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Lepidocrocite, as a ubiquitous iron mineral, is widely detected in different morphologies in natural environments, controlling the mobility and availability of heavy metal ions (HMIs).

The inessential heavy metal/loids cadmium (Cd) and arsenic (As), which often co-occur in polluted paddy soils, are toxic to rice. Silicon (Si) treatment is known to reduce Cd and As toxicity in rice plants. To better understand the shared mechanisms by which Si alleviates Cd and As stress, rice seedlings were hydroponically exposed to Cd or As, then treated with Si. The addition of Si significantly ameliorated the inhibitory effects of Cd and As on rice seedling growth. Si supplementation decreased Cd and As translocation from roots to shoots, and significantly reduced Cd- and As-induced reactive oxygen species generation in rice seedlings. Transcriptomics analyses were conducted to elucidate molecular mechanisms underlying the Si-mediated response to Cd or As stress in rice. The expression patterns of the differentially expressed genes in Cd- or As-stressed rice roots with and without Si application were compared. The transcriptomes of the Cd- and As-stressed rice roots were similarly and profoundly reshaped by Si application, suggesting that Si may play a fundamental, active role in plant defense against heavy metal/loid stresses by modulating whole genome expression. We also identified two novel genes, Os01g0524500 and Os06g0514800, encoding a myeloblastosis (MYB) transcription factor and a thionin, respectively, which may be candidate targets for Si to alleviate Cd and As stress in rice, as well as for the generation of Cd- and/or As-resistant plants. This study provides valuable resources for further clarification of the shared molecular mechanisms underlying the Si-mediated alleviation of Cd and As toxicity in rice.

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How metal-reducing bacteria transfer electrons during dissimilatory energy generation under electron acceptor-limited conditions is poorly understood. Here, we incubated the iron and manganese-reducing bacterium Shewanella oneidensis MR-1 without electron acceptors. Removal of soluble extracellular organic compounds (EOCs) dramatically retarded transfer of electrons to an experimental electron acceptor, Cr(VI), by MR-1. However, the return of either high MW (>3000 Da) or low MW (

Anaerobic microorganisms play a key role in the biological mercury (Hg) cycle due to their ability to produce bioaccumulative neurotoxic methylmercury (MeHg). However, despite recent advances, how ...

Enrichments of REY (rare earth + yttrium) and other trace metals (Co and Ni) in deep-sea ferromanganese (Fe Mn) micronodules have received increasing attention in both deep-sea research and mineral exploration. Due to the presence of multiple, easily-crushed and poorly-crystallized phases in micronodules, the genesis of micronodules and their adsorption of various trace elements are poorly understood. To address this gap, we examined the spatial distributions of elements in cross-sections of micronodules from the western tropical North Pacific Ocean using high-resolution (HR) LA-ICP-MS raster mapping coupled with laser Raman and X-ray photoelectron spectroscopy (XPS). The ferromanganese micronodules we studied are dominated by Fe and Mn oxides with minor carbonate minerals, such as siderite, rhodochrosite and calcite. LA-ICP-MS maps show that these micronodules consist of a Mn-rich core and a Fe-rich rim. The Fe-enriched rim is enriched in As and surrounds a Mg, Mn, Cu, Co and Ni concreted core. Laser Raman maps show that the micronodule core contains more birnessite, an important scavenger of trace metals in deep sea sediments, than the rim. The birnessite filled core of these micronodules does not have elevated REY. Indeed, birnessite line channels may feed metal-rich fluid containing REY to adjacent minerals, including well-crystallized bio-apatite and zeolite, as high Ce and Y levels are spatially correlated with these minerals. The observed element profiles and XPS observations showing the coexistence of multiple oxidation states of Mn (+2, +3 and +4), Fe (+2 and +3) and Ce (+3, +4) demonstrate that the Fe Mn phases of these micronodules are of a diagenetic origin and that they are sites of redox-driven metal enrichment in deep-sea sediment.

