Dimethylsulfide Production Research Articles

Bacterial Dimethylsulfoniopropionate Degradation Genes in the Oligotrophic North Pacific Subtropical Gyre

Dimethylsulfoniopropionate (DMSP) is an organic sulfur compound that is rapidly metabolized by marine bacteria either by cleavage to dimethylsulfide (DMS) or demethylation to 3-methiolpropionate. The abundance and diversity of genes encoding bacterial DMS production (dddP) and demethylation (dmdA) were measured in the North Pacific subtropical gyre (NPSG) between May 2008 and February 2009 at Station ALOHA (22°45'N, 158°00'W) at two depths: 25 m and the deep chlorophyll maximum (DCM; ∼100 m). The highest abundance of dmdA genes was in May 2008 at 25 m, with ∼16.5% of cells harboring a gene in one of the eight subclades surveyed, while the highest abundance of dddP genes was in July 2008 at 25 m, with ∼2% of cells harboring a gene. The dmdA gene pool was consistently dominated by homologs from SAR11 subclades, which was supported by findings in metagenomic data sets derived from Station ALOHA. Expression of the SAR11 dmdA genes was low, with typical transcript:gene ratios between 1:350 and 1:1,400. The abundance of DMSP genes was statistically different between 25 m and the DCM and correlated with a number of environmental variables, including primary production, photosynthetically active radiation, particulate DMSP, and DMS concentrations. At 25 m, dddP abundance was positively correlated with pigments that are diagnostic of diatoms; at the DCM, dmdA abundance was positively correlated with temperature. Based on gene abundance, we hypothesize that SAR11 bacterioplankton dominate DMSP cycling in the oligotrophic NPSG, with lesser but consistent involvement of other members of the bacterioplankton community.

Macroscale patterns of the biological cycling of dimethylsulfoniopropionate (DMSP) and dimethylsulfide (DMS) in the Northwest Atlantic

The influence of the seasonal development of microplankton communities on the cycling of dimethylsulfide (DMS) and its precursor dimethylsulfoniopropionate (DMSP) was investigated along a South–North gradient (36–59°N) in the Northwest (NW) Atlantic Ocean. Three surveys allowed the sampling of surface mixed layer (SML) waters at stations extending from the subtropical gyre to the Greenland Current during May, July and October 2003. Pools and transformation rates of DMSP and DMS were quantified and related to prevailing physical and biochemical conditions, phytoplankton abundance and taxonomic composition, as well as bacterioplankton abundance and leucine uptake. The South–North progression of the diatom bloom, a prominent feature in the NW Atlantic, did not influence the production of DMS whereas conditions in the N Atlantic Drift lead to a persistent bloom of DMSP-rich flagellate-dominated phytoplankton community and high net DMS production rates. Macroscale patterns of the observed variables were further explored using principal component analysis (PCA). The first axis of the PCA showed a strong association between the spatio-temporal distribution of DMSP and the abundance of several phytoplankton groups including dinoflagellates and prymnesiophytes, as well as with microbial-mediated DMSPd consumption and yields and rates of the conversion of DMSP into DMS. The second axis revealed a strong association between concentrations of DMS and SML depth and photosynthetically active radiation, a result supporting the prominent role of solar radiation as a driver of DMS dynamics.

Decreased marine dimethyl sulfide production under elevated CO2 levels in mesocosm and in vitro studies

