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Thousands of man-made synthetic chemicals are released to oceans and compose the anthropogenic dissolved organic carbon (ADOC). Little is known about the effects of this chronic pollution on marine microbiome activities. In this study, we measured the pollution level at three sites in the Northeast Subarctic Pacific Ocean (NESAP) and investigated how mixtures of three model families of ADOC at different environmentally relevant concentrations affected naturally occurring marine bacterioplankton communities' structure and metabolic functioning. The offshore northernmost site (North) had the lowest concentrations of hydrocarbons, as well as organophosphate ester plasticizers, contrasting with the two other continental shelf sites, the southern coastal site (South) being the most contaminated. At North, ADOC stimulated bacterial growth and promoted an increase in the contribution of some Gammaproteobacteria groups (e.g. Alteromonadales) to the 16 rRNA pool. These groups are described as fast responders after oil spills. In contrast, minor changes in South microbiome activities were observed. Gene expression profiles at Central showed the coexistence of ADOC degradation and stress-response strategies to cope with ADOC toxicities. These results show that marine microbial communities at three distinct domains in NESAP are influenced by background concentrations of ADOC, expanding previous assessments for polar and temperate waters., This publication is dedicated to the memory of our missed friend, mentor and colleague, Prof. Ronald P. Kiene. The authors thank the Capitan and crew of the R/V Oceanus. This work was funded by the Spanish MEIC through projects ISOMICS (CTM2015-65691-R) and SENTINEL (CTM2015-70535-P). The authors sincerely thank J.M. Gasol for support with flow cytometry counts and I. Forn for microscopy counts. The research group of Global Change and Genomic Biogeochemistry is supported by the Catalan Government (2017SGR800). IDAEA-CSIC is a Centre of Excellence Severo Ochoa (Spanish Ministry of Science and Innovation, Project CEX2018-000794-S)., With the funding support of the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000928-S), of the Spanish Research Agency (AEI).

Unlike biologically available nitrogen and phosphorus, which are often at limiting concentrations in surface seawater, sulfur in the form of sulfate is plentiful and not considered to constrain marine microbial activity. Nonetheless, in a model system in which a marine bacterium obtains all of its carbon from co-cultured phytoplankton, bacterial gene expression suggests that at least seven dissolved organic sulfur (DOS) metabolites support bacterial heterotrophy. These labile exometabolites of marine dinoflagellates and diatoms include taurine, N-acetyltaurine, isethionate, choline-O-sulfate, cysteate, 2,3-dihydroxypropane-1-sulfonate (DHPS), and dimethylsulfoniopropionate (DMSP). Leveraging from the compounds identified in this model system, we assessed the role of sulfur metabolites in the ocean carbon cycle by mining the Tara Oceans dataset for diagnostic genes. In the 1.4 million bacterial genome equivalents surveyed, estimates of the frequency of genomes harboring the capability for DOS metabolite utilization ranged broadly, from only 1 out of every 190 genomes (for the C2 sulfonate isethionate) to 1 out of every 5 (for the sulfonium compound DMSP). Bacteria able to participate in DOS transformations are dominated by Alphaproteobacteria in the surface ocean, but by SAR324, Acidimicrobiia, and Gammaproteobacteria at mesopelagic depths, where the capability for utilization occurs in higher frequency than in surface bacteria for more than half the sulfur metabolites. The discovery of an abundant and diverse suite of marine bacteria with the genetic capacity for DOS transformation argues for an important role for sulfur metabolites in the pelagic ocean carbon cycle.

Dimethylsulfoniopropionate (DMSP) is an abundant organic sulfur metabolite produced by many phytoplankton species and degraded by bacteria via two distinct pathways with climate-relevant implications. We assessed the diversity and abundance of bacteria possessing these pathways in the context of phytoplankton community composition over a 3-week time period spanning September-October, 2014 in Monterey Bay, CA. The dmdA gene from the DMSP demethylation pathway dominated the DMSP gene pool and was harboured mostly by members of the alphaproteobacterial SAR11 clade and secondarily by the Roseobacter group, particularly during the second half of the study. Novel members of the DMSP-degrading community emerged from dmdA sequences recovered from metagenome assemblies and single-cell sequencing, including largely uncharacterized gammaproteobacteria and alphaproteobacteria taxa. In the DMSP cleavage pathway, the SAR11 gene dddK was the most abundant early in the study, but was supplanted by dddP over time. SAR11 members, especially those harbouring genes for both DMSP degradation pathways, had a strong positive relationship with the abundance of dinoflagellates, and DMSP-degrading gammaproteobacteria co-occurred with haptophytes. This in situ study of the drivers of DMSP fate in a coastal ecosystem demonstrates for the first time correlations between specific groups of bacterial DMSP degraders and phytoplankton taxa.

The objective of this study was to assess experimentally the potential impact of anthropogenic pH perturbation (ApHP) on concentrations of dimethyl sulfide (DMS) and dimethylsulfoniopropionate (DMSP), as well as processes governing the microbial cycling of sulfur compounds. A summer planktonic community from surface waters of the Lower St. Lawrence Estuary was monitored in microcosms over 12 days under three pCO2 targets: 1 × pCO2 (775 µatm), 2 × pCO2 (1,850 µatm), and 3 × pCO2 (2,700 µatm). A mixed phytoplankton bloom comprised of diatoms and unidentified flagellates developed over the course of the experiment. The magnitude and timing of biomass buildup, measured by chlorophyll a concentration, changed in the 3 × pCO2 treatment, reaching about half the peak chlorophyll a concentration measured in the 1 × pCO2 treatment, with a 2-day lag. Doubling and tripling the pCO2 resulted in a 15% and 40% decline in average concentrations of DMS compared to the control. Results from 35S-DMSPd uptake assays indicated that neither concentrations nor microbial scavenging efficiency of dissolved DMSP was affected by increased pCO2. However, our results show a reduction of the mean microbial yield of DMS by 34% and 61% in the 2 × pCO2 and 3 × pCO2 treatments, respectively. DMS concentrations correlated positively with microbial yields of DMS (Spearman’s ρ = 0.65; P < 0.001), suggesting that the impact of ApHP on concentrations of DMS in diatom-dominated systems may be strongly linked with alterations of the microbial breakdown of dissolved DMSP. Findings from this study provide further empirical evidence of the sensitivity of the microbial DMSP switch under ApHP. Because even small modifications in microbial regulatory mechanisms of DMSP can elicit changes in atmospheric chemistry via dampened efflux of DMS, results from this study may contribute to a better comprehension of Earth’s future climate.

The oceanic frontal region above the Chatham Rise east of New Zealand was investigated during the late austral summer season in February and March 2012. Despite its potential importance as a source of marine-originating and climate-relevant compounds, such as dimethyl sulfide (DMS) and its algal precursor dimethylsulfoniopropionate (DMSP), little is known of the processes fuelling the reservoirs of these sulfur (S) compounds in the water masses bordering the subtropical front (STF). This study focused on two opposing short-term fates of DMSP-S following its uptake by microbial organisms (either its conversion into DMS or its assimilation into bacterial biomass) and has not considered dissolved non-volatile degradation products. Sampling took place in three phytoplankton blooms (B1, B2, and B3) with B1 and B3 occurring in relatively nitrate-rich, dinoflagellate-dominated subantarctic waters, and B2 occurring in nitrate-poor subtropical waters dominated by coccolithophores. Concentrations of total DMSP (DMSPt) and DMS were high across the region, up to 160 and 14.5 nmol L−1, respectively. Pools of DMSPt showed a strong association with overall phytoplankton biomass proxied by chlorophyll a (rs = 0.83) likely because of the persistent dominance of dinoflagellates and coccolithophores, both DMSP-rich taxa. Heterotrophic microbes displayed low S assimilation from DMSP (less than 5 %) likely because their S requirements were fulfilled by high DMSP availability. Rates of bacterial protein synthesis were significantly correlated with concentrations of dissolved DMSP (DMSPd, rs = 0.86) as well as with the microbial conversion efficiency of DMSPd into DMS (DMS yield, rs = 0.84). Estimates of the potential contribution of microbially mediated rates of DMS production (0.1–27 nmol L−1 day−1) to the near-surface concentrations of DMS suggest that bacteria alone could not have sustained DMS pools at most stations, indicating an important role for phytoplankton-mediated DMS production. The findings from this study provide crucial information on the distribution and cycling of DMS and DMSP in a critically under-sampled area of the global ocean, and they highlight the importance of oceanic fronts as hotspots of the production of marine biogenic S compounds.

