Effect of Electron Donor to Sulfate Ratio on Mercury Methylation in Floodplain Sediments under Saturated Flow Conditions

A column transport experiment was conducted to examine the release and methylation of Hg using Hg contaminated sediment from the floodplain of the South River near Waynesboro, Virginia. Three input solutions were sequentially introduced into the column. Input 1 was unamended South River water, Input 2 was river water amended with 100 mg L^?1 SO₄ and 3600 mg L^?1 lactate, and Input 3 was river water amended with 500 mg L^?1 SO₄ and 340 mg L^?1 lactate. During the first stage of the experiment (Input 1) the effluent Hg concentration was initially 4 µg L^?1 and peaked at 21 µg L^?1 and after 21 pore volumes stabilized at 13 µg L^?1. During the second stage, at high lactate to SO₄ ratios, elevated concentrations of acetic and propionic acids were detected, indicating that fermentative bacteria were dominant. During the third stage, at high SO₄ to lactate ratios, a decrease in SO₄ and an increase in H₂S concentrations were detected in the column effluent indicating that SO₄ reduction was occurring. Concentrations of methyl Hg (MeHg) in the effluent were variable over the duration of the experiment. During the first phase, concentrations of MeHg remained <3.3 ng L^?1. During the fermentative stage, concentrations of MeHg increased to a maximum value of 32 ng L^?1, and during the sulfate-reducing stage to a maximum value of 266 ng L^?1. When the column was deconstructed both molecular and cultural techniques indicated that sulfate reducing bacteria were most dominant near the influent port. These results indicate that the formation of MeHg in the sediment is not limited by the availability of Hg and that the bacterial community that contributes to mercury methylation can respond quickly to changes in the abundances of electron donors and acceptors.     

A Michaelis–Menten type equation for describing methylmercury dependence on inorganic mercury in aquatic sediments

Methylation of mercury (Hg) is the crucial process that controls Hg biomagnification along the aquatic food chains. Aquatic sediments are of particular interest because they constitute an essential reservoir where inorganic divalent Hg (Hg^II) is methylated. Methylmercury (MeHg) concentrations in sediments mainly result from the balance between methylation and demethylation reactions, two opposite natural processes primarily mediated by aquatic microorganisms. Thus, Hg availability and the activity of methylating microbial communities control the MeHg abundance in sediments. Consistently, some studies have reported a significant positive correlation between MeHg and Hg^II or total Hg (Hg_T), taken as a proxy for Hg^II, in aquatic sediments using enzyme-catalyzed methylation/demethylation mechanisms. By compiling 1,442 published and unpublished Hg_T–MeHg couples from lacustrine, riverine, estuarine and marine sediments covering various environmental conditions, from deep pristine abyssal to heavily contaminated riverine sediments, we show that a Michaelis–Menten type relationship is an appropriate model to relate the two parameters: MeHg = aHg_T/(K _m  + Hg_T), with a = 0.277 ± 0.011 and K _m  = 188 ± 15 (R ² = 0.70, p < 0.001). From K _m variations, which depend on the various encountered environmental conditions, it appears that MeHg formation and accumulation are favoured in marine sediments compared to freshwater ones, and under oxic/suboxic conditions compared to anoxic ones, with redox potential and organic matter lability being the governing factors.     

