Cross-shelf processes of terrigenous organic matter drive mercury speciation on the east siberian shelf in the Arctic Ocean

The cross-shelf distributions of total mercury (THg), methylmercury (MeHg) and organic and inorganic matter, as well as the presence of the hgcA gene were investigated on the East Siberian Shelf (ESS) to understand the processes underlying the speciation of sedimentary Hg. Samples were collected from 12 stations grouped into four zones based on water depth: inner shelf (5 stations), mid-shelf (3 stations), outer shelf (2 stations), and slope (2 stations). The THg concentration in the surface sediment increased from the inner shelf (0.25 ± 0.023 nmol g-1) toward the slope (0.52 nmol g-1), and, when normalized to total organic carbon content, the THg showed a positive correlation with the clay-to-sand ratio (r2 = 0.48, p = 0.012) and degree of chemical weathering (r2 = 0.79, p = 0.0001). The highest MeHg concentrations (3.0 ± 1.8 pmol g-1), as well as peaks in the S/C ratio (0.012 ± 0.002) of sediment-leached organic matter, were found on the mid-shelf, suggesting that the activities of sulfate reducers control the net Hg(II) methylation rates in the sediment. This was supported by results from a principal component analysis (PCA) performed with Hg species concentrations and sediment-leached organic matter compositions. The site-specific variation in MeHg showed the highest similarity with that of CHONS compounds in the PCA, where Deltaproteobacteria were projected to be putative Hg(II) methylators in the gene analysis. In summary, the hydrodynamic sorting of lithogenic particles appears to govern the cross-shelf distribution of THg, and in situ methylation is considered a major source of MeHg in the ESS sediment.

Mercury and other trace elements distribution and profiling of microbial community in the surface sediments of East Siberian Sea

In this study, total mercury (THg), methylmercury (MeHg), various trace elements, and microbial communities were measured in surface sediments of the East Siberian Sea (ESS). The results showed that the average values of THg and MeHg were 58.8 ± 15.21 μg/kg and 0.50 ± 0.22 μg/kg, respectively. The notable levels of trace elements present in both surface sediment and porewater were Al, Fe, and Mn. The enrichment factor and geoaccumulation index analyses found that both natural phenomena and anthropogenic activities contributed to elevated concentrations of metals in the ESS. The redox proxy metals, pH, and SO₄^2- were the major factors influencing the THg and MeHg distributions. Microbial profiles were substantially affected by metals and other abiotic factors. Proteobacteria and Thaumarchaeota were the most abundant phyla. Overall, the findings presented here facilitate the understanding of the current status of metal contamination, its influencing factors, and metal-microbiota-interactions in ESS.

The impact of the abnormal salinity enrichment in pore water on the thermodynamic stability of marine natural gas hydrates in the Arctic region

In this study, the thermodynamic and structural characteristics of natural gas hydrates (NGHs) retrieved from gas hydrate mounds (ARAON Mound 03 (AM03) and ARAON Mound 06 (AM06)) in the Chukchi Sea in the Arctic region were investigated. The gas compositions, crystalline structure, and cage occupancy of the NGHs at AM03 and AM06 were experimentally measured using gas chromatography (GC), ¹³C nuclear magnetic resonance (NMR), Raman spectroscopy, and powder X-ray diffraction (PXRD). In the NGHs from AM03 and AM06, a significantly large fraction of CH₄ (> 99%) and a very small amount of H₂S were enclathrated in small (5¹²) and large (5¹²6²) cages of sI hydrate. The NGHs from AM03 and AM06 were almost identical in composition, guest distributions, and existing environment to each other. The salinity of the residual pore water in the hydrate-bearing sediment (AM06) was measured to be 50.32‰, which was much higher than that of seawater (34.88‰). This abnormal salinity enrichment in the pore water of the low-permeability sediment might induce the dissociation of NGHs at a lower temperature than expected. The saturation changes in the NGHs that corresponded with an increase in the seawater temperature were also predicted on the basis of the salinity changes in the pore water. The experimental and predicted results of this study would be helpful for understanding the thermodynamic stability of NGHs and potential CH₄-releasing phenomena in the Arctic region.