The abundant marine organosulfur compound, dimethylsulfoniopropionate (DMSP) can be degraded to acrylate and dimethylsulfide (DMS), with some DMS further oxidized to dimethylsulfoxide (DMSO). Despite intensive study of DMSP and DMS in a variety of marine settings, the processes affecting acrylate and DMSO concentrations in marine waters are poorly known, particularly their loss from the dissolved phase through biological uptake. We measured the concentrations of dissolved acrylate (acrylated) and DMSO (DMSOd) in coastal and open-ocean waters of the northern Gulf of Mexico during non-bloom conditions and quantified the rates and kinetics of their biological uptake using 14C labeled substrates. Acrylated concentrations and uptake rates ranged from 0.8–2.1 nmol L−1 and 0.07–1.8 nmol L−1 d−1, respectively. Somewhat higher uptake rates were observed for DMSOd (0.27–3.9 nmol L−1 d−1) owing to higher DMSOd concentrations (5.5–14 nmol L−1). Both compounds were taken up by the microbial community with high affinity uptake systems, with similar Ks and Vmax values to those for other well-studied biological substrates including amino acids and monosaccharides. However, median turnover times were relatively slow, 4.8 d for acrylated and 7.4 d for DMSOd. The slow acrylated turnover points to low supply rates of this compound to the dissolved phase, a finding consistent with previous observations that the microbial DMSP lyase pathway accounts for only a small fraction of dissolved DMSP degradation (and therefore acrylate production) in the Gulf of Mexico.

Metagenomic and metatranscriptomic time-series data covering a 52-day period in the fall of 2016 provide an inventory of bacterial and archaeal community genes, transcripts, and taxonomy during an intense dinoflagellate bloom in Monterey Bay, CA, USA. The dataset comprises 84 metagenomes (0.8 terabases), 82 metatranscriptomes (1.1 terabases), and 88 16S rRNA amplicon libraries from samples collected on 41 dates. The dataset also includes 88 18S rRNA amplicon libraries, characterizing the taxonomy of the eukaryotic community during the bloom. Accompanying the sequence data are chemical and biological measurements associated with each sample. These datasets will facilitate studies of the structure and function of marine bacterial communities during episodic phytoplankton blooms., Design Type(s)transcription profiling design • sequence assembly objective • biodiversity assessment objectiveMeasurement Type(s)transcription profiling assay • marine metagenome • microbial communityTechnology Type(s)RNA sequencing • DNA sequencing • amplicon sequencingFactor Type(s)assay protocol • temporal_instantSample Characteristic(s)marine metagenome • Monterey Bay • ocean biome Machine-accessible metadata file describing the reported data (ISA-Tab format)

Dimethylsulfoniopropionate (DMSP) is produced by many species of marine phytoplankton and has been reported to provide a variety of beneficial functions including osmoregulation. Dinoflagellates are recognized as major DMSP producers; however, accumulation has been shown to be highly variable in this group. We explored the effect of hyposaline transfer in Gambierdiscus belizeanus between ecologically relevant salinities (36 and 31) on DMSP accumulation, Chl a, cell growth, and cell volume, over 12 d. Our results showed that G. belizeanus maintained an intracellular DMSP content of 16.3 pmol cell-1 and concentration of 139 mM in both salinities. Although this intracellular concentration was near the median reported for other dinoflagellates, the cellular content achieved by G. belizeanus was the highest reported of any dinoflagellate thus far, owing mainly to its large size. DMSP levels were not significantly affected by salinity treatment but did change over time during the experiment. Salinity, however, did have a significant effect on the ratio of DMSP:Chl a, suggesting that salinity transfer of G. belizeanus induced a physiological response other than DMSP adjustment. A survey of DMSP content in a variety of Gambierdiscus species and strains revealed relatively high DMSP concentrations (1.0-16.4 pmol cell-1 ) as well as high intrageneric and intraspecific variation. We conclude that, although DMSP may not be involved in long-term (3-12 d) osmoregulation in this species, G. belizeanus and other Gambierdiscus species may be important contributors to DMSP production in tropical benthic microalgal communities due to their large size and high cellular content.

Coral holobionts, including their symbionts, are known to produce large amounts of dimethylsulfoniopropionate (DMSP), a compound which some corals use to mitigate oxidative stress; however, very little work has been done on the presence and use of DMSP in early life history stages of coral, such as in eggs and larvae. This study shows that eggs and larvae, from Acropora palmata, and brooded larvae, from Porites astreoides, also contain high amounts of DMSP. Eggs and larvae of wild A. palmata were collected and contained extremely high levels of DMSP: 1.2 μmol per larva and 359 μmol per 100 μL eggs. Larvae of P. astreoides were collected in flow-through laboratory tanks, from wild-collected parent colonies, over the course of the larval release cycle. In brooded larvae of P. astreoides, the amount of DMSP in larvae ranged from 11.6 to 1510 nmol per larva; larval DMSP peaked by the second night of release and then decreased over the following release nights. These high levels in aposymbiotic eggs and larvae of A. palmata and the peaking trend in symbiotic larvae of P. astreoides are suggestive of parental provisioning. Given the large amounts of DMSP found in eggs and larvae, this study provides further evidence that even early life stages of the coral holobiont may benefit from the presence of DMSP.

A methane surplus relative to the atmospheric equilibrium is a frequently observed feature of ocean surface water. Despite the common fact that biological processes are responsible for its origin, the formation of methane in aerobic surface water is still poorly understood. We report on methane production in the central Arctic Ocean, which was exclusively detected in Pacific derived water but not nearby in Atlantic derived water. The two water masses are distinguished by their different nitrate to phosphate ratios. We show that methane production occurs if nitrate is depleted but phosphate is available as a P source. Apparently the low N:P ratio enhances the ability of bacteria to compete for phosphate while the phytoplankton metabolite dimethylsulfoniopropionate (DMSP) is utilized as a C source. This was verified by experimentally induced methane production in DMSP spiked Arctic sea water. Accordingly we propose that methylated compounds may serve as precursors for methane and thermodynamic calculations show that methylotrophic methanogenesis can provide energy in aerobic environments.

Methionine (Met) and dimethylsulfoniopropionate (DMSP) are 2 important substrates that can serve as sources of sulfur and carbon to microbial communities in the sea. We studied the vertical and die ...

