Dimethylsulfide Production Research Articles

Impact of Arctic meltdown on the microbial cycling of sulphur

The Arctic is warming faster than any other region in the world. The resultant large-scale shift in sea ice cover could increase oceanic emissions of dimethylsulphide, a climate-relevant trace gas generated by ice algae and phytoplankton. The Arctic is warming faster than any other region in the world. Among the changes already witnessed, the loss of seasonal sea ice is by far the most striking. This large-scale shift in sea-ice cover could affect oceanic emissions of dimethylsulphide — a climate-relevant trace gas generated by ice algae and phytoplankton. During the spring melt period, conditions at the margin of Arctic sea ice favour the growth of these organisms. As a result, high levels of dimethylsulphide can accumulate at the bottom of the ice, in leads, and in the water column at the ice edge during the spring melt season. Production of dimethylsulphide is not limited to the sea-ice edge, however. Significant concentrations have also been detected in the seasonal ice-free zone in spring and summer. Preliminary observations, together with model results, suggest that the production and emission of dimethylsulphide will increase in the Arctic as seasonal sea-ice cover recedes. If it escapes to the atmosphere, this newly generated dimethylsulphide could potentially cool the Arctic climate.

Strong relationship between dimethyl sulfide and net community production in the western subarctic Pacific

Although much attention has been paid to describing the distribution of oceanic dimethyl sulfide (DMS) concentrations, establishing robust relationships between DMS concentrations and biological, physical, and chemical variables is still challenging. Previous studies have proposed semiempirical parameterizations by combining multiple physical and biogeochemical parameters to better understand and reproduce the global distribution of sea surface DMS. However, none of these parameterization schemes could reconcile regionally elevated DMS peaks found in high‐resolution DMS measurements made in the western subarctic Pacific. Here we found that DMS concentrations are highly correlated with the net community production, a parameter that integrates biological activity over time. We anticipate that this relationship may be exportable to other regions with high primary productivity, such as the Southern Ocean or upwelling regions, and can be used as an important parameterization scheme, combined with solar radiation dose relationship.

Effect of Grazing-Mediated Dimethyl Sulfide (DMS) Production on the Swimming Behavior of the Copepod Calanus helgolandicus

Chemical interactions play a fundamental role in the ecology of marine foodwebs. Dimethyl sulfide (DMS) is a ubiquitous marine trace gas that acts as a bioactive compound by eliciting foraging behavior in a range of marine taxa including the copepod Temora longicornis. Production of DMS can rapidly increase following microzooplankton grazing on phytoplankton. Here, we investigated whether grazing-induced DMS elicits an increase in foraging behavior in the copepod Calanus helgolandicus. We developed a semi-automated method to quantify the effect of grazing-mediated DMS on the proportion of the time budget tethered females allocate towards slow swimming, typically associated with feeding. The pooled data showed no differences in the proportion of the 25 min time budget allocated towards slow swimming between high (23.6 ± 9.74%) and low (29.1 ± 18.33%) DMS treatments. However, there was a high degree of variability between behavioral responses of individual copepods. We discuss the need for more detailed species-specific studies of individual level responses of copepods to chemical signals at different spatial scales to improve our understanding of chemical interactions between copepods and their prey.

Diel patterns of oceanic dimethylsulfide (DMS) cycling: Microbial and physical drivers

