Dissociation constants of protonated methionine species in seawater media

The dissociation constants (pK₁ and pK₂) for methionine have been measured in artificial seawater as a function of salinity (S = 5 to 35) and temperature (5 to 45 °C). The seawater pK₂ values were lower than the values in NaCl at the same ionic strength while the pK₁ values in seawater were lower only at S = 35. In an attempt to understand these differences, we have made measurements of the constants in Na–Mg–Cl solutions at 25 °C. The measured values have been used to determine the formation of Mg²⁺ complexes and Pitzer interaction parameters for Mg²⁺ with methionine. The seawater model with the interaction parameters accounts for the differences between the value of pK₁ and pK₂ between NaCl and seawater. This study demonstrates that it is important to consider all of the ionic interactions in natural waters when examining the dissociation of organic acids.     

Dissociation constants of protonated cysteine species in seawater media

The dissociation constants (pK₁, pK₂ and pK₃) for cysteine have been measured in seawater as a function of temperature (5 to 45 °C) and salinity (S = 5 to 35). The seawater values were lower than the values in NaCl at the same ionic strength. In an attempt to understand these differences, we have made measurements of the constants in Na–Mg–Cl solutions at 25 °C. The measured values have been compared to those calculated from the Pitzer ionic interaction model. The lower values of pK₃ in the Na–Mg–Cl solutions have been attributed to the formation of Mg²⁺ complexes with Cys²⁻ anions

Mg²⁺ + Cys²⁻ = MgCys

The stability constants have been fitted to

after corrections are made for the interaction of Mg²⁺ with H⁺.The pK₁ seawater measurements indicate that H₃Cys⁺ interacts with SO₄²⁻. The Pitzer parameters β⁰(H₃CysSO₄), β¹(H₃CysSO₄) and C(H₃CysSO₄) have been determined for this interaction. The formation of CaCys as well as MgCys are needed to account for the values of pK₂ and pK₃ in seawater.The consideration of the formation of MgCys and CaCys in seawater yields model calculated values of pK₁, pK₂ and pK₃ that agree with the measured values to within the experimental error of the measurements. This study shows that it is important to consider all of the ionic interactions in natural waters when examining the dissociation of organic acids.     

Parameterization of iron and manganese cycling in the Black Sea suboxic and anoxic environment

New and published data on the distribution and speciation of manganese and iron in seawater are analyzed to identify and parameterize major biogeochemical processes of their cycling within the suboxic (15.6σ_t16.2) and anoxic layers (σ_t16.2) of the Black Sea. A steady-state transport-reaction model is applied to reveal layering and parameterize kinetics of redox and dissolution/precipitation processes. Previously published data on speciation of these elements in seawater are used to specify the nature of the transformations. Two particulate species of iron (Fe(III) hydroxide and Fe(II) sulfide) are necessary to adequately parameterize the vertical profile of suspended iron, while three particulate species (hydrous Mn(IV) oxide, Mn(II) sulfide, and Mn(II) carbonate) are necessary to describe the profile of suspended manganese. In addition to such processes as mixing and advection, precipitation, sinking, and dissolution of manganese carbonate are found to be essential in maintaining the observed vertical distribution of dissolved Mn(II). These results are used to interpret the observed difference in the form of vertical distribution for dissolved Mn(II) and Fe(II). Redox transformations of iron and manganese are coupled via oxidation of dissolved iron by sinking suspended manganese at σ_t16.2±0.2 kg m⁻³. The particulate manganese, necessary for this reaction, is supplied through oxidation of dissolved Mn(II). The best agreement with observations is achieved when nitrate, rather than oxygen, is set to oxidize dissolved Mn(II) in the lower part of the suboxic layer (15.90σ_t16.2). The results support the idea that, after sulfides of these metals are formed, they sink with particulate organic matter. The sinking rates of the particles and specific rates of individual redox and dissolved-particulate transformations have been estimated by fitting the vertical profile of the net rate.     