The organic sulfur compound 3-mercaptopropionic acid (3-MPA) is an important thiol intermediate in organic sulfur metabolism in natural environments. It is generated during degradation of sulfur-containing amino acids (e.g. methionine) and from demethylation of dimethylsulfoniopropionate (DMSP). This pathway is an alternative enzymatic process in the DMSP catabolism that routes sulfur away from the climatically-active dimethyl sulfide (DMS). 3-MPA detection and subsequent quantification in different matrices is difficult due to its extreme reactivity. We therefore developed a sensitive method for determination of 3-MPA based on pre-column derivatization with monobromobimane and analysis by high-performance liquid chromatography (HPLC) with fluorescence detection. This methodology was first tested with 3-MPA standards under low (0.005-0.2μmolL(-1)) and high (1-25μmolL(-1)) concentrations. For the optimization of the reaction, CHES and, alternatively, Tris-HCl buffers were evaluated in the derivatization step, with Tris-HCl showing more effective separation of thiol derivatives and a better 3-MPA peak shape. The detection limit was 4.3nmolL(-1) with a 10μL sample injection, and mean recoveries of 3-MPA ranged from 97 to 105% in estuarine waters with different salinities (0.17 and 35.9ppt). The linearity (r>0.99) and repeatability of detector response, with intra- and inter-day precision (% CV) of 2.68-7.01% and 4.86-12.5%, respectively, confirmed the reliability of the method. Previous 3-MPA analytical methods required immediate analysis due to unstable derivatives, but in this method we achieved high stability of the derivatized samples when stored at 4°C, with only a 3-5% loss after more than one year of storage. This method was successfully applied to measure 3-MPA concentrations and rates of 3-MPA production in a variety of intertidal estuarine sediment slurries. Dissolved 3-MPA concentrations in these sediment slurries varied between 2 and 237μmolL(-1) and, 3-MPA net fluxes ranged in wet sediments between -3.6±1.7 and 30±5μmolL(-1)g(-1)h(-1). Thus, the application of this optimized methodology showed an efficient performance for measuring 3-MPA in environmental samples, with a straightforward sample derivatization and a simple analysis of stable 3-MPA derivatives.

This study reports on the temporal variations in algal and bacterial metabolism of dissolved dimethylsulfoniopropionate (DMSPd) in Arctic ice-covered waters in response to the release of organic matter (OM) from the sea ice and the onset of under-ice phytoplankton growth. Sampling took place between 21 May and 21 June 2012 at a station located in Resolute Passage. A snow and ice melt event was accompanied by an important release of OM and total DMSP from the bottom ice to the water column. This input of OM coincided with increases in DMSPd and DMSPd loss rate constant at the ice-water interface and, 2 days later, with increases in DMSPd and bacter- ial dimethylsulfide (DMS) yields from DMSPd at 0.5 m under the ice. The different microbial re - sponses suggest that DMSPd-rich brines were released first, followed by the release of sympagic algae due to ice melt. In both cases, the changes in DMSPd metabolism resulted in an increase in gross DMS production from 0.15 to 1.9 nmol l �1 d �1 . The initiation of phytoplankton growth resulted in increases in bacterial abundance, DMSPd loss-rate constant and DMSP-sulfur assimilation. In contrast, DMS yield remained low during the onset of phytoplankton growth, indicating that bac- teria used DMSP as a carbon and sulfur source. These results show that ice DMSPd can be rapidly (

Carbon gas fluxes in tidal marshes vary spatially and temporally because of vegetation cover, subsurface biogeochemical processes, and environmental forcing. The objective of this study was to examine how ecosystem carbon gas exchange changes along an estuarine salinity gradient. We measured carbon dioxide (CO2) and methane (CH4) gas fluxes from three marshes representing a salinity gradient (0–32 ppt) in the Mobile Bay estuary, Alabama, USA. CH4 flux was relatively small with no significant differences across sites despite salinity differences. Interestingly, sediment porewater CH4 concentrations were significantly higher at the high salinity salt marsh and decreased with decreasing salinity. Midday net ecosystem exchange (where a positive rate indicates net carbon assimilated through photosynthesis) was greatest at the most fresh site (4.8 ± 0.3 µmol CO2 m−2 s−1), followed by the saline (2.8 ± 1.0 µmol CO2 m−2 s−1) and brackish (1.4 ± 0.6 µmol CO2 m−2 s−1) sites. However, net ecosystem exchange integrated diurnally revealed each marsh to be a net CO2 source to the atmosphere as a result of high ecosystem respiration with the freshwater marsh emitting more CO2 (−893.4 ± 187.9 g C m−2 year−1) than the brackish (−517.8 ± 85.2 g C m−2 year−1) and salt marsh (−410.2 ± 98.2 g C m−2 year−1). This finding leads to the conclusion that either the marshes are losing carbon or that they receive a subsidy of respirable carbon, possibly via tidal deposition. The extent to which sedimentation from tidal deposition contributes carbon to these ecosystems, however, remains unknown. Without such a subsidy, marshes in the study area will not be able to keep up with sea level rise.

The oceanic frontal region above the Chatham Rise east of New Zealand was investigated during the late austral summer season in February and March 2012. Despite its potential importance as a source of marine-originating and climate-relevant compounds, such as dimethylsulfide (DMS) and its algal precursor dimethylsulfoniopropionate (DMSP), little is known of the processes fuelling the reservoirs of these sulfur (S) compounds in the water masses bordering the Subtropical Front (STF). This study focused on the two opposing fates of DMSP-S following its uptake by microbial organisms: either its conversion into DMS, or its assimilation into bacterial biomass. Sampling took place in three phytoplankton blooms (B1, B2 and B3) with B1 and B3 occurring in relatively nitrate-rich, dinoflagellate-dominated Subantarctic waters, and B2 occurring in nitrate-poor Subtropical waters dominated by coccolithophores. Concentrations of total DMSP (DMSPt) and DMS were high across the region, up to 160 nmol L−1 and 14.5 nmol−1, respectively. Pools of DMSPt measured in this study showed a strong association with overall phytoplankton biomass proxied by chlorophyll a (rs = 0.83) likely because of the persistent dominance of dinoflagellates and coccolithophores, both DMSP-rich taxa. Heterotrophic microbes displayed low S assimilation from DMSP (less than 5 %) likely because their S requirements were fulfilled by high DMSP availability. Rates of bacterial protein synthesis were significantly correlated with concentrations of dissolved DMSP (DMSPd, rs = 0.86) as well as with the microbial conversion efficiency of DMSPd into DMS (DMS yield, rs = 0.84). Estimates of the potential contribution of microbially-mediated rates of DMS production (0.1–27 nmol L−1 d−1) to the near-surface concentrations of DMS suggest that bacteria alone could not have sustained DMS pools at most stations, indicating an important role for phytoplankton-mediated DMS production. The findings from this study provide crucial information on the distribution and cycling of DMS and DMSP in a critically under-sampled area of the global ocean, and they highlight the importance of oceanic fronts as hotspots of the production of marine biogenic S compounds and as potential sources of aerosols particularly in regions of low anthropogenic perturbations such as the frontal waters of the Southern Hemisphere.

Dimethylsulphoxide (DMSO) is a sulphur compound that can result from the oxidation of biogenic dimethylsulphide (DMS) in marine algae and bacteria; with dimethylsulphoniopropionate (DMSP) being the main precursor of DMS. The two most commonly used methods for the analysis of DMSO in seawater and biological samples consist of its chemical reduction to DMS by either titanium trichloride (TiCl3) or sodium borohydride (NaBH4), with subsequent measurement of derived DMS by gas chromatography. Here, these two methods have been compared for the quantitative analysis of DMSO in the zooxanthellate coral Acropora aspera and in two species of marine algae (Ulva intestinalis and Ulva lactuca) using headspace analysis on DMSO-derived DMS. Reduction by NaBH4 or TiCl3 in biological samples yielded highly linear calibrations (R2 ≥ 0.99) and excellent repeatability (RSD = 6.17% and 4.32% for TiCl3 and NaBH4 respectively, n = 10). In coral samples, although a strong linear correlation was generally obtained between the two reduction methods (R2 = 0.8464, p 2 μM. The reasons for these significant differences remain unclear at this stage and we therefore cannot draw conclusions on the preferential suitability of one reducing agent over the other for coral DMSO analysis. In macroalgae samples, significantly lower DMSO concentrations were obtained with NaBH4 than with TiCl3 for DMSO concentrations averaging 0.6 μM and 0.8 μM for U. intestinalis and U. lactuca respectively. The difference between reduction methods in the analysis of DMSO across macroalgae and coral samples was interpreted as a difference in taxa or in sample preparation, although this needs to be further investigated. Corals were found to contain more DMSO than macroalgae with similar DMSP concentrations.