AbstractDimethylsulfide (DMS) is a biogenic gas with potential climatic effects, and its marine emission depends on the interplay between microbial activity and physical forcing in the oceanic upper mixed layer. We investigated the diel cycling patterns of DMS and its precursor dimethylsulfoniopropionate (DMSP) in four experiments (28 to 48 h long) performed in mesotrophic to ultraoligotrophic Mediterranean and Sargasso Sea waters. Samples taken every 4 or 6 h were analyzed for dimethylated sulfur pools and incubated to measure DMS and DMSP cycling rates, as well as primary and bacterial production. In all four experiments, DMS budgets showed pronounced day versus night variability. In the three summer experiments, gross community DMS production (GPDMS) increased by twofold to threefold from nighttime to daytime, peaking 0–4 h after solar noon. This excess GPDMS was balanced by higher photochemical and microbial sinks during the day, effectively buffering DMS concentrations. In the only winter experiment, GPDMS exhibited opposed temporal dynamics and peaked at nighttime in parallel to total DMSP consumption. Community DMSP to DMS conversion yields were generally <10% throughout the winter experiment and at night in summer, and increased to >15% (even >50%) during the day in summer, presumably due to phytoplankton radiative stress. Our data suggest that (1) diel variability should be taken into account in process studies, diagnostic, and prognostic models of DMS cycling and (2) the community DMS yield is a key variable that defines characteristic DMS cycling regimes.

오미자 추출물의 구강병원균에 대한 항균효과 및 구취억제 효과

This study was conducted to investigate the antimicrobial and anti-oral malodor effect of Schizandra chinensis extracts against oral pathogens. S. chinensis methanol (MeOH) extract showed better antimicrobial activities against oral pathogens than the S. chinensis hot water extract did. In particular, ethylacetate (EA) fraction of S. chinensis MeOH extract had the greatest effects against Streptococcus mutans, S. sanguinis, S. salivaris subsp. thermophols and Porphyromons gingivalis. The anti-malodor effect of S. chinensis against the generation of volatile sulfur compounds (VSC), such as hydrogen sulfide, methyl mercaptan, and dimethyl sulfide, was estimated using oral chroma in a practical study. The results showed that 2% MeOH extract of S. chinensis against the production of hydrogen sulfide, methyl mercaptan, and dimethyl sulfide had inhibitions of 91.15, 78.72 and 71.58%, respectively. These results showed that Schizandra chinensis extract had antimicrobial activity against several oral pathogens and exhibited significant effects on the inhibition of VSCs. Thus, Schizandra chinensis extract could be an alternative to available synthetic chemicals and oral hygiene products.

Linking TFT-LCD wastewater treatment performance to microbial population abundance of Hyphomicrobium and Thiobacillus spp.

This study investigated the linkage between performance of two full-scale membrane bioreactor (MBR) systems treating thin-film transistor liquid crystal display (TFT-LCD) wastewater and the population dynamics of dimethylsulfoxide (DMSO)/dimethylsulfide (DMS) degrading bacteria. High DMSO degradation efficiencies were achieved in both MBRs, while the levels of nitrification inhibition due to DMS production from DMSO degradation were different in the two MBRs. The results of real-time PCR targeting on DMSO/DMS degrading populations, including Hyphomicrobium and Thiobacillus spp., indicated that a higher DMSO oxidation efficiency occurred at a higher Hyphomicrobium spp. abundance in the systems, suggesting that Hyphomicrobium spp. may be more important for complete DMSO oxidation to sulfate compared with Thiobacillus spp. Furthermore, Thiobacillus spp. was more abundant during poor nitrification, while Hyphomicrobium spp. was more abundant during good nitrification. It is suggested that microbial population of DMSO/DMS degrading bacteria is closely linking to both DMSO/DMS degradation efficiency and nitrification performance.

Spectral irradiance dependence of sunlight effects on plankton dimethylsulfide production

We investigated the short‐term effects of variable solar irradiance and spectrum on the gross biological production of dimethylsulfide (DMS), a trace gas with potential climatic effects, in eight experiments performed at different times of the year in a northwest Mediterranean coastal site. Experimentally determined net community DMS production, DMS photolysis, and dark microbial DMS consumption rates were used to calculate gross community DMS production by budgeting. In addition, the composition of the bacterioplankton and phytoplankton communities in the initial samples, and the photoinhibition of bulk bacterial heterotrophic activity and phytoplankton photosynthetic efficiency were monitored. Our results show that: (1) gross DMS production is irradiance dependent, with a maximum short‐term stimulation factor of 2‐ to 6‐fold compared to dark incubations; (2) its spectral shape is variable but generally similar to that of phytoplankton photoinhibition or photodamage, with more effective stimulation at shorter ultraviolet wavelengths; and (3) stronger stimulation occurs when samples are overexposed with respect to their prior exposure. Remarkably, the photoresponse of gross DMS production was in most cases strong enough to (at least) compensate the photochemical DMS loss at the water subsurface. Such response would prevent DMS depletion in stratified and highly irradiated waters. Since the initial microbial communities were representative of meso‐ to oligotrophic conditions, our observations should apply to a wide variety of oceanic regimes.

