasons for these differences remain to be elucidated.

Occurrence and turnover of DMSP and DMS in deep waters of the Ross Sea, Antarctica

High concentrations of the phytoplankton metabolite dimethylsulfoniopropionate (DMSP) and its degradation product dimethylsulfide (DMS) are associated with blooms of Phaeocystis antarctica in the Ross Sea, Antarctica. Episodic and rapid vertical export of Phaeocystis biomass to deep water has been reported for the Ross Sea, therefore we examined the distribution and microbial consumption rates of DMSP and DMS throughout the sub-euphotic water column. Total DMSP (dissolved+particulate; DMSPt) was present at 0.5–22 nM at depths between 70 and 690 m during both the early bloom (November) and the late bloom (January). Sub-euphotic peaks of DMSP were sometimes associated with mid-water temperature maxima, and elevated DMSP below 70 m was found mainly in water masses characterized as Modified Circumpolar Deep Water or Antarctic Shelf Water. Overall, 50–94% of the integrated water-column DMSPt was found below the euphotic zone. At one station during the early bloom, local maxima of DMSPt (14 nM) and DMS (20 nM) were observed between 113 and 240 m and these maxima corresponded with high chlorophyll a concentrations, P. antarctica cell numbers, and Fv/Fm (the quantum yield of photosystem II). During the late bloom, a sub-euphotic maximum of DMSPt (15.8 nM) at 250 m cooccurred with peaks of chlorophyll a concentration, DMSP lyase activity, bacterial production and dissolved DMSP consumption rates. DMSP turnover contributed ∼12% of the bacterial carbon demand between 200 and 400 m. DMS concentrations peaked at 286 m but the maximum concentration (0.42 nM) was far lower than observed during the early bloom, probably because of relatively rapid biological consumption of DMS (1–3 turnovers per day) which, in turn, contributed to elevated dissolved dimethylsulfoxide (DMSO) concentrations. Relatively stable DMSPt distributions at some sites suggest that rapid sinking of Phaeocystis biomass is probably not the major mechanism responsible for mesopelagic DMSP accumulations. Rather, subduction of near-surface water masses, lateral advective transport or trapping of slowly sinking P. antarctica biomass in intermediate water masses are more likely mechanisms. We found that a culture of P. antarctica maintained cellular integrity during 34 days of darkness, therefore the presence of intact cells (and DMSP) at depth can be explained even under a slow sinking/advection scenario. Whatever the mechanism, the large pools of DMSP and DMS below the euphotic zone suggest that export exerts a control on potential DMS emission from the surface waters of the Ross Sea.

Molecular genetic analysis of a dimethylsulfoniopropionate lyase that liberates the climate‐changing gas dimethylsulfide in several marine α‐proteobacteria and Rhodobacter sphaeroides

Molecular genetic analysis of a dimethylsulfoniopropionate lyase that liberates the climate‐changing gas dimethylsulfide in several marine α‐proteobacteria and <i>Rhodobacter sphaeroides</i>

Molecular genetic analysis of a dimethylsulfoniopropionate lyase that liberates the climate‐changing gas dimethylsulfide in several marine α‐proteobacteria and Rhodobacter sphaeroides

The alpha-proteobacterium Sulfitobacter EE-36 makes the gas dimethylsulfide (DMS) from dimethylsulfoniopropionate (DMSP), an abundant antistress molecule made by many marine phytoplankton. We screened a cosmid library of Sulfitobacter for clones that conferred to other bacteria the ability to make DMS. One gene, termed dddL, was sufficient for this phenotype when cloned in pET21a and introduced into Escherichia coli. Close DddL homologues exist in the marine alpha-proteobacteria Fulvimarina, Loktanella Oceanicola and Stappia, all of which made DMS when grown on DMSP. There was also a dddL homologue in Rhodobacter sphaeroides strain 2.4.1, but not in strain ATCC 17025; significantly, the former, but not the latter, emits DMS when grown with DMSP. Escherichia coli containing the cloned, overexpressed dddL genes of R. sphaeroides 2.4.1 and Sulfitobacter could convert DMSP to acrylate plus DMS. This is the first identification of such a 'DMSP lyase'. Thus, DMS can be made either by this DddL lyase or by a DMSP acyl CoA transferase, specified by dddD, a gene that we had identified in several other marine bacteria.