Environmental contextAs atmospheric CO2 levels rise due to human activities, more of the gas dissolves in the oceans, increasing their acidity. The effect of these seawater changes on marine organisms is largely unknown. We examine the consequences of higher CO2 levels on the production by plankton of dimethyl sulfide, a climatically active gas. We find that higher CO2 levels leads to lower concentrations of dimethyl sulfide in the seawater, which has potentially important implications for the future climate. AbstractThe oceans have absorbed approximately half of the CO2 produced by human activities and it is inevitable that surface seawaters will become increasingly acidified. The effect of lower pH on marine organisms and ocean–atmosphere exchanges is largely unknown but organisms with CaCO3 structural components are likely to be particularly affected. Because calcifying phytoplankton are significant producers of dimethyl sulfide (DMS), it is vital to understand how lower seawater pH may affect DMS production and emission to the atmosphere. Here we show, by mesocosm (Raunefjorden, Norway, April–May 2003) and in vitro studies, that the net production of DMS and its cellular precursor dimethylsulfoniopropionate (DMSP) is approximately halved in microbial communities subjected to doubled CO2 levels. Our findings provide evidence that the amount of DMS entering the atmosphere could decrease in the future. Because atmospheric oxidation of DMS can lead to climate cooling by increasing cloud albedo, a consequence of reduced DMS emissions from a lower pH ocean would be an enhancement in global warming.

Influence of salinity on dimethyl sulfide and methanethiol formation in estuarine sediments and its side effect on nitrous oxide emissions

We investigated the regulatory effect of salinity on the production of dimethylsulfide (DMS) and methanethiol (MeSH) in estuarine sediments and the potential interactions with the nitrous oxide (N2O) reductase step of the denitrification pathway. This was achieved by monitoring DMS, MeSH and N2O accumulation in sediment slurries retrieved from a temperate estuary (Ave, NW Portugal). Treatments were performed with and without amendments of potential sulfur gas precursors, DMSP (0–50 μM) or methionine (0–500 μM) at different salinities (0, 15 and 30 ppt). Experimental increases of salinity inhibited DMS accumulation under both oxic and anoxic incubation conditions, and the pattern was observed whether DMSP or methionine was added or not, i.e. lower salinities stimulated DMS net production. In contrast, MeSH tended to accumulate to higher concentrations in higher salinity treatments (15 and 30 ppt). Our results also suggest that while salinity had a direct influence on N2O accumulation, it also may modulated N2O production through its regulatory effect on the formation of MeSH, a compound previously shown to inhibit N2O reduction activity. Overall, our results suggest that changes in salinity may have an important regulatory role in net production of DMS, MeSH and N2O and their potential emissions to the atmosphere.

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., r2 = 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.

High concentrations and turnover rates of DMS, DMSP and DMSO in Antarctic sea ice

[1] The vast Antarctic sea-ice zone (SIZ) is a potentially significant source of the climate-active gas dimethylsulfide (DMS), yet few data are available on the concentrations and turnover rates of DMS and the related compounds dimethylsulfoniopropionate (DMSP) and dimethylsulfoxide (DMSO) in sea ice environments. Here we present new measurements characterizing the spatial variability of DMS, DMSP, and DMSO concentrations across the Antarctic SIZ, and results from tracer experiments quantifying the production rates of DMS from various sources. We observed extremely high concentrations (>200 nM) and turnover rates (>100 nM d−1) of DMS in sea-ice brines, indicating intense cycling of DMS/P/O. Our results demonstrate a previously unrecognized role for DMSO reduction as a major pathway of DMS production in Antarctic sea ice.

Irradiance-mediated dimethylsulphoniopropionate (DMSP) responses of red coralline algae

Red coralline algae produce significant quantities of dimethylsulphoniopropionate (DMSP), whose breakdown products include the important climate gas dimethylsulphide (DMS) but little is known about how environmental factors influence this DMS(P) production. The effect of photosynthetically active radiation (PAR) on intracellular DMS(P) concentrations in the red coralline algae Lithothamnion glaciale was investigated using short (30 min) and longer-term (up to 507 h) acclimatory responses and control and high-PAR light regimes. Longer-term acclimatory intracellular DMS(P) concentrations were significantly reduced following exposure to high-PAR (220–250 μmol m −2 s −1). No short-term acclimatory effects were observed. We conclude that while DMS(P) content in L. glaciale does respond to changes in irradiance, the effect takes place over hours – days rather than minutes, suggesting a continued turnover of DMS(P) to combat oxidative stress induced by prolonged high-PAR exposure. Immediate short-term acclimatory responses do not appear to occur.