The photochemical reduction of Hg(II) is an important pathway in the environmental Hg cycle because it competes with Hg methylation and potentially limits the formation of bioaccumulative methylmercury. Hg stable isotope systematics have proven to be an effective tool for investigating the transport, transformation, and bioaccumulation of Hg. The dominant cause of mass independent isotope fractionation (MIF) of Hg in nature is the photochemical reduction of various species of Hg(II). However, it is difficult to fully interpret Hg stable isotope signatures due to the lack of mechanistic information about which Hg compounds are susceptible to MIF and why. This study investigates Hg isotope fractionation during the photochemical reduction of Hg(II) complexed to organic ligands, which are representative of the available binding sites in natural dissolved organic matter. The photochemical reduction of Hg(II) in the presence of cysteine resulted in both negative and positive MIF in residual Hg(II), where the sign depended on pH and dissolved oxygen level. In the presence of serine, either nuclear volume or magnetic isotope effects were observed depending on the wavelength of light and the extent of Hg(II) complexation by serine. In the presence of ethylenediamine, MIF was negative. Our Hg stable isotope results suggest that MDF and MIF are induced at different steps in the overall photochemical reduction reaction and that MIF does not depend on the rate-determining step but instead depends on photophysical aspects of the reaction such as intersystem crossing and hyperfine coupling. The behavior of Hg isotopes reported here will allow for a better understanding of the underlying reaction mechanisms controlling the Hg isotope signatures recorded in natural samples.

Acid mine drainage (AMD), a source of considerable environmental pollution worldwide, has prompted the development of many strategies to alleviate its effects. Unfortunately, the methods available for remedial treatment of AMD and the damage it cause are generally costly, labor-intensive, and time-consuming. Furthermore, such treatments may result in secondary pollution. Alternatively, treating the AMD problem at its source through pyrite surface passivation has become an important topic for research because it has the potential to reduce or prevent the generation of AMD and associated pollution. This review summarizes various pyrite anti-corrosion technologies, including the formation of various passivating coatings (inorganic, organic and organosilane) and carrier-microencapsulation. Several effective long-term passivators are identified, although many of them currently have important deficiencies that limit their practical application. Combining the mechanisms of existing passivation agents or new artificial materials, while considering environmental conditions, costs, and long-term passivation performance, is a feasible direction for future research.

Soot and mercury (Hg) are two notorious air pollutants, and the fate and transport of Hg may be affected by soot at various scales in the environment as soot may be both a carrier and a reactant for active Hg species. This study was designed to quantify photoreduction of Hg(II) in the presence of soot and the associated Hg isotope fractionation under both atmospheric aerosol and aqueous conditions (water-saturated). Photoreduction experiments were conducted with diesel soot particulate matter under controlled temperature and relative humidity (RH) conditions using a flow-through semibatch reactor system. Mass-dependent fractionation resulted in the enrichment of heavier Hg isotopes in the remaining Hg(II) with enrichment factors (e202Hg) of 1.48 ± 0.02‰ (±2 standard deviation) to 1.75 ± 0.05‰ for aerosol-phase reactions (RH 28-68%) and from 1.26 ± 0.11 to 1.50 ± 0.04‰ for aqueous-phase reactions. Positive odd mass-independent fractionation (MIF) was observed in aqueous-phase reactions, resulting in Δ199Hg values for reactant Hg(II) as high as 5.29‰, but negative odd-MIF occurred in aerosol-phase reactions, in which Δ199Hg values of reactant Hg(II) varied from -1.02 to 0‰. The average ratio of Δ199Hg/Δ201Hg (1.1) indicated that under all conditions, MIF was dominated by magnetic isotope effects during photoreduction of Hg(II). Increasing RH resulted in higher reduction rates but lower extents of negative MIF in the aerosol-phase experiments, suggesting that the reduction of soot particle-bound Hg(II) was responsible for the observed negative odd-MIF. Our results suggest that mass-independent Hg isotope fractionation during Hg(II) photoreduction varies with soot aerosol water content and that Hg-stable isotope ratios may be used to understand the transformational histories of aerosol-bound Hg(II) in the environment.

Significance Bluefin tuna (BFT) is an apex predatory, long-lived, migratory pelagic fish that is widely distributed throughout the world's oceans. These fish have very high concentrations of neurotoxic methylmercury (MeHg) in their tissues, which increase with age. Our study shows that Hg accumulation rates (MARs) in BFT as a global pollution index can reveal global patterns of Hg pollution and bioavailability in the oceans and further reflect both natural and anthropogenic emissions and regional environmental features. Overall, MARs provide a means to compare Hg bioavailability among geographically distinct populations of upper trophic level marine fish across ocean subbasins, to investigate trophic dynamics of Hg in marine food webs, and furthermore, to improve public health risk assessments of Hg exposure from seafood., Bluefin tuna (BFT), highly prized among consumers, accumulate high levels of mercury (Hg) as neurotoxic methylmercury (MeHg). However, how Hg bioaccumulation varies among globally distributed BFT populations is not understood. Here, we show mercury accumulation rates (MARs) in BFT are highest in the Mediterranean Sea and decrease as North Pacific Ocean > Indian Ocean > North Atlantic Ocean. Moreover, MARs increase in proportion to the concentrations of MeHg in regional seawater and zooplankton, linking MeHg accumulation in BFT to MeHg bioavailability at the base of each subbasin's food web. Observed global patterns correspond to levels of Hg in each ocean subbasin; the Mediterranean, North Pacific, and Indian Oceans are subject to geogenic enrichment and anthropogenic contamination, while the North Atlantic Ocean is less so. MAR in BFT as a global pollution index reflects natural and human sources and global thermohaline circulation.