The shipboard analysis of trace levels of sulfur hexafluoride, chlorofluorocarbon-11 and chlorofluorocarbon-12 in seawater

Methods are described for the rapid (11 min) automated shipboard analysis of dissolved sulfur hexafluoride (SF₆) in small volume (200 cm³) seawater samples. Estimated precision for the SF₆ measurements is 2% or 0.02 fmol kg⁻¹ (whichever is greater). The method also allows for the simultaneous measurement of chlorofluorocarbon-11 (CFC11) and chlorofluorocarbon-12 (CFC12) on the same water sample, with significantly improved sensitivity over previous analytical methods.     

Particulate organic carbon–Th relationships in particles separated by settling velocity in the northwest Mediterranean Sea

In order to better understand the relationship between the natural radionuclide ²³⁴Th and particulate organic carbon (POC), marine particles were collected in the northwestern Mediterranean Sea (spring/summer, 2003 and 2005) by sediment traps that separated them according to their in situ settling velocities. Particles also were collected in time-series sediment traps. Particles settling at rates of >100 m d⁻¹ carried 50% and 60% of the POC and ²³⁴Th fluxes, respectively, in both sampling years. The POC flux decreased with depth for all particle settling velocity intervals, with the greatest decrease (factor of 2.3) in the slowly settling intervals (0.68–49 m d⁻¹) over trap depths of 524–1918 m, likely due to dissolution and decomposition of material. In contrast the flux of ²³⁴Th associated with the slowly settling particles remained constant with depth, while ²³⁴Th fluxes on the rapidly settling particles increased. Taking into account decay of ²³⁴Th on the settling particles, the patterns of ²³⁴Th flux with depth suggest that either both slow and fast settling particles scavenge additional ²³⁴Th during their descent or there is significant exchange between the particle classes. The observed changes in POC and ²³⁴Th flux produce a general decrease in POC/²³⁴Th of the settling particles with depth. There is no consistent trend in POC/²³⁴Th with settling velocity, such as might be expected from surface area and volume considerations. Good correlations are observed between ²³⁴Th and POC, lithogenic material and CaCO₃ for all settling velocity intervals. Pseudo-K_ds calculated for ²³⁴Th in the shallow traps (2005) are ranked as lithogenic material opal <calcium carbonate <organic carbon. Organic carbon contributes 33% to the bulk K_d, and for lithogenic material, opal and CaCO₃, the fraction is 22% each. Decreases in POC/²³⁴Th with depth are accompanied by increases in the ratio of ²³⁴Th to lithogenic material and opal. No change in the relationship between ²³⁴Th and CaCO₃ was evident with depth. These patterns are consistent with loss of POC through decomposition, opal through dissolution and additional scavenging of ²³⁴Th onto lithogenic material as the particles sink.     

Uncertainties in the preparation of Ra Mn fiber standards

Delayed coincidence counters (RaDeCC), used for measuring ²²³Ra and ²²⁴Ra preconcentrated from water onto MnO₂-impregnated acrylic fiber (“Mn-fiber”), require a standard Mn-fiber column that has a precisely known activity of ²²⁴Ra for calibration. This may be done by adding an aged ²²⁸Th standard solution to adsorb both ²²⁸Th and its daughter ²²⁴Ra quantitatively onto a Mn fiber. We used both seawater and deionized water (DIW) for testing the adsorption efficiency of Th and Ra onto Mn fibers. Our experimental results show that more than 50% of thorium (²³²Th and ²²⁸Th) breaks through the Mn-fiber column when DIW is used as a medium. However, near quantitative recoveries are obtained if filtered (0.45 μm) seawater is used to prepare the standard. In the case of pure DIW, the pH (initial pH  5.3) rises to > 10 after passing through the column while seawater (initial pH  7.8) changes to  7.2. Thus, the lack of thorium adsorption in DIW may be attributed to this huge increase of pH and the consequent formation of Th(OH)₄ and polyhydroxyl colloids. Based on these observations, we recommend that one should use either artificial seawater or natural seawater (which has negligible ²²⁴Ra and ²²⁸Th) as a loading solution after 0.45 μm filtration. In addition, the thorium adsorption efficiency should be confirmed either by thorium analysis of the effluent solution or long-term monitoring of the supported ²²⁴Ra on the Mn fiber using the RaDeCC. Similar cautions are likely necessary for making ²²³Ra standards by adsorption of ²²⁷Ac onto Mn fibers.     