Behavior of mercury in the Patuxent River estuary

An overview of a comprehensive study of the behavior and fate of mercury in the estuarine Patuxent River is presented. Total Hg (Hg_T) and methylmercury (MeHg) exhibited weakly non-conservative behavior in the estuary. Total Hg concentrations ranged from 6 ng L^-1 in the upper reaches of the sub-urbanized tidal freshwater river to <0.5 ng L^-1 in the mesohaline lower estuary. Filterable (0.2 µm) Hg_T ranged from 0.2 to 1.5 ng L^-1. On average, MeHg accounted for <5% of unfiltered Hg_T and <2% of filterable Hg_T. Dissolved gaseous section Hg (DGHg) concentrations were highest (up to 150 pg L^-1) in the summer in the mesohaline, but were not well correlated with primary production or chlorophyll a, demonstrating the complex nature of Hg⁰ formation and cycling in an estuarine environment. Organic matter content appeared to control the Hg_T content of sediments, while MeHg in sediments was positively correlated with Hg_T and organic matter, and negatively correlated with sulfide. MeHg in sediments was low (0.1 to 0.5% of Hg_T). Preliminary findings suggest that net MeHg production within sediments exceeds net accumulation. Although Hg_T in pore waters increased with increasing sulfide, bulk MeHg concentrations decreased. The concentration of MeHg in sediments was not related to the concentration of Hg_T in pore waters. These observations support the hypothesis that sulfide affects the speciation and therefore bioavailability of dissolved and/or solid-phase Hg for methylation. Comparison with other ecosystems, and the negative correlation between pore water sulfide and sediment MeHg, suggest that sulfide limits production and accumulation of MeHg in this system.     

Photosynthetic based algal-bacterial combined treatment of mixtures of organic pollutants and CO2 mitigation in a continuous photobioreactor

An algal-bacterial microcosm was synthetically constructed of Chlorella vulgaris MMl and Pseudomonas MTl. This microcosm was able to treat simulated wastewater supplemented with mixtures of phenol and pyridine up to 4.6 and 4.4 mM, respectively, in a continuous stirred tank bioreactor (CSTR) using photosynthetic oxygenation. Complete pollutant removal and detoxification and 82 % removal of introduced chemical oxygen demand (COD) were achieved at a hydraulic retention time (HRT) of 2.7 days. Increasing the influent load to 5.3 and 6.3 mM reduced the removal of phenol, pyridine and COD to 78, 21 and 59 %, respectively. Fertilization of the photobioreactor with 24 mM NaHCO₃ restored the treatment and detoxification efficiencies. The system was able to additionally mitigate up to 72 mM NaHCO₃ at the same HRT. Although the fertilization increased the system treatment efficiency, the settleability of the algal-bacterial microcosm was significantly reduced. When the photobioreactor was operated at HRT of 2.7 days in a 12/12 h of dark/light cycle, complete removal of 4.7 mM phenol was recorded but only 11 % of 5.7 mM pyridine was removed. The COD removal efficiency and CO₂ mitigation were also reduced to 65 and 86 %, respectively, and the effluent retained significant toxicity where 73 % inhibition was recorded. Elongation of the illumination time to 48 h (HRT of 4 days at 12/12 h dark/light cycle) restored the treatment and detoxification efficiencies.     

Complete anaerobic mineralization of pentachlorophenol (PCP) under continuous flow conditions by sequential combination of PCP‐dechlorinating and phenol‐degrading consortia

Complete mineralization of 50 µM of pentachlorophenol (PCP) was achieved anaerobically under continuous flow conditions using two columns connected in series with a hydraulic retention time of 14.2 days, showing the highest reported mineralization rate yet of 3.5 µM day^?1. The first column, when injected with a reductive PCP dechlorinating consortium, dechlorinated PCP to mainly phenol and traces of 3‐chlorophenol (3‐CP) using lactate supplied continuously as an electron donor. The second column, with an anaerobic phenol degrading consortium, decomposed phenol and 3‐CP under iron‐reducing conditions with substantial fermentative degradation of organic compounds. When 20 mM of lactate was introduced into the first column, the phenol degradation activity of the second column was lost in a short period of time, because the amorphous Fe(III) oxide (FeOOH) that had been packed in the column before use was depleted by lactate metabolites, such as acetate and propionate, flowing into the second column from the first column. The complete mineralization of PCP was maintained for a long period by reducing the lactate concentration to 4 mM, effectively extending the longevity of second‐column activity with no depletion of FeOOH for more than 200 pore volumes (corresponding to 3,000 days). The carbon balance showed that 50 µM PCP and 4 mM lactate in the influent had transformed to CO₂ (81%) and CH₄ (3%) and had contributed to biomass growth (8%). A comparison of the microbial consortia introduced into the columns and those flowing out from the columns suggested that the introduced population did not flow out during the experiments, although the microbial composition of the phenol column was considered to be affected by the inflow of microbes from the PCP dechlorination column. These results suggest that a sequential combination of reductive dechlorinating and anaerobic oxidizing consortia is useful for anaerobic remediation of chlorinated aromatic compounds in a microbial permeable reactive barrier. Biotechnol. Bioeng. 2010;107: 775–785. © 2010 Wiley Periodicals, Inc.     