Bacterial degradation of dimethylsulfoniopropionate (DMSP) represents one of the main sources of the climatically–active trace gas dimethylsulfide (DMS) in the upper ocean. Short-term enrichment studies to stimulate specific pathways of DMSP degradation in oligotrophic waters from the Sargasso Sea were used to explore regulatory connections between the different bacterial DMSP degradation steps and determine potential biological controls on DMS formation in the open ocean. Experiments were conducted with surface water at the BATS station in the western North Atlantic Ocean. We added selected organic substrates (25 nmol L−1 final concentration) to induce different steps of DMSP degradation in the microbial community, and then measured DMSP dynamics (assimilation and turnover rates), DMS yields (using 35sulfur-DMSP tracer), and bacterial production rates. In most treatments, the main fate of consumed S-DMSP was excretion as a non-volatile S product. 35S-DMSP tracer turnover rates (accumulation + assimilation + excretion of transformed products as DMS or others) increased upon addition of DMSP and glucose, but not acrylate, methymercaptopropionate (MMPA), methanethiol, DMS or glycine betaine. DMS yields from 35S-DMSP never exceeded 16 % except in a short term DMSP enrichment, for which the yield reached 45 % (±17 %). Results show that availability of non-sulfur containing labile C sources (glucose, acrylate) decreased bacterial DMS production while stimulating bacterial heterotrophic production, and suggest an influence of bacterial sulfur demand in controlling DMS-yielding pathways. However, regulatory effects on 35S-DMSP fate were not consistent across all reduced sulfur compounds (i.e., methanethiol or MMPA), and may reflect alternate roles of DMSP as a bacterial energy source and osmolyte.

Biogenic dimethylsulfide (DMS) and its main precursors, dimethylsulfoniopropionate (DMSP) and dimethylsulfoxide (DMSO), are potential scavengers of reactive oxygen species in marine algae, and these dimethylated sulfur compounds (DSC) could take part in the algal antioxidant system. In this study, a link between the DSC production and the antioxidant capacity (AOC) of Acropora aspera reef coral was investigated under a range of environmental factors (temperature, light, salinity, and air exposure) that can lead to oxidative stress in the coral holobiont. Enhanced DMS(P)(O) production occurred under experimental conditions, indicating that DSC are potential biomarkers of stress level in coral tissue. Differences in concentrations and partitioning as a response to different treatments suggest that DSC production and turnover undergo different biochemical pathways depending on the type and severity of environmental stress. Osmotic pressure and light depletion led to an upregulation of the coral AOC that was correlated with a significant increase in DMSO:DSC ratio. These results, combined with a positive correlation between the AOC and DMSO concentrations under these two treatments, suggest that the DMSP-based antioxidant system is involved in the overall antioxidant regulation of the coral holobiont. Enhanced DMS production coupled with an increased DMS:DSC ratio under increased temperature indicated that thermal stress triggers DMS formation in coral tissue. Considering the role that DMS can have in both climate regulation and the DMSP-based antioxidant system, our findings highlight the need to further examine the fate of DSC in coral reef environments under scenarios of increasing sea surface temperatures.

Dimethylsulfoniopropionate (DMSP) lyases are present in a wide variety of marine phytoplankton and are responsible for converting the osmolyte DMSP into dimethylsulfide (DMS) and acrylate. The physiological functions of DMSP lyases are not well understood, but they have received considerable attention because of the role of volatile DMS in trophic dynamics and ocean–atmosphere sulfur exchange. Marine phytoplankton of the genus Phaeocystis are important DMSP producers in the ocean and play a pivotal role in global biogeochemical cycles by forming massive blooms and emitting large amounts of DMS to the atmosphere (~ 0.05 Tmol DMS year− 1). In this study, we used an in vitro, whole-cell extract assay to examine the pH and temperature dependence as well as substrate-specific kinetics of DMSP lyase activity in five different strains of Phaeocystis, including colonial growth forms of Phaeocystis antarctica CCMP 1871, Phaeocystis globosa CCMP 627 and P. globosa CCMP 628, and single cell growth forms of Phaeocystis jahnii CCMP 2496 and Phaeocystis cordata CCMP 3104. All of the tested strains had optimum lyase activity at pH 5. At this pH, the highest Vmax and lowest Km values were recorded at 20 °C for P. antarctica; 25 °C for P. globosa 627, P. jahnii and P. cordata; and 30 °C for P. globosa 628. Under optimal conditions, Vmax and Km ranged from 22.2 to 56.4 nmol DMS min− 1 μg Chl a− 1 and 2.11 to 7.12 mM, respectively, and higher Vmax values were found in colonial Phaeocystis species (P. antarctica and P. globosa) as compared with the single-celled species (P. cordata and P. jahnii). Substrate specificity tests indicated that the lyases of all five Phaeocystis strains cleaved the DMSP analogs, diethylsulfoniopropionate (DESP) and 2-chloroDMSP with liberation of diethylsulfide (DES) and DMS, respectively. However, the lyases of all five Phaeocystis strains had at least 1.5 to 3.4 times higher affinity towards DMSP compared to its analogs 2-chloroDMSP and DESP, indicating moderate specificity of the enzyme for DMSP. Chlorophyll a-normalized lyase activities in Phaeocystis species are in the upper range of what has been measured in field samples, highlighting the potential importance of Phaeocystis spp. in oceanic DMS production.

Members of the marine phytoplankton genus Phaeocystis (Prymnesiophyceae) pro- duce large amounts of the intracellular osmolyte DMSP and they are known to also produce lyase enzymes that cleave DMSP into the biogeochemically important trace gas DMS. The functional characteristics of DMSP lyase activity in Phaeocystis spp. are not well known. We characterized DMSP lyase activity in extracts from 2 ecologically important species from this genus, the mesophile P. globosa (strain CCMP629) and the psychrophile P. antarctica (strain CCMP1374). Results from whole cell extracts showed that both algal species were potent producers of DMSP lyase, with Michaelis-Menten constant (Km) and maximum reaction velocity (Vmax) values of 1.77 mM and 17.3 nmol DMS min �1 mg protein �1 , respectively, for P. globosa, and 2.31 mM and 28.2 nmol DMS min �1 mg protein �1 , respectively, for P. antarctica. The optimal DMSP lyase activity was recorded at pH 4 and 30°C for P. globosa, and at pH 5 and 20°C for P. antarctica. The half-life of the DMSP lyase of P. globosa was 210 min at 25°C, which was longer than that of the P. antarctica enzyme (61.9 min). First-order kinetic analysis of DMSP lyase thermal denaturation demonstrated that the activation energy, free energy, enthalpy and entropy of denaturation in P. antarctica extracts were lower than for P. globosa extracts, confirming that the P. antarctica DMSP lyase was more thermolabile than the lyase from the temperate strain. Inhibitor tests with metals, a chelator (EDTA) and a serine binding agent (PMSF) suggested that the DMSP lyases from both Phaeocystis species were metalloenzymes with serine and sulfhydryl groups at the active site. The acidic pH optima for the Phaeocystis strains are consistent with findings from other Prymnesiophyceae, and we speculate that this may reflect adaptation to an acidic sub-cellular location for the DMSP lyase.

We investigated the possible role of dimethylsulfoniopropionate (DMSP) as an antioxidant in Spartina alterniflora (Smooth cordgrass). Experimentally applied oxidative stress caused by application of the herbicides, 1,1′-dimethyl-4,4′-bipyridinediium dichloride (Paraquat), and 1,1-dimethyl-3-(3,4-dichlorophenyl)urea(DCMU)to the leaves of S. alterniflora resulted in increased oxidation of DMSP to dimethylsulfoxide (DMSO). We did not see an increase in DMSP synthesis and accumulation in response to herbicide induced oxidative stress. The role of DMSP in oxidative stress protection was further investigated via the exogenous application of Paraquat and DMSP + acrylate to leaf discs of the non-DMSP producing grass Panicum commutatum (Panic grass). Inclusion of DMSP + acrylate in the treatment medium resulted in significantly less tissue damage as indicated by decreased tissue necrosis, and chlorophyll bleaching. While the lack of enhanced synthesis of DMSP suggests that DMSP may not function solely as an antioxidant in S. alterniflora , ourresults show that the ability of S. alterniflora to synthesize DMSP may partially explain its ability to thrive in the stressful low marsh environment.

