Dimethylsulfoniopropionate Production Research Articles

Influence of temperature and elevated carbon dioxide on the production of dimethylsulfoniopropionate and glycine betaine by marine phytoplankton

The sulfur metabolite dimethylsulfoniopropionate (DMSP) is the most important precursor of the climate relevant metabolite dimethylsulfide (DMS). It has thus gained interest in the context of climate change and several studies investigated the influence of elevated temperature and/or CO 2 on DMSP in complex plankton communities. However, only little information about changes in response to these factors in single species is available. Therefore, we analyzed DMSP in different phytoplankton cultures ( Thalassiosira pseudonana, Phaeodactylum tricornutum, Emiliania huxleyi) under the influence of increased temperature by 6 °C and elevated CO 2 to 790 ppmv. In addition, we addressed glycine betaine (GBT) that fulfills a similar function as osmolyte like DMSP. In all cultures GBT concentrations increased at higher temperature and decreased at elevated CO 2. In contrast, diatoms and prymnesiophytes revealed opposite trends for DMSP. In diatoms increased CO 2 and temperature led to decreased DMSP concentrations, while rather elevated levels of this metabolite under the influence of these parameters were observed for E. huxleyi.

Modulation of dimethylsulfoniopropionate (DMSP) concentration in an Alexandrium minutum (Dinophyceae) culture by small-scale turbulence: A link to toxin production?

Some marine dinoflagellates produce important amounts of dimethylsulfoniopropionate (DMSP), a common compatible solute, and its cleavage product dimethylsulfide (DMS), a climatically active trace gas. In the field, dinoflagellate proliferations appear to be favored by calm weather and water column stability; indeed, small-scale turbulence is a physical factor that directly affects ecophysiological aspects of this phytoplankton group, including toxin production. Here we report the effect of experimentally generated turbulence on DMSP production by a paralytic shellfish poisoning (PSP) toxin producing strain of Alexandrium minutum, a widespread bloomforming dinoflagellate species. With respect to still conditions, the populations exposed to turbulence grew at a slower growth rate and yielded low cell numbers turbulence. Concurrently, the cellular DMSP concentration increased by ca. 20% (from 0.22 ± 0.01 to 0.27 ± 0.03 fmol μm −3 on a cell volume basis) in the shaken cultures. DMSP was preferentially synthesized during the light period in both treatments. During the night, a slowdown of the division process caused DMSP accumulation in the cells exposed to shaking. The study suggests the existence of a tight link between the dynamics of DMSP concentration and other cell processes entrained by circadian rhythms in dinoflagellates. The observed effects of small-scale turbulence on the DMSP dynamics supports the suggested role of this compound as an overflow mechanism in metabolically unbalanced cells. Furthermore, considering all the effects on the physiology of A. minutum exposed to the same experimental setup, we propose a possible link between the DMSP and the PSP metabolisms.

Phaeocystis globosa Scherffel, a harmful microalga, and its production of dimethylsulfoniopropionate

The production of dimethylsulfoniopropionate (DMSP) and its cleavage products (DMS) are well studied in phytoplankton worldwide. However, less is known about their sources, distributions, and impacts in the coast of China. We examined the production of DMSP and DMS in Phaeocystis globosa Scherffel and other benthic macroalgae from the South China coast in relation to environmental conditions. P. globosa was a harmful marine microalgal species and its bloom took place in the eutrophic waters along the South China Sea frequently. It also produced high content of DMSP at different growth stages, with the highest concentration usually observed in the stationary period. Moreover, the production of DMSP in P. globosa was significantly affected by salinity and temperature with the highest contents associated with high salinity (e.g. 40) and low temperature (e.g. 20°C). In field benthic macroalgae, there was also a marked difference in the DMSP of various species or different samples of the same species. Chlorophyll a contents were also determined for each macroalgal species. The highest chlorophyll a (238.7 ng/g fresh weight) was recorded in Chlorophyta Ulva lactuca at Guishan Island (Zhuhai), while the lowest value (1.5 ng/g fresh weight) was found in Rhodophyta Gracilaria tenuistipitata in Zhanjiang. Further correlation analysis indicated that there was no significant relationship between the content of DMSP and chl-a in macroalgae samples (P > 0.05). All the results suggested that the production of DMSP in marine algae was not only species- and stage-related, but also greatly affected by various environmental factors.

