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.     

Biogenic production of DMSP and its degradation to DMS—their roles in the global sulfur cycle

Dimethyl sulfide(DMS) is the most abundant form of volatile sulfur in Earth's oceans, and is mainly produced by the enzymatic clevage of dimethylsulfoniopropionate(DMSP). DMS and DMSP play important roles in driving the global sulfur cycle and may affect climate. DMSP is proposed to serve as an osmolyte, a grazing deterrent, a signaling molecule, an antioxidant, a cryoprotectant and/or as a sink for excess sulfur. It was long believed that only marine eukaryotes such as phytoplankton produce DMSP. However, we recently discovered that marine heterotrophic bacteria can also produce DMSP, making them a potentially important source of DMSP. At present, one prokaryotic and two eukaryotic DMSP synthesis enzymes have been identified.Marine heterotrophic bacteria are likely the major degraders of DMSP, using two known pathways: demethylation and cleavage.Many phytoplankton and some fungi can also cleave DMSP. So far seven different prokaryotic and one eukaryotic DMSP lyases have been identified. This review describes the global distribution pattern of DMSP and DMS, the known genes for biosynthesis and cleavage of DMSP, and the physiological and ecological functions of these important organosulfur molecules, which will improve understanding of the mechanisms of DMSP and DMS production and their roles in the environment.     

Influence of dimethyl sulfide on the carbon cycle and biological production

Dimethyl sulfide (DMS) is a significant source of marine sulfate aerosol and plays an important role in modifying cloud properties. Fully coupled climate simulations using dynamic marine ecosystem and DMS calculations are conducted to estimate DMS fluxes under various climate scenarios and to examine the sign and strength of phytoplankton-DMS-climate feedbacks for the first time. Simulation results show small differences in the DMS production and emissions between pre-industrial and present climate scenarios, except for some areas in the Southern Ocean. There are clear changes in surface ocean DMS concentrations moving into the future, and they are attributable to changes in phytoplankton production and competition driven by complex spatially varying mechanisms. Comparisons between parallel simulations with and without DMS fluxes into the atmosphere show significant differences in marine ecosystems and physical fields. Without DMS, the missing subsequent aerosol indirect effects on clouds and radiative forcing lead to fewer clouds, more solar radiation, and a much warmer climate. Phaeocystis, a uniquely efficient organosulfur producer with a growth advantage under cooler climate states, can benefit from producing the compound through cooling effects of DMS in the climate system. Our results show a tight coupling between the sulfur and carbon cycles. The ocean carbon uptake declines without DMS emissions to the atmosphere. The analysis indicates a weak positive phytoplankton-DMS-climate feedback at the global scale, with large spatial variations driven by individual autotrophic functional groups and complex mechanisms. The sign and strength of the feedback vary with climate states and phytoplankton groups. This highlights the importance of a dynamic marine ecosystem module and the sulfur cycle mechanism in climate projections.     

164 Chemical Antiherbivore Defenses of Temperate Green Macroalgae

Many temperate green macroalgae contain secondary meatbolites that provide protection from grazing by some herbivores. These include the production of dopamine hydrochloride by the ulvoid green alga Ulvaria obscura and the production of dimethylsulfoniopropionate (DMSP) by many species of Ulvales and Caulerpales. The dopamine hydrochloride defense was isolated using bioassay‐guided fractionation and is effective against sea urchins (Strongylocentrotus droebachiensis) and littorinid snails (Littorina sitkana). The DMSP activated defense system involves enzymatic cleavage of DMSP into dimethyl sulfide (DMS) and acrylic acid. It is found in many of the Ulvales and several species of Codium in the northeastern Pacific and Australasian regions. Many green algae such as Ulva fenestrata and Enteromorpha linza are avoided by urchins, which are deterred by DMS and acrylic acid in laboratory assays. However, these algae are often preferred foods of snails, which are deterred by DMS and acrylic acid. Snails may preferentially consume ulvoid green algae, despite being deterred by DMS and acrylic acid, because these algae contain relatively high nitrogen concentrations.     

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     

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.     

Metabolism of dimethylsulfoniopropionate and glycine betaine by a marine bacterium

Abstract The metabolism of the methylated osmolytes glycine betaine (GB) and dimethylsulfoniopropionate (DMSP) was studied in a bacterium (strain MD 14–50) isolated from a colony of the cyanobacterium Trichodesmium . MD 14–50 when grown on DMSP cleaved dimethylsulfide (DMS) from DMSP and oxidized acrylate. In contrast to DMSP, GB was metabolized by sequential N-demethylations. Low concentrations (100 μM) of DMSP or GB allowed the growth of MD 14–50 on glucose at higher salinities than in their absence. At elevated salinities, DMSP was accumulated intracellularly with less catabolism and DMS production. Thus, DMSP and GB were catabolized by different mechanisms but functioned interchangeably as osmolytes.     