Plants typically take up Fe through either strategy I or strategy II, whereas rice uses both. Stable Fe isotopes potentially reveal pathways of Fe uptake in rice plants. In this study we measured Fe isotopic compositions of rice grown under different conditions, i.e. paddy soil with deficient Fe supply and Fe 3+ -EDTA aqueous solution with sufficient Fe supply, to investigate whether Fe isotope fractionation is distinct under Fe-deficient and Fe-sufficient conditions as well as their possible controlling mechanisms. Our results show that rice grown in the Fe 3+ -EDTA aqueous solutions with sufficient Fe supply preferentially takes up light Fe isotopes (Δ 56 Fe bulk plant-nutrient = −1.36‰) and accumulates Fe in different parts of plant (roots, stems, leaves, husks, and grains) with large Fe isotope fractionation. Under such a growing condition, rice takes up Fe through strategy I and the reduction of Fe 3+ to Fe 2+ results in plants enriching isotopically light Fe. Within the plant, the transportation of Fe is accompanied with changes in redox state, which thus causes significant Fe isotope fractionation among different plant tissues. In contrast, rice plants grown in soils with deficient Fe supply are slightly enriched in heavy Fe isotopes (Δ 56 Fe bulk plant-soil solution = 0.27‰) and accumulate Fe in different plant tissues with little isotope fractionation. Under this growing condition, the rice plant takes up Fe through strategy II. Plants take up Fe from soils as Fe 3+ -phytosiderophores (Fe 3+ -PS) complex and transport Fe as Fe 3+ -nicotianamine (Fe 3+ -NA) complex throughout the plant, which does not involve changes in redox state and thus results in very limited isotope fractionation. The observed difference in Fe isotope fractionation indicates two distinct Fe uptake and translocation strategies for rice grown under the two different conditions: strategy I under Fe-sufficient conditions, and strategy II under Fe-deficient conditions. Our results demonstrate that Fe isotope ratios may be used to distinguish Fe-sufficient versus Fe-deficient conditions and to elucidate Fe biogeochemical processes during rice growth.

Size scaling of the accumulation of four trace metals was examined in the infaunal clam Amiantis umbonella in Kuwait Bay. In clams of varying shell length (2.5–5 cm), soft tissue growth in A. umbonella from a contaminated site was inhibited compared with clams from a less contaminated reference site. Body burdens of all four metals were positively correlated with clam soft tissue wet weight, but for Cd, Cu, and Pb, correlations were stronger in clams from the contaminated site (r2 = 0.6–0.9, p

Cadmium (Cd) and arsenic (As) are nonessential and toxic elements in rice that often occur together in contaminated paddy field soils. To understand whether rice has a common molecular response mechanism against Cd and As toxicity, 30-day seedlings (Oryza sativa L. indica) were exposed separately to Cd and As3+ in hydroponic cultures for up to 7 days. Root transcriptomic analysis of plants exposed to Cd and As for 3 days revealed that a total of 2224 genes in rice roots responded to Cd stress, while 1503 genes responded to As stress. Of these, 841 genes responded to both stressors. The genes in common to Cd and As stress were associated with redox control, stress response, transcriptional regulation, transmembrane transport, signal transduction, as well as biosynthesis and metabolism of macromolecules and sulfur compounds. In plants exposed to Cd and As separately or in combination for 3 and 7 days, qRT-PCR verification revealed that the glutathione metabolism associated gene Os09g0367700 was significantly up-regulated with respect to unexposed controls and had a positive synergistic effect under combined Cd and As stress. In addition, the redox control related genes Os06g0216000, Os07g0638300 and Os01g0294500, the glutathione metabolism related gene Os01g0530900, the cell wall biogenesis related genes Os05g0247800, Os11g0592000 and Os03g0416200, the expression regulation related genes Os07g0597200 and Os02g0168200, and the transmembrane transport related genes Os04g0524500, also varied significantly with respect to an unexposed control and displayed synergistic effects after 7 days of simultaneous exposure to Cd and As. Our identification of a novel set of genes in rice which responded to both Cd and As3+ stress may be of value in mitigating the toxicity of co-contaminated soils. These results also provide a deeper understanding of the molecular mechanisms involved in response to multi-metal/loids stress, and may be used in the genetic improvement of rice varieties.