The relative significance of viral lysis and microzooplankton grazing as pathways of dimethylsulfoniopropionate (DMSP) cleavage: An Emiliania huxleyi culture study

Dimethylsulfoniopropionate (DMSP) cleavage was investigated during culture studies of grazing by the microzooplankter Oxyrrhis marina and viral lysis by Emiliania huxleyi virus 86 (EhV‐86) on two axenic strains of E. huxleyi. The cleavage products of DMSP, dimethyl sulfide (DMS) and acrylic acid (AA), accumulated during viral infection of both strains, confirming that viral lysis of algae can lead directly to DMSP cleavage. AA and DMS accumulated in parallel with compromised E. huxleyi cells, indicating that DMSP cleavage occurred during the physical disruption of the infected cells. This is in agreement with the hypothesis that DMSP and DMSP lyase ([DL] the enzyme responsible for cleaving DMSP) are segregated in healthy or undamaged cells. During grazing, the concentrations of DMS and AA produced per eaten cell were an order of magnitude higher than the concentrations resulting from cell death caused by viral infection, suggesting that grazing is the quantitatively more significant pathway of DMS production in E. huxleyi. Levels of DL activity decreased in infected cultures to a minimum of 0.00065 fmol cell−1 min−1 as compared with an average of 0.09 fmol cell−1 min−1 in the control cultures, indicating that reduced DL activity in virally infected cells was responsible for the lower levels of DMSP cleavage observed during viral lysis.

Structural and Regulatory Genes Required to Make the Gas Dimethyl Sulfide in Bacteria

Dimethyl sulfide (DMS) is a key compound in global sulfur and carbon cycles. DMS oxidation products cause cloud nucleation and may affect weather and climate. DMS is generated largely by bacterial catabolism of dimethylsulfoniopropionate (DMSP), a secondary metabolite made by marine algae. We demonstrate that the bacterial gene dddD is required for this process and that its transcription is induced by the DMSP substrate. Cloned dddD from the marine bacterium Marinomonas and from two bacterial strains that associate with higher plants, the N(2)-fixing symbiont Rhizobium NGR234 and the root-colonizing Burkholderia cepacia AMMD, conferred to Escherichia coli the ability to make DMS from DMSP. The inferred enzymatic mechanism for DMS liberation involves an initial step in which DMSP is modified by addition of acyl coenzyme A, rather than the immediate release of DMS by a DMSP lyase, the previously suggested mechanism.

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.

Latitudinal and vertical distributions of particle-associated dimethylsulfoniopropionate (DMSP) lyase activity in the western North Atlantic Ocean

Dimethylsulfoniopropionate (DMSP) lyase enzymes cleave phytoplankton-derived DMSP into dimethylsulfide (DMS) and acrylate. We measured particle-associated (&gt;0.7 µm) DMSP lyase activity (DLA) in diverse waters from the Gulf of Maine to the Sargasso Sea. DLA fell within a relatively narrow range (0.63–5.4 nmol DMS·L–1·min–1), with no significant geographic trend. DLA was, however, generally highest near the surface and decreased with depth within the euphotic zone. When normalized to chlorophyll a (Chl a), DLA was significantly higher in oligotrophic surface waters of the Sargasso Sea (DLA:Chl a = 33–53 nmol DMS·min–1·µg Chl a–1) compared with more productive Gulf of Maine waters (DLA:Chl a = 0.5–7.9 nmol DMS·min–1·µg Chl a–1). In optically clear waters of the Sargasso Sea, DLA:Chl a was generally highest near the surface and decreased with depth, following a similar trend to that of the photoprotective pigment diadinoxanthin. When ammonium and phosphate were added to oligotrophic water from the Sargasso Sea, Chl a increased exponentially, whereas DLA:Chl a decreased by 83%. Our results suggest that Chl a specific DLA is highest when plankton are exposed to high solar radiation and low nutrients, consistent with the idea that DMSP lyases may be involved in oxidative stress protection.