Diversity patterns in macrobenthos across a continental slope in the NE Atlantic

Different estimates were used to assess the diversity of the total macrofauna and its major taxonomic groups separately from a broad bathymetric range at a site in the NE Atlantic. In the Goban Spur region, a transect was sampled from the shelf to the abyssal plain over a depth range from 200 to 4500 m and in the Porcupine Sea Bight two stations were sampled (at 3670 m and 4115 m). Species diversity (the number of species per number of individuals) increased with increasing water depth, both when expressed as Hurlbert's E(S_n) and as Shannon's H′log e. The expected number of species in a 100-individual sample E(S₁₀₀) of total macrofauna increased from 30 on the shelf to 68 on the abyssal plain. Evenness (the proportional abundance of species), estimated with Shannon's J′, also increased with water depth from 0.66 to 0.91, whereas dominance (Simpson's D) decreased from 0.09 to 0.01. Species richness (the number of species per unit of area), however, showed a parabolic pattern with a peak at the upper slope. The largest number of species was found at the slope station at 1425 m (232 species within 0.66 m²). It is argued that species richness is not a synonym of species diversity, but that species richness depends both on species density (which decreases with increasing water depth) and on species diversity. Across the whole bathymetric range (200 to 4500 m) a total of 696 species within 8327 specimens in a total sampled area of 4.12 m² were counted, yielding mean values of 12 individuals per species and 169 species per m². Different communities were found to exist on the shelf, slope and abyss. It is suggested that this could have been caused by different selection processes. Differences in life-history strategies and organic-matter supply could (at least partly) explain the different community structures and diversity patterns found along the depth gradient.     

Determination of picomolar iron in seawater by double Mg(OH)2 precipitation isotope dilution high-resolution ICPMS

A low-blank pre-concentration procedure is described for the analysis of picomolar iron (Fe) in seawater by isotope dilution high-resolution inductively coupled plasma mass-spectrometry (HR-ICPMS). The procedure uses a two-step Mg(OH)₂ co-precipitation procedure to extract Fe from a 50 ml seawater sample into a 100 μl 4% nitric acid (HNO₃) solution followed by HR-ICPMS measurement. The high pre-concentration ratio ( 500:1) achieved by the procedure minimizes the Fe blank due to ICPMS instrumental Fe background and results in a detection limit of  2 pM and a precision of  4% at the 50 pM Fe level. The measurement of a low-Fe seawater sample spiked with gravimetric Fe standard shows that the method can clearly distinguish 0.01 nM Fe from 0.02 nM Fe in seawater with high accuracy. The method is demonstrated by the analysis of dissolved Fe in the equatorial Pacific Ocean.     

Geometry of scour hole around, and the influence of the angle of attack on the burial of finite cylinders under combined flows

Experiments were conducted to investigate the geometry of the scour hole and flow structure around short cylinders under the action of waves alone (WA) and combined flows (CF). The study is aimed at better understanding the dynamics of isolated objects on a sandy floor under oscillatory flows as occurs in shallow water regions in coastal areas. Flow velocities within the fluid core were recorded and 3D mapping of the bottom was performed with sub-aquatic acoustic sensors. Experiments were conducted for cylinder Reynolds wave number and Keulegan-Carpenter number within the ranges 10⁴R_e1.7×10⁵ and 2KC71, respectively. The present experimental evidence shows that the geometric characteristics of the scour hole (length and width) depend primarily on the Keulegan-Carpenter number (KC) and the cylinder aspect ratio (a_r=L_c/D). The effect of variation in the angle of attack of the flow with respect to the cylinder main axis was also investigated. Initial orientations of zero and ninety degrees were found to be stable while cylinders with intermediate initial orientations tended to orientate their main axes perpendicular to the flow direction. The final angle of orientation was found to be primarily a function of the Shields parameter, θ, and the initial angle of attack, α_i.     

