John W. H. Dacey

Oceanic dimethylsulfide:Production during zooplankton grazing on phytoplankton

About half the biogenic sulfur flux to the earths atmosphere each year arises from the oceans. Dimethylsulfide (DMS), which constitutes about 90% of this marine sulfur flux, is presumed to originate from the decomposition of dimethylsulfoniopropionate produced by marine organisms, particularly phytoplankton. The rate of DMS release by phytoplankton is greatly increased when the phytoplankton are subjected to grazing by zooplankton. DMS production associated with such grazing may be the major mechanism of DMS production in many marine settings.

SAR11 marine bacteria require exogenous reduced sulphur for growth

Sulphur is a universally required cell nutrient found in two amino acids and other small organic molecules. All aerobic marine bacteria are known to use assimilatory sulphate reduction to supply sulphur for biosynthesis, although many can assimilate sulphur from organic compounds that contain reduced sulphur atoms. An analysis of three complete ‘Candidatus Pelagibacter ubique’ genomes, and public ocean metagenomic data sets, suggested that members of the ubiquitous and abundant SAR11 alphaproteobacterial clade are deficient in assimilatory sulphate reduction genes. Here we show that SAR11 requires exogenous sources of reduced sulphur, such as methionine or 3-dimethylsulphoniopropionate (DMSP) for growth. Titrations of the algal osmolyte DMSP in seawater medium containing all other macronutrients in excess showed that 1.5 × 108 SAR11 cells are produced per nanomole of DMSP. Although it has been shown that other marine alphaproteobacteria use sulphur from DMSP in preference to sulphate, our results indicate that ‘Cand. P. ubique’ relies exclusively on reduced sulphur compounds that originate from other plankton.

Environmental turbulent mixing controls on air-water gas exchange in marine and aquatic systems

[1] Air-water gas transfer influences CO 2 and other climatically important trace gas fluxes on regional and global scales, yet the magnitude of the transfer is not well known. Widely used models of gas exchange rates are based on empirical relationships linked to wind speed, even though physical processes other than wind are known to play important roles. Here the first field investigations are described supporting a new mechanistic model based on surface water turbulence that predicts gas exchange for a range of aquatic and marine processes. Findings indicate that the gas transfer rate varies linearly with the turbulent dissipation rate to the 1/4 power in a range of systems with different types of forcing - in the coastal ocean, in a macro-tidal river estuary, in a large tidal freshwater river, and in a model (i.e., artificial) ocean. These results have important implications for understanding carbon cycling.

Carbon dioxide flux techniques performed during GasEx-98

A comprehensive study of air–sea interactions focused on improving the quantification of CO2 fluxes and gas transfer velocities was performed within a large open ocean CO2 sink region in the North Atlantic. This study, GasEx-98, included shipboard measurements of direct covariance CO2 fluxes, atmospheric CO2 profiles, atmospheric DMS profiles, water column mass balances of CO2, and measurements of deliberate SF6–3He tracers, along with air–sea momentum, heat, and water vapor fluxes. The large air–sea differences in partial pressure of CO2 caused by a springtime algal bloom provided high signals for accurate CO2 flux measurements. Measurements were performed over a wind speed range of 1–16 m s−1 during the three-week process study. This first comparison between the novel air-side and more conventional water column measurements of air–sea gas transfer show a general agreement between independent air–sea gas flux techniques. These new advances in open ocean air–sea gas flux measurements demonstrate the progress in the ability to quantify air–sea CO2 fluxes on short time scales. This capability will help improve the understanding of processes controlling the air–sea fluxes, which in turn will improve our ability to make regional and global CO2 flux estimates.

Annual Carbon Mineralization and Belowground Production of Spartina Alterniflora in a New England Salt Marsh

The annual rates and depth distribution of organic carbon mineralization to CO₂ were determined in sediments supporting stands of short Spartina alterniflora. Carbon dioxide production was estimated by two independent techniques. We constructed a CO₂ budget based on measurements of CO₂ emission from the sediment to the atmosphere, export of dissolved inorganic carbon in porewater exchange, and changes in the porewater pool of dissolved inorganic carbon throughout the year. We also measured CO₂ production in salt marsh sediments by monitoring changes in total inorganic carbon in cores. The estimates obtained by the two methods were similar, giving a total annual CO₂ production of between 67 and 70 mol°m— ²°yr— ¹. We also measured the losses of organic matter as methane (O.1—0.3 mol°m— ²°yr— ¹) and dissolved organic carbon (0—3 mol°m— ²°yr— ¹) and burial (7.4 mol°m— ²°yr— ¹) in order to construct a carbon budget for the sediments. These data, when combined with the estimates of carbon mineralization, gave an estimate for the organic carbon loading to the sediments of 68—78 mol°m— ²°yr— ¹. About 95% of the annual carbon input either decomposes to CO₂ in situ or is buried, and <5% is exported from the sediment. We estimated that belowground C production in short S. alterniflora in this Massachusetts marsh is 58—75 mol°m— ²°yr— ¹.

