Mercury Reduction and Oxidation by Reduced Natural Organic Matter in Anoxic Environments

Natural organic matter (NOM)-mediated redox cycling of elemental mercury Hg(0) and mercuric Hg(II) is critically important in affecting inorganic mercury transformation and bioavailability. However, these processes are not well understood, particularly in anoxic water and sediments where NOM can be reduced and toxic methylmercury is formed. We show that under dark anoxic conditions reduced organic matter (NOM_re) simultaneously reduces and oxidizes Hg via different reaction mechanisms. Reduction of Hg(II) is primarily caused by reduced quinones. However, Hg(0) oxidation is controlled by thiol functional groups via oxidative complexation, which is demonstrated by the oxidation of Hg(0) by low-molecular-weight thiol compounds, glutathione, and mercaptoacetic acid, under reducing conditions. Depending on the NOM source, oxidation state, and NOM:Hg ratio, NOM reduces Hg(II) at initial rates ranging from 0.4 to 5.5 h^–1, which are about 2 to 6 times higher than those observed for photochemical reduction of Hg(II) in open surface waters. However, rapid reduction of Hg(II) by NOM_re can be offset by oxidation of Hg(0) with an estimated initial rate as high as 5.4 h^–1. This dual role of NOM_re is expected to strongly influence the availability of reactive Hg and thus to have important implications for microbial uptake and methylation in anoxic environments

Mercury Stable Isotopes in Ornithogenic Deposits As Tracers of Historical Cycling of Mercury in Ross Sea, Antarctica

Production of methylmercury (MeHg) in ocean waters and its bioaccumulation in marine organisms are critical processes controlling the fate and toxicity of mercury (Hg). However, these processes are not well understood in the Antarctic, where high levels of MeHg are observed in the subsurface ocean (100–1000 m). We explored the use of Hg stable isotope compositions in historical and modern biological deposits as a new approach for discerning Hg sources and tracing MeHg cycling in the ocean and bioaccumulation in marine biota. We found similar mass independent isotope fractionation (MIF) of Hg between a sediment profile containing historical penguin and seal feces deposits from coastal Antarctica and modern penguin and seal feces, suggesting that penguin and seal feces were the dominant sources of Hg to the sediments at different time periods. Furthermore, sediments dominated by seal feces displayed a significantly lower MIF slope (Δ¹⁹⁹Hg/Δ²⁰¹Hg) than those dominated by penguin feces despite similar extents of MIF. Since seals forage at greater depths (>400 m) than penguins (<100 m), the high MIF values and lower Δ¹⁹⁹Hg/Δ²⁰¹Hg in seal feces suggest that a significant fraction of MeHg accumulated by seals was produced in situ in the subsurface ocean from residual inorganic Hg­(II) that sank from the euphotic zone after partial photoreduction. Our results suggest that in situ Hg methylation can be an important source of MeHg for marine biota, and Hg isotope compositions in biological archives can be valuable tracers of MeHg cycling

Variations in Stable Isotope Fractionation of Hg in Food Webs of Arctic Lakes

Biotic and abiotic fractionation of mercury (Hg) isotopes has recently been shown to occur in aquatic environments. We determined isotope ratios (IRs) of Hg in food webs (zooplankton, chironomids, Arctic char) and sediments of 10 Arctic lakes from four regions and investigated the extent of Hg isotope fractionation. Hg IRs were analyzed by multicollector inductively coupled plasma mass spectrometry (MC-ICP/MS). Hg mass independent fractionation (MIF; Δ199Hg) and mass dependent fractionation (MDF; δ202Hg) were calculated and compared among samples. IRs of Hg in sediment were characterized mainly by MDF and low MIF (Δ199Hg −0.37 to 0.74‰). However, all biota showed evidence of MIF, most pronounced in zooplankton (Δ199Hg up to 3.40 ‰) and char (Δ199Hg up to 4.87‰). Zooplankton takes up highly fractionated MeHg directly from the water column, while benthic organisms are exposed to sedimentary Hg, which contains less fractionated Hg. As evidenced by δ13C measurements, benthic chironomids make up a large proportion of char diet, explaining in part why MIFchar zooplankton in lakes, where both samples were measured. Hg IRs in char varied among regions, while char from lakes from each region showed similar degrees of MIF. A MIF-offset was derived representing the mean MIF difference between sediment and fish, and indicated that fish in two regions retain sediment signatures altered by a consistent offset. Due to its minimal lake-to-catchment area and very high water retention time (∼330 years), the meteor impact crater lake (Pingualuk) reflects a “pure” atmospheric Hg signature, which is modified only by aqueous in-lake processes. All other lakes are also affected by terrestrial Hg inputs and sediment processes

Effect of FLNA on PC2 degradation.