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.

Linking atmospheric dimethyl sulfide and the Arctic Ocean spring bloom

AbstractWe measured atmospheric dimethyl sulfide (DMS) mixing ratios at approximately hourly intervals over a 1 year period (April 2010 to March 2011) in the Atlantic sector of the Arctic Ocean (Svalbard, Norway; 78.5°N, 11.8°E). The mixing ratios varied by several orders of magnitude over time scales of less than several days, and occasionally reached 200−300 parts per trillion by volume during the major phytoplankton growth period (May to September), whereas during the winter months (October to April) the mixing ratios were on the order of a few parts per trillion by volume. Our results, based on analyses using multiple data products (atmospheric DMS mixing ratios, satellite‐derived ocean colors, and meteorological datasets), indicated that weekly variability in the DMS mixing ratios at Svalbard was highly correlated with variability in the chl‐a concentration in waters in the vicinity of Svalbard (r = 0.89). Hourly‐to‐daily variability in the DMS mixing ratios were satisfactorily explained by changes in the trajectory, altitude, and speed of air masses passing the DMS sources prior to reaching Svalbard. The observed coupling between DMS mixing ratios and chl‐a concentration is surprising, and indicates that the variability in chl‐a concentrations in the study area represents the change in the abundance of phytoplankton capable of producing DMS. The intensive monitoring of DMS levels at Svalbard enabled us to identify in situ production and the flux of oceanic DMS over the Arctic region. It thus constitutes a useful analytical tool for detecting changes in DMS production associated with variations in phytoplankton productivity resulting from changes in sea ice extent as a consequence of Arctic seasonality and warming.

Regression modeling of the North East Atlantic Spring Bloom suggests previously unrecognized biological roles for V and Mo.

In order to identify the biogeochemical parameters controlling pCO2, total chlorophyll a, and dimethyl sulfide (DMS) concentrations during the North East Atlantic Spring Bloom (NASB), we used previously unpublished particulate and dissolved elemental concentrations to construct several linear regression models; first by hypothesis-testing, and then with exhaustive stepwise linear regression followed by leave-one-out cross-validation. The field data was obtained along a latitudinal transect from the Azores Islands to the North Atlantic, and best-fit models (determined by lowest predictive error) of up to three variables are presented. Total chlorophyll a is predicted best by biomass (POC, PON) parameters and by pigments characteristic of picophytoplankton for the southern section of the sampling transect (from the Azores to the Rockhall-Hatton Plateau) and coccolithophores in the northern portion (from the Rockhall-Hatton Plateau to the Denmark Strait). Both the pCO2 and DMS models included variables traditionally associated with the development of the NASB such as mixed-layer depth and with Fe, Si, and P-deplete conditions (dissolved Fe, dissolved and biogenic silica, dissolved PO3−4). However, the regressions for pCO2 and DMS also include intracellular V and Mo concentrations, respectively. Mo is involved in DMS production as a cofactor in dimethylsulfoxide reductase. No significant biological role for V has yet been determined, although intracellular V is significantly correlated (p-value <0.05) with biogenic silica (R2 = 0.72) and total chlorophyll a (R2 = 0.49) while the same is not true for its biogeochemical analogue Mo, suggesting active uptake of V by phytoplankton. Our statistical analysis suggests these two lesser-studied metals may play more important roles in bloom dynamics than previously thought, and highlights a need for studies focused on determining their potential biological requirements and cell quotas.