Sandaracinobacter neustonicus sp. nov., isolated from the sea surface microlayer in the Southwestern Pacific Ocean, and emended description of the genus Sandaracinobacter

A Gram-stain-negative, non-motile, facultatively anaerobic and rod-shaped bacterial strain, designated PAMC 28131^T, was isolated from a sea surface microlayer sample in the open water of the Pacific Ocean. Phylogenetic analysis of the 16S rRNA gene sequence of strain PAMC 28131^T revealed an affiliation to the genus Sandaracinobacter with the closest species Sandaracinobacter sibiricus RB16-17^T (sequence similarity of 98.2 %). Strain PAMC 28131^T was able to grow optimally with 0.5-1.0 % NaCl and at pH 6.5-7.0 and 30 °C. The polar lipids were phosphatidylglycerol, phosphatidylethanolamine, two unidentified phospholipids, an unidentified aminolipid, an unidentified glycolipid and an unidentified lipid. The major cellular fatty acids (>10 %) were C_18 : 1  ω6c and/or C_18 : 1  ω7c, (42.6 %), C_17 : 1  ω6c (19.3 %) and C_16 : 1  ω6c and/or C_16 : 1  ω7c (15.8 %), and the respiratory quinone was Q-10. The genomic DNA G+C content was 65.3 mol%. The phylogenetic, phenotypic and chemotaxonomic data showed that strain PAMC 28131^T could be clearly distinguished from S. sibiricus RB16-17^T. Thus, strain PAMC 28131^T should be classified as representing a novel species in the genus Sandaracinobacter, for which the name Sandaracinobacter neustonicus sp. nov. is proposed. The type strain is PAMC 28131^T (=KCCM 43127^T=JCM 30734^T).

Mass Budget of Methylmercury in the East Siberian Sea: The Importance of Sediment Sources

Biological concentrations of methylmercury (MeHg) are elevated throughout the Arctic Ocean; however, to date, the major sources and the spatial variability of MeHg are not well quantified. To identify the major inputs and outputs of MeHg to the Arctic shelf water column, we measured MeHg concentrations in the seawater and sediment samples from the East Siberian Sea collected from August to September 2018. We found that the MeHg concentrations in seawater and pore water were higher on the slope than on the shelf, while the MeHg concentrations in the sediment were higher on the shelf than on the slope. We created a mass budget for MeHg and found that the benthic diffusion and resuspension largely exceed other sources, such as atmospheric deposition and river water input. The major sinks of MeHg in the water column were dark demethylation and evasion. When we extrapolated our findings on benthic diffusion to the entire Arctic shelf system, the annual MeHg diffusion from the shelf sediments was estimated to be 23,065 ± 939 mol yr^-1, about 2 times higher than previously proposed river discharges. Our study suggests that the MeHg input from shelf sediments in the Arctic Ocean is significant and has been previously underestimated.

Chemical characteristics of submicron aerosols observed at the King Sejong Station in the northern Antarctic Peninsula from fall to spring

The water-soluble ions and carbonaceous compounds of PM₁ were measured at the King Sejong Station (KSG) in the northern part of Antarctic Peninsula from March to November in 2009. As the sum of all measured species including organic matter [OM; organic carbon (OC)*1.9], the PM₁ mass reached a maximum of 936 ng m^-3 with the mean of 686 ± 226 ng m^-3. The most abundant constituents were OM (389 ± 109 ng m^-3) and sea-salts (Na⁺ and Cl^-, 193 ± 122 ng m^-3), which comprised 85% of the PM₁ mass. In contrast, the contribution of SO₄^2- was below 1% and its depletion relative to Na⁺ was prevalent particularly during winter, which was attributed to the frost flowers on newly formed sea-ice surface. The OC concentration was the highest in fall and its subcomponents OC2 and OC3 were moderately correlated with sea-salts (r = 0.5), indicating the marine biogenic source for OC. The elemental carbon (EC) concentration was much lower than OC, leading to the mean OC/EC ratio over 10. While the charred fraction of EC (EC1) was elevated by the long-range transport of biomass burning plume from nearby continent, the mass fraction of soot-EC (EC23) was increased concurrently with enhanced NO₃^-, suggesting EC23 as a good indicator for local influence in pristine environments like Antarctic region.