Distribution and microbial metabolism of dimethylsulfoniopropionate and dimethylsulfide during the 2007 Arctic ice minimum

[1] The distribution and biological cycling of the climate active trace gas dimethylsulfide (DMS) and its algal precursor dimethylsulfoniopropionate (DMSP) were characterized at 20 stations across the Canadian High Arctic during fall 2007. Transformation rates of DMSP and production rates of DMS from dissolved DMSP (DMSPd) were measured during 3 h onboard incubations with radioactively labeled 35S-DMSP. Particulate DMSP (DMSPp) in surface waters varied between 2 and 39 nmol L−1 and increased with chlorophyll a (Chl a) concentrations (r = 0.84). DMS concentrations in surface waters ranged from 0.05 to 0.8 nmol L−1 and were positively correlated with DMSPp (r = 0.89) and Chl a (r = 0.74). The DMSPd loss rate constant varied from 0.01 to 0.14 h−1 and was also positively correlated with Chl a concentrations (r = 0.67). The turnover time of the DMSPd pool varied between 0.3 and 3.4 days (mean = 0.96 day). Bacterial DMS production varied between 0.01 and 0.51 nmol L−1 d−1 (mean = 0.14 nmol L−1 d−1). Assuming local steady state conditions at the time scale of a day, the turnover time of the DMS pool based only on production from DMSPd was ∼6 days at the sampling stations. This long turnover time suggests that DMS production was dominated by nonbacterial processes during our study. Our results show that DMS production could persist at low rates in late fall under ice-free conditions. The magnitude of this production appears to be limited by the low algal and bacterial production prevailing at that time.

Diagnostic modeling of dimethylsulfide production in coastal water west of the Antarctic Peninsula

The rate of gross biological dimethylsulfide (DMS) production at two coastal sites west of the Antarctic Peninsula, off Anvers Island, near Palmer Station, was estimated using a diagnostic approach that combined field measurements from 1 January 2006 through 1 March 2006 and a one-dimensional physical model of ocean mixing. The average DMS production rate in the upper water column (0–60m) was estimated to be 3.1±0.6nMd−1 at station B (closer to shore) and 2.7±0.6nMd−1 at station E (further from shore). The estimated DMS replacement time was on the order of 1d at both stations. DMS production was greater in the mixed layer than it was below the mixed layer. The average DMS production normalized to chlorophyll was 0.5±0.1(nMd−1)/(mgm−3) at station B and 0.7±0.2(nMd−1)/(mgm−3) at station E. When the diagnosed production rates were normalized to the observed concentrations of total dimethylsulfoniopropionate (DMSPt, the biogenic precursor of DMS), we found a remarkable similarity between our estimates at stations B and E (0.06±0.02 and 0.04±0.01(nMDMSd−1)/(nMDMSP), respectively) and the results obtained in a previous study from a contrasting biogeochemical environment in the North Atlantic subtropical gyre (0.047±0.006 and 0.087±0.014(nMDMSd−1)/(nMDMSP) in a cyclonic and anticyclonic eddy, respectively). We propose that gross biological DMS production normalized to DMSPt might be relatively independent of the biogeochemical environment, and place our average estimate at 0.06±0.01(nMDMSd−1)/(nMDMSPt). The significance of this finding is that it can provide a means to use DMSPt measurements to extrapolate gross biological DMS production, which is extremely difficult to measure experimentally under realistic in situ conditions.

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.