The effect of chronic and acute low pH on the intracellular DMSP production and epithelial cell morphology of red coralline algae

The release of dimethylsulphoniopropionate (DMSP) by marine algae has major impacts on the global sulphur cycle and may influence local climate through the formation of dimethylsulphide (DMS). However, the effect of global change on DMSP/DMS (DMS(P)) production by algae is not well understood. This study examined the effect of low pH on DMS(P) production and epithelial cell morphology of the free-living red coralline alga Lithothamnion glaciale. Three pH treatments were used in the 80-day experiment: (1) current pH level (8.18, control), (2) low, chronic pH representing a 2100 ocean acidification (OA) scenario (7.70) and (3) low, acute pH (7.75, with a 3-day spike to 6.47), representing acute variable conditions that might be associated with leaks from carbon capture and storage infrastructure, at CO2 vent sites or in areas of upwelling. DMS(P) production was not significantly enhanced under low, stable pH conditions, indicating that red coralline algae may have some resilience to OA. However, intracellular and water column DMS(P) concentrations were significantly higher than the control when pH was acutely spiked. Cracks were observed between the cell walls of the algal skeleton in both low pH treatments. It is proposed that this structural change may cause membrane damage that allows DMS(P) to leak from the cells into the water column, with subsequent implications for the cycling of DMS(P) in coralline algae habitats.

On-line detection of root-induced volatiles in Brassica nigra plants infested with Delia radicum L. root fly larvae

Plants emit various volatile organic compounds (VOCs) upon herbivore attack. These VOC emissions often show temporal dynamics which may influence the behavior of natural enemies using these volatiles as cues. This study analyzes on-line VOC emissions by roots of Brassica nigra plants under attack by cabbage root fly larvae, Delia radicum. Root emitted VOCs were detected using Proton-Transfer-Reaction Mass Spectrometry (PTR-MS) and Gas Chromatography-Mass Spectrometry (GC-MS). These analyses showed that several sulfur containing compounds, such as methanethiol, dimethyl sulfide (DMS), dimethyl disulfide (DMDS), dimethyl trisulfide (DMTS) and glucosinolate breakdown products, such as thiocyanates (TC) and isothiocyanates (ITC), were emitted by the roots in response to infestation. The emissions were subdivided into early responses, emerging within 1-6 h after infestation, and late responses, evolving only after 6-12 h. The marker for rapid responses was detected at m/z 60. The ion detected at m/z 60 was identified as thiocyanic acid, which is also a prominent fragment in some TC or ITC spectra. The emission of m/z 60 stopped when the larvae had pupated, which makes it an excellent indicator for actively feeding larvae. Methanethiol, DMS and DMDS levels increased much later in infested roots, indicating that activation of enzymes or genes involved in the production of these compounds may be required. Earlier studies have shown that both early and late responses can play a role in tritrophic interactions associated with Brassica species. Moreover, the identification of these root induced responses will help to design non-invasive analytical procedures to assess root infestations.

Influence of transport and ocean ice extent on biogenic aerosol sulfur in the Arctic atmosphere

The recent decline in sea ice cover in the Arctic Ocean could affect the regional radiative forcing via changes in sea ice–atmosphere exchange of dimethyl sulfide (DMS) and biogenic aerosols formed from its atmospheric oxidation, such as methanesulfonic acid (MSA). This study examines relationships between changes in total sea ice extent north of 70°N and atmospheric MSA measurement at Alert, Nunavut, during 1980–2009; at Barrow, Alaska, during 1997–2008; and at Ny‐Ålesund, Svalbard, for 1991–2004. During the 1980–1989 and 1990–1997 periods, summer (July–August) and June MSA concentrations at Alert decreased. In general, MSA concentrations increased at all locations since 2000 with respect to 1990 values, specifically during June and summer at Alert and in summer at Barrow and Ny‐Ålesund. Our results show variability in MSA at all sites is related to changes in the source strengths of DMS, possibly linked to changes in sea ice extent as well as to changes in atmospheric transport patterns. Since 2000, a late spring increase in atmospheric MSA at the three sites coincides with the northward migration of the marginal ice edge zone where high DMS emissions from ocean to atmosphere have previously been reported. Significant negative correlations are found between sea ice extent and MSA concentrations at the three sites during the spring and June. These results suggest that a decrease in seasonal ice cover influencing other mechanisms of DMS production could lead to higher atmospheric MSA concentrations.