Characterization of a DMSP-degrading bacterial isolate from the Sargasso Sea

A bacterium which cleaves dimethylsulfoniopropionate (DMSP) to form dimethylsulfide (DMS) was isolated from surface Sargasso Sea water by a DMSP enrichment technique. The isolate, here designated LFR, is a Gram-negative, obligately aerobic, rod-shaped, carotenoid-containing bacterium with a DNA G+C content of 70%. Sequencing and comparison of its 16S ribosomal ribonucleic acid (rRNA) with that of known eubacteria revealed highest similarity (91% unrestricted sequence similarity) to Roseobacter denitrificans (formerly Erythrobacter species strain OCh114), an aerobic, bacteriochlorophyll-containing marine representative of the α-Proteobacteria. However, physiological differences between the two bacteria, and the current lack of other characterized close relatives, preclude assignment of strain LFR to the Roseobacter genus. Screening of fifteen characterized marine bacteria revealed only one, Pseudomonas doudoroffii, capable of degrading DMSP to DMS. Strain LFR is deposited with the American Type Culture Collection (ATCC 51258) and 16S rRNA sequence data are available under GenBank accession number 15345.

Biogeochemistry of dimethylsulfide in a seasonally stratified coastal salt pond

Abstract Dimethylsulfide (DMS) is the major volatile reduced organic sulfur compound in the water column of coastal Salt Pond, Cape Cod, MA. DMS concentration and vertical distributions vary seasonally in response to changing biogeochemical processes in the pond. When the pond is thermally stratified in summer, maximum DMS concentrations of up to 60 nmol/1 were found in the oxygen-deficient metalimnion. DMS concentrations in the epilimnion (typically 5–10 nmol/1) were always an order of magnitude higher than in the hypolimnion (

Short-term endproducts of sulfate reduction in a salt marsh: Formation of acid volatile sulfides, elemental sulfur, and pyrite☆

Rates of sulfate reduction, oxygen uptake and carbon dioxide production in sediments from a short Spartina alterniflora zone of Great Sippewissett Marsh were measured simultaneously during late summer. Surface sediments (0–2 cm) were dominated by aerobic metabolism which accounted for about 45% of the total carbon dioxide production over 0–15 cm. Rates of sulfate reduction agreed well with rates of total carbon dioxide production below 2 cm depth indicating that sulfate reduction was the primary pathway for sub-surface carbon metabolism. Sulfate reduction rates were determined using a radiotracer technique coupled with a chromous chloride digestion and carbon disulfide extraction of the sediment to determine the extent of formation of radiolabelled elemental sulfur and pyrite during shortterm (48 hr) incubations. In the surface 10 cm of the marsh sediments investigated, about 50% of the reduced radiosulfur was recovered as dissolved or acid volatile sulfides, 37% as carbon disulfide extractable sulfur, and only about 13% was recovered in a fraction operationally defined as pyrite. Correlations between the extent of sulfate depletion in the marsh sediments and the concentrations of dissolved and acid volatile sulfides supported the results of the radiotracer work. Our data suggest that sulfides and elemental sulfur may be major short-term end-products of sulfate reduction in salt marshes.

Water‐air flux of dimethylsulfide

The water-air exchange of dimethylsulfide (DMS) has been measured in a laboratory wind-wave tank in fresh and seawater. To understand the transport behavior of DMS, its exchange was measured simultaneously with that of O2, SF6, Ne, CH4, and He under varying wind speeds and hydrodynamic conditions. No unpredictable differences between fresh and seawater were found, indicating that DMS surface affinities do not exist in seawater. Results also indicate that Schmidt number corrections can be applied to DMS gas exchange. While the rate of transfer of relatively insoluble or sparingly soluble permanent gases between the ocean and the atmosphere is primarily controlled by the rate of flux through the aqueous boundary layer, interfacial mass balances indicate that increased DMS solubility increases the significance of the airside control of flux. The hypothesis that DMS transport across the water-air interface is subject to waterside control for moderate environmental conditions is supported. However, for ocean-atmospheric DMS exchange with low sea surface temperature or moderate wind speeds, there may be a significant influence by the atmospheric mass boundary layer. An atmospheric gradient fraction γa applied to the waterside air-sea gas transfer velocity will correct for these effects. Estimates of ocean-atmospheric DMS transfer velocities for these conditions are provided.

Relaxed eddy accumulation measurements of the sea-to-air transfer of dimethylsulfide over the northeastern Pacific

[1] Gas transfer rates were determined from relaxed eddy accumulation ( REA) measurements of the flux of dimethylsulfide (DMS) over the northeastern Pacific Ocean. This first application of the REA technique for the measurement of DMS fluxes over the open ocean produced estimates of the gas transfer rate that are on average higher than those calculated from commonly used parameterizations. The relationship between the total gas transfer rate and wind speed was found to be gas k(gas) = 0.53 (+/-0.05) U-10(2). Because of the effect of the airside resistance, the waterside transfer rate was up to 16% higher than kgas. Removal of the airside transfer component from the total transfer rate resulted in a relation between wind speed and waterside transfer of k(660) = 0.61 (+/-0.06) U-10(2). However, DMS fluxes showed a high degree of scatter that could not readily be accounted for by wind speed and atmospheric stability. It has to be concluded that these measurements do not permit an accurate parameterization of gas transfer as a function of wind speed.