Methylmercury Mass Budgets and Distribution Characteristics in the Western Pacific Ocean

Methylmercury (MeHg) accumulation in marine organisms poses serious ecosystem and human health risk, yet the sources of MeHg in the surface and subsurface ocean remain uncertain. Here, we report the first MeHg mass budgets for the Western Pacific Ocean estimated based on cruise observations. We found the major net source of MeHg in surface water to be vertical diffusion from the subsurface layer (1.8-12 nmol m^-2 yr^-1). A higher upward diffusion in the North Pacific (12 nmol m^-2 yr^-1) than in the Equatorial Pacific (1.8-5.7 nmol m^-2 yr^-1) caused elevated surface MeHg concentrations observed in the North Pacific. We furthermore found that the slope of the linear regression line for MeHg versus apparent oxygen utilization in the Equatorial Pacific was about 2-fold higher than that in the North Pacific. We suggest this could be explained by redistribution of surface water in the tropical convergence-divergence zone, supporting active organic carbon decomposition in the Equatorial Pacific Ocean. On the basis of this study, we predict oceanic regions with high organic carbon remineralization to have enhanced MeHg concentrations in both surface and subsurface waters.

Oceanic source strength of carbon monoxide on the basis of basin-wide observations in the Atlantic

We measured the carbon monoxide (CO) concentrations in the marine boundary layer and the surface waters of the Atlantic Ocean from 50°N to 50°S during the UK Atlantic Meridional Transect expedition (AMT-7) in October 1998, covering the open ocean and coastal regions. Throughout the cruise track, atmospheric CO concentrations continually decreased southwards in the northern hemisphere with sporadic low and high concentrations encountered. South of the intertropical convergence zone (ITCZ) atmospheric CO was enhanced by ∼10 ppb compared to north of the ITCZ due likely to biomass burning emissions prevailing in the tropical continents. The remainder of the southern hemisphere remains nearly invariable except for the vicinity of Rio de la Plata. The surface seawater was supersaturated everywhere along the track and its saturation anomaly oscillated up to 90, exhibiting a typical diurnal cycle. The maximal dissolved CO concentration in the diurnal cycle appeared 2-5 hours behind the local maximum of solar insolation in the open ocean and the time lag further increased in the coastal region. The global ocean flux of CO to the atmosphere was estimated to be 14 Tg(CO) a(-1) within the range of 4-24 Tg(CO) a(-1). This is within uncertainty almost identical to what was estimated on the basis of the basin-wide observations in the Pacific and the Atlantic, but more than ∼4 times lower than the values appeared in the Intergovernmental Panel on Climate Change (IPCC) reports.

Rhodococcus aerolatus sp. nov., isolated from subarctic rainwater

A Gram-stain-positive, rod-shaped and non-motile strain, designated PAMC 27367(T), was isolated from rainwater collected on the Bering Sea. Analysis of the 16S rRNA gene sequence of the strain showed an affiliation with the genus Rhodococcus. Phylogenetic analyses revealed that strain PAMC 27367(T) formed a robust clade with the type strains of Rhodococcus rhodnii, Rhodococcus aetherivorans and Rhodococcus ruber with 16S rRNA gene sequence similarities of 96.3 %, 95.8 % and 95.5 %, respectively. Cells of the strain grew optimally at 25 °C and at pH 6.5-7.0 in the presence of 0-2 % (w/v) sea salts. The major polar lipids were diphosphatidylglycerol, phosphatidylglycerol, phosphatidylethanolamine, phosphatidylinositol, phosphatidylinositol mannoside and three unknown phospholipids. The major cellular fatty acids (>10 %) were iso-C16 : 0, C17 : 1ω8c and 10-methyl C17 : 0. Cell wall analysis showed that strain PAMC 27367(T) contained meso-diaminopimelic acid. The genomic DNA G+C content was 77.1 mol%. Based on the phylogenetic, chemotaxonomic and phenotypic data presented here, we propose a novel species with the name Rhodococcus aerolatus sp. nov., with PAMC 27367(T) ( = KCTC 29240(T) = JCM 19485(T)) as the type strain.