Combined effects of inorganic carbon and light on &amp;lt;i&amp;gt;Phaeocystis globosa&amp;lt;/i&amp;gt; Scherffel (Prymnesiophyceae)

Abstract. Phaeocystis globosa (Prymnesiophyceae) is an ecologically dominating phytoplankton species in many areas around the world. It plays an important role in both the global sulfur and carbon cycles, by the production of dimethylsulfide (DMS) and the drawdown of inorganic carbon. Phaeocystis globosa has a polymorphic life cycle and is considered to be a harmful algal bloom (HAB) forming species. All these aspects make this an interesting species to study the effects of increasing carbon dioxide (CO2) concentrations, due to anthropogenic carbon emissions. Here, the combined effects of three different dissolved carbon dioxide concentrations (CO2(aq)) (low: 4 μmol kg−1, intermediate: 6–10 μmol kg−1 and high CO2(aq): 21–24 μmol kg−1) and two different light intensities (low light, suboptimal: 80 μmol photons m−2 s−1 and high light, light saturated: 240 μmol photons m−2 s−1) are reported. The experiments demonstrated that the specific growth rate of P. globosa in the high light cultures decreased with increasing CO2(aq) from 1.4 to 1.1 d−1 in the low and high CO2 cultures, respectively. Concurrently, the photosynthetic efficiency (FV/FM) increased with increasing CO2(aq) from 0.56 to 0.66. The different light conditions affected photosynthetic efficiency and cellular chlorophyll a concentrations, both of which were lower in the high light cultures as compared to the low light cultures. These results suggest that in future inorganic carbon enriched oceans, P. globosa will become less competitive and feedback mechanisms to global change may decrease in strength.

Phylogenetic diversity of the dddP gene for dimethylsulfoniopropionate-dependent dimethyl sulfide synthesis in mangrove soils

The dddP gene encodes an enzyme that cleaves dimethylsulfoniopropionate (DMSP) into dimethyl sulfide (DMS) plus acrylate and has been identified in various marine bacteria and some fungi. The diversity of dddP genes was investigated by culture-independent PCR-based analysis of metagenomic DNA extracted from 4 mangrove soils in Southern China. A phylogenetic tree of 144 cloned dddP sequences comprised 7 groups, 3 of which also included dddP genes from previously identified Ddd(+) (DMSP-dependent DMS production) bacteria. However, most (69%) of the DddP sequences from the mangroves were in 4 other subgroups that did not include sequences from known bacteria, demonstrating a high level of diversity of this gene in these environments. Each clade contained clones from all of the sample sites, suggesting that different dddP types are widespread in mangroves of different geographical locations. Furthermore, it was found the dddP genotype distribution was remarkably influenced by the soil properties pH, available sulfur, salt, and total nitrogen.