Schwertmannites are metastable and often contaminated by chromium and arsenic. However, when schwertmannite is subjected to reducing conditions, the effect of reductive dissolution of schwertmannite coupled to the potential reduction of Cr(VI) or As(V) on the behavior of chromium and arsenic are less known. The present study systematically explored Fe(II)-induced transformation of schwertmannite with adsorbed CrO2-4 or AsO3-4 at pH 6.5, and the consequent redistribution of chromium and arsenic. The results show that adsorption of oxyanions obviously facilitated the release of schwertmannite-bound SO42− from both the mineral surface and the lattice structure via SO42--exchange. Adsorbed chromate and arsenate both inhibited the Fe(II)-induced transformation of schwertmannite. CrO2-4 -schwertmannite persisted as the dominant mineral phase for up to 30 d as a consequence of Fe(II) consumption and coverage of Cr(III)-Fe(III) hydroxide. In contrast, AsO3-4-schwertmannite transformed to poorly crystalline goethite via an intermediate lepidocrocite in 4 mM Fe(II) treatment. Both sorption and redox processes of Cr(VI) imprisoned chromium on schwertmannite. Transformation of AsO3-4-schwertmannite caused no re-mobilization of arsenic into solution, indicating the repartition to secondary minerals. Reduction of Cr(VI) to Cr(III) by Fe(II) would greatly lower the environmental hazard of chromium, and the persistence of As(V) would limit the toxicity of arsenic. These findings will help to understand the geochemical behavior of the important toxic Cr(VI) and As(V) specifically in the environmental systems with cyclic redox conditions.

Schwertmannite is a ferric oxyhydroxy-sulfate mineral, which is commonly found in acid sulfate systems. In this study, the liberation of chromium from Cr(VI)-substituted schwertmannite in the presence of the siderophore desferrioxamine B was studied. In addition, mineral transformation was examined using X-ray diffraction, scanning electron microscopy, and X-ray absorption spectroscopy. Based on solid phase analyses, the transformation of schwertmannite incubated at pH 6.5 in the presence of 100 mg/L desferrioxamine B proceeded through a ligand-reductive induced pathway that released dissolved sulfate, Fe(III), and Cr(III), and generated goethite. Results of XAFS showed that CrO42− substituted for SO42− in the schwertmannite framework. Kinetic analysis using the Kabai and shrinking core models showed that Cr suppressed the transformation of schwertmannite into goethite and the liberation of Cr was controlled by diffusion through the secondary mineral's layer of dissolution. The information provided in this study will be useful in predicting the effects of the interaction between dissolved organic matter and minerals on the mobilization of Cr in acid mine drainage systems.

Mercury (Hg) bound to fine aerosols (PM 2.5 -Hg) may undergo photochemical reaction that causes isotopic fractionation and obscures the initial isotopic signatures. In this study, we quantified Hg isotopic compositions for 56 PM 2.5 samples collected between 15 September and 16 October 2015 from Beijing, China, among which 26 were collected during daytime (between 08:00 and 18:30 LT) and 30 during night (between 19:00 and 07:30 LT). The results show that diel variation was statistically significant ( p ) for Hg content, Δ199Hg and Δ200Hg , with Hg content during daytime ( 0.32±0.14 µ g g −1 ) lower than at night ( 0.48±0.24 µ g g −1) and Δ199Hg and Δ200Hg values during daytime (mean of 0.26 ‰±0.40 ‰ and 0.09 ‰±0.06 ‰ , respectively) higher than during nighttime ( 0.04 ‰±0.22 ‰ and 0.06 ‰±0.05 ‰ , respectively), whereas PM 2.5 concentrations and δ202Hg values showed insignificant ( p > 0.05 ) diel variation. Geochemical characteristics of the samples and the air mass backward trajectories (PM 2.5 source related) suggest that diel variation in Δ199Hg values resulted primarily from the photochemical reduction of divalent PM 2.5 -Hg, rather than variations in emission sources. The importance of photoreduction is supported by the strong correlations between Δ199Hg and (i) Δ201Hg (positive, slope = 1.1), (ii) δ202Hg (positive, slope = 1.15), (iii) content of Hg in PM 2.5 (negative), (iv) sunshine durations (positive) and (v) ozone concentration (positive) observed for consecutive day–night paired samples. Our results provide isotopic evidence that local, daily photochemical reduction of divalent Hg is of critical importance to the fate of PM 2.5 -Hg in urban atmospheres and that, in addition to variation in sources, photochemical reduction appears to be an important process that affects both the particle mass-specific abundance and isotopic composition of PM 2.5 -Hg.