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.

High‐resolution dimethyl sulfide and dimethylsulfoniopropionate time series profiles in decaying summer first‐year sea ice at Ice Station Polarstern, western Weddell Sea, Antarctica

High‐resolution profiles of ice dimethyl sulfide (DMS) and dimethylsulfoniopropionate (DMSP) concentrations were measured together with a suite of ancillary physical and biological properties during a time series of decaying summer‐level first‐year sea ice throughout December 2004 during the Ice Station Polarstern drift experiment (western Weddell Sea, Antarctica). Ice DMSP and DMS concentrations were always maximum at the bottom of the ice sheet (636–2627 and 292–1430 nM, respectively) where the highest chlorophyll a levels were also found (15–30 μg L−1). Throughout the observation period, the autotrophic surface community (32–205 μg C L−1) was dominated by Phaeocystis sp. while the bottom community (1622–3830 μg C L−1) mainly consisted of pennate diatoms. This illustrates that, although being known for lower DMSP‐to‐chlorophyll a ratios than Phaeocystis sp., diatoms dominated the overall DMSP production because of their much larger biomass. Decreasing DMSP concentrations and increasing DMS‐to‐DMSP ratios in the bottom layers with time suggested active DMSP‐to‐DMS conversion in a slowly degrading environment. Drastic temporal brine volume and brine salinity changes associated with the decaying sea ice cover are shown to directly impact (1) the migration of DMSP and DMS through the brine network, (2) the DMSP‐to‐DMS conversion processes within the ice interior, and (3) the physiological response of the ice algae. First‐order flux estimates show that decaying summer‐level first‐year sea ice alone can significantly contribute to the regional sulfur budget of the Weddell Sea with an estimated average loss rate of 5.7 μmol DMS(P) m−2 d−1) toward the atmosphere and the ocean.

Spatial and temporal variation in DMSP content in the invasive seaweed Codium fragile ssp. fragile: effects of temperature, light and grazing

The methionine-derived secondary metabolite dimethylsulfoniopropionate (DMSP) is believed to function in cryoprotection, defense against herbivory, and reduction of oxidative stress in some seaweeds. The ability to produce DMSP, and modify its concentration according to environ- mental conditions, may confer adaptive advantages to these algae. We studied the spatial and tem- poral distribution of DMSP content in Codium fragile off the Atlantic coast of Nova Scotia. Levels of DMSP in C. fragile were highest in boreal spring (~2% of total dry wt) and lowest in fall (~1%), and negatively related to seawater temperature. DMSP content differed among sites and tended to decrease or remain constant with depth. In tide pools, partially bleached algae contained less DMSP than undamaged individuals, consistent with the putative function of DMSP in response to oxidative stress. Results of field- and laboratory-based experiments, designed to examine plasticity in DMSP production by C. fragile, indicate that it increases with light intensity, exposure to sea urchin grazing, and decreasing temperature.

Spatial and temporal variability of the dimethylsulfide to chlorophyll ratio in the surface ocean: an assessment based on phytoplankton group dominance determined from space