Microbial sulfate-reducing activities in anoxic sediment from Marine Lake Grevelingen: screening of electron donors and acceptors

Sulfate-reducing bacteria in marine sediments mainly utilize sulfate as a terminal electron acceptor with different organic compounds as electron donors. This study investigated microbial sulfate-reducing activity of coastal sediment from Marine Lake Grevelingen (MLG), the Netherlands using different electron donors and electron acceptors. All four electron donors (ethanol, lactate, acetate and methane) showed sulfate-reducing activity with sulfate as electron acceptor, suggesting the presence of an active sulfate-reducing bacterial population in the sediment, even at dissolved sulfide concentrations exceeding 12 mM. Ethanol showed the highest sulfate reduction rate of 55 µmol g _VSS ^?1 day^?1 compared to lactate (32 µmol g _VSS ^?1 day^?1), acetate (26 µmol g _VSS ^?1 day^?1) and methane (4.7 µmol g _VSS ^?1 day^?1). Sulfide production using thiosulfate and elemental sulfur as electron acceptors and methane as the electron donor was observed, however, mainly by disproportionation rather than by anaerobic oxidation of methane coupled to sulfate reduction. This study showed that the MLG sediment is capable of performing sulfate reduction by using diverse electron donors, including the gaseous and cheap electron donor methane.     

Mercury inputs and outputs at a small lake in northern Minnesota

Storages and cycling of total mercury (Hg_T), methylmercury (MeHg), and Hg⁰ are described for Spring Lake, a small bog lake in the Marcell Experimental Forest in north-central Minnesota. We quantified photoredox transformations, MeHg photolysis, burial to the sediments, and internal and external loadings of Hg_T and MeHg. Atmospheric deposition was the main input of Hg_T; MeHg was supplied by a combination of atmospheric, near-shore wetland, and biotic (methylation) sources. Hg_T outputs were dominated by burial (67%), and Hg⁰ evasion accounted for 26% of Hg_T outputs. The watershed of Spring Lake is small (3.7× lake surface area), and accordingly, bog and upland runoff were minor contributors to both Hg_T and MeHg inputs. Wet deposition was ∼9% of total MeHg input, and other external inputs (runoff, sediment porewater) provided only an additional 7%, indicating that internal production of MeHg was occurring in the lake. Photolysis of MeHg, measured in the field and laboratory, removed ∼3× the lake mass of MeHg (20 mg) annually, and was the dominant sink for MeHg. Residence times of MeHg and Hg_T in the lake were 48 and 61 days, respectively, during the open-water season, compared with only 8 days for the residence time of MeHg on settling particles (seston). Photoreduction of Hg²⁺ to Hg⁰ was greater than the reverse reaction (Hg⁰ photooxidation), and the residence time of Hg⁰ in the photic zone was short (hours). Data from this study show active cycling of all the measured species of mercury (Hg_T, MeHg, and Hg⁰) and the importance of MeHg photolysis and photo-redox processes.     