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.

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

Summary Bacterioplankton consume a large proportion of pho- tosynthetically fixed carbon in the ocean and control its biogeochemical fate. We used an experimental metatranscriptomics approach to compare bacterial activities that route energy and nutrients during a phytoplankton bloom compared with non-bloom con- ditions. mRNAs were sequenced from duplicate bloom and control microcosms 1 day after a phy- toplankton biomass peak, and transcript copies per litre of seawater were calculated using an internal mRNA standard. Transcriptome analysis revealed a potential novel mechanism for enhanced efficiency during carbon-limited growth, mediated through membrane-bound pyrophosphatases (V-type H(+)- translocating; hppA); bloom bacterioplankton partici- pated less in this metabolic energy scavenging than non-bloom bacterioplankton, with possible implica- tions for differences in growth yields on organic sub- strates. Bloom bacterioplankton transcribed more copies of genes predicted to increase cell surface adhesiveness, mediated by changes in bacterial sig- nalling molecules related to biofilm formation and motility; these may be important in microbial aggre- gate formation. Bloom bacterioplankton also tran- scribed more copies of genes for organic acid utilization, suggesting an increased importance of this compound class in the bioreactive organic matter released during phytoplankton blooms. Transcription patterns were surprisingly faithful within a taxon regardless of treatment, suggesting that phylogeny broadly predicts the ecological roles of bacterial groups across 'boom' and 'bust' environmental backgrounds.

The algal metabolite, dimethylsulfoniopropionate (DMSP), is known to be stable in acid solution and acidification of seawater samples to pH Phaeocystis antarctica bloom in the Ross Sea, Antarctica, whereas little or no loss of DMSP occurred in other waters where Phaeocystis spp . were minor components of the phytoplankton community. Tests with cultures of colonial P . globosa showed that up to 68% of culture DMSP was lost during the first minutes after acid addition. After initial losses, DMSP remained constant, confirming the chemical stability of DMSP at pH P. globosa cultures suggesting that DMSP was degraded via lyase enzymes. Major acid-mediated DMSP losses were unique to colonial Phaeocystis spp., as no significant DMSP losses occurred with solitary P. globosa cells or other phytoplankton genera tested, although a small amount of DMS production (equivalent to Emiliania huxleyi (CCMP 373). Exogenous dissolved 35 S-DMSP was not lost during acidification of colonial P. globosa cultures indicating that intracellular rather than dissolved DMSP was cleaved during the time between acid addition and lyase enzyme inactivation. Experiments with the differentially-permeable DMSP lyase inhibitors, p-CMB and p-CMBS, showed that only the membrane permeable p-CMB stopped DMSP loss under acidified conditions, confirming that DMSP loss occurred intracellularly. These results highlight the remarkable potential activity of DMSP lyases in Phaeocystis spp. and suggest that acidification alone is inadequate for preservation of samples containing colonial Phaeocystis spp.

Over half of the bacterioplankton cells in ocean surface waters are capable of carrying out a demethylation of the phytoplankton metabolite dimethylsulfoniopropionate (DMSP) that routes the sulfur moiety away from the climatically active gas dimethylsulfide (DMS). In this study, we tracked changes in dmdA , the gene responsible for DMSP demethylation, over the course of an induced phytoplankton bloom in Gulf of Mexico seawater microcosms. Analysis of >91,000 amplicon sequences indicated 578 different dmdA sequence clusters at a conservative clustering criterion of ≥90% nucleotide sequence identity over the 6-day study. The representation of the major clades of dmdA , several of which are linked to specific taxa through genomes of cultured marine bacterioplankton, remained fairly constant. However, the representation of clusters within these major clades shifted significantly in response to the bloom, including two Roseobacter -like clusters and a SAR11-like cluster, and the best correlate with shifts of the dominant dmdA clades was chlorophyll a concentration. Concurrent 16S rRNA amplification and sequencing indicated the presence of Roseobacter , SAR11, OM60, and marine Rhodospirillales populations, all of which are known to harbor dmdA genes, although the largest taxonomic change was an increase in Flavobacteriaceae , a group not yet demonstrated to have DMSP-demethylating capabilities. Sequence heterogeneity in dmdA and other functional gene populations is becoming increasingly evident with the advent of high-throughput sequencing technologies, and understanding the ecological implications of this heterogeneity is a major challenge for marine microbial ecology.

Dimethylsulfoniopropionate (DMSP) is produced by a wide variety of marine phytoplankton, and it can be enzymatically cleaved to dimethylsulfide (DMS) and acrylate by DMSP lyases. DMS formation in the sea plays an important role in the global sulfur cycle, yet the factors regulating production of DMS are poorly understood. We evaluated various procedures used for in vitro assays of DMSP lyase activity (DLA) of cells captured on filters. We also compared in vitro DLA from plankton material collected from diverse water samples, as well as from cultures of different phytoplankton species. The type of filter used to collect plankton material affected the apparent in vitro DLA, with glass fiber filters (Whatman GF/F) yielding higher rates than polycarbonate filters. Treatment of filtered plankton with Tris buffer (200 mM in 500 mM NaCl), with vigorous mixing, appeared to permeablize cells allowing maximal DLA. We found that buffer pH and dithiothreitol (DTT) affected DLA positively or negatively depending on the phytoplankton species or water sample tested. In general, natural seawater plankton and cultured dinoflagellates had higher DLA with higher pH buffers (pH range 6.5 - 8.5). In contrast, the prymnesiophytes Emiliania huxleyi and Phaeocystis antarctica had higher in vitro DLA at lower pH (pH range 6.5 - 8.5), and DTT and Triton X-100 significantly increased the apparent enzyme activities. Not all phytoplankton contained detectable DLA, and DLA varied greatly among strains of the same species. DLA was however, detectable in all samples of particulate materials collected from a wide range of marine surface waters, including those from the coastal Gulf of Mexico, North Atlantic and Southern Ocean, and these samples showed patterns of high Chl a normalized DLA where high irradiances and low nutrients occur. Consistent with this observation, the phytoplankton cultures isolated from such environments also showed high DLA. Thus, light and nutrients may be important factors determining DLA. However, not all the variations in DLA were explained by light and nutrients, and these variations may be due to the different functions of DMSP lyases such as osmotic regulation, anti-grazing, and antioxidant activity.

Microbes play critical roles in the biogeochemical cycling of nitrogen and sulfur in aquatic environments. Here we investigated the interaction between the naturally occurring organic sulfur compound methanethiol (MeSH) and the final step of the denitrification pathway, the reduction of nitrous oxide (N2 O) to dinitrogen (N2 ) gas, in sediment slurries from the temperate Douro and Ave estuaries (NW Portugal) and in pure cultures of the marine bacterium Ruegeria pomeroyi. Sediment slurries and cell suspensions were amended with a range of concentrations of either MeSH (0-120 µM) or methionine (0-5 mM), a known precursor of MeSH. MeSH or methionine additions caused N2 O to accumulate and this accumulation was linearly related to MeSH concentrations in both coastal sediments (R(2) = 0.7-0.9, P

Summary The fraction of dissolved dimethylsulfoniopropionate (DMSPd) converted by marine bacterioplankton into the climate-active gas dimethylsulfide (DMS) varies widely in the ocean, with the factors that determine this value still largely unknown. One current hypothesis is that the ratio of DMS formation : DMSP demethylation is determined by DMSP availability, with ‘availability’ in both an absolute sense (i.e. concentration in seawater) and in a relative sense (i.e. proportionally to other labile organic S compounds) proposed as the critical factor. We investigated these models during an experimentally induced phytoplankton bloom using a taxon-specific microarray targeting DMSP-related gene transcription in members of the Roseobacter clade, a group hypothesized to play an important role in the surface ocean sulfur cycle and well represented by genome sequences. The array consisted of 1578 probes to 431 genes and was designed to target diverse Roseobacter communities in natural seawater by using hierarchical probe design based on 13 genome sequences. The prevailing pattern of Roseobacter gene transcription showed relative depletion of DMSP-related transcripts during the peak of the bloom, despite increasing absolute concentrations and flux of DMSP-related compounds. DMSPd thus appeared to be assimilated by Roseobacter populations in proportion to its relative abundance in the organic matter pool (the ‘relative sense’ hypothesis), rather than assimilated in preference to other labile organic sulfur or carbon compounds produced during the bloom. The relative investment of the Roseobacter community in DMSP demethylation was not useful for predicting the formation of DMS, however, suggesting a complex regulatory process that may involve multiple taxa and alternative fates of DMSPd.