Abstract. Dimethylsulfoniopropionate (DMSP) is produced in surface seawater by phytoplankton. Phytoplankton culture experiments have shown that nanoeucaryotes (NANO) display much higher mean DMSP-to-Carbon or DMSP-to-Chlorophyll (Chl) ratios than Prochlorococcus (PRO), Synechococcus (SYN) or diatoms (DIAT). Moreover, the DMSP-lyase activity of algae which cleaves DMSP into dimethylsulfide (DMS) is even more group specific than DMSP itself. Ship-based observations have shown at limited spatial scales, that sea surface DMS-to-Chl ratios (DMS:Chl) are dependent on the composition of phytoplankton groups. Here we use satellite remote sensing of Chl (from SeaWiFS) and of Phytoplankton Group Dominance (PGD from PHYSAT) with ship-based sea surface DMS concentrations (8 cruises in total) to assess this dependence on an unprecedented spatial scale. PHYSAT provides PGD (either NANO, PRO, SYN, DIAT, Phaeocystis (PHAEO) or coccolithophores (COC)) in each satellite pixel (1/4° horizontal resolution). While there are identification errors in the PHYSAT method, it is important to note that these errors are lowest for NANO PGD which we typify by high DMSP:Chl. In summer, in the Indian sector of the Southern Ocean, we find that mean DMS:Chl associated with NANO + PHAEO and PRO + SYN + DIAT are 13.6±8.4 mmol g−1 (n=34) and 7.3±4.8 mmol g−1 (n=24), respectively. That is a statistically significant difference (P<0.001) that is consistent with NANO and PHAEO being relatively high DMSP producers. However, in the western North Atlantic between 40° N and 60° N, we find no significant difference between the same PGD. This is most likely because coccolithophores account for the non-dominant part of the summer phytoplankton assemblages. Meridional distributions at 22° W in the Atlantic, and 95° W and 110° W in the Pacific, both show a marked drop in DMS:Chl near the equator, down to few mmol g−1, yet the basins exhibit different PGD (NANO in the Atlantic, PRO and SYN in the Pacific). In tropical and subtropical Atlantic and Pacific waters away from the equatorial and coastal upwelling, mean DMS:Chl associated with high and low DMSP producers are statistically significantly different, but the difference is opposite of that expected from culture experiments. Hence, in a majority of cases PGD is not of primary importance in controlling DMS:Chl variations. We therefore conclude that water-leaving radiance spectra obtained simultaneously from ocean color sensor measurements of Chl concentrations and dominant phytoplankton groups can not be used to predict global fields of DMS.

Experimental studies on dimethylsulfide (DMS) and dimethylsulfoniopropionate (DMSP) production by four marine microalgae

The production of dimethylsulfide (DMS) and dimethylsulfoniopropionate (DMSP) by marine microalgae was investigated to elucidate more on the role of marine phytoplankton in ocean-atmosphere interactions in the global biogeochemical sulfur cycle. Axenic laboratory cultures of four marine microalgae-Isochrysis galbana 8701, Pavlova viridis, Platymonas sp. and Chlorella were tested for DMSP production and conversion into DMS. Among these four microalgae, Isochrysis galbana 8701 and Pavlova viridis are two species of Haptophyta, while Chlorella and Platymonas sp. belong to Chlorophyta. The results demonstrate that the four algae can produce various amounts of DMS(P), and their DMS(P) production was species specific. With similar cell size, more DMS was released by Haptophyta than that by Chlorophyta. DMS and dissolved DMSP (DMSPd) concentrations in algal cultures varied significantly during their life cycles. The highest release of DMS appeared in the senescent period for all the four algae. Variations in DMSP concentrations were in strong compliance with variations in algal cell densities during the growing period. A highly significant correlation was observed between the DMS and DMSPd concentrations in algal cultures, and there was a time lag for the variation trend of the DMS concentrations as compared with that of the DMSPd. The consistency of variation patterns of DMS and DMSPd implies that the DMSPd produced by phytoplankton cells has a marked effect on the production of DMS. In the present study, the authors’ results specify the significant contribution of the marine phytoplankton to DMS(P) production and the importance of biological control of DMS concentrations in oceanic water.

Microbial dimethylsulfoniopropionate (DMSP) dynamics along a natural iron gradient in the northeast subarctic Pacific