<p>A, <i>left panel</i>, representative data from WB showing PC2degradation in M2 and A7 cells using cycloheximide (CHX) to block protein synthesis for various time (in hr). <i>Right panel</i>, data on PC2 degradation from 4 independent experiments were normalized, averaged and fitted with exponential decay curves Y(t) = 2^-t/τ, where τ represents the protein half-life. B, representative data showing degradation of GFP-PC2 (<i>upper panel)</i>, GFP-PC2C (<i>middle panel</i>) and GFP-PC2N (<i>lower panel</i>) transiently expressed in A7 cells with FLNA KD or control (Ctrl). C, representative data showing proteasome-dependent inhibition of degradation of GFP-PC2 stably expressed in A7 and M2 cells by MG-132. Cells were treated with MG-132(10 μM) or DMSO (negative control) for 4 hr before proceeding to WB with FLNA and PKD2 antibodies.</p

Oxidation of Dissolved Elemental Mercury by Thiol Compounds under Anoxic Conditions

Mercuric ion, Hg²⁺, forms strong complexes with thiolate compounds that commonly dominate Hg­(II) speciation in natural freshwater. However, reactions between dissolved aqueous elemental mercury (Hg(0)_aq) and organic ligands in general, and thiol compounds in particular, are not well studied although these reactions likely affect Hg speciation and cycling in the environment. In this study, we compared the reaction rates between Hg(0)_aq and a number of selected organic ligands with varying molecular structures and sulfur (S) oxidation states in dark, anoxic conditions to assess the role of these ligands in Hg(0)_aq oxidation. Significant Hg(0)_aq oxidation was observed with all thiols but not with ligands containing no S. Compounds with oxidized S (e.g., disulfide) exhibited little or no reactivity toward Hg(0)_aq either at pH 7. The rate and extent of Hg(0)_aq oxidation varied greatly depending on the chemical and structural properties of thiols, thiol/Hg ratios, and the presence or absence of electron acceptors. Smaller aliphatic thiols and higher thiol/Hg ratios resulted in higher Hg(0)_aq oxidation rates than larger aromatic thiols at lower thiol/Hg ratios. The addition of electron acceptors (e.g., humic acid) also led to substantially increased Hg(0)_aq oxidation. Our results suggest that thiol-induced oxidation of Hg(0)_aq is important under anoxic conditions and can affect Hg redox transformation and bioavailability for microbial methylation

Effect of FLNAC on the FLNA-PC2 interaction.

<p>All data are from three or more independent experiments. A, representative data from co-IP showing interaction of GFP-PC2 with FLNA and FLNAC in M2 and A7 cells. B, representative data showing the effect of blocking peptide FLNAC on the FLNA-PC2 interaction in A7 cells. C, Far WB showing the competition of FLNAC with PC2C for binding to FLNA. Lysates of M2 and A7 cells (stably expressing GFP-PC2) were separated by SDS-PAGE and transferred to nitrocellulose membrane. Proteins were denatured, renatured and then incubated with purified GST-PC2C and none (<i>left panel</i>) or His-FLNAC (<i>right panel</i>). Bound protein was detected by PC2 (H-280) antibody. The arrow (←) indicates PC2C signal detected at the site of FLNA. The star (*) indicates stably expressed GFP-PC2 signal, as a control.</p

Role of FLNA in, and effect of FLNAC on, the interaction of PC2 with actin.