Effects of metal pollution and macronutrient enrichment on the photoproduction of hydroxyl radicals in seawater by the alga Dunaliella salina

The marine microalga Dunaliella salina was used as a model organism in this study. Hydroxyl radicals (OH) were determined using HPLC with sodium benzoate as a probe, and the photochemical activity of marine alga in the formation of OH was confirmed for the first time. Coastal organisms are often exposed to both metal pollution and macronutrient enrichment, and so the effects of algal concentration, exposure time, macronutrient (nitrate and phosphate) additions and metal pollution (5.0 μg/L Pb(II) and 0.1 μg/L methylmercury) on the photoproduction of OH were examined. Photoproduction was increased with increasing algal concentration and with exposure time. It could be increased greatly, with or without the presence of D. salina, by the addition of Pb(II), or Pb(II) and methylmercury, but was decreased by the addition of methylmercury only. Photoproduction of OH was positively correlated with the amount of basic functional groups on the cell's surface and also with the chlorophyll a content per cell. The influence of macronutrient additions on the photoproduction of OH resulted from the photolysis of nitrate and their effects on the photochemical activity of D. salina.     

Determination of gas bubble fractionation rates in the deep ocean by laser Raman spectroscopy

A new deep-sea laser Raman spectrometer (DORISS—Deep Ocean Raman In Situ Spectrometer) is used to observe the preferential dissolution of CO₂ into seawater from a 50%–50% CO₂–N₂ gas mixture in a set of experiments that test a proposed method of CO₂ sequestration in the deep ocean. In a first set of experiments performed at 300 m depth, an open-bottomed 1000 cm³ cube was used to contain the gas mixture; and in a second set of experiments a 2.5 cm³ funnel was used to hold a bubble of the gas mixture in front of the sampling optic. By observing the changing ratios of the CO₂ and N₂ Raman bands we were able to determine the gas flux and the mass transfer coefficient at 300 m depth and compare them to theoretical calculations for air–sea gas exchange. Although each experiment had a different configuration, comparable results were obtained. As expected, the ratio of CO₂ to N₂ drops off at an exponential rate as CO₂ is preferentially dissolved in seawater. In fitting the data with theoretical gas flux calculations, the boundary layer thickness was determined to be  42 μm for the gas cube, and  165 μm for the gas funnel reflecting different boundary layer turbulence. The mass transfer coefficients for CO₂ are k_L = 2.82 × 10^− 5 m/s for the gas cube experiment, and k_L = 7.98 × 10^− 6 m/s for the gas funnel experiment.     

Physical dynamics and biological implications of Cyclone Noah in the lee of Hawaii during E-Flux I

Quasi-synoptic observations of the horizontal and vertical structure of a cold-core cyclonic mesoscale eddy feature (Cyclone Noah) were conducted in the lee of Hawai’i from November 4–22, 2004 as part of the E-Flux interdisciplinary collaborative research program. Cyclone Noah appears to have spun up to the southwest of the ‘Alenuihaha Channel (between Maui and Hawai’i) as a result of strong and persistent northeasterly trade winds through the channel. Shipboard hydrographic surveys 2.5 months later suggest that Noah weakened and was in a hypothesized spin-down phase of its life cycle. Although the initial surface expression of Noah was limited in scale to 40 km in diameter and, as evidenced by surface temperatures, 2–3 °C cooler than the surrounding waters, depth profiles revealed a fully developed semi-elliptical shallow feature (200 m), 144 km long and 90 km wide (based on sigma-t=23 kg m⁻³) with tangential speeds of 40–80 cm s⁻¹, and substantial isopycnal doming. Potential vorticity distribution of Noah suggests that radial horizontal flow of the core water was inhibited from the surface to depths of 75 m, with high vorticity confined above the sigma-t=23.5 kg m⁻³ isopycnal surface. Upward displacements of isopycnal surfaces in the eddy's center (50 m) were congruent with enhanced pigment concentrations (0.50 mg m⁻³). Comparisons of the results obtained for E-Flux I (Noah) and E-Flux III (Opal) suggest that translation characteristics of cyclonic Hawaiian lee eddies may be important in establishing the biogeochemical and biological responses of the oligotrophic ocean to cyclonic eddies.     