Aspects of Bioavailability of Mercury for Methylation in Pure Cultures of Desulfobulbus propionicus (1pr3)

We have previously hypothesized that sulfide inhibits Hg methylation by decreasing its bioavailability to sulfate-reducing bacteria (SRB), the important methylators of Hg in natural sediments. With a view to designing a bioassay to test this hypothesis, we investigated a number of aspects of Hg methylation by the SRB Desulfobulbus propionicus, including (i) the relationship between cell density and methylmercury (MeHg) production, (ii) the time course of Hg methylation relative to growth stage, (iii) changes in the bioavailability of an added inorganic Hg (Hg_I) spike over time, and (iv) the dependence of methylation on the concentration of dissolved Hg_I present in the culture. We then tested the effect of sulfide on MeHg production by this microorganism. These experiments demonstrated that under conditions of equal bioavailability, per-cell MeHg production was constant through log-phase culture growth. However, the methylation rate of a new Hg spike dramatically decreased after the first 5 h. This result was seen whether methylation rate was expressed as a fraction of the total added Hg or the filtered Hg_I concentration, which suggests that Hg bioavailability decreased through both changes in Hg complexation and formation of solid phases. At low sulfide concentration, MeHg production was linearly related to the concentration of filtered Hg_I. The methylation of filtered Hg_I decreased about fourfold as sulfide concentration was increased from 10⁻⁶ to 10⁻³ M. This decline is consistent with a decrease in the bioavailability of Hg_I, possibly due to a decline in the dissolved neutral complex, HgS⁰.     

Environmental risk assessment of manganese and its associated heavy metals in a stream impacted by manganese mining in South China

In South China, high manganese content in the drinking water source influenced by upstream manganese mine drainage has become a major concern. To investigate the extent of metal pollution and environmental risk in upstream sediments and native aquatic macrophytes, a study was conducted on a manganese mining-impacted river named the Heishui River. The results indicated that streambed sediments collected were polluted by Mn and other metals with the highest contents of Mn 43349.4 mg kg^?1, Pb 128.6 mg kg^?1, Zn 502.9 mg kg^?1, and Cu 107.2 mg kg^?1. The level of Mn in all sediments was higher than the consensus-based Probable Effect Concentration, indicating that adverse effects on sediment-dwelling organisms were likely to occur frequently. Among the studied metals, Mn had the highest bioavailability and ecological risk, followed by Zn. Native aquatic macrophytes accumulate large amounts of the studied metals. A significantly positive correlation was found between exchangeable fractions of the studied metals in sediments and in aquatic macrophytes. The risk assessment code showed the following risk levels of metals in sediments in descending order: Mn > Zn > Cu > Pb. In conclusion, the river impacted by manganese mining drainage poses a high risk to both the local ecosystem and downstream drinking water.     

Methylmercury concentrations and production rates across a trophic gradient in the northern Everglades

Methylmercury (MeHg) concentrations and production rates were examined along with sulfur biogeochemistry in Everglades sediments in March, July and December, 1995, as part of a large, multi-investigator study, the Aquatic Cycling of Mercury in the Everglades (ACME) project. The sites examined constitute a trophic gradient, generated from agricultural runoff, across the Everglades Nutrient Removal (ENR) Area, which is a re-constructed wetland, and Water Conservation Areas (WCA) 2A, 2B and 3 in the northern Everglades. MeHg concentrations and %MeHg (MeHg as a percent of total Hg) were lowest in the more eutrophic areas and highest in the more pristine areas in the south. MeHg concentrations ranged from <0.1 ng gdw^-1 sediment in the ENR to 5 ng gdw^-1 in WCA3 sediments; and MeHg constituted <0.2% of total Hg (Hg_T) in ENR, but up to about 2% in two sites in WCA2B and WCA3. Methylation rates in surficial sediments, estimated using tracer-level injections of²⁰³ Hg(II) into intact sediment cores, ranged from 0 to 0.12 d^-1, or about 1 to 10 ng g^-1 d^-1when the per day values are multiplied by the ambient total Hg concentration. Methylation was generally maximal at or within centimeters of the sediment surface, and was never observed in water overlying cores. The spatial pattern of MeHg production generally matched that of MeHg concentration. The coincident distributions of MeHg and its production suggest that in situ production controls concentration, and that MeHg concentration can be used as an analog for MeHg production. In addition, the spatial pattern of MeHg in Everglades sediments matches that in biota, suggesting that MeHg bioaccumulation may be predominantly a function of the de novo methylation rate in surficial sediments.Sulfate concentrations in surficial pore waters (up to 400 µm), microbial sulfate-reduction rates (up to 800 nm cc^-1 d^-1) and resultant pore water sulfide concentrations (up to 300 µm) at the eutrophic northern sites were all high relative to most freshwater systems. All declined to the south, and sulfate concentrations in WCA2B and in central WCA3 resembled those in oligotrophic lakes (50–100 µm). MeHg concentration and production were inversely related to sulfate reduction rate and pore water sulfide. Control of MeHg production in the northern Everglades appears to mimic that in an estuary, where sulfate concentrations are high and where sulfide produced by microbial sulfate reduction inhibits MeHg production.     