The effect of added iron on bacterial cycling of the climate-active gas dimethylsulfide (DMS) and its precursor dimethylsulfoniopropionate (DMSP) was tested during the second Subarctic Pacific Iron Experiment for Ecosystem Dynamics Study (SEEDS II) from 19 July to 21 August 2004 aboard the R/V Hakuho-Maru. The study area in the northwest Pacific Ocean (48°N 165°E) was enriched with Fe and the conservative tracer, SF6, allowing the fertilized patch to be tracked. Microbial DMSP cycling rates were determined in the surface mixed layer (5 m) during incubations using the 35S-DMSP technique. The addition of iron resulted in a 4-fold increase in concentrations of chlorophyll a (chl a) within the surface mixed layer (5 m depth), and the length of the sampling period allowed the observation of both bloom and post-bloom conditions. Inside the fertilized patch, the alleviation of resource limitation gave rise to the concurrent increase in bacterial abundance and production. Changes in the phytoplankton community within the Fe-enriched patch translated into a sustained decrease in chl a-normalized particulate DMSP (DMSPp) concentrations, suggesting a preferential stimulation of the growth of DMSPp-poor phytoplankton species. Despite short-lived peaks of DMSPp within the Fe-enriched area, concentrations of DMSPp generally remained stable during the entire sampling period inside and outside the fertilized patch. During the Fe-induced bloom, microbial DMSP-sulfur (DMSP-S) assimilation efficiency increased 2.6-fold inside the Fe-enriched area, which indicated that as bacterial production increased, a greater proportion of DMSP-S was assimilated and possibly diverted away from the bacterial cleavage pathway (i.e. production of DMS). Our results suggest that iron-induced stimulation of weak DMSPp-producers and DMSP-assimilating bacteria may diminish the potential production of DMS and thus limit its flux towards the atmosphere over the subarctic Pacific Ocean.

High concentrations of the phytoplankton metabolite dimethylsulfoniopropionate (DMSP) and its degradation product dimethylsulfide (DMS) are associated with blooms of Phaeocystis antarctica in the Ross Sea, Antarctica. Episodic and rapid vertical export of Phaeocystis biomass to deep water has been reported for the Ross Sea, therefore we examined the distribution and microbial consumption rates of DMSP and DMS throughout the sub-euphotic water column. Total DMSP (dissolved+particulate; DMSPt) was present at 0.5–22 nM at depths between 70 and 690 m during both the early bloom (November) and the late bloom (January). Sub-euphotic peaks of DMSP were sometimes associated with mid-water temperature maxima, and elevated DMSP below 70 m was found mainly in water masses characterized as Modified Circumpolar Deep Water or Antarctic Shelf Water. Overall, 50–94% of the integrated water-column DMSPt was found below the euphotic zone. At one station during the early bloom, local maxima of DMSPt (14 nM) and DMS (20 nM) were observed between 113 and 240 m and these maxima corresponded with high chlorophyll a concentrations, P. antarctica cell numbers, and Fv/Fm (the quantum yield of photosystem II). During the late bloom, a sub-euphotic maximum of DMSPt (15.8 nM) at 250 m cooccurred with peaks of chlorophyll a concentration, DMSP lyase activity, bacterial production and dissolved DMSP consumption rates. DMSP turnover contributed ∼12% of the bacterial carbon demand between 200 and 400 m. DMS concentrations peaked at 286 m but the maximum concentration (0.42 nM) was far lower than observed during the early bloom, probably because of relatively rapid biological consumption of DMS (1–3 turnovers per day) which, in turn, contributed to elevated dissolved dimethylsulfoxide (DMSO) concentrations. Relatively stable DMSPt distributions at some sites suggest that rapid sinking of Phaeocystis biomass is probably not the major mechanism responsible for mesopelagic DMSP accumulations. Rather, subduction of near-surface water masses, lateral advective transport or trapping of slowly sinking P. antarctica biomass in intermediate water masses are more likely mechanisms. We found that a culture of P. antarctica maintained cellular integrity during 34 days of darkness, therefore the presence of intact cells (and DMSP) at depth can be explained even under a slow sinking/advection scenario. Whatever the mechanism, the large pools of DMSP and DMS below the euphotic zone suggest that export exerts a control on potential DMS emission from the surface waters of the Ross Sea.

Dimethylsulfoxide (DMSO) is an abundant but poorly understood methylated sulfur compound in the marine environment. One potentially significant loss pathway for DMSO is through its biological reduction to dimethylsulfide (DMS), which has been documented in a number of organisms, most notably bacteria. Here we present the first detailed study of DMSO reduction by several marine phytoplankton in axenic cultures. Reduction of DMSO was observed in four algal classes, with in vivo reduction rates ranging from 0.006 to 1.5 mmol [L cell volume]21 s21 at 1.0 mmol L21 DMSO. Corresponding turnover times for measured intracellular DMSO pools varied from hours to days. Michaelis–Menton kinetic parameters were estimated for Isochrysis galbana, Thalassiosira pseudonana, and Amphidinium carterae. The half-saturation constant (Km) and maximal rate (Vmax) for DMSO reduction ranged between 0.96 and 2.7 mmol [L cell volume]21 and 17–118 nmol [L cell volume]21 s21, respectively. Our results suggest that DMSO reduction is a universal activity in marine phytoplankton, even in algae with no detectable dimethylsulfoniopropionate (DMSP). Although reduction of DMSO by marine eukaryotes may not contribute significantly to removal of DMSO from the dissolved phase, this reduction is likely to be a major source of DMS in species lacking detectable DMSP lyase activity. The ability of marine phytoplankton to reduce DMSO to DMS should allow algae to cycle these compounds as part of an antioxidant system.

Dimethylsulfoxide (DMSO) is an important degradation product of the climate-influencing gas dimethylsulfide (DMS). In the Ross Sea, Antarctica, dissolved DMSO (DMSOd) concentrations exhibited substantial seasonal and vertical variations. Surface water DMSOd concentrations in pre-bloom waters were very low ( < 1 nM) but increased rapidly up to 41 nM during the spring Phaeocystis antarctica bloom (late November). Increases in DMSOd concentrations lagged by several days increases in DMS concentrations. Although DMSOd concentrations reached relatively high levels during the spring bloom, concentrations were generally higher (36.3-60.6 nM) during summer (January), even though phytoplankton biomass and DMS concentrations had decreased by that time. During both seasons, DMSOd concentrations were substantially higher within the surface mixed layer than below it. DMSOd production from biological DMS consumption (BDMSC) was higher during late November (3,4-5,2 nMd -1 ) than during the summer (0.7-2.4 nM d -1 therefore, production via BDMSC alone could not explain the higher DMSOd concentrations encountered during the summer. Mixed layer-integrated DMSOd production from BDMSC was 2.5-13.7 times greater than production from dissolved DMS photolysis during the P. antarctica bloom, while photolysis contributed 1.3 times more DMSO than BDMSC before the bloom. The DMSO yield from BDMSC was consistently higher within the upper mixed layer than at depths below. Experimental incubations with water from the mixed layer showed that exposure to full spectrum sunlight for 72 h caused an increase in the DMSO yield whereas exposure to only photosynthetically active radiation did not. This suggests that ultraviolet radiation is a potential factor shifting the fate of biologically consumed DMS toward DMSO. In general, the highest DMSO yields from BDMSC were in samples with slow biological DMS turnover, whereas fast turnover favored sulfate rather than DMSO as a major end-product. This study provides the first detailed information about DMSOd distribution and production in the Ross Sea and points to DMSOd as an important biological and photochemical degradation product of DMS and a major reservoir of methylated sulfur in these polar surface waters.