Influence of dinoflagellate diurnal vertical migrations on dimethylsulfoniopropionate and dimethylsulfide distribution and dynamics (St. Lawrence Estuary, Canada)

The influence of the diurnal vertical migration of the dinoflagellates Alexandrium tamarense and Scrippsiella trochoidea on dimethylsulfoniopropionate (DMSP) and dimethylsulfide (DMS) dynamics was studied during a 34-h Lagrangian experiment in the St. Lawrence Estuary in July 2000. Particulate DMSP (DMSPp), dissolved DMSP (DMSPd), and DMS exhibited diel patterns with minimum concentrations during the night and maximum concentrations around noon. DMSPp concentrations were correlated with the abundance of the two vertically migrating DMSP-rich dinoflagellates. The DMSPp:Chl a ratio exhibited similar diel variations, suggesting a light-induced de novo DMSP synthesis during the day. Diel variations of the DMS:Chl a ratio suggest that the accumulation of DMS around noon resulted from physiological responses of the algae and (or) bacteria to light. Biological gross DMS production and bacterial DMS consumption were decoupled, leading to rapid fluctuations in DMS. These results show that in systems dominated by DMSP-rich dinoflagellates containing DMSP lyases, DMS concentrations may vary by as much as a factor of 10 over a 24-h period. Such diel variations must be considered when estimating the contribution of such systems to the DMS sea to air flux.

Chemical defense in the microplankton I: Feeding and growth rates of heterotrophic protists on the DMS‐producing phytoplankter Emiliania huxleyi

In this study, the hypothesis that Emiliania huxleyi, a cosmopolitan, bloom‐forming coccolithophorid, produces chemical defenses against protist grazers was tested using four axenic strains of the alga. The putative chemical defense involves the cleavage of dimethylsulfoniopropionate (DMSP) by the enzyme DMSP lyase to yield dimethylsulfide (DMS) and acrylate. Out of six tested protist grazer species, five (including ciliates and heterotrophic dinoflagellates) showed lower feeding rates on E. huxleyi strains with high DMPS lyase activity than on low‐lyase strains. Reductions in population growth rate were consistent with feeding reductions. These results suggest that high levels of DMSP lyase activity somehow promote reduced palatability, although covariation of E. huxleyi protein and carbohydrate content with DMSP lyase activity meant that prey “nutritional quality” could not be ruled out as a contributing factor. Additional experiments with the heterotrophic dinoflagellate Amphidinium longum demonstrated that (1) individuals need not ingest E. huxleyi cells to receive the “don't eat me” cue and (2) exposure to high‐lyase E. huxleyi cells confers no harmful consequences in terms of the ability to feed and grow on alternate phytoplankton prey. Thus, factors promoting reduced grazing on high‐lyase E. huxleyi are hypothesized to constitute signals rather than acute toxins. Furthermore, although cell surface carbohydrates (as assayed by lectin binding) showed few differences among strains, results of feeding studies indicate that the deterrent signal resides on the cell surface or in the near‐cell dissolved phase. Behavior responses to such cues might play an important role in the outcome of encounters between protist grazers and their prey, with profound consequences for planktonic community structure.

Dimethylsulfide release during macroinvertebrate grazing and its role as an activated chemical defense

Almost half the biogenic sulfur emitted globally originates as dimethylsulfide (DMS) in marine algae. Yet, despite its importance to climate and the global sulfur cycle, the reasons why many diverse algal taxa produce DMS remain unclear. DMS is produced when dimethylsulfoniopro- pionate (DMSP) is cleaved by the enzyme DMSP lyase into DMS and acrylic acid. Because acrylic acid can deter feeding by some herbivores, the production of DMSP has been postulated to be part of an antiherbivore chemical defense system, with DMS being a by-product of the production of the defensive compound acrylic acid. However, we found that DMS plays a more direct role in herbivore deterrence. Using laboratory bioassays, we demonstrated that DMS functions as a potent feeding deterrent against sea urchins Strongylocentrotus droebachiensis at a wide range of concentrations. The defensive function of DMSP cleavage is further supported by the results of a multiple-choice feeding preference assay in which we offered urchins a choice of 8 algal species. In these assays, algae containing DMSP tended to be consumed at lower rates than species that lacked it. By measur- ing headspace DMS concentrations during grazing, we showed that DMS is produced when urchins feed on algae containing DMSP, but not when urchins feed on algae without it. DMS production occurred while the urchins were feeding on the algae and for 18 h after the algae were removed, suggesting that post-ingestive processes might be playing a role in activation. These results demon- strate that DMS is produced when urchins feed on algae containing DMSP and that it functions as an herbivore deterrent, causing species that produce DMSP to be avoided by some herbivores.

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.

