DMS Production Research Articles

Biological sources and sinks of dimethylsulfide disentangled by an induced bloom experiment and a numerical model

AbstractDimethylsulfide (DMS) is a climatically active trace gas promoting cloud formation. The biochemical precursor of DMS, dimethylsulfoniopropionate (DMSP), is a phytoplankton metabolite and a source of reduced sulfur for many microbial species. Because of the complex interactions between their many producers and consumers, the dynamics of DMSP and DMS in the ocean are still poorly constrained. In this study we measure particulate DMSP, dissolved DMSP (), and DMS concentrations in seven mesocosms where two consecutive phytoplankton blooms (first, pico‐ and nano‐algae; second, Emiliania huxleyi) were induced by nutrient addition, and we build a mechanistic numerical model to identify the sources and sinks that best account for the observations. The mesocosms were designed as replicates but differ from each other by their E. huxleyi virus abundance due to stochastic differences in initial conditions. The model shows that heterotrophic bacteria cannot be the only consumers of . A fraction of dissolved must be consumed by phytoplankton to avoid excessive accumulation during the first bloom. The induced blooms increase DMS concentration by 220% on average, until an increase in the abundance of DMS‐consuming bacteria brings DMS concentration back to its pre‐bloom value, after 3 weeks of experiment. Therefore phytoplankton blooms can increase DMS emission to the atmosphere, but only during a transient regime of a few weeks. The model also shows that the DMS yield, production and emission are increased when the coccolithophore bloom is terminated by a viral infection, but decreased if the infection occurs several days before the bloom can reach its maximum.

The Effect of Zooplankton on the Distributions of Dimethyl Sulfide and Dimethylsulfoniopropionate in the Bohai and Yellow Seas

AbstractDimethyl sulfide (DMS) and its precursor dimethylsulfoniopropionate (DMSP) are ubiquitous sulfur compounds in the ocean. DMS is emitted into the atmosphere and has potential climatic effects. The distributions of DMS and DMSP are affected by various biological factors (i.e., bacterial catabolism and phytoplankton and zooplankton community composition). The horizontal and vertical distributions of DMSP, DMSP lyase activity (DLA), DMS, and the abundances of bacteria, DMSP‐consuming bacteria, dimethyl sulfoxide‐consuming bacteria, and picophytoplankton were investigated in the Bohai Sea (BS) and Yellow Sea (YS) during autumn 2020. DLA was significantly correlated with chlorophyll a, DMS, and dissolved DMSP concentrations. Our data show that bacteria Clade_I SAR11 was a significant contributor to DLA. A dilution experiment indicated that the highest microzooplankton grazing rate coincided with the highest DMS concentration and DMS production rate. A proportion of 16%–62% of the DMSPt was converted to DMS in the dilution experiment. Copepods dominated the mesozooplankton community. Calanus sinicus was the predominant copepod in the BS and YS. C. sinicus grazing stimulated DMS production. DMS concentration increased 299% after C. sinicus grazing on physically broken algal cells for 48 hr. These results will help with a better understanding of the control of DMS and DMSP concentrations by zooplankton and the DMS release mechanisms that occur through zooplankton grazing.

Light stress induced DMS(P) production in Skeletonema costatum: An experimental approach and field observation

Dimethylsulphide is a dominant biogenic sulphur anti-greenhouse gas produced by marine phytoplankton. A non-axenic culture of Skeletonema costatum was studied to comprehend the effects of different growth stages and light stress on DMSP/DMS production. The intracellular DMSP concentration increased during late exponential to mid-stationary phase and attained a maximum (0.59 pg S cell−1) during the stationary phase, indicating more contribution from actively dividing smaller cells. Likewise, exposure to first light after a 12-hour dark phase caused stress, invariably leading to elevated levels of DMS (~9 fold). These observations were upheld by additional laboratory and field experiments, and a field time-series observation, which recorded higher DMS concentrations during exposure to first light after a dark cycle and during early mornings, respectively. While our study depicts the variable DMSP and DMS concentrations during different growth stages of S. costatum, it gives new information on the effect of light stress on DMS production.