Alkali-labile precursors of dimethyl sulfide in marine benthic cyanobacteria

By the method of cold alkali hydrolysis, 29 marine benthic cyanobacteria were screened for production of alkali-labile precursors of dimethyl sulfide (DMS) including dimethylsulfoniopropionate (DMSP), a compound of significant importance in marine environments. Concentrations of DMS precursors ranged from undetectable to 0.8 mmol (g Chl a)^–1. The data correspond to some previous investigations concerning DMSP content of marine cyanobacteria and suggest that marine benthic cyanobacteria are only minor producers of DMSP. Received: 3 July 1997 / Accepted: 21 October 1997     

The quantitative role of microzooplankton grazing in dimethylsulfide (DMS) production in the NW Mediterranean

The ubiquitous, biogenic trace gas dimethylsulfide (DMS) represents the largest natural source of atmospheric sulfur. Given DMS involvement in cloud formation and climate, understanding and parameterizing the oceanic DMS source and cycling processes is a necessary challenge. We report DMS cycling rates from microzooplankton dilution grazing experiments conducted monthly during 1 year in coastal northwestern Mediterranean waters. Concentrations of DMS, its algal precursor dimethylsulfoniopropionate (DMSPt) and chlorophyll a (Chla) ranged 0.9–11 nmol L^?1, 10–71 nmol L^?1, and 0.2–1.5 µg L^?1, respectively. By comparing the growth and stock production rates of the DMSP-producing algae to those of total phytoplankton, we estimated that 3?±?4% (range 0.4–12%) of the carbon primary production was invested in DMSP biosynthesis. Microzooplankton grazing rates on DMSP-producing phytoplankton (0.46–1.45 day^?1) were generally higher than those on the bulk assemblage (0.08–0.99 day^?1), except in midsummer months. This could have been due to the smaller size of most DMSP producers. There was no indication of micrograzer selection against DMSP-containing phytoplankton, since they were not grazed at lower rates than the bulk phytoplankton assemblage. A proportion of 6–20% of the grazed DMSP was converted into DMS, and this grazing-derived production accounted for 32–96% of dark gross DMS production by the total community. Bacteria consumed daily?≤?14–100% of the gross DMS production, which resulted in biological DMS turnover times of 1 to?≥?10 days. Throughout the year, grazing-mediated DMS production explained 73% of the variance in the DMS concentration, implying that microzooplankton grazing plays a major role in controlling DMS concentration in surface waters across a broad range of environmental and productivity conditions in the Mediterranean Sea. These findings should help improve the representation of herbivore grazing in prognostic models to predict the distribution and dynamics of the global DMS emission and its feedback response to changing climate.     

A Highlights from MBoC Selection: Age-dependent Preferential Dense-Core Vesicle Exocytosis in Neuroendocrine Cells Revealed by Newly Developed Monomeric Fluorescent Timer Protein

Although it is evident that only a few secretory vesicles accumulating in neuroendocrine cells are qualified to fuse with the plasma membrane and release their contents to the extracellular space, the molecular mechanisms that regulate their exocytosis are poorly understood. For example, it has been controversial whether secretory vesicles are exocytosed randomly or preferentially according to their age. Using a newly developed protein-based fluorescent timer, monomeric Kusabira Green Orange (mK-GO), which changes color with a predictable time course, here we show that small GTPase Rab27A effectors regulate age-dependent exocytosis of secretory vesicles in PC12 cells. When the vesicles were labeled with mK-GO–tagged neuropeptide Y or tissue-type plasminogen activator, punctate structures with green or red fluorescence were observed. Application of high [K⁺] stimulation induced exocytosis of new (green) fluorescent secretory vesicles but not of old (red) vesicles. Overexpression or depletion of rabphilin and synaptotagmin-like protein4-a (Slp4-a), which regulate exocytosis positively and negatively, respectively, disturbed the age-dependent exocytosis of the secretory vesicles in different manners. Our results suggest that coordinate functions of the two effectors of Rab27A, rabphilin and Slp4-a, are required for regulated secretory pathway.     

Mechanistic insight into acrylate metabolism and detoxification in marine dimethylsulfoniopropionate‐catabolizing bacteria

Dimethylsulfoniopropionate (DMSP) cleavage, yielding dimethyl sulfide (DMS) and acrylate, provides vital carbon sources to marine bacteria, is a key component of the global sulfur cycle and effects atmospheric chemistry and potentially climate. Acrylate and its metabolite acryloyl‐CoA are toxic if allowed to accumulate within cells. Thus, organisms cleaving DMSP require effective systems for both the utilization and detoxification of acrylate. Here, we examine the mechanism of acrylate utilization and detoxification in Roseobacters. We propose propionate‐CoA ligase (PrpE) and acryloyl‐CoA reductase (AcuI) as the key enzymes involved and through structural and mutagenesis analyses, provide explanations of their catalytic mechanisms. In most cases, DMSP lyases and DMSP demethylases (DmdAs) have low substrate affinities, but AcuIs have very high substrate affinities, suggesting that an effective detoxification system for acylate catabolism exists in DMSP‐catabolizing Roseobacters. This study provides insight on acrylate metabolism and detoxification and a possible explanation for the high K_m values that have been noted for some DMSP lyases. Since acrylate/acryloyl‐CoA is probably produced by other metabolism, and AcuI and PrpE are conserved in many organisms across all domains of life, the detoxification system is likely relevant to many metabolic processes and environments beyond DMSP catabolism.     

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.     

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     

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.     

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     

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.     

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.