DMS flux over the Antarctic sea ice zone

This study presents concentrations of dimethylsulphide (DMS) and its precursor compound dimethylsulphoniopropionate (DMSP), and chlorophyll a in a variety of sea ice and seawater habitats from pack and fast ice in the Antarctic sea ice zone (ASIZ) of eastern Antarctica. Estimated “hot spot” seawater DMS concentrations released during sea ice melting in October and derived from a total DMSP (DMSPt)-sea ice depth relationship and application of a DMSPt:DMS conversion ratio, predict that sea ice derived DMS of the order of 20–50nM is associated with DMSPt released from melting sea ice of 0.4m thickness. In November and December, melting of large areas of ice less than 0.6m thick could release larger amounts of DMS around 50–80nM and 150–270nM, respectively. In addition, DMSPt released from 1.0 to 1.2m thick ice in December could result in “hot spot” concentrations of DMS of around 100nM. DMS flux in November and mid to late December was highest in pack ice>fast ice>ice edge (pack ice=54μmolm−2d−1; (range 1–325); fast ice=16μmolm−2d−1; (range 10–50); ice edge=12 (range 1.2–26) and 23μmolm−2d−1). In ice-free seawater in the Davis area from January to February, although dissolved DMS concentrations were low, DMS fluxes were high due to high wind speeds (DMS=3nM; DMS flux=27μmolm−2d−1 (range 1–101)). DMS concentrations and flux from a fast ice tide crack (DMS=12nM; DMS flux 6–81μmolm−2d−1), and two days following fast ice breakout from the Davis region DMS concentrations (12nM) and DMS flux (45–84μmolm−2d−1) also indicated that fast ice was a significant source of DMS to the atmosphere of this region. In contrast DMS flux from a fast ice melt-pool was low (DMS flux 0.5–7μmolm−2d−1). These measurements support the suggestion that during sea ice melting during late spring to early summer (November–December), the ASIZ is an area of high DMS (P) production from various sources, leading to very high and variable fluxes of DMS to the atmosphere of this region. As sea ice melts in late November voids in sea ice become larger enabling sea ice to vent DMS directly to the atmosphere, as well as being released from sea ice melting. At most sea ice sites DMS often displayed non-detectable to elevated concentrations of DMS (mean 11nM) in the surface 0–0.1m. Using average DMS concentrations from a range of studies we estimate that DMS flux from ASIZ in November is about 23μmolm−2d−1, much lower than what we have estimated from pack ice using a DMSPt:DMS conversion ratio technique (54μmolm−2d−1). The difference (31μmolm−2d−1) could reflect venting or degassing of DMS directly from pack ice to the Antarctic atmosphere. Depth-integrated sea ice DMS concentrations varied with time of day, with elevated DMS concentrations (18–30nM) in the cores during the later part of the day (i.e. 16.00–20.00h), whilst close to mid-day (11.00–12.00h) concentrations were much lower (2–9nM). The lower depth-integrated DMS concentrations found in pack ice in November could suggest that large amounts of DMS are vented from pack ice close to mid-day when temperatures and wind speeds often increase. Varying DMSPt–chlorophyll a correlations in the sea ice highlight how varying biological activity affects DMSP concentrations, and suggest that variations in biological activity in sea ice also influence DMS flux.

Production of dimethylsulphide during the seasonal anoxia off Goa

Subsurface waters over the western Indian continental shelf experience seasonal anoxia towards the end of the southwest monsoon season. During a 3-day study carried out at the Candolim time series site (off the coast of Goa), dimethylsulphide (DMS) concentrations showed a 40-fold increase to a maximum of 442 nM at 25 m depth compared to the oxygenated surface waters. This extremely high DMS was found to be associated with relatively low chlorophyll a, low phytoplankton cell counts and a high concentration of hydrogen sulphide. However, total dimethylsulphoniopropionate, total dimethylsulphoxide and methanethiol concentrations were quite low and unlikely to account for the DMS build-up through presently known pathways of DMS production. While there are several possible mechanisms for the observed accumulation of DMS, we were unable to pinpoint the exact pathway of DMS production. Future work will involve investigation of the source of DMS through sediment slurry experiments, to explore this interesting link between the carbon and sulphur cycles under anoxic conditions.