We characterized the effect of an inshore‐offshore gradient in Fe in the northeast subarctic Pacific on the bacterioplankton and phytoplankton assemblages and on the microbial cycling of particulate and dissolved dimethylsulfoniopropionate (DMSPp and DMSPd) and dimethylsulfide (DMS). Averaged concentrations of total dissolved Fe (TDFe) decreased linearly with increasing water density along the transect, from 3.4 nmol L−1 at the two inshore stations to 1.0 nmol L−1 at the offshore stations, as a result of the vertical and lateral mixing between the Fe‐rich coastal water and the Fe‐poor Alaska Current. The Fe‐rich inshore stations were dominated by diatoms and characterized by low DMSPp : chlorophyll a (Chl a) ratios (ca. 26 nmol [g−1) and bacterial DMS yield (< 4%). In contrast, the Fe‐poor offshore stations were dominated by prymnesiophytes and exhibited high DMSPp : Chl a ratios (ca. 84 nmol [g−1) and bacterial DMS yield (8%). Chl a, DMSPp, and the abundance of total bacteria and three bacterial clades (Gammaproteobacteria, Roseobacter, and Betaproteobacteria) were positively correlated with the TDFe gradient. At the Fe‐poor offshore stations, the positive correlation found between TDFe and the DMSPp : Chl a ratios suggests that Fe supplied by mixing stimulated DMSP production in the prymnesiophyte‐dominated assemblage, a response similar to that generally observed during the first days of most of the large‐scale ocean iron fertilizations (OIFs). These results suggest that the stimulation of DMSP production takes place whatever the Fe supply mode: atmospheric dust deposition, as simulated by OIFs, or mixing, as reported in this study.

Variability in dimethylsulfoniopropionate (DMSP) concentrations in Spartina alterniflora and the effect on Littoraria irrorata

We measured concentrations of dimethylsulfoniopropionate (DMSP) in Spartina alterni- flora (Loisel.) in response to the plant hormones abscisic acid (ABA), jasmonic acid (JA), and salicylic acid (SA) to determine whether DMSP concentration is linked to any of their signaling pathways. DMSP concentrations were also measured in plants at a salt marsh dieback site in Georgetown County, South Carolina (USA), to determine whether dieback conditions affect foliar DMSP concen- trations. We found elevated levels of DMSP in plants receiving SA treatment compared to a control treatment, but we found no treatment effect for either of the other 2 hormone treatments. This sug- gests that DMSP production or transformation is linked to the SA signaling pathway. Diminished lev- els of DMSP were observed in plants nearest the dieback edge compared to those farthest from the dieback in an apparently healthy marsh, which suggests that DMSP concentration responds to envi- ronmental conditions connected with salt marsh dieback. Using a ring assay, we found that Littoraria irrorata snails, potential herbivores of S. alterniflora, were attracted to DMSP. However, we found no correlation between DMSP concentration and L. irrorata density at the acute dieback site. These results suggest that factors such as plant biomass may play a more important role than DMSP in determining L. irrorata distribution in dieback areas.

In vivo DMSP‐biosynthesis measurements using stable‐isotope incorporation and proton‐transfer‐reaction mass spectrometry (PTR‐MS)

Assessing turnover rates of dimethylsulfoniopropionate (DMSP), precursor of the climate‐active gas dimethylsulfide (DMS), under different environmental conditions is fundamental to accurately modeling the marine sulfur cycle. Current gas chromatographic methods only provide net results of all processes acting on the different sulfur pools and are unable to measure fluxes, leaving room for speculation on the controlling factors and magnitudes of turnover processes. Here we present a new method in which stable isotope additions (D2O or NaH13CO3) are used to follow the in vivo production of DMSP. Incorporation of deuterium or 13C into DMSP was monitored using a proton‐reaction‐transfer mass spectrometer (PTR‐MS). A growth model was developed to calculate specific DMSP synthesis rates from the progress in mass ratio of labeled versus nonlabeled DMSP over time. Instead of measuring the amount of incorporated isotope, our method uses ratios of molecule masses. This approach has the advantage that the effect of the isotope addition on the change in mass ratio is amplified by the number of positions the label can occupy within the molecule. Application of the method was demonstrated in cultures of the ubiquitous DMSP producer Emiliania huxleyi and further developed for use in short‐time incubation studies with water from the Sargasso Sea during July 2004. Very low specific synthesis rates between 0.05 and 0.2 d−1 could be measured. The method is the first to measure in vivo DMSP production rates. It is relatively easy to apply, highly sensitive, and may also be applicable to other biogenic compounds.

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.