A dynamic equation for the potential energy anomaly for analysing mixing and stratification in estuaries and coastal seas

In this paper, a time-dependent dynamic equation for the potential energy anomaly, , is rigorously derived from dynamic equations for potential temperature and salinity, the continuity equation and the equation of state for sea water. The terms locally changing are (A) the -advection, (B) the depth-mean straining, (C) the non-mean straining, (D) the vertical advection, (E) the vertical mixing, (F) surface and bottom density fluxes, (G) inner sources of density e.g. due to absorption of solar radiation and the non-linearity of the equation of state, and (H) horizontal divergence of horizontal turbulent density fluxes. In order to derive the equation in concise form, a vertical velocity (linearly varying with depth) with respect to depth-proportional vertical coordinates had to be defined. The evaluation of the terms in the -equation is then carried out for a one-dimensional tidal straining study and a two-dimensional estuarine circulation study. Comparisons to empirical estimates for these terms are made for the one-dimensional study. It is concluded that the -equation provides a general reference for empirical bulk parameterisations of stratification and mixing processes in estuaries and coastal seas and that it is a tool for complete analysis of the relevant terms from numerical models.     

Benthic remineralization at the land–ocean interface: A case study of the Rhône River (NW Mediterranean Sea)

Biogeochemical processes in sediments under the influence of the Rhône River plume were studied using both in situ microelectrodes and ex situ sediment core incubations. Organic carbon (OC) and total nitrogen (TN) content as well as stable carbon isotopic composition of OC (δ¹³C_OC) were analysed in 19 surface sediments to determine the distribution and sources of organic matter in the Rhône delta system. Large spatial variations were observed in both the total O₂ uptake (5.2 to 29.3 mmol m⁻² d⁻¹) and NH₄⁺ release (−0.1 to −3.5 mmol m⁻² d⁻¹) rates at the sediment–water interface. The highest fluxes were measured near the Rhône River mouth where sedimentary OC and TN contents reached 1.81% and 0.23% respectively. Values of δ¹³C_OC ranged from −26.83‰ to −23.88‰ with a significant seawards enrichment tracing the dispersal of terrestrial organic matter on the continental shelf. The amount of terrestrial-derived OC reaches 85% in sediments close to the Rhône mouth decreasing down to 25% in continental shelf sediments. On the prodelta, high terrestrial OC accumulation rates support high oxygen uptake rates and thus indicating that a significant fraction of terrestrial OC is remineralized. A particulate organic carbon (POC) mass balance indicates that only 3% of the deposited POC is remineralized in prodelta sediments while 96% is recycled on the continental shelf. It was calculated that a large proportion of the Rhône POC input is either buried (52%) or remineralized (8%), mostly on the prodelta area. The remaining fraction (40%) is either mineralized in the water or exported outside the Rhône delta system in dissolved or particulate forms.     