Effect of visible light on dimethylsulfoniopropionate assimilation and conversion to dimethylsulfide in the North Pacific Subtropical Gyre

The aim of the present study was to assess the effect of photosynthetically active radiation (PAR, 400 to 700 nm) on the utilization of dissolved dimethylsulfoniopropionate (DMSPd) by microbial communities in the oligotrophic North Pacific Subtropical Gyre, using 35 S-labeled substrate. Rates of DMSPd-sulfur (DMSPd-S) assimilation into macromolecules of microorganisms in surface mixed layer seawater were 39 to 78% higher in samples that were incubated in the light than in dark controls. There was no photoinhibition in the light range tested (up to 1200 µmol pho- tons m �2 s �1 ). Leucine assimilation, an index of bacterial protein production, was also photostimu- lated and was significantly correlated (r = 0.93) to DMSPd-S assimilation, suggesting that cells can respond to an increase in S requirements for protein synthesis by assimilating DMSPd-S. Light- driven changes in DMSPd-S assimilation were inversely and significantly correlated (r = �0.57) to changes in the corresponding dimethylsulfide (DMS) yield (i.e. DMS produced per DMSP con- sumed), in support of the hypothesis that DMS is produced only after cellular sulfur requirements are met. While light-driven changes in DMSPd-S assimilation appear to affect DMS yield, more than half of the DMSPd product pool remained unidentified; thus, in addition to assimilation, there may be other metabolic processes or products that can regulate DMS production.

Effect of elevated pCO2 on the production of dimethylsulphoniopropionate (DMSP) and dimethylsulphide (DMS) in two species of Ulva (Chlorophyceae)

Concentrations of the secondary algal metabolite dimethylsulphoniopropionate (DMSP) and its breakdown product, the climate-active trace gas dimethylsulphide (DMS), are sensitive to changes in pCO2. Data on the response of marine macroalgae to future pCO2 levels are lacking. Here we report the first measurements of DMSP and DMS production in two species of chlorophyte macroalgae (Ulva lactuca and U. clathrata). Laboratory cultures were grown in pH–stated medium that received pulses of CO2 to create pCO2 conditions ranging from ambient (432 μatm) to future (1514 μatm). Intracellular concentration of DMSP remained unaffected in both species (101 ± 21 and 69 ± 20 mmol g−1 FW in U. lactuca and U. clathrata, respectively) but significant differences in extracellular production of DMSP and DMS were observed for U. lactuca: Whilst production of total DMSP (the sum of external DMSP and DMS) was different between replicated experiments, the percentage of total DMSP produced throughout each experiment increased significantly by up to 65% with increasing pCO2 to 1514 μatm. In contrast, DMS production decreased from 0.4 to 0.25 nmol g−1 FW h−1. This decrease was not a linear function of pCO2 but an almost 50% step-wise loss of DMS production was indicated between 635 and 884 μatm, a pCO2 predicted for the next 100 years. Since Ulva spp. form massive harmful blooms (‘green tides’) that can occur free-floating and faraway (>100 km) from the coast, they may provide a locally significant source of DMS to the remote marine boundary layer.

Environmental, biochemical and genetic drivers of DMSP degradation and DMS production in the Sargasso Sea

Dimethylsulfide (DMS) is a climatically relevant trace gas produced and cycled by the surface ocean food web. Mechanisms driving intraannual variability in DMS production and dimethylsulfoniopropionate (DMSP) degradation in open-ocean, oligotrophic regions were investigated during a 10-month time-series at the Bermuda Atlantic Time-series Study site in the Sargasso Sea. Abundance and transcription of bacterial DMSP degradation genes, DMSP lyase enzyme activity, and DMS and DMSP concentrations, consumption rates and production rates were quantified over time and depth. This interdisciplinary data set was used to test current hypotheses of the role of light and carbon supply in regulating upper-ocean sulfur cycling. Findings supported UV-A-dependent phytoplankton DMS production. Bacterial DMSP degraders may also contribute significantly to DMS production when temperatures are elevated and UV-A dose is moderate, but may favour DMSP demethylation under low UV-A doses. Three groups of bacterial DMSP degraders with distinct intraannual variability were identified and niche differentiation was indicated. The combination of genetic and biochemical data suggest a modified 'bacterial switch' hypothesis where the prevalence of different bacterial DMSP degradation pathways is regulated by a complex set of factors including carbon supply, temperature and UV-A dose.