Dimethyl sulfide production in a saline eutrophic lake, Salton Sea, California

The concentrations and distribution of volatile organic sulfur compounds were quantified over a 13-month period in the Salton Sea, a warm eutrophic saline lake in Southern California, U.S.A. The concentrations of dimethyl sulfide (DMS) appear to be the highest reported thus far for a natural body of water, with an average surface (0–2 m) concentration of 2.5 µmol L−1. DMS concentrations as high as 11 µmol L−1 were measured, and the concentrations of DMS correlated strongly with chlorophyll a (r2 = 0.62, n = 265, p < 0.05). Dimethyl disulfide was also measured; concentrations were much lower than DMS and often below detection (<0.01-0.32 µmol L−1). Carbon disulfide concentrations were low (<0.03 µmol L−1) and associated with strongly reduced conditions. Very high concentrations of dimethylsulfoniopropionate (DMSP), an osmolyte in marine algae, were also measured (average total DMSP of 2.4 µmol L−1), with concentrations strongly correlated with chlorophyll a (r2 = 0.88, n = 36, p < 0.05). The biomass of the Salton Sea is composed mostly of marine phytoplankton species that are high DMSP producers; based on the correlations of DMS, chlorophyll a, and DMSP, it appears that the DMS in the Salton Sea is directly linked to algal biomass through DMSP. As a result of its very high DMS concentrations, the average estimated volatilization at the Salton Sea (480 µmol m−2 d−1) was greater than estimates for other lakes and the open ocean. We calculate ~9.6 × 105 mol of DMS was volatilized off the surface of the Sea during the course of this study.

Plankton origin of particulate dimethylsulfoniopropionate in a Mediterranean oligotrophic coastal and shallow ecosystem

We report here dimethylsulfide (DMS) and dimethylsulfoniopropionate (DMSP) levels as a function of plankton communities and abiotic factors over a 12-month cycle in the Mediterranean oligotrophic coastal and shallow ecosystem of Niel Bay (N.W. Mediterranean Sea, France). Total particulate DMSP (DMSP p ) and DMS concentrations were highly seasonal, peaking during a spring (April) bloom at 8.9 nM and 73.9 nM, respectively. Significant positive correlations were found between total DMSP p concentration and the abundance or biomass of the dinoflagellate Prorocentrum compressum (Spearman's rank correlation test: r = 0.704; p = 0.011). Similarly, DMS concentrations peaked during the development of blooms of P. compressum and Gymnodinium sp. There seemed to be a positive relationship between the chlorophyll a to pheopigment ratio and DMS concentrations, suggesting that DMS was released during phytoplankton growth. High DMS levels recorded in the shallow Niel Bay may also result from the activity of benthic macroalgae, and/or macrophytes such as Posidonia spp., or the resuspension of sulfur species accumulating in sediments. The fractionation of particulate DMSP into three size classes (>90 μm, 5–90 μm and 0.2–5 μm) revealed that 5–90 μm DMSP-containing particles made the greatest contribution to the total DMSP p pool (annual mean contribution = 62%), with a maximal contribution in April (96%). This size class consisted mainly of dinoflagellates (annual mean contribution = 68%), with P. compressum and Gymnodinium sp. the predominant species, together accounting for up to 44% of the phytoplankton present. The positive correlation between DMSP concentration in the 5–90 μm size class and the abundance of P. compressum (Spearman's rank correlation test: r = 0.648; p = 0.023) suggests that this phytoplankton species would be the major DMSP producer in Niel Bay. The DMSP collected in the >90 μm fraction was principally associated with zooplankton organisms, dominated by copepods (nauplii and copepodites). DMSP >90, not due to a specific zooplankton production, resulted from the phytoplankton cells ingested during grazing. The concomitant peaks of DMS concentration and zooplankton abundance suggest that zooplankton may play a role in releasing DMSP and/or DMS through sloppy feeding.