Benthic foraminifera living in Gulf of Mexico bathyal and abyssal sediments: Community analysis and comparison to metazoan meiofaunal biomass and density

Benthic foraminiferal biomass, density, and species composition were determined at 10 sites in the Gulf of Mexico. During June 2001 and 2002, sediment samples were collected with a GoMex box corer. A 7.5-cm diameter subcore was taken from a box core collected at each site and sliced into 1-cm or 2-cm sections to a depth of 2 or 3 cm; the >63-μm fraction was examined shipboard for benthic foraminifera. Individual foraminifers were extracted for adenosine triphosphate (ATP) using a luciferin–luciferase assay, which indicated the total ATP content per specimen; that data was converted to organic carbon. Foraminiferal biomass and density varied substantially (2–53 mg C m⁻²; 3600–44,500 individuals m⁻², respectively) and inconsistently with water depth: although two 1000-m deep sites were geographically separated by only 75 km, the foraminiferal biomass at one site was relatively low (9 mg C m⁻²) while the other site had the highest foraminiferal biomass (53 mg C m⁻²). Although most samples from Sigsbee Plain (>3000 m) had low biomass, one Sigsbee site had >20 mg foraminiferal C m⁻². The foraminiferal community from all sites (i.e. bathyal and abyssal locales) was dominated by agglutinated, rather than calcareous or tectinous, species. Foraminiferal density never exceeded that of metazoan meiofauna at any site. Foraminiferal biomass, however, exceeded metazoan meiofaunal biomass at 5 of the 10 sites, indicating that foraminifera constitute a major component of the Gulf's deep-water meiofaunal biomass.     

Probability distributions of wave heights in bimodal seas in an offshore basin

This study aims at assessing the adequacy for describing bimodal sea states of different non-linear probability distributions that have been developed for single sea states. It is based on data collected at an offshore test basin. The measurements represent three bimodal sea states with individual unidirectional wave systems propagating at 60, 90 and 120 from each other. The wave spectra are separated into swell and wind sea components and the relative energy ratio between the areas under the associated spectral curves is estimated and is related with the statistics of the time series considered. Dependence is found between the normalized high order cumulants, which describe the non-Gaussian surface, and the predominant contribution of the wind sea energy. Furthermore, the probabilities of exceedance of the individual wave heights are estimated and compared with the Rayleigh model and with other models that take into account either the effect of spectral bandwidth or the effect of wave nonlinearities. The results are discussed with respect to three classes of sea states that reflect the relative contribution of swell and wind sea energy.     

Development of a fluorescence quenching assay to measure the fraction of organic carbon present in self-assembled gels in seawater

The dissolved organic carbon pool (DOC) is among the largest reservoir of reduced carbon on our planet. The demonstration that DOC polymers remain in assembly/dispersion equilibrium forming microscopic hydrogels has a broad range of critical implications. Previous studies estimate that  10% of DOC could be assembled as gels, yielding values of  7 × 10¹⁶ g of organic carbon present as microscopic hotspots of high substrate concentration. This huge mass of reduced carbon emphasizes the need to develop reliable methods to systematically investigate the budget of self-assembled marine gels (SAG), and their role in biogeochemical cycling. However, a quantitative method to measure SAG in seawater has not been available. Here we present the validation of a simple assay to measure the mass of organic carbon assembled as microgels in native seawater. This method is based on the ratio of Chlortetracycline (CTC) fluorescence quenching between Ca bound to non-assembled organic molecules and molecules assembled as microgels. This assay can be readily implemented on board using a low cost fluorometer and provisions to measure TOC.     

Seasonal cycles and long-term trends of plankton in shelf and oceanic habitats of the Norwegian Sea in relation to environmental variables

The plankton abundance data of the Continuous Plankton Recorder (CPR) route from Bergen or Rotterdam to Weather Station Mike (6444^′N, 2E) from 1949 to 1981 were analysed for long-term trends and seasonal production cycles, and were related to environmental data. The data were explored using the canonical correlation analysis and nonparametric techniques like the Nadaraya–Watson regression. While large copepods such as Calanus spp. and Metridia lucens did not show any temporal trends, a sharp decrease in the abundances of smaller copepods and phytoplankton was observed after 1960. The temporal trends were not related to the NAO, but did show a correlation with the wind direction. Seasonal abundance curves showed that production of both phytoplankton and zooplankton taxa started earlier in coastal water compared to Atlantic water. From the 1950s to the 1970s most taxa showed a delay in the start of the seasonal production cycles, indicating a reduction in the length of the productive cycle. This may to some extent explain the reduced abundance of smaller copepods, phytoplankton and other species during the 1960s and 1970s.     