Diversity of DMSP transport in marine bacteria, revealed by genetic analyses

The enzyme product of the dddD gene, found in several different marine bacteria, acts on dimethylsulfoniopropionate (DMSP), liberating dimethyl sulfide (DMS) and generating 3-OH-propionate as the initially detected C3 product. In many bacteria, dddD is near genes whose sequence suggests that they encode a DMSP transporter. These are of two very different types, in the BCCT (betaine-carnitine-choline transporter) family or resembling members of the ABC super-family that import betaines. Even within these two families, the amino acid sequences of these putative transporters are not particularly similar to each other. Genes for the predicted DMSP transporters of Halomonas and Marinomonas (both BCCT type) and of Burkholderia ambifaria AMMD (ABC-type) were each cloned and introduced into an Escherichia coli mutant (MKH13) that is defective in betaine uptake, and so fails to catabolise DMSP even when a cloned dddD gene was present, due to the failure of the substrate to be imported. DMSP-dependent DMS production (Ddd+ phenotype) was restored by introducing any of these cloned transporters into MKH13 containing dddD. Other marine bacteria use a range of enzymes, called DddL, DddP, DddQ, DddW and DddY, to cleave DMSP, but the various ddd genes that encode them are usually unlinked to any that are predicted to encode betaine transporters. We identified one gene in Sulfitobacter sp. EE-36 and two in Roseovarius nubinhibens ISM, which, when cloned and introduced into E. coli MKH13, overcame its osmotic sensitivity when it was grown with DMSP or other exogenous betaines. These genes all encoded BCCT transporters, but were unlinked to any known genes involved in DMSP catabolism in these two strains of α-proteobacteria.

Secondary organic aerosol formation from photooxidation of a mixture of dimethyl sulfide and isoprene

Abstract Secondary organic aerosol (SOA) created from the photooxidation of a mixture of isoprene and dimethyl sulfide (DMS) was studied at different NO x concentrations (40–220 ppb) and humidities (12%, 42% and 80%) using a Teflon film indoor chamber. To study the effect of isoprene on DMS products, the major DMS photooxidation products, such as sulfuric acid, methanesulfonic acid (MSA) and methanesulfinic acid (MSIA), were quantified in both the presence and the absence of isoprene using a Particle-Into-Liquid-Sampler coupled with Ion Chromatography (PILS-IC). The resulting PILS-IC data showed that the DMS aerosol yield significantly decreased due to the photooxidation of isoprene. A 35.2% DMS aerosol yield reduction was observed due to 800 ppb isoprene in 185 ppb NO x and 140 ppb DMS. Among the aerosol-phase DMS oxidation products, MSA was the most sensitive to the presence of isoprene (e.g., 46% reduction). The DMS aerosol product analysis indicates that isoprene oxidation affects the pathways of MSA formation on the aerosol surface. Using a new approach that implements an Organic Carbon (OC) analyzer, the isoprene SOA yield ( Y iso ) in the DMS/isoprene/NO x system was also estimated. The OC data showed that Y iso increased significantly with DMS compared to the Y iso without DMS. For example, Y iso with 80 ppb NO x and 840 ppb isoprene was increased by 124.6% due to 100 ppb DMS at RH = 42%. Our study suggests that the heterogeneous reactions of isoprene oxidation products with the highly acidic products (e.g., MSA and sulfuric acid) from DMS photooxidation can considerably contribute to the Y iso increase.

Grazing-induced production of DMS can stabilize food-web dynamics and promote the formation of phytoplankton blooms in a multitrophic plankton model

Volatile infochemicals including climatically relevant dimethylsulphide (DMS) have been suggested to play important roles in the structuring and functioning of marine food webs. Experimenting with complex natural plankton communities or several trophic levels in laboratory microcosms is challenging and, as a result, empirical data confirming the role of DMS in trophic interactions is lacking. Models are a suitable tool to provide insight into such complex interactions. Here we consider a model of the interactions between three trophic levels of plankton: phytoplankton, grazing microzooplankton and predatory mesozooplankton. We show that the inclusion of a grazing-induced DMS production term has a stabilizing effect on the system dynamics under the assumption that DMS acts as an info-chemical and increases the rate of mesozooplankton predation on grazing microzooplankton. We further demonstrate how this feedback between trophic levels can potentially lead to the formation of a phytoplankton bloom. The model provides a suitable framework for further study into the possible role of DMS in the ecology of marine food webs beyond its recognised role as a climate-cooling gas.