The impact of the decline of the vernal bloom on the bacterial metabolism of dimethyl- sulfoniopropionate (DMSP), the precursor of dimethyl- sulfide (DMS), was investigated during a 7 d Lagran- gian study conducted in the Northwest Atlantic in spring 2003. Daily variations in bacterial leucine incor- poration, dissolved DMSP (DMSPd) uptake and DMS production rates were measured in the surface mixed layer (SML) and in the deep chlorophyll a maximum (DCM) that formed as the bloom collapsed. Seawater samples were amended with 35 S-DMSPd, and the prod- ucts of bacterial DMSPd degradation were measured during 3 h on-board incubations. The gradual decrease in phytoplankton biomass and diatom abundance measured in the SML was accompanied by a sharp doubling of the bacterial abundance and a peak in leucine incorporation rate on Day 2, suggesting that bacteria responded to a transient pulse in dissolved organic matter. Bacterial DMSPd uptake and DMS production were highest on Days 1 and 2 (1.2 and 0.10 nmol l -1 h -1 , respectively), but rapidly decreased by Day 3, suggesting that DMSPd was becoming a less important substrate for the growing bacterial assem- blage as other substrates became available. Bacterial DMSPd uptake and DMS production rates were also low in the DCM despite very high DMS yields (40 to 50%), showing that neither the decline of the diatom spring bloom in the SML nor the accumulation of cells in the DCM resulted in a stimulation of bacterial DMSP metabolism or accumulation of DMS. The pre- sent study provides new field evidence for the po- tential uncoupling between bacterial production and DMS dynamics likely due to variations in the availabil- ity of other S-containing organic compounds released during the decay of phytoplankton blooms.

14 pages,11 figures The major source of reduced sulfur in the remote marine atmosphere is the biogenic compound dimethylsulfide (DMS), which is ubiquitous in the world’s oceans and released through food web interactions. Relevant fluxes and concentrations of DMS, its phytoplankton-produced precursor, dimethylsulfoniopropionate (DMSP) and related parameters were measured during an intensive Lagrangian field study in two mesoscale eddies in the Sargasso Sea during July–August 2004, a period characterized by high mixed-layer DMS and low chlorophyll—the so-called ‘DMS summer paradox’. We used a 1-D vertically variable DMS production model forced with output from a 1-D vertical mixing model to evaluate the extent to which the simulated vertical structure in DMS and DMSP was consistent with changes expected from field-determined rate measurements of individual processes, such as photolysis, microbial DMS and dissolved DMSP turnover, and air–sea gas exchange. Model numerical experiments and related parametric sensitivity analyses suggested that the vertical structure of the DMS profile in the upper 60 m was determined mainly by the interplay of the two depth variable processes—vertical mixing and photolysis—and less by biological consumption of DMS. A key finding from the model calibration was the need to increase the DMS(P) algal exudation rate constant, which includes the effects of cell rupture due to grazing and cell lysis, to significantly higher values than previously used in other regions. This was consistent with the small algal cell size and therefore high surface area-to-volume ratio of the dominant DMSP-producing group—the picoeukaryotes. We gratefully acknowledge the financial assistance provided through NSF Biocomplexity funding (OPP-0083078) and an Australian Research Council Discovery Grant. We are grateful to the comments by D.J. Kieber. We recognize the participation and help of K. Bailey, J. Bisgrove, B. Blomquist, I. Forn, H. Harada, B. Huebert, D. Jones, L. Maroney, A. Neely, S. Riseman, C. Smith, J. Stefels, K. Tinklepaugh, M. Vila-Costa, G. Westby, H. Zemmelink and the R/V Seward Johnson crew. DiTullio et al., 2001; Simo ́ and Dachs, 2002; Simon and Azam, 1989; Zemmelink et al., 2005

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).

Methane in situ production occurs frequently in the oxygenated upper ocean. A principal pathway by which methane can be formed is methylotrophic methanogenesis, while an important methylated substrate is DMSP (dimethylsulfoniopropionate) produced by marine phytoplankton. Here we report on an in situ methane production/consumption cycle during a summer phytoplankton bloom and a potential link to DMSP concentration in Storfjorden (Svalbard Archipelago) – a polar shelf region. The study is based on measurements of δ 13 C CH4 values, concentrations of methane, chlorophyll-a, particulate and dissolved DMSP, as well as water temperature and salinity along four transects in August 2005. Freshwater input creates a stable surface layer in Storfjorden during summer, below which a denser subsurface layer is found. A methane surplus (between 5 and 55 nM) in relation to the atmospheric equilibrium concentration (about 3.5 nM) is detected in the water column and the carbon isotopic signatures of dissolved methane (− 52 to − 24‰ PDB) deviate from those of atmospheric methane (− 47‰ PDB). The methane plumes observed in the surface and subsurface water differed from each other, suggesting that they are generated independently. The subsurface water in summertime contained methane that was released from sediments during winter, and oxidized over time, leaving the residual methane 13 C-enriched. The surface water, on the other hand, contained recently produced, 13 C-depleted methane. We propose that methane in situ production occurs during the summer phytoplankton bloom. The concentration of methane increases up to a certain threshold value, above which methane consumption begins. A methane production-removal cycle is established, which is reflected in the varying methane concentrations and δ 13 C CH4 values. DMSP and methane are inversely correlated suggesting that DMSP could be a potential substrate for the methylotrophic methanogenesis.

Dimethylsulfoxide (DMSO) is an abundant form of methylated sulfur in marine systems and it is known to be produced from dimethylsulfide (DMS). Using radiolabeled 35S-DMS and gas chromatography techniques, we quantified the dissolved DMSO (DMSOd) produced from photo- and biological oxidation of dissolved DMS and compared the DMSOd production from these pathways with the net change in DMSOd concentrations in unfiltered seawater samples. The net change in DMSOd in light-exposed treatments exceeded DMSOd production from photo- plus biological oxidation of dissolved DMS. This indicated that DMSOd was produced by one or more light-driven processes likely associated with particulate material. Results from in situ incubation arrays showed that the relative importance of DMSOd production processes was dependent on irradiation depth, with the unidentified particle-associated process and dissolved DMS photooxidation the main DMSOd sources close to the surface (0–10 m) and biological oxidation of dissolved DMS the main process at depths at which the light level was low (.10 m). Deckboard and in situ incubations revealed that DMSOd production from the particle-associated process was stimulated by ultraviolet radiation. Higher particle-associated production of DMSOd in samples more prone to suffer light-induced stress supports the hypothesis that this process was related to phytoplanktonic biosynthesis and release of DMSO because of oxidative stress. Our results suggest that particle-associated DMSOd production is an important source of DMSOd in surface waters of the Ross Sea and also help to explain why DMSOd is periodically the main organosulfur compound detected in the upper water column.