Phytoplankton Sources and Sinks of Dimethylsulphoniopropionate (DMSP) in Temperate Coastal Waters of Australia.

The ecologically important organic sulfur compound, dimethylsulfoniopropionate (DMSP), is ubiquitous in marine environments. Produced by some species of phytoplankton and bacteria, it plays a key role in cellular responses to environmental change. Recently, uptake of DMSP by non-DMSP-producing phytoplankton species has been demonstrated, highlighting knowledge gaps concerning DMSP distribution through the marine microbial food web. In this study, we traced the uptake and distribution of DMSP through a natural marine microbial community collected from off the eastern coastline Australia. We found a diverse phytoplankton community representing six major taxonomic groups and conducted DMSP-enrichment experiments both on the whole community, and the community separated into large (≥8.0 µm), medium (3.0–8.0 µm), and small (0.2–3.0 µm) size fractions. Our results revealed active uptake of DMSP in all three size fractions of the community, with the largest fraction (>8 µm) forming the major DMSP sink, where enrichment resulted in an increase of DMSPp by 144%. We observed evidence for DMSP catabolism in all size fractions with DMSP enrichment, highlighting loss from the system via MeSH or DMS production. Based on taxonomic diversity, we postulate the sources of DMSP were the dinoflagellates, Phaeocystis sp., and Trichodesmium sp., which were present in a relatively high abundance, and the sinks for DMSP were the diatoms and picoeucaryotes in this temperate community. These findings corroborate the role of hitherto disregarded phytoplankton taxa as potentially important players in the cycling of DMSP in coastal waters of Australia and emphasize the need to better understand the fate of accumulated DMSP and its significance in cellular metabolism of non-DMSP producers.

Bacterial Composition Associated With Giant Colonies of the Harmful Algal Species Phaeocystis globosa.

The cosmopolitan algae Phaeocystis globosa forms harmful algal blooms frequently in a number of tropical and subtropical coastal regions in the past two decades. During the bloom, the giant colony, which is formed by P. globosa, is the dominant morphotype. However, the microenvironment and the microbial composition in the intracolonial fluid are poorly understood. Here, we used high-throughput 16S rRNA amplicon sequencing to examine the bacterial composition and predicted functions in intracolonial fluid. Compared with the bacterial consortia in ambient seawater, intracolonial fluids possessed the lower levels of microbial richness and diversity, implying selectivity of bacteria by the unique intracolonial microenvironment enclosed within the P. globosa polysaccharide envelope. The bacterial consortia in intracolonial fluid were dominated by Balneola (48.6% of total abundance) and Labrezia (28.5%). The bacteria and microbial function enriched in intracolonial fluid were involved in aromatic benzenoid compounds degradation, DMSP and DMS production and consumption, and antibacterial compounds synthesis. We suggest that the P. globosa colonial envelope allows for the formation of a specific microenvironment; thus, the unique microbial consortia inhabiting intracolonial fluid has close interaction with P. globosa cells, which may benefit colony development.

Dimethyl sulfide and its role in aerosol formation and growth in the Arctic summer – a modelling study