Neutrophilic, nitrate-dependent, Fe(II) oxidation by a Dechloromonas species

A species of Dechloromonas, strain UWNR4, was isolated from a nitrate-reducing, enrichment culture obtained from Wisconsin River (USA) sediments. This strain was characterized for anaerobic oxidation of both aqueous and chelated Fe(II) coupled to nitrate reduction at circumneutral pH. Dechloromonas sp. UWNR4 was incubated in anoxic batch reactors in a defined medium containing 4.5–5 mM NO₃ ^?, 6 mM Fe²⁺ and 1–1.8 mM acetate. Strain UWNR4 efficiently oxidized Fe²⁺ with 90 % oxidation of Fe²⁺ after 3 days of incubation. However, oxidation of Fe²⁺ resulted in Fe(III)-hydroxide-encrusted cells and loss of metabolic activity, suggested by inability of the cells to utilize further additions of acetate. In similar experiments with chelated iron (Fe(II)-EDTA), encrusted cells were not produced and further additions of acetate and Fe(II)-EDTA could be oxidized. Although members of the genus Dechloromonas are primarily known as perchlorate and nitrate reducers, our findings suggest that some species could be members of microbial communities influencing iron redox cycling in anoxic, freshwater sediments. Our work using Fe(II)-EDTA also demonstrates that Fe(II) oxidation was microbially catalyzed rather than a result of abiotic oxidation by biogenic NO₂ ^?.     

Biogeochemical processes at the sediment–water interface, Bombah Broadwater, Myall Lakes

Myall Lakes has experienced algal blooms in recent years which threaten water quality. Biomarkers, benthic fluxes measured with chambers, and pore water metabolites were used to identify the nature and reactivity of organic matter (OM) in the sediments of Bombah Broadwater (BB), and the processes controlling sediment-nutrient release into the overlying waters. The OM in the sediments was principally from algal sources although terrestrial OM was found near the Myall River. Terrestrial faecal matter was identified in muddy sediments and was probably sourced via runoff from farm lands. The reactive OM which released nutrients into the overlying waters was from diatoms, dinoflagellates and probably cyanobacteria. Microcystis filaments were observed in surface sediments. OM degradation rates varied between 5.3 and 47.1 mmol m^?2 day^?1 (64–565 mg m^?2 day^?1), were highest in the muddy sediments and sulphate reduction rates accounted for 20–40% of the OM degraded. Diatoms, being heavy sink rapidly, and are an important vector to transport catchment N and P to sites of denitrification and P-trapping in the sediments. Denitrification rates (mean ～4 mmol N m^?2 day^?1), up to 7 mmol N m^?2 day^?1 (105 mg N m^?2 day^?1) were measured, and denitrification efficiencies were highest (mean = 86 ± 4%) in the sandy sediments (～20% of the area of BB), but lower in the muddy sediments (mean = 63 ± 15%). These differences probably result from higher OM loads and anaerobic respiration in muddy sediments. Most DIP (>70%) from OM degradation was not released into overlying waters but remained trapped in surface sediments. Biophysical (advective) processes were responsible for the measured metabolite (O₂, CO₂, DSi, DIN and DIP) fluxes across the sediment–water interface.     