DMS dynamics in the most oligotrophic subtropical zones of the global ocean

The influences of physico-chemical and biological processes on dimethylsulfide (DMS) dynamics in the most oligotrophic subtropical zones of the global ocean were investigated. As metrics for the dynamics of DMS and the so-called ‘summer DMS paradox’ of elevated summer concentrations when surface chlorophyll a (Chl) and particulate organic carbon (POC) levels are lowest, we used the DMS-to-Chl and DMS-to-POC ratios in the context of three independent and complementary approaches. Firstly, field observations of environmental variables (such as the solar radiation dose, phosphorus limitation of phytoplankton and bacterial growth) were used alongside discrete DMS, Chl and POC estimates extracted from global climatologies (i.e., a ‘station based’ approach). We then used monthly climatological data for DMS, Chl, and POC averaged over the biogeographic province wherein a given oligotrophic subtropical zone resides (i.e., a ‘province based’ approach). Finally we employed sensitivity experiments with a new DMS module coupled to the ocean general circulation and biogeochemistry model PISCES to examine the influence of various processes in governing DMS dynamics in oligotrophic regions (i.e., a ‘model based’ approach). We find that the ‘station based’ and ‘province based’ approaches yield markedly different results. Interestingly, the ‘province based’ approach suggests the presence of a ‘summer DMS paradox’ in most all of the oligotrophic regions we studied. In contrast, the ‘station based’ approach suggests that the ‘summer DMS paradox’ is only present in the Sargasso Sea and eastern Mediterranean. Overall, we found the regional differences in the absolute and relative concentrations of DMS between 5 of the most oligotrophic regions of the world’s oceans were better accounted for by their nutrient dynamics (specifically phosphorus limitation) than by physical factors often invoked, e.g., the solar radiation dose. Our ‘model based’ experiments suggest that it is the limitation of phytoplankton/bacterial production and bacterial consumption of DMS by pervasive phosphorus limitation that is responsible for the ‘summer DMS paradox’.

Stimulation of gross dimethylsulfide (DMS) production by solar radiation

Oceanic gross DMS production (GP) exerts a fundamental control on the concentration and the sea‐air flux of this climatically‐active trace gas. However, it is a poorly constrained process, owing to the complexity of the microbial food web processes involved and their interplay with physical forcing, particularly with solar radiation. The “inhibitor method”, using dimethyldisulfide (DMDS) or other compounds to inhibit bacterial DMS consumption, has been frequently used to determine GP in dark incubations. In the work presented here, DMDS addition was optimized for its use in light incubations. By comparing simultaneous dark and light measurements of GP in meso‐ to ultraoligotrophic waters, we found a significant enhancement of GP in natural sunlight in 7 out of 10 experiments. Such stimulation, which was generally between 30 and 80% on a daily basis, occurred throughout contrasting microbial communities and oceanographic settings.

DddW, a third DMSP lyase in a model Roseobacter marine bacterium, Ruegeria pomeroyi DSS-3

Ruegeria pomeroyi DSS-3 is a model Roseobacter marine bacterium, particularly regarding its catabolism of dimethylsulfoniopropionate (DMSP), an abundant anti-stress molecule made by marine phytoplankton. We found a novel gene, dddW, which encodes a DMSP lyase that cleaves DMSP into acrylate plus the environmentally important volatile dimethyl sulfide (DMS). Mutations in dddW reduced, but did not abolish DMS production. Transcription of dddW was greatly enhanced by pre-growth of cells with DMSP, via a LysR-type regulator. Close DddW homologs occur in only one other Roseobacter species, and there are no close homologs and only a few related sequences in metagenomes of marine bacteria. In addition to DddW, R. pomeroyi DSS-3 had been shown to have two other, different, DMSP lyases, DddP and DddQ, plus an enzyme that demethylates DMSP, emphasizing the importance of this substrate for this model bacterium.