Abstract. Atmospheric dimethyl sulfide, DMS(g), is a climatically important sulfur compound and is the main source of biogenic sulfate aerosol in the Arctic atmosphere. DMS(g) production and emission to the atmosphere increase during the summer due to the greater ice-free sea surface and higher biological activity. We implemented DMS(g) in the Environment and Climate Change Canada’s (ECCC) online air quality forecast model, GEM-MACH (Global Environmental Multiscale–Modelling Air quality and CHemistry), and compared model simulations with DMS(g) measurements made in Baffin Bay and the Canadian Arctic Archipelago in July and August 2014. Two seawater DMS(aq) datasets were used as input for the simulations: (1) a DMS(aq) climatology dataset based on seawater concentration measurements (Lana et al., 2011) and (2) a DMS(aq) dataset based on satellite detection (Galí et al., 2018). In general, GEM-MACH simulations under-predict DMS(g) measurements, which is likely due to the negative biases in both DMS(aq) datasets. However, a higher correlation and smaller bias were obtained with the satellite dataset. Agreement with the observations improved when climatological values were replaced by DMS(aq) in situ values that were measured concurrently with atmospheric observations over Baffin Bay and the Lancaster Sound area in July 2014. The addition of DMS(g) to the GEM-MACH model resulted in a significant increase in atmospheric SO2 for some regions of the Canadian Arctic (up to 100 %). Analysis of the size-segregated sulfate aerosol in the model shows that a significant increase in sulfate mass occurs for particles with a diameter smaller than 200 nm due to the formation and growth of biogenic aerosol at high latitudes (>70∘ N). The enhancement in sulfate particles is most significant in the size range from 50 to 100 nm; however, this enhancement is stronger in the 200–1000 nm size range at lower latitudes (<70∘ N). These results emphasize the important role of DMS(g) in the formation and growth of fine and ultrafine sulfate-containing particles in the Arctic during the summertime.

Impacts of Shifts in Phytoplankton Community on Clouds and Climate via the Sulfur Cycle

AbstractDimethyl sulfide (DMS), primarily produced by marine organisms, contributes significantly to sulfate aerosol loading over the ocean after being oxidized in the atmosphere. In addition to exerting a direct radiative effect, the resulting aerosol particles act as cloud condensation nuclei, modulating cloud properties and extent, with impacts on atmospheric radiative transfer and climate. Thus, changes in pelagic ecosystems, such as phytoplankton physiology and community structure, may influence organosulfur production, and subsequently affect climate via the sulfur cycle. A fully coupled Earth system model, including explicit marine ecosystems and the sulfur cycle, is used here to investigate the impacts of changes associated with individual phytoplankton groups on DMS emissions and climate. Simulations show that changes in phytoplankton community structure, DMS production efficiency, and interactions of multielement biogeochemical cycles can all lead to significant differences in DMS transfer to the atmosphere. Subsequent changes in sulfate aerosol burden, cloud condensation nuclei number, and radiative effect are examined. We find the global annual mean cloud radiative effect shifts up to 0.21 W/m2, and the mean surface temperature increases up to 0.1 °C due to DMS production changes associated with individual phytoplankton group in simulations with radiative effects at the 2,100 levels under an 8.5 scenario. However, changes in DMS emissions, radiative effect, and surface temperature are more intensive on regional scales. Hence, we speculate that major uncertainties associated with future marine sulfur cycling will involve strong region‐to‐region climate shifts. Further understanding of marine ecosystems and the relevant phytoplankton‐aerosol‐climate linkage are needed for improving climate projections.

Evaluating the relationship between cell viability and volatile organic compound production following DMSO treatment of cultured human cells.

Methylsulfinylmethane (dimethyl sulfoxide; DMSO) is widely used in clinical treatment and bioresearch. Moreover, there is bioconversion between methylsulfanylmethane (dimethyl sulfide; DMS), DMSO, and methylsulfonylmethane (DMSO2) in mammalian metabolism. Due to the real-time detection limits for volatile compounds, most research has focused on DMSO2 as a stable byproduct of DMSO. Therefore, details about the production of DMS as a byproduct of DMSO metabolism remain to be elucidated. Here, we report the characterization of trace-level volatile organic compounds (VOCs) produced following DMSO treatment of cultured human cells using an ultrasensitive vacuum ultraviolet photoionization mass spectrometer (VUV-PIMS). Using this approach, 24 h after DMSO treatment we detected 16.9 and 21 parts per billion by volume (ppbv) DMS in the atmosphere above the cells (headspace) within HeLa and 293T tissue culture flasks, respectively. When simultaneously exposed to 50 nM paclitaxel (PTX), 17.6 and 22.3 ppbv DMS were detected in the headspace of HeLa and 293T culture flasks, respectively. Nevertheless, at doses of PTX more or less than 50 nM, the detectable levels of DMS were reduced to as low as 8.4 ppbv. Our experimental results demonstrate that by co-administering 5 to 10 nM PTX with DMSO, it is possible to moderate the production of DMS considerably. However, at higher doses of PTX, increased apoptosis was observed that likely contributed to higher DMS production by cells.