Kinetics of nitrate and sulfate removal using a mixed microbial culture with or without limited-oxygen fed

The biological degradation of nitrate and sulfate was investigated using a mixed microbial culture and lactate as the carbon source, with or without limited-oxygen fed. It was found that sulfate reduction was slightly inhibited by nitrate, since after nitrate depletion the sulfate reduction rate increased from 0.37 mg SO₄ ^2?/mg VSS d to 0.71 mg SO₄ ^2?/mg VSS d, and the maximum rate of sulfate reduction in the presence of nitrate corresponded to 56 % of the non-inhibited sulfate reduction rate determined after nitrate depleted. However, simultaneous but not sequential reduction of both oxy-anions was observed in this study, unlike some literature reports in which sulfate reduction starts only after depletion of nitrate, and this case might be due to the fact that lactate was always kept above the limiting conditions. At limited oxygen, the inhibited effect on sulfate reduction by nitrate was relieved, and the sulfate reduction rate seemed relatively higher than that obtained without limited-oxygen fed, whereas kept almost constant (0.86–0.89 mg SO₄ ^2?/mg VSS d) cross the six R_OS states. In contrast, nitrate reduction rates decreased substantially with the increase in the initial limited-oxygen fed, showing an inhibited effect on nitrate reduction by oxygen. Kinetic parameters determined for the mixed microbial culture showed that the maximum specific sulfate utilization rate obtained (0.098?±?0.022 mg SO₄ ^2?/(mg VSS h)) was similar to the reported typical value (0.1 mg SO₄ ^2?/(mg VSS h)), also indicating a moderate inhibited effect by nitrate.     

MMHg production and export from intertidal sediments to the water column of a tidal lagoon (Arcachon Bay,France)

Hg cycling in biologically productive coastal areas is of special importance given the potential for bioaccumulation of monomethylmercury (MMHg) into aquatic organisms. Field experiments were performed during three different seasons in Arcachon Bay, a mesotidal lagoon (SW France), to assess the variability of the water column concentrations, sediment–water exchanges and potential formation and degradation of MMHg. The objectives were to evaluate the contribution of intertidal mudflats to MMHg production and the various pathways of Hg species export. Dissolved and bulk concentrations of Hg species in the water column downstream of tidal flats were measured throughout several tidal cycles. The Hg benthic fluxes at the sediment–water interface were determined by means of benthic chambers for three different stations. Hg methylation and demethylation potentials were determined in surficial sediments and the water column using isotopic tracers. The tidal surveys demonstrated that benthic remobilization of Hg occurs primarily in association with sediment erosion and advection during ebb tide. However, elevated dissolved Hg concentrations observed at low tide were found to be caused by a combination of pore-waters seeping, benthic fluxes and methylation in the water column. Benthic fluxes were more intense during late winter conditions (median MMHg and inorganic Hg (IHg) fluxes: 64 and 179 pmol m^?2 h^?1, respectively) and subsequently decreased in spring (median 0.7 and ?5 pmol m^?2 h^?1, respectively) and fall (median ?0.4 and ?1.3 pmol m^?2 h^?1, respectively). The trends in methylation and demethylation potentials were at the opposite of the fluxes, two times lower during winter than for spring or fall conditions. In this tidal environment, MMHg production in surface sediments and its subsequent release is estimated to be the major source of MMHg to the water column during winter and spring time. However, during the more productive summer period, the Hg methylation extent in the water column may be very significant and equivalent to the sediment contribution.     