Nuclear magnetic resonance analysis of [1-13C]dimethylsulfoniopropionate (DMSP) and [1-13C]acrylate metabolism by a DMSP lyase-producing marine isolate of the alpha-subclass of Proteobacteria.

The prominence of the alpha-subclass of Proteobacteria in the marine bacterioplankton community and their role in dimethylsulfide (DMS) production has prompted a detailed examination of dimethylsulfoniopropionate (DMSP) metabolism in a representative isolate of this phylotype, strain LFR. [1-(13)C]DMSP was synthesized, and its metabolism and that of its cleavage product, [1-(13)C]acrylate, were studied using nuclear magnetic resonance (NMR) spectroscopy. [1-(13)C]DMSP additions resulted in the intracellular accumulation and then disappearance of both [1-(13)C]DMSP and [1-(13)C]beta-hydroxypropionate ([1-(13)C]beta-HP), a degradation product. Acrylate, the immediate product of DMSP cleavage, apparently did not accumulate to high enough levels to be detected, suggesting that it was rapidly beta-hydroxylated upon formation. When [1-(13)C]acrylate was added to cell suspensions of strain LFR it was metabolized to [1-(13)C]beta-HP extracellularly, where it first accumulated and was then taken up in the cytosol where it subsequently disappeared, indicating that it was directly decarboxylated. These results were interpreted to mean that DMSP was taken up and metabolized by an intracellular DMSP lyase and acrylase, while added acrylate was beta-hydroxylated on (or near) the cell surface to beta-HP, which accumulated briefly and was then taken up by cells. Growth on acrylate (versus that on glucose) stimulated the rate of acrylate metabolism eightfold, indicating that it acted as an inducer of acrylase activity. DMSP, acrylate, and beta-HP all induced DMSP lyase activity. A putative model is presented that best fits the experimental data regarding the pathway of DMSP and acrylate metabolism in the alpha-proteobacterium, strain LFR.

PHYTOPLANKTON CHEMICAL SIGNALS INFLUENCE HERBIVORY BY PROTIST GRAZERS

Strom, S. L.1 &amp; Wolfe, G. V.2 1Shannon Point Marine Center, Western Washington University, Anacortes WA 98221 USA; 2Department of Biological Sciences, California State University Chico, Chico CA 95929 USADeterrence of herbivores through chemical signaling ‐ plant chemical defense ‐ is known to be widespread throughout terrestrial and marine benthic ecosystems. The occurrence and significance of chemical defenses produced by unicellular marine phytoplankton has not, however, been systematically explored. We are investigating the role of the phytoplankton‐produced compounds dimethylsulfo‐niopropionate (DMSP) and its cleavage products dimethylsulfide (DMS) and acrylate in reducing herbivory by protist grazers. DMSP is produced in high intracellular concentrations by numerous phytoplankton species, especially notorious bloom‐forming dinoflagellates and prymnesiophytes, and is cleaved by the enzyme DMSP lyase during grazing, physiological stress or cell lysis. Using four different strains of Emiliania huxleyi, a coccolithophorid (prymnesiophyte), we have shown that strains with high DMSP lyase activities experience consistently reduced levels of herbivory in comparison with low lyase strains. Curiously, the products of the cleavage reaction (DMS and acrylate) do not affect rates of herbivory, indicating that this is not an activated chemical defense. Feeding rates are, however, proportionally and substantially reduced by the reaction precursor, DMSP. We also have preliminary evidence that high lyase strains release more DMSP to seawater than do low lyase strains. DMSP, while not evidently toxic to protist grazers, appears to act as a chemical signal that reduces grazing. We hypothesize that these unicellular herbivores may have evolved to recognize and respond to a compound (DMSP) that is a precursor of the potentially deleterious cleavage product acrylate. This and related chemical signaling processes may be important in promoting formation of phytoplankton blooms in the sea.