Contrasted sensitivity of DMSP production to high light exposure in two Arctic under-ice blooms

In polar regions, low-light acclimated phytoplankton thriving under sea ice may be suddenly exposed to high irradiance when ice pack breaks or surface currents carry them into adjacent ice-free areas. Here we experimentally determined how rapid shifts in light regime affect the phytoplankton and the production of dimethylsulfoniopropionate (DMSP), an algal compound with antioxidant properties. Springtime experiments were carried out on under-ice phytoplankton blooms that exhibited different taxonomic assemblages in 2010 and 2011 within the Canadian Arctic Archipelago. Phytoplankton samples collected under-ice were incubated for 24h or less on the ice at irradiances mimicking what they would experience in an ice-free upper mixed layer. We observed contrasting responses between the two years. In 2010, exposure of the under-ice centric diatom bloom to high irradiance consistently resulted in a rapid chlorophyll a decrease accompanied by a quasi-total release of the intracellular DMSP. In 2011, only the first experiment conducted with a low biomass flagellate-dominated assemblage resulted in a similar response. During the subsequent under-ice pennate diatom bloom, neither a chlorophyll a decrease nor a consistent increase in dissolved DMSP concentrations were observed during the experiments. The results demonstrate that the response of under-ice phytoplankton to the irradiance prevailing in ice-free water may vary depending on the developmental stage and taxonomic composition of the under-ice bloom. The rapid chlorophyll a decrease during the centric diatom bloom in 2010 and the associated DMSP exudation may potentially lead to a significant increase in DMS production via the microbial pathway at the ice edge.

800-year ice-core record of nitrogen deposition in Svalbard linked to ocean productivity and biogenic emissions

Abstract. We present the records of the two nitrogen species nitrate (NO3-) and ammonium (NH4+) analysed in a new ice core from Lomonosovfonna, Svalbard, in the Eurasian Arctic covering the period 1222–2009. Changes in melt at the Lomonosovfonna glacier are assumed to have a negligible effect on the decadal variations of the investigated compounds. Accordingly, we use decadal records to investigate the major emission sources of NO3- and NH4+ precursors and find that during the twentieth century both records are influenced by anthropogenic pollution from Eurasia. In pre-industrial times NO3- is highly correlated with methane sulfonate (MSA), which we explain by a fertilising effect. We propose that enhanced atmospheric NO3- concentrations and the corresponding nitrogen input to the ocean trigger the growth of dimethyl-sulfide-(DMS)-producing phytoplankton. Increased DMS production results in elevated fluxes to the atmosphere where it is oxidised to MSA. Eurasia was presumably the main source area also of pre-industrial NO3-, but a more exact source apportionment could not be performed based on our data. This is different for NH4+, where biogenic ammonia (NH3) emissions from Siberian boreal forests were identified as the dominant source of pre-industrial NH4+.

Enhancement of dimethylsulfide production by anoxic stress in natural seawater

AbstractDimethylsulfide (DMS) is produced by phytoplankton in the ocean and plays an important role in biogeochemical cycles and climate system of the Earth. Previous field studies reported a possible relationship between DMS enhancement and anoxic condition, although the governing processes are still to be identified. Here we show the first direct evidence for the enhancement of DMS production by natural planktonic assemblages caused by anoxic stress. Under the anoxic condition, DMS production was considerably enhanced and DMS bacterial consumption was inhibited, resulting in an eightfold higher rate of gross DMS production than that under the oxic condition. Our results demonstrated that anoxic stress is one of important “environmental factors” in the marine DMS dynamics, suggesting the possible global importance due to ubiquity of anoxic conditions in the coastal oceans. This process would become more important in the future due to expansion of coastal hypoxic and anoxic zones by global warming.