Seasonal dimethylsulfoniopropionate (DMSP) variability in Dona Paula bay

Data on temporal variations of total dimethylsulfoniopropionate (DMSP t) and the environmental factors that influence DMSP t concentrations are important in understanding the biogeochemical cycling of organic sulfur compounds. Annual and diurnal variations of DMSP t were investigated in relation to environmental variables at a fixed station in Dona Paula bay (west coast of India). DMSP concentrations were high in the day and low at night and ranged from 3.69 to 84 nM with a maximum at 17.00 h. The high concentrations of DMSP t during daytime closely followed that of Chl a concentrations. The DMSP utilizers averaged 0.8 ± 0.3 × 10 3 cells l −1 during night and 0.4 ± 0.1 × 10 3 cells l −1 during the day. The diel variation of DMSP t was influenced more by biological variables than hydrographic parameters. In the year-round study, the concentrations ranged from 0.69 to 15.8 nM. It was fourfold higher during the southwest monsoon season (13.4 ± 2 nM) and threefold higher during the post-monsoon season (9.96 ± 5 nM) compared to the pre-monsoon season (3.1 ± 1 nM). DMSP t concentrations showed temporal variability, both during diurnal and annual studies. Diatoms were identified as producers of DMSP in Dona Paula bay. Dinoflagellates also contributed during the non-monsoon seasons. Another factor involved in the variability of DMSP t was DMSP utilizing bacteria, which ranged from 1 to 10% of the total heterotrophic count.

Dimethylsulfide production in Sargasso Sea eddies

Abstract Lagrangian time series of dimethylsulfide (DMS) concentrations from a cyclonic and an anticyclonic eddy in the Sargasso Sea were used in conjunction with measured DMS loss rates and a model of vertical mixing to estimate gross DMS production in the upper 60 m during summer 2004. Loss terms included biological consumption, photolysis, and ventilation to the atmosphere. The time- and depth (0–60 m)-averaged gross DMS production was estimated to be 0.73±0.09 nM d −1 in the cyclonic eddy and 0.90±0.15 nM d −1 in the anticyclonic eddy, with respective DMS replacement times of 5±1 and 6±1 d. The higher estimated rate of gross production and lower measured loss rate constants in the anticyclonic eddy were equally responsible for this eddy's 50% higher DMS inventory (0–60 m). When normalized to chlorophyll and total dimethylsulfoniopropionate (DMSP), estimated gross production in the anticyclonic eddy was about twice that in the cyclonic eddy, consistent with the greater fraction of phytoplankton that were DMSP producers in the anticyclonic eddy. Higher rates of gross production were estimated below the mixed layer, contributing to the subsurface DMS maximum found in both eddies. In both eddies, gas exchange, microbial consumption, and photolysis were roughly equal DMS loss terms in the surface mixed layer (0.2–0.4 nM d −1 ). Vertical mixing was a substantial source of DMS to the surface mixed layer in both eddies (0.2–0.3 nM d −1 ) owing to the relatively high DMS concentrations below the mixed layer. Estimated net biological DMS production rates (gross production minus microbial consumption) in the mixed layer were substantially lower (by almost a factor of 3) than those estimated in a previous study of the Sargasso Sea, which may explain the relatively low mixed-layer DMS concentrations found here during July 2004 (∼3 nM) compared to previous summers (∼4–6 nM).

A dynamic model of oceanic sulfur (DMOS) applied to the Sargasso Sea: Simulating the dimethylsulfide (DMS) summer paradox