Uptake and metabolism of arsenate by anexic cultures of the microalgae Dunaliella tertiolecta and Phaeodactylum tricornutum

Axenic cultures of the microalgae species, Dunaliella tertiolecta and Phaeodactylum tricornutum were grown at arsenic (As) concentrations typically found in uncontaminated marine environments ( 2 µg L^− 1) under different phosphorus concentrations. D. tertiolecta accumulated higher arsenic concentrations (mean: 13.7 ± 0.7 µg g^− 1 dry mass) than P. tricornutum (mean: 1.9 ± 0.2 µg g−1 dry mass). Media phosphorus concentrations (0.6–3 mg/L) had little influence on microalgae growth rates or arsenic accumulation. Arsenic was present as lipid bound (29–38%; 4.2–9.5%), water-soluble (20–29%; 26–34%) and residue bound (41–45%; 57–69%) arsenic species in D. tertiolecta and P. tricornutum respectively. Hydrolysed lipids contained mostly glycerol arsenoribose (OH- ribose), dimethylarsinate (DMA) and inorganic arsenic (As(V)) moieties. Water-soluble species of microalgae were very different. D. tertiolecta contained inorganic arsenic (54–86%) with variable amounts of DMA (7.4–20%), arsenoriboses (5–25%) and traces of methylarsonate (MA) ( 1%). P. tricornutum contained mostly DMA (32–56%) and phosphate arsenoribose (PO₄-ribose, 23–49%) and small amounts of OH-ribose (3.8–6.5%) and As(V) (9–16%). Both microalgae contained an unknown cationic arsenic species. The residue fractions of both microalgae contained predominately inorganic arsenic (99–100%). These results show that at natural seawater arsenic concentrations, both algae take up substantial amounts of inorganic arsenic that is complexed with structural elements or sequestered in vacuoles as stable complexes. A significant portion is also incorporated into lipids. Arsenic is metabolised to simple methylated species and arsenoriboses.     

Isolation of colloidal monomethyl mercury in natural waters using cross-flow ultrafiltration techniques

A series of experiments was conducted to evaluate the appropriateness of cross-flow ultrafiltration (CFUF) techniques for the determination of the phase speciation of monomethyl mercury (MeHg) in natural waters. Spiral-wound cartridge (Amicon S1Y1) and Miniplate (Amicon) were evaluated for their nominal molecular weight cut-offs of 1 and 10 kDa, respectively. The ultrafiltration behavior of standard macromolecules showed that the permeation of high molecular weight (HMW) organic macromolecules was not significant when a concentration factor (CF)>15 was used. The retention of low molecular weight (LMW) molecules was significant, especially at a low CF<5, suggesting that the use of a high CF (15) will minimize the retention of LMW molecules. Sorptive losses of MeHg in the solution phase to the 1 kDa membrane were negligible, but MeHg bound to HMW macromolecules was still retained (20%), even with a preconditioned membrane. The mass balance recovery of MeHg during ultrafiltration averaged 101±15% (n=7) and 105±14% (n=5) for the 1 and 10 kDa membranes, respectively. Sample storage over 24 h caused significant coagulation (47%) of the <10 kDa MeHg into the 10 kDa–0.45 μm colloidal or the particulate MeHg pool. The 1 kDa–0.45 μm colloidal MeHg in Galveston Bay and the Trinity River water samples accounted for 40–48% of the filter-passing MeHg, although the most abundant fraction (52–60%) of MeHg was the truly dissolved fraction (<1 kDa). The partition coefficient between the colloidal (1 kDa–0.45 μm) and truly dissolved MeHg (average log K_C=5.2) was higher than the partition coefficient based on particle/filter-passing (average log K_D=4.6) or particle/truly dissolved MeHg (average log K_P=4.8), suggesting that MeHg has stronger affinity for natural colloids than macroparticulate materials (>0.45 μm).