Inflammation triggers production of dimethylsphingosine from oligodendrocytes

Neuropathic pain is a chronic, refractory condition that arises after damage to the nervous system. We previously showed that an increased level of the endogenous metabolite N,N-dimethylsphingosine (DMS) in the central nervous system (CNS) is sufficient to induce neuropathic pain-like behavior in rats. However, several important questions remain. First, it has not yet been demonstrated that DMS is produced in humans and its value as a therapeutic target is therefore unknown. Second, the cell types within the CNS that produce DMS are currently unidentified. Here we provide evidence that DMS is present in human CNS tissue. We show that DMS levels increase in demyelinating lesions isolated from patients with multiple sclerosis, an autoimmune disease in which the majority of patients experience chronic pain. On the basis of these results, we hypothesized that oligodendrocytes may be a cellular source of DMS. We show that human oligodendrocytes produce DMS in culture and that the levels of DMS increase when oligodendrocytes are challenged with agents that damage white matter. These results suggest that damage to oligodendrocytes leads to increased DMS production which in turn drives inflammatory astrocyte responses involved in sensory neuron sensitization. Interruption of this pathway in patients may provide analgesia without the debilitating side effects that are commonly observed with other chronic pain therapies.

Relationship between methanesulfonate (MS−) in atmospheric particulate and remotely sensed phytoplankton activity in oligo-mesotrophic central Mediterranean Sea

The coupling between oceanic and atmospheric sulfur cycles is fundamental for the understanding of the role of sulfate particles as potential climate regulators. We discuss existing relationships among methanesulfonate (MS− – one of the end products of oxidation of biogenic dimethylsulfide – DMS) in the atmospheric particulate, phytoplankton biomass, and remotely-sensed activity in the central Mediterranean. The MS− concentration in the aerosol particles is based on PM10 sampling (from 2005 to 2008) of atmospheric aerosols at the island of Lampedusa (35.5°N, 12.6°E) in the central Mediterranean Sea.The marine primary production in the sea sector surrounding the sampling site is obtained by using Ocean Color remote sensed data (SeaWiFS, MODIS-Aqua). In particular, primary production is calculated using a bio-optical model of sea reflectance and a Wavelength-Depth-Resolved Model (WDRM), fed by elaborated satellite data (chlorophyll concentration in the euphotic layer – Chl, sea surface temperature) and daily solar surface irradiance measurements.The multi-year evolution of MS− atmospheric concentration shows a well-defined seasonal cycle with a summer maximum, corresponding to the annual peak of solar radiation and a minimum of phytoplankton biomass (expressed as Chl).Statistically significant linear relationships between monthly means of atmospheric MS− and both the phytoplankton productivity index PB (r2 = 0.84, p < 0.001) and the solar radiation dose (SRD; r2 = 0.87, p < 0.001) in the upper mixed layer of the sea around Lampedusa are found. These correlations are mainly driven by the common seasonal pattern and suggest that DMS production in the marine surface layer is mainly related to the phytoplankton physiology. High values of PB are also the expression of stressed cells. The main stress factors in Mediterranean Sea during summer are high irradiance and shallow depth of the upper mixed layer, which lead to enhanced DMS emissions and higher MS− amounts in the atmosphere.During spring 2005 high biomass and primary productivity values are observed in February and April, just one month before the peaks of atmospheric MS− (March and May). The occurrence of anomalously high values at this time is hypothesized to be related to the negative phase of the North Atlantic Oscillation, and to related oceanic and atmospheric processes. The possible role of the taxonomic composition of phytoplankton assemblages is also discussed.