Activated defense systems in marine macroalgae: evidence for an ecological role for DMSP cleavage

Activated defenses against herbivores and predators are defenses whereby a precursor compound is stored in an inactive or mildly active form. Upon damage to the prey, the precursor is enzymatically converted to a more potent toxin or feeding deterrent. In marine systems, activated defenses are only known to exist in a few species of tropical macroalgae. In this study, we examined an activated defense system in temperate marine macroalgae in which the osmolyte dimethylsulfo- niopropionate (DMSP) is converted to acrylic acid or acrylate, depending upon the pH, and dimethyl sulfide (DMS) by the enzyme DMSP lyase upon damage to the alga. We surveyed 39 species of red, green, and brown algae from the Washington and Oregon coasts, and found high concentrations of DMSP in the chlorophytes Acrosiphonia coalita, Codium fragile, Enteromorpha intestinalis, E. linza, Ulva californica, U. fenestrata, and U. taeniata, and in the rhodophyte Polysiphonia hendryi. Con- centrations of DMSP ranged from 0.04% of the alga's fresh mass (FM) to 1.8% FM. We found signif- icant DMSP lyase activity in 1 green alga, U. fenestrata, and 1 red alga, P. hendryi, with DMSP cleav- age rates approaching 300 mmol kg -1 FM min -1 . Loss of DMSP and the production of DMS when the tissues of U. californica and P. hendryi were crushed suggested that physical damage results in DMSP cleavage. In laboratory feeding preference experiments, acrylic acid deterred feeding by the sea urchin Strongylocentrotus droebachiensis at concentrations of 0.1 to 2% FM and by S. purpura- tus at 0.25 to 2% FM, while the precursor DMSP functioned as a feeding attractant to both sea urchins. In contrast, feeding by the isopod Idotea wosnesenskii was not deterred by acrylic acid even at concentrations as high as 8% FM. Our data suggest that DMSP may function as a precursor in an activated defense system in diverse species of temperate macroalgae and may possibly contribute to the widespread success of the Ulvophyceae. This chemical system is also found in unicellular phyto- plankton, and presents an opportunity to compare and contrast the ecological role of chemical defense among micro- and macroorganisms.

New and important roles for DMSP in marine microbial communities

The algal osmolyte dimethylsulfoniopropionate (DMSP) is recognised as the major precursor of marine dimethylsulfide (DMS), a volatile sulfur compound that affects atmospheric chemistry and global climate. Recent studies, using 35S-DMSP tracer techniques, suggest that DMSP may play additional very important roles in the microbial ecology and biogeochemistry of the surface ocean. DMSP may serve as an intracellular osmolyte in bacteria that take up phytoplankton-derived DMSP from seawater. In addition, DMSP appears to support from 1 to 13% of the bacterial carbon demand in surface waters, making it one of the most significant single substrates for bacterioplankton so far identified. Furthermore, the sulfur from DMSP is efficiently incorporated into bacterial proteins (mostly into methionine) and DMSP appears to be a major source of sulfur for marine bacterioplankton. Assimilatory metabolism of DMSP is via methanethiol (MeSH) that is produced by a demethylation/demethiolation pathway which dominates DMSP degradation in situ. Based on the linkage between assimilatory metabolism of DMSP and bacterial growth, we offer a hypothesis whereby DMSP availability to bacteria controls the production of DMS by the competing DMSP lyase pathway. Also linked with the assimilatory metabolism of DMSP is the production of excess MeSH which, if not assimilated into protein, reacts to form dissolved non-volatile compounds. These include sulfate and DOM–metal–MeSH complexes, both of which represent major short-term end-products of DMSP degradation. Because production rates of MeSH in seawater are high (3–90nMd−1), reaction of MeSH with trace metals could affect metal availability and chemistry in seawater. Overall, results of recent studies provide evidence that DMSP plays important roles in the carbon, sulfur and perhaps metal and DOM cycles in marine microbial communities. These findings, coupled with the fact that the small fraction of DMSP converted to DMS may influence atmospheric chemistry and climate dynamics, draws attention to DMSP as a molecule of central importance to marine biogeochemical and ecological processes.

Metabolism of acrylate to beta-hydroxypropionate and its role in dimethylsulfoniopropionate lyase induction by a salt marsh sediment bacterium, Alcaligenes faecalis M3A.