<p>A, effect of FLNA KD on the PC2-actin interaction revealed by co-IP with anti-GFP (EU4) antibody in A7 PC2 stable cells. B, effect of FLNA KD on the PC2-actin interaction revealed by co-IP with PC2 (H-280) antibody in HeLa cells. C, localization of PC2, filamin-A and actin in A7 (<i>upper panel</i>) and IMCD <i>(lower panel)</i> cells, and co-localization of PC2 and Na/K ATPase in A7 cells were determined by immunofluorescence assays, as previously described [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0123018#pone.0123018.ref021" target="_blank">21</a>]. GFP-PC2 are stably expressed in A7 and IMCD cell lines. Primary antibodies against FLNA (H-300), β-actin (C-4), and Na/K ATPase (H-300) were used. Rabbit Cy3- and mouse Cy5-conjugated secondary antibodies were used to detect FLNA (or Na/K ATPase) and actin, respectively. Images were acquired using AIVI spinning disc confocal microscopy with x60 objective. D, effect of FLNAC on the FLNA-mediated PC2-actin interaction using A7 and M2 (as control, no FLNA) cells revealed by biotinylation assays that showed recruitment of actin to the PM.</p

Effects of FLNA on PC2 protein expression and PC2 synthesis.

<p>A, WB showing endogenous PC2 level in HeLa (<i>Left panel</i>) and HEK293 (<i>Right panel</i>) cells with FLNA KD by siRNA 5601, 7116 and 371, respectively. The numbers indicate the nucleotide positions in the FLNA mRNA open reading frame where the siRNA sequence starts. β-actin was used as a loading control. B, <i>left panel</i>, data from WB showing the expression of PC2 in M2 and A7 cells stably expressing GFP-PC2. GFP (B-2) antibody was used to detect GFP-PC2. <i>Middle panel</i>, PC2 expression in M2 and A7 cells transiently expressing GFP-PC2. <i>Right panel</i>, comparison between averaged GFP-PC2 levels normalized by HSP60 in M2 and A7 cells under stable and transient expression conditions. C, <i>left panel</i>, representative data from M2 and A7 cells showing GFP-PC2 synthesis assessed by ³⁵S pulse labeling. Anti-GFP (EU4) was used to precipitate GFP-PC2. <i>Right panel</i>, comparison between averaged GFP-PC2 syntheses in M2 and A7 cells (N = 4; p = 0.004).</p

Table1_Shale Heavy Metal Isotope Records of Low Environmental O2 Between Two Archean Oxidation Events.XLSX

Evidence of molecular oxygen (O2) accumulation at Earth’s surface during the Archean (4.0–2.5 billion years ago, or Ga) seems to increase in its abundance and compelling nature toward the end of the eon, during the runup to the Great Oxidation Event. Yet, many details of this late-Archean O2 story remain under-constrained, such as the extent, tempo, and location of O2 accumulation. Here, we present a detailed Fe, Tl, and U isotope study of shales from a continuous sedimentary sequence deposited between ∼2.6 and ∼2.5 Ga and recovered from the Pilbara Craton of Western Australia (the Wittenoom and Mt. Sylvia formations preserved in drill core ABDP9). We find a progressive decrease in bulk-shale Fe isotope compositions moving up core (as low as δ56Fe = –0.78 ± 0.08‰; 2SD) accompanied by invariant authigenic Tl isotope compositions (average ε205TlA = –2.0 ± 0.6; 2SD) and bulk-shale U isotope compositions (average δ238U = –0.30 ± 0.05‰; 2SD) that are both not appreciably different from crustal rocks or bulk silicate Earth. While there are multiple possible interpretations of the decreasing δ56Fe values, many, to include the most compelling, invoke strictly anaerobic processes. The invariant and near-crustal ε205TlA and δ238U values point even more strongly to this interpretation, requiring reducing to only mildly oxidizing conditions over ten-million-year timescales in the late-Archean. For the atmosphere, our results permit either homogenous and low O2 partial pressures (between 10−6.3 and 10−6 present atmospheric level) or heterogeneous and spatially restricted O2 accumulation nearest the sites of O2 production. For the ocean, our results permit minimal penetration of O2 in marine sediments over large areas of the seafloor, at most sufficient for the burial of Fe oxide minerals but insufficient for the burial of Mn oxide minerals. The persistently low background O2 levels implied by our dataset between ∼2.6 and ∼2.5 Ga contrast with the timeframes immediately before and after, where strong evidence is presented for transient Archean Oxidation Events. Viewed in this broader context, our data support the emerging narrative that Earth’s initial oxygenation was a dynamic process that unfolded in fits-and-starts over many hundreds-of-millions of years.</p