Revisiting the dilution technique to quantify the role of microzooplankton in DMS(P) cycling: laboratory and field tests

With the aim of re-examining the suitability of the dilution technique in studies of dimethylated sulfur cycling in the sea, we conducted two types of experiments: one with a simplified food chain of cultured organisms, and secondly with field-sampled natural communities. In the case of Isocrhysis galbana being grazed by Oxyrrhis marina, algal growth (μ) and grazing mortality (m) rates calculated using either chlorophyll a (Chla) or particulate dimethylsulfoniopropionate (DMSPp) were very similar. By means of a parallel incubation of prey with no predator, the regression-derived phytoplankton μ was validated. Net 24-h changes in DMS concentration showed a linear trend against DMSP grazing rates, with a slope that allowed estimation of the rate of DMS production due to grazing. When the method was applied to natural communities from coastal NW Mediterranean waters, μ values of the bulk Chla-containing phytoplankton assemblage were higher than that of the DMSP-containing algae, but m were not significantly different. The grazing-mediated DMS production rate was successfully estimated, and parallel incubations with and without a specific inhibitor of bacterial DMS metabolism allowed us to estimate the gross DMS production by the whole community and the bacterial DMS consumption rate. A further experiment showed that bacterial consumption was not affected by dilution.

Molecular diversity of bacterial production of the climate-changing gas, dimethyl sulphide, a molecule that impinges on local and global symbioses

This paper describes the ddd genes that are involved in the production of the gas dimethyl sulphide from the substrate dimethylsulphoniopropionate (DMSP), an abundant molecule that is a stress protectant in many marine algae and a few genera of angiosperms. What is known of the arrangement of the ddd genes in different bacteria that can undertake this reaction is reviewed here, stressing the fact that these genes are probably subject to horizontal gene transfer and that the same functions (e.g. DMSP transport) may be accomplished by very different mechanisms. A surprising number of DMS-emitting bacteria are associated with the roots of higher plants, these including strains of Rhizobium and some rhizosphere bacteria in the genus Burkholderia. One newly identified strain that is predicted to make DMS is B. phymatum which is a highly unusual beta-proteobacterium that forms N(2)-fixing nodules on some tropical legumes, in this case, the tree Machaerium lunatum, which inhabits mangroves. The importance of DMSP catabolism and DMS production is discussed, not only in terms of nutritional acquisition by the bacteria but also in a speculative scheme (the 'messy eater' model) in which the bacteria may make DMS as an info-chemical to attract other organisms, including invertebrates and other plankton.

DMSP and DMS in the Northwest Atlantic: Late-summer distributions, production rates and sea-air fluxes

DMSP and DMS were measured along a set of transects in the Northwest Atlantic during September, 1999. Six 24 h Lagrangian stations were occupied between 36° and 61° N latitude, covering subtropical to polar water types. Profiles of total DMSP (DMSPt), DMS, chl a, and oceanographic variables were determined at each station. Phytoplankton abundance and species assemblage were determined in surface waters and at the depth of the Chl a maximum in all profiles. Between profile stations, DMSPt and DMS samples were collected by a pump while the vessel was moving. Chl a and DMSPt were most abundant in the northern regions, with very low levels in subtropical waters. There was no direct correlation between DMSPt and Chl a. Maximum DMSPt concentrations reached 203 nM in coastal waters and 112 nM in the open ocean. A strong correlation was observed between DMSPt and the abundance of dinoflagellates (Spearman r=0.91; p <0.0001; n=13) and prymnesiophytes (Spearman r=0.91; p<0.0001; n=13). Cryptophytes also showed a weak but significant correlation (Spearman r=0.58; p=0.039; n=13). The waters around Greenland were the only site dominated by diatoms and their abundance was not correlated with DMSPt concentrations. DMS concentrations were low and fairly uniform, with maximum levels of 4.7 nM in coastal waters and 2.2 nM in the open ocean. DMS fluxes from surface waters were calculated based on observed sea-surface concentrations and wind speeds and showed a strong peak associated with a storm event, although no depletion of DMS resulting from the storm was observed. In situ incubation experiments showed DMSP consumption and DMS production rates to be relatively high, notwithstanding the generally low phytoplankton biomass.