Dimethylsulfoniopropionate (DMSP) is degraded to dimethylsulfide (DMS) and acrylate by the enzyme DMSP lyase. DMS or acrylate can serve as a carbon source for both free-living and endophytic bacteria in the marine environment. In this study, we report on the mechanism of DMSP-acrylate metabolism by Alcaligenes faecalis M3A. Suspensions of citrate-grown cells expressed a low level of DMSP lyase activity that could be induced to much higher levels in the presence of DMSP, acrylate, and its metabolic product, beta-hydroxypropionate. DMSP was degraded outside the cell, resulting in an extracellular accumulation of acrylate, which in suspensions of citrate-grown cells was then metabolized at a low endogenous rate. The inducible nature of acrylate metabolism was evidenced by both an increase in the rate of its degradation over time and the ability of acrylate-grown cells to metabolize this molecule at about an eight times higher rate than citrate-grown cells. Therefore, acrylate induces both its production (from DMSP) and its degradation by an acrylase enzyme. (1)H and (13)C nuclear magnetic resonance analyses were used to identify the products resulting from [1-(13)C]acrylate metabolism. The results indicated that A. faecalis first metabolized acrylate to beta-hydroxypropionate outside the cell, which was followed by its intracellular accumulation and subsequent induction of DMSP lyase activity. In summary, the mechanism of DMSP degradation to acrylate and the subsequent degradation of acrylate to beta-hydroxypropionate in the aerobic beta-Proteobacterium A. faecalis has been described.

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 &mgr;M, 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.

Turnover of dimethylsulfoniopropionate (DMSP) by the purple sulfur bacterium Thiocapsa roseopersicina M11: ecological implications

The use of dimethylsulfoniopropionate (DMSP) by the anoxygenic phototrophic purple sulfur bacterium Thiocapsa roseopersicina M11 under different environmental conditions was studied. Under anoxic/light conditions DMSP cleavage occurred both at low and intermediate salinities but at different growth phases: at low salinity (5‰ S) DMSP cleavage was observed in the exponential growth phase, whereas at intermediate salinity (35‰ S) cleavage was not observed until the stationary phase was reached. At higher salinities DMSP accumulated intracellularly, prior to cleavage, suggesting usage as compatible solute. Circumstantial evidence suggests that under anoxic/light conditions acrylate, one of the DMSP cleavage products, was reduced to propionate at the expense of intracellular sulfur oxidation, and subsequently used as carbon source. The addition of acrylate had an inhibitory effect on growth, the reduction of acrylate to propionate can thus play an additional role as a detoxification mechanism. Dimethyl sulfide (DMS), also a DMSP cleavage product, was not used under anoxic conditions. However, under oxic conditions both DMS and acrylate metabolization resulted in protein production. DMSP lyase in cell suspensions was induced by acrylate and activity was enhanced in the light. The latter suggests that DMSP cleavage is an energy dependent process. The potential anaerobic DMS production and aerobic consumption in T. roseopersicina M11 is in agreement with previously observed diurnal DMS dynamics in coastal marine sediments, the natural habitat of this anoxygenic phototrophic bacterium. This is the first report in which a bacterium is described that is able to cleave DMSP with subsequent metabolization of DMS.

Turnover of dimethylsulfoniopropionate (DMSP) by the purple sulfur bacterium Thiocapsa roseopersicina M11: ecological implications

The use of dimethylsulfoniopropionate (DMSP) by the anoxygenic phototrophic purple sulfur bacterium Thiocapsa roseopersicina M11 under different environmental conditions was studied. Under anoxic/light conditions DMSP cleavage occurred both at low and intermediate salinities but at different growth phases: at low salinity (5‰ S) DMSP cleavage was observed in the exponential growth phase, whereas at intermediate salinity (35‰ S) cleavage was not observed until the stationary phase was reached. At higher salinities DMSP accumulated intracellularly, prior to cleavage, suggesting usage as compatible solute. Circumstantial evidence suggests that under anoxic/light conditions acrylate, one of the DMSP cleavage products, was reduced to propionate at the expense of intracellular sulfur oxidation, and subsequently used as carbon source. The addition of acrylate had an inhibitory effect on growth, the reduction of acrylate to propionate can thus play an additional role as a detoxification mechanism. Dimethyl sulfide (DMS), also a DMSP cleavage product, was not used under anoxic conditions. However, under oxic conditions both DMS and acrylate metabolization resulted in protein production. DMSP lyase in cell suspensions was induced by acrylate and activity was enhanced in the light. The latter suggests that DMSP cleavage is an energy dependent process. The potential anaerobic DMS production and aerobic consumption in T. roseopersicina M11 is in agreement with previously observed diurnal DMS dynamics in coastal marine sediments, the natural habitat of this anoxygenic phototrophic bacterium. This is the first report in which a bacterium is described that is able to cleave DMSP with subsequent metabolization of DMS.