Production of DMS and DMSP in different physiological stages and salinity conditions in two marine algae

Dimethylsulfide (DMS) and dimethylsulfoniopropionate (DMSP) production by Scrippsiella trochoidea and Prorocentrum minimum was investigated to characterize the effects of physiological stage and salinity on DMS and DMSP pools of these two marine phytoplankton species. Axenic laboratory cultures of the two marine algae were tested for DMSP production and its conversion into DMS. The results demonstrated that both algal species could produce DMS, but the average concentration of DMS per cell in S. trochoidea (12.63 fmol/L) was about six times that in P. minimum (2.01 fmol/L). DMS and DMSP concentrations in algal cultures varied significantly at different growth stages, with high release during the late stationary growth phase and the senescent phase. DMS production induced by three salinities (22, 28, 34) showed that the DMS concentrations per cell in the two algal cultures increased with increasing salinity, which might result from intra-cellular DMSP up-regulation with the change of osmotic stress. Our study specifies the distinctive contributions of different physiological stages of marine phytoplankton on DMSP and DMS production, and clarifies the influence of salinity conditions on the release of DMS and DMSP. As S. trochoidea and P. minimum are harmful algal bloom species with high DMS production, they might play an additional significant role in the sulfur cycle when a red tide occurs.

Influence of salinity and nitrogen content on production of dimethylsulfoniopropionate (DMSP) and dimethylsulfide (DMS) by Skeletonema costatum

The effects of changing salinity and nitrogen limitation on dimethylsulfoniopropionate (DMSP) and dimethylsulfide (DMS) concentrations were investigated in batch cultures of coastal diatom Skeletonema costatum, an ecologically important species. Changes in salinity from 20–32 caused no measurable variation in cell growth or culture yield, but increased intracellular DMSP per cell by 30%. Nitrogen limitation caused up to a two-fold increase in total DMSP per cell and up to a three-fold increase in DMS per cell. These changes in DMSP and DMS per cell in the Skeletonema costatum cultures with nitrogen limitation and changing salinity were primarily attributed to the physiological functions of DMSP as an osmolyte and an antioxidant. The data obtained in this study indicated that nitrogen limitation and salinity may play an important role in climate feedback mechanisms involving biologically derived DMS.

Effect of oxidoreduction potential on aroma biosynthesis by lactic acid bacteria in nonfat yogurt

The aim of this study was to investigate the effect of oxidoreduction potential (Eh) on the biosynthesis of aroma compounds by lactic acid bacteria in non-fat yogurt. The study was done with yogurts fermented by Lactobacillus bulgaricus and Streptococcus thermophilus. The Eh was modified by the application of different gaseous conditions (air, nitrogen, and nitrogen/hydrogen). Acetaldehyde, dimethyl sulfide, diacetyl, and pentane-2,3-dione, as the major endogenous odorant compounds of yogurt, were chosen as tracers for the biosynthesis of aroma compounds by lactic acid bacteria. Oxidative conditions favored the production of acetaldehyde, dimethyl sulfide, and diketones (diacetyl and pentane-2,3-dione). The Eh of the medium influences aroma production in yogurt by modifying the metabolic pathways of Lb. bulgaricus and Strep. thermophilus. The use of Eh as a control parameter during yogurt production could permit the control of aroma formation.

DddY, a periplasmic dimethylsulfoniopropionate lyase found in taxonomically diverse species of Proteobacteria

The abundant compatible solute dimethylsulfoniopropionate (DMSP) is made by many marine algae. Different marine bacteria catabolise DMSP by various mechanisms, some of which liberate the environmentally important gas dimethyl sulfide (DMS). We describe an enzyme, DddY, which cleaves DMSP into DMS plus acrylate and is located in the bacterial periplasm, unlike other DMSP lyases that catalyse this reaction. There are dddY-like genes in strains of Alcaligenes, Arcobacter and Shewanella, in the β-, ɛ- and γ-proteobacteria, respectively. In Alcaligenes, dddY is in a cluster of ddd and acu genes that resemble, but also have significant differences to, those in other bacteria that catabolise both DMSP and acrylate. Although production of DMS and transcription of Alcaligenes dddY are both apparently inducible by pre-growth of cells with DMSP, this substrate must be catabolised to form acrylate, the bona fide coinducer.