Changes in DMS production and flux in relation to decadal shifts in ocean circulation

Changes in DMS production and flux in relation to decadal shifts in ocean circulation

Determination of apparent quantum yield spectra of DMS photo‐degradation in an in situ iron‐induced Northeast Pacific Ocean bloom

The wavelength dependence of the photochemical removal efficiency for DMS was studied for samples from an iron‐induced bloom in the Northeastern Pacific Ocean. In July 2002, a 64 km2 patch of ocean was iron‐fertilized near Ocean Station Papa (50°12′N; 144°45′W). Only small changes in pseudo‐first‐order apparent quantum yield (AQY*DMS(λ)) were observed outside the iron‐patch. However, inside the patch, AQY*DMS(λ) decreased considerably over the two weeks following the initial iron injection. A positive strong correlation was found between pseudo‐first‐order apparent quantum yield determined at 330 nm (AQY*DMS(330 nm)) and NO3− concentrations. We propose that NO3−‐photolysis has a substantial influence on DMS photo‐degradation rates in oceanic waters. This finding demonstrates that in addition to control DMS production, marine phytoplankton could indirectly influence the DMS photochemical loss rate via its control on NO3− distribution.

A pseudo-Lagrangian study of the sulfur budget in the remote Arctic marine boundary layer

The atmospheric sulfur cycle of the remote Arctic marine boundary layer is studied using trajectories and measurements of sulfur compounds from the International Arctic Ocean Expedition 1991, along with a pseudo-Lagrangian approach and an analytical model. The dimethyl sulfide [DMS(g)] turnover time was 59⁺¹⁸ ^-9h. Only 25⁺¹¹ _-9% of DMS(g) followed reaction paths to sulfur dioxide [SO₂(g)], sub-micrometre aerosol non-seasalt sulfate (nss-SO₄ ²⁻) or methane sulfonate (MSA). During the first 3 d of transport over the pack ice, fog deposition and drizzle resulted in short turnover times; 16⁺⁸ _-6 h for SO₂(g), 18⁺⁴ _-4h for MSA and 18⁺² ^-3 h for nss-SO₄ ²⁻. Therefore, DMS(g) will, owing to its origin along or south of the ice edge and longer turnover time, survive the original sub-micrometre sulfur aerosol mass and gradually replace it with new biogenic sulfur aerosol mass. The advection of DMS(g) along with heat and moisture will influence the clouds and fogs over the Arctic pack ice through the formation of cloud condensation nuclei (CCN). If the pack ice cover were to decrease owing to a climate change, the total Arctic Ocean DMS production would change, and potentially there could be an ice–DMS–cloud–albedo climate feedback effect, but it would be accompanied by changes in the fog aerosol sink.

Seawater DMS in a perturbed coastal ecosystem

DMS concentrations, chlorophyll a concentrations, abiotic parameters of water quality and quantitative samples of plankton were carried out once a month from January to December 1997 into two zones of a semi-enclosed french littoral ecosystem (Toulon Bay, NW Mediterranean sea). This bay is divided into two subecosystems by an artificial breakwater: the inner bay (polluted zone, P) is largely influenced by anthropogenic perturbations and the outer bay (less polluted, LP) is much less polluted. We found greater concentrations of DMS and chlorophyll a, of phytoplankton and zooplankton densities and biomasses in the polluted zone (P) than in the less polluted zone (LP) of the bay. The DMS concentration and phytoplankton biomass were strongly correlated, and a high degree of eutrophication may contribute, in connection with other factors, to a greater production of phytoplankton which in turn enhances the DMS production. The DMS concentration in coastal polluted zones is then greatly higher than in open sea (around three times) and this greater production should be taken into account for the global estimation, at least on a local scale, of DMS production in seawater, which is a key factor for the biogenic sulfur cycle.