A new one‐dimensional model of DMSP/DMS dynamics (DMOS) is developed and applied to the Sargasso Sea in order to explain what drives the observed dimethylsulfide (DMS) summer paradox: a summer DMS concentration maximum concurrent with a minimum in the biomass of phytoplankton, the producers of the DMS precursor dimethylsulfoniopropionate (DMSP). Several mechanisms have been postulated to explain this mismatch: a succession in phytoplankton species composition towards higher relative abundances of DMSP producers in summer; inhibition of bacterial DMS consumption by ultraviolet radiation (UVR); and direct DMS production by phytoplankton due to UVR‐induced oxidative stress. None of these hypothetical mechanisms, except for the first one, has been tested with a dynamic model. We have coupled a new sulfur cycle model that incorporates the latest knowledge on DMSP/DMS dynamics to a preexisting nitrogen/carbon‐based ecological model that explicitly simulates the microbial‐loop. This allows the role of bacteria in DMS production and consumption to be represented and quantified. The main improvements of DMOS with respect to previous DMSP/DMS models are the explicit inclusion of: solar‐radiation inhibition of bacterial sulfur uptakes; DMS exudation by phytoplankton caused by solar‐radiation‐induced stress; and uptake of dissolved DMSP by phytoplankton. We have conducted a series of modeling experiments where some of the DMOS sulfur paths are turned “off” or “on,” and the results on chlorophyll‐a, bacteria, DMS, and DMSP (particulate and dissolved) concentrations have been compared with climatological data of these same variables. The simulated rate of sulfur cycling processes are also compared with the scarce data available from previous works. All processes seem to play a role in driving DMS seasonality. Among them, however, solar‐radiation‐induced DMS exudation by phytoplankton stands out as the process without which the model is unable to produce realistic DMS simulations and reproduce the DMS summer paradox.

Spatial variations of dimethylsulfide and dimethylsulfoniopropionate in the surface microlayer and in the subsurface waters of the South China Sea during springtime

Spatial variations in dimethylsulfide (DMS) and dimethylsulfoniopropionate (DMSP) were surveyed in the surface microlayer and in the subsurface waters of the low productivity South China Sea in May 2005. Overall, average subsurface water concentrations of DMS and DMSP of dissolved (DMSPd) and particulate (DMSPp) fractions were 1.74 (1.00-2.50), 3.92 (2.21-6.54) and 6.06 (3.40-8.68) nM, respectively. No enrichment in DMS and DMSPp was observed in the microlayer. In contrast, the microlayer showed a DMSPd enrichment, with an average enrichment factor (EF, defined as the ratio of the microlayer concentration to subsurface water concentration) of 1.40. In the study area, none of the sulfur components were correlated with chlorophyll a. An important finding in this study was that DMS, DMSP and chlorophyll a concentrations in the surface microlayer were respectively correlated with those in the subsurface water, suggesting a close linkage between these two water bodies. The ratios of DMS:Chl-a and DMSPp:Chl-a showed a gradually increasing trend from North to South. This might be due to changes in the proportion of DMSP producers in the phytoplankton community with the increased surface seawater temperature. A clear diurnal variation in the DMS and DMSP concentrations was observed at an anchor station with the highest concentrations appearing during the day and the lowest concentrations during the night. The higher DMS and DMSP concentrations during daytime might be attributed to the light-induced increase in both algal synthesis and exudation of DMSP and biological production of DMS. The mean flux of DMS from the investigated area to the atmosphere was estimated to be 2.06 micromo lm(-2)d(-1). This low DMS emission flux, together with the low DMS surface concentrations was attributed to the low productivity in this sea.

Anti-grazing activity and seasonal variation of dimethylsulfoniopropionate-associated compounds in the invasive alga Codium fragile ssp. tomentosoides

In this study, we present evidence that the invasive alga Codium fragile ssp. tomentosoides is chemically defended against grazing by a wound-activated chemical defense involving dimethylsulfoniopropionate (DMSP) and the products of its cleavage, dimethylsulfide (DMS), and acrylic acid (AA). DMSP in C. fragile ssp. tomentosoides was present throughout the year, but concentrations varied seasonally and were highest in the winter. Intra-thallus variation was neither large, nor consistent over time. High DMSP concentrations were uncommon among northwestern Atlantic macrophytes. Of 26 other species tested, only two, Ulva lactuca and Polysiphonia harveyi contained concentrations comparable to, or higher than, those of C. fragile ssp. tomentosoides. DMS and AA, both individually and together, deterred grazing by the green sea urchin Strongylocentrotus droebachiensis at “natural” concentrations. These results suggest that DMS and AA contribute to the avoidance of C. fragile ssp. tomentosoides by S. droebachiensis. As a result, the production of DMSP and its subsequent cleavage, upon injury, may reduce herbivory on C. fragile ssp. tomentosoides and contribute to its success.