Isotope analysis of hydrocarbons: trapping, recovering and archiving hydrocarbons and halocarbons separated from ambient air

It is argued that isotope analysis of atmospheric non-methane hydrocarbons (NMHCs) and, in particular, the analysis of the deuterium/hydrogen (D/H) ratio is valuable because the dominant self-cleansing property of the troposphere is based on the OH radical which removes, e.g., CH4 and other alkanes by H-atom abstraction, which induces large kinetic isotope effects. The major obstacle in applying D/H isotope analysis to atmospheric NMHCs is not only the low abundance of D itself but, in particular, the low concentrations of NMHCs in the parts per trillion range. We show how a selection of NMHCs can be quantitatively separated from 300 L air samples together with CO2 as carrier gas matrix, by using high efficiency cryogenic traps. After diluting the extracted NMHC mixtures with hydrocarbon free air, and determining the mixing ratios, good agreement with original whole air sample analysis exists for alkanes and several halocarbons. For unsaturated hydrocarbons and some other halocarbons the extraction and recovery yield under the given conditions fell considerably, as a function of boiling point. Furthermore, the mixture of NMHCs in the CO2 matrix is proven to remain unchanged over several years when conveniently stored in glass ampoules. The 'extracts' or 'concentrates' of condensables extracted from larger air samples will enable the D/H isotope analysis of ultra trace gases in the atmosphere.

Continuous-flow isotope analysis of the deuterium/hydrogen ratio in atmospheric hydrogen

A convenient method is described for analyzing the deuterium/hydrogen (D/H) ratio of atmospheric molecular hydrogen (H(2)) based on mass spectrometric isotope-ratio monitoring. The method requires small amounts of air ( approximately 300 mL STP), is operated on-line, and comprises four steps: (1). the condensation of the air matrix at approximately 40 K; (2). the collection of the non-condensed components of the air sample (H(2), Ne, He, and traces of N(2)) in a 5 A molecular sieves pre-concentration trap at approximately 63 K; (3). gas chromatographic purification of H(2) in a flow of He; and (4) quantification of the D/H ratio in an isotope-ratio mass spectrometer. The precision of the determination of the D/H ratio is better than 2 per thousand, which is comparable to, or better than, that obtained by conventional duel-inlet off-line analysis. There are, however, discrepancies relative to the D/H ratios determined by conventional duel-inlet analysis. This is due to differences in peak shape between reference and sample air, depending on the amount of H(2) injected. Consequently, calibration runs are required. After the calibration of the system, we obtained an accuracy of 1.5 per thousand, so that the accumulated uncertainty is estimated to be less than 4 per thousand. The method also allows determination of the H(2) concentration, with an uncertainty estimated to be 2%.

Inositol 1,3,4,5-tetrakisphosphate as a mediator of neuronal death in ischemic hippocampus

Selective death of CA1 pyramidal neurons after transient forebrain ischemia has attracted interest for its possible relation to the pathogenesis of memory deficits and dementia. Using whole cell patch-clamp recording from CA1 pyramidal neurons in hippocampal slices of gerbils after ischemia we studied the intracellular signaling mechanisms related to the phosphoinositide cycle. Intracellular application of an antibody against phosphatidylinositol 4,5-bisphosphate rescued ischemic neurons from stimulus-induced irreversible depolarization. Furthermore, application of inositol 1,3,4,5-tetrakisphosphate in normal cells caused an irreversible depolarization in response to synaptic input, which mimicked the deterioration of ischemic neurons. Depolarization of both ischemic and normal neurons in the presence of inositol 1,3,4,5-tetrakisphosphate was prevented by the addition of the Ca2+ chelator, 1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetra-acetate. Application of antibody against inositol 1,4,5-triphosphate 3-kinase, which blocks formation of inositol 1,3,4,5-tetrakisphosphate, also protected against cell deterioration. Our results suggest that the vulnerability of hippocampal pyramidal neurons following ischemia is caused by a disturbed phosphoinositide cascade, with one metabolite, inositol 1,3,4,5-tetrakisphosphate, playing a key role in the induction of Ca2+ accumulation, which leads to neuronal death.