Concentrations of total dimethylsulfoniopropionate (DMSPt) and its degradation products, dissolved dimethylsulfide (DMS) and dimethylsulfoxide (DMSOd), were measured in surface waters along three transects between 49 to 76°S latitude (November 2003 & 2005 and December 2004) in the New Zealand sector of the Southern Ocean. Most water samples were collected from the ship’s underway pump system, and concentrations of DMSPt, DMS and DMSOd obtained with this method showed excellent agreement with Niskin bottle-collected samples. Dissolved DMSP (DMSPd), on the other hand, was significantly higher in pump samples. Biological consumption rates for DMSPd and DMS were also measured in Niskin-collected surface waters at selected stations. Concentrations of DMSPt (12 to 52 nM) and DMS (0.6 to 3.2 nM) were moderate in open waters north of the seasonal sea ice (north of 63°S), and very low ( 80% (65 – 73°S; November transects). High concentrations of DMSPt (up to 95 nM) and DMS (up to 30 nM), and high DMS:DMSPt ratios, were observed on the northern boundaries of the seasonal sea ice (63 to 68°S) and in the northern Ross Sea (74 – 76°S). Surface water DMSOd concentrations were variable (1 – 55 nM), but generally higher in ice melt zones and the northern Ross Sea, especially in summer (December 2004). Rates of biological DMS and DMSPd consumption were elevated in ice melt zones, but were generally quite low (

Nitrate and nitrite solar actinometers or chemical ‘light meters’ were used to quantify light doses in photochemical and photobiological experiments involving dimethylsulfide (DMS) and dimethylsulfoniopropionate (DMSP) cycling. Light doses were calculated based on the photochemical production of salicylic acid (SA) from benzoic acid in these actinometers, with SA quantified by either spectrofluorometry or high performance liquid chromatography. Nitrate and nitrite actinometers were modified for deployment at low temperatures in Antarctic waters by addition of sodium chloride as a freezing point depressant. The addition of salt did not affect the solar response of the actinometers; however, the solar response did change slightly with latitude. In the Antarctic, peak response wavelengths (and bandwidths) for the Mylar D-wrapped actinometers in quartz tubing were 326 nm (319 – 333 nm) and 353 nm (325 – 380 nm) for nitrate and nitrite, respectively, and these were 2 – 5 nm blue shifted compared to the peak response wavelengths and bandwidths observed in the Sargasso Sea. Excellent agreement was observed when comparing the integrated irradiance determined with the actinometers to that determined with a spectroradiometer. Likewise, diffuse attenuation coefficients for downwelling irradiance (Kd(λ)) calculated from water column actinometer measurements agreed well with Kd(λ) values calculated from irradiance measurements determined with a Biospherical PUV-511 profiling radiometer. Actinometers were used to measure light doses in experiments involving DMS and DMSP transformations during several field campaigns in the Ross Sea, Antarctica and the Sargasso Sea. Based on actinometer measurements, it was determined that DMS photolysis was dependent on UV irradiation between approximately 325 – 380 nm, while biological consumption rates of DMS and DMSP were inhibited by radiation at wavelengths less than approximately 333 nm. When DMS photolysis rate constants were expressed in terms of light dose rather than time, it was possible to 1) directly determine photolysis rate constants in the water column and 2) directly compare photolysis rate constants across diverse oceanographic regions.

Author Posting. © Oceanography Society, 2007. This article is posted here by permission of Oceanography Society for personal use, not for redistribution. The definitive version was published in Oceanography 20, 2 (2007): 117-123.

Spartina alterniflora is unique among salt marsh macrophytes in its synthesis of dimethylsulfoniopropionate (DMSP). One potential degradation product of DMSP is dimethylsulfoxide (DMSO). The concentrations of DMSP and DMSO were determined for S. alterniflora plants collected from Mobile Bay, Alabama. Although the distribution of DMSO within healthy S. alterniflora tissues mirrored somewhat that of DMSP, with the highest concentrations found in leaf tissue followed by stem and root tissue, the DMSO/DMSP ratio in root tissue was more than twice that in leaf or stem tissue. This is likely the result of a higher rate of DMS(P) oxidation in roots. It has been proposed that DMSP functions as an antioxidant in certain marine phytoplankton, and a similar function may be operational in S. alterniflora. Enhanced oxidative stress in the S. alterniflora root zone may help explain the high DMSO/DMSP ratio. In support of this idea was the finding that the DMSO/DMSP ratio was higher in tissues presumably experiencing increased oxidative stress such as yellowing S. alterniflora leaves and senescing segments of individual leaves compared to healthy tissue. The occurrence of DMSO in tissues of S. alterniflora is consistent with DMS(P) functioning as an antioxidant in this plant. Higher DMSO/DMSP ratios in stressed leaves further supports that role. However, specific roles for DMSP, DMS, or DMSO in oxidative stress protection in S. alterniflora remain to be demonstrated.

Biological consumption is a major sink for dimethylsulfide (DMS) in the surface ocean, but the fate of DMS is poorly known. We determined the fate of sulfur from biologically consumed DMS in samples from the upper 60 m of the Sargasso Sea during July 2004. Using tracer levels of 35S-DMS in dark incubations we found that DMS was transformed into three identifiable non-volatile, sulfur-containing product pools: dimethylsulfoxide (DMSO), sulfate, and particle-associated macromolecules. Together, DMSO and sulfate accounted for most (81–93%) of the non-volatile sulfur products. Only a small fraction (∼ 2%) of the consumed DMS-sulfur was recovered in cellular macromolecules, leaving 5–17% of the metabolic products of DMS consumption unidentified. The relative importance of the two major products varied with depth. DMSO was the main sulfur product (∼ 72%) from DMS metabolism in the surface mixed layer, whereas sulfate was the most important product (∼ 74%) below the mixed layer. Changes in temperature and photosynthetically-active radiation (PAR) did not cause shifts in DMS fate in short term incubations (7–12 h), however these or other factors (e.g., exposure to ultraviolet radiation), operating over longer time scales, could potentially influence the observed pattern of DMS fate with depth. Biological DMSO production rates ranged from 0.07 to 0.33 nM day− 1, with the highest rate found at 30 m, just below the surface mixed layer. With DMSO concentrations ranging from 4.0 to 8.6 nM, turnover times for DMSO were long (15–61 days) when only the biological production from DMS was considered. Identification of the main sulfur containing products from DMS metabolism improves understanding of this important process in the marine sulfur cycling. Detection and quantification of DMSO production from biological DMS consumption also provides a more complete picture of DMSO biogeochemistry in the ocean.

12 pages, 5 figures, 2 tables, supplementary material http://onlinelibrary.wiley.com/doi/10.1111/j.1462-2920.2006.01102.x/suppinfo, Microbial consumption is one of the main processes, along with photolysis and ventilation, that remove the biogenic trace gas dimethylsulfide (DMS) from the surface ocean. Although a few isolates of marine bacteria have been studied for their ability to utilize DMS, little is known about the characteristics or phylogenetic affiliation of DMS consumers in seawater. We enriched coastal and open-ocean waters with different carbon sources to stimulate different bacterial communities (glucose-consuming bacteria, methyl group-consuming bacteria and DMS consumers) in order to test how this affected DMS consumption and to examine which organisms might be involved. Dimethylsulfide consumption was greatly stimulated in the DMS addition treatments whereas there was no stimulation in the other treatments. Analysis of microbial DNA by two different techniques (sequenced bands from DGGE gels and clone libraries) showed that bacteria grown specifically with the presence of DMS were closely related to the genus Methylophaga. We also followed the fate of consumed DMS in some of the enrichments. Dimethylsulfide was converted mostly to DMSO in glucose or methanol enrichments, whereas it was converted mostly to sulfate in DMS enrichments, the latter suggesting use of DMS as a carbon and energy source. Our results indicate that unlike the biochemical precursor of DMS, dimethylsulfoniopropionate (DMSP), which is consumed by a broad spectrum of marine microorganisms, DMS seems to be utilized as a carbon and electron source by specialists. This is consistent with the usual observation that DMSP turns over at much higher rates than DMS, This work was supported by a PhD fellowship from the Spanish Ministry of Education to M.V. Field work was supported by NSF through the Biocomplexity in the Environment program (OPP 0221748; P. Matrai, P.I.) and by the Spanish MEC through project CAOS (CTM2004-20022-E). The molecular work was funded by projects MICRODIFF (REN2001-2120/MAR), BASICS (EVK3-CT-2002-00078) and GENµMAR (CTM2004-02586/MAR)