Patterns of Bacterial Diversity Along a Long-Term Mercury-Contaminated Gradient in the Paddy Soils

Mercury (Hg) pollution is usually regarded as an environmental stress in reducing microbial diversity and altering bacterial community structure. However, these results were based on relatively short-term studies, which might obscure the real response of microbial species to Hg contamination. Here, we analysed the bacterial abundance and community composition in paddy soils that have been potentially contaminated by Hg for more than 600 years. Expectedly, the soil Hg pollution significantly influenced the bacterial community structure. However, the bacterial abundance was significantly correlated with the soil organic matter content rather than the total Hg (THg) concentration. The bacterial alpha diversity increased at relatively low levels of THg and methylmercury (MeHg) and subsequently approached a plateau above 4.86 mg kg^?1 THg or 18.62 ng g^?1 MeHg, respectively. Contrasting with the general prediction of decreasing diversity along Hg stress, our results seem to be consistent with the intermediate disturbance hypotheses with the peak biological diversity under intermediate disturbance or stress. This result was partly supported by the inconsistent response of bacterial species to Hg stress. For instance, the relative abundance of Nitrospirae decreased, while that of Gemmatimonadetes increased significantly along the increasing soil THg and MeHg concentrations. In addition, the content of SO₄ ^2?, THg, MeHg and soil depth were the four main factors influencing bacterial community structures based on the canonical correspondence analysis (CCA). Overall, our findings provide novel insight into the distribution patterns of bacterial community along the long-term Hg-contaminated gradient in paddy soils.     

Denitrification, Nitrate Reduction, and Oxygen Consumption in Coastal and Estuarine Sediments

Denitrification and consumption of oxygen and nitrate in sediments from Tama Estuary, Odawa Bay, and Tokyo Bay were measured in an experimental sediment-water system. Filtered seawater containing [¹⁵N]nitrate flowed continuously over undisturbed sediments, and the concentrations of O₂, nitrate, and nitrite in the influent and effluent and of ¹⁵N₂ in the effluent were monitored. Under steady-state conditions, the rate of nitrate consumption was the same order of magnitude as the rate of oxygen consumption in Tama Estuary sediments, whereas the former rate was one order of magnitude lower than the latter rate in Odawa Bay and Tokyo Bay sediments. Denitrification accounted for 27 to 57% of the nitrate consumption.     

Eicosanoid Signaling and Vascular Dysfunction: Methylmercury-Induced Phospholipase D Activation in Vascular Endothelial Cells

Mercury, especially methylmercury (MeHg), is implicated in the etiology of cardiovascular diseases. Earlier, we have reported that MeHg induces phospholipase D (PLD) activation through oxidative stress and thiol-redox alteration. Hence, we investigated the mechanism of the MeHg-induced PLD activation through the upstream regulation by phospholipase A₂ (PLA₂) and lipid oxygenases such as cyclooxygenase (COX) and lipoxygenase (LOX) in the bovine pulmonary artery endothelial cells (BPAECs). Our results showed that MeHg significantly activated both PLA₂ (release of [³H]arachidonic acid, AA) and PLD (formation of [³²P]phosphatidylbutanol) in BPAECs in dose- (0–10 μM) and time-dependent (0–60 min) fashion. The cPLA₂-specific inhibitor, arachidonyl trifluoromethyl ketone (AACOCF₃), significantly attenuated the MeHg-induced [³H]AA release in ECs. MeHg-induced PLD activation was also inhibited by AACOCF₃ and the COX- and LOX-specific inhibitors. MeHg also induced the formation of COX- and LOX-catalyzed eicosanoids in ECs. MeHg-induced cytotoxicity (based on lactate dehydrogenase release) was protected by PLA₂-, COX-, and LOX-specific inhibitors and 1-butanol, the PLD-generated PA quencher. For the first time, our studies showed that MeHg activated PLD in vascular ECs through the upstream action of cPLA₂ and the COX- and LOX-generated eicosanoids. These results offered insights into the mechanism(s) of the MeHg-mediated vascular endothelial cell lipid signaling as an underlying cause of mercury-induced cardiovascular diseases.     

Electron Donors Utilized by Sulfate-Reducing Bacteria in Eutrophic Lake Sediments

Mineralization rates of ¹⁴C-labeled substrates were determined in the presence and absence of Na₂MoO₄, an inhibitor of sulfate reduction, in the profundal sediments of a shallow eutrophic lake. Sulfate reduction was inhibited by Na₂MoO₄ at all concentrations tested (0.2 to 200 mM), whereas methane production was inhibited at Na₂MoO₄ concentrations greater than 20 mM. Initial mineralization rates of glucose were unaffected by Na₂MoO₄; however, Na₂MoO₄ decreased the mineralization rates of lactate (58%), propionate (52%), an amino acid mixture (85%), and acetate (14%). These decreases in the rates of mineralization were attributed to inhibition of sulfate reduction. Hydrogen stimulated the reduction of ³⁵SO₄²⁻ 2.5- to 2.8-fold, demonstrating potential hydrogen oxidation by sulfate-reducing bacteria. These results indicate that sulfate reducers utilize an array of substrates as electron donors and are of potential significance to the in situ mineralization of lactate, propionate, and free amino acids in these sediments.     

Mercury dynamics in Tivoli South Bay, a freshwater tidal mudflat wetland in the Hudson River

The accumulation of total mercury (Hg_T) andmethylmercury (MeHg) was evaluated in sediments ofTivoli South Bay, a freshwater tidal mudflat wetlandin the Hudson River National Estuarine ResearchReserve system. Hg_T concentrations in sedimentcores were measured to evaluate the spatialvariability of Hg_T deposition, and to establisha chronology of Hg_T accumulation. Cores takenfrom the northern, middle, and southern sections ofthe bay had similar distribution patterns andconcentrations of Hg_T, suggesting a common sourceof Hg_T throughout the bay. Sedimentconcentrations ranged from 190 to 1040 ng Hg g^–1,2 to 10 times greater than concentrations expected insediments from non-anthropogenic sources. Hg_Tdeposition rates were similar in different regions ofthe bay, and increased from 200 ng Hg cm^–2yr^–1in the 1930s to a maximum of 300 ngHg cm^–2 yr^–1 in the 1960s. Deposition rateshave steadily declined since the 1970s and arecurrently at 80 ng Hg cm^–2 yr^–1. Transportof Hg_T by tidal waters from the Hudson River islikely the main source of Hg_T in the bay.Distribution patterns and absolute concentrations ofMeHg in sediment cores were similar throughout thebay, with concentrations ranging from 0.43 to 2.95ng g^–1. Maxima in MeHg concentration profilesoccurred just below the sediment-water interface andat a depth of 30 cm. The maximum at 30 cm wascoincident with maximum Hg_T concentrations. MeHgconcentrations in suspended particulate matter (SPM)from the Hudson River suggest that MeHg in the baycould be derived from riverine SPM rather than formedin situ.     

An enzyme kinetics study of the pH dependence of chloride activation of oxygen evolution in photosystem II

Oxygen evolution by photosystem II (PSII) involves activation by Cl^? ion, which is regulated by extrinsic subunits PsbQ and PsbP. In this study, the kinetics of chloride activation of oxygen evolution was studied in preparations of PSII depleted of the PsbQ and PsbP subunits (NaCl-washed and Na₂SO₄/pH 7.5-treated) over a pH range from 5.3 to 8.0. At low pH, activation by chloride was followed by inhibition at chloride concentrations >100 mM, whereas at high pH activation continued as the chloride concentration increased above 100 mM. Both activation and inhibition were more pronounced at lower pH, indicating that Cl^? binding depended on protonation events in each case. The simplest kinetic model that could account for the complete data set included binding of Cl^? at two sites, one for activation and one for inhibition, and four protonation steps. The intrinsic (pH-independent) dissociation constant for Cl^? activation, K _S, was found to be 0.9 ± 0.2 mM for both preparations, and three of the four pK _as were determined, with the fourth falling below the pH range studied. The intrinsic inhibition constant, K _I, was found to be 64 ± 2 and 103 ± 7 mM for the NaCl-washed and Na₂SO₄/pH7.5-treated preparations, respectively, and is considered in terms of the conditions likely to be present in the thylakoid lumen. This enzyme kinetics analysis provides a more complete characterization of chloride and pH dependence of O₂ evolution activity than has been previously presented.