Metal sulfides, which are important indicators of sulfur cycling, are usually divided into two categories according to sulfur chemical valence: (1) monosulfides (S2-) and (2) disulfides (S22-). The two sulfur species are separated and quantified by a sequential-extraction method. Specifically, monosulfides are extracted as acid-volatile sulfide (AVS) using 6 M HCl prior to the extraction of disulfides using acidic CrCl2, which is defined as chromium-reducible sulfur (CRS). However, the conventional AVS procedure does not result in the quantitative extraction of S2- from the acid-insoluble metal monosulfide, copper sulfide (CuS). Consequently, residual sulfur in CuS (CuS-S) may be extracted as CRS resulting in the inaccurate separation of these two sulfur species. In this study, we used stannous chloride (SnCl2) to improve CuS-S recovery in the AVS procedure and permit the separate extraction of sulfur from CuS and pyrite (FeS2), the most abundant disulfide in nature. Our results show that the addition of SnCl2 increased the recovery of CuS-S as AVS from less than 36% to as high as 92% in the absence of pyrite and Fe3+ and 89% in the presence of pyrite and Fe3+. In addition, based on the observed correlation between the concentration of SnCl2 and the dissolution of FeS2, we identified the appropriate concentration of SnCl2 needed to avoid the dissolution of FeS2 in the AVS procedure. SnCl2 also minimized the oxidation of CuS-S by Fe3+ released from ferric minerals during the extraction of AVS. Based on the results of a series of sequential-extraction experiments, we show that an amendment of SnCl2 in the AVS procedure followed by CRS permits the quantitative separation of CuS-S and FeS2-S while also preventing interference by Fe3+. Our method will find application in research concerned with the fate of metals and the biogeochemistry of sulfur in the environment.

Anaerobic bacteria are known to produce neurotoxic methylmercury [MeHg] when elemental mercury [Hg(0)] is provided as the sole mercury source. In this study, we examined the formation of MeHg in anaerobic incubations of sediment collected from the San Jacinto River estuary (Texas, USA) amended with aqueous Hg(0) to investigate the microbial communities involved in the conversion of Hg(0) to MeHg. The results show that the addition of the methanogen inhibitor 2-bromoethanesulfonate (BES) significantly decreased MeHg production. The mercury methylation gene, hgcA, was detected in these sediments using archaeal specific primers, and 16S rRNA sequencing showed that a member of the Methanosarcinaceae family of methanogens was active. These results suggest that methanogenic archaea play an underappreciated role in the production of MeHg in estuarine sediments contaminated with Hg(0).

How metal-reducing bacteria transfer electrons during dissimilatory energy generation under electron acceptor-limited conditions is poorly understood. Here, we incubated the iron and manganese-reducing bacterium

Arsenic–rich schwertmannite may cause arsenic (As) release during phase transition. In this study, microbial sulfidogenesis on As(V)–loaded schwertmannite (As-Sch) and associated As mobility at different SO42− concentrations were investigated under anaerobic conditions by Desulfosporosinus meridiei (D. meridiei). For biotic treatments, the more SO42− was added, the more Fe3+ was reduced to Fe2+, and the more As(V) was released during the reductive dissolution of As-Sch. The reduction of As(V) to As(III) by D. meridiei resulted in a higher concentration, toxicity, solubility and mobility of As than the corresponding abiotic treatments. However, compared with the abiotic treatments, a variety of new minerals (such as mackinawite, vivianite, sulfur, As2S3, and parasymplesite) were generated in the biotic treatments, and the As concentration in aqueous solution was less than 1 μM at the end of the incubation period regardless of the presence of SO42−. The results of continuous extraction of different species of As from secondary minerals showed that the effect of microorganisms decreased As content of amorphous iron oxide-bound phase, while increasing that bound on the surface of iron oxide surface-bound phase, thus increasing As fluidity. Our findings indicated that under anaerobic conditions, D. meridiei sulfidogenesis can trigger significant As mobilization in the early stage and remove As from the aqueous solutions when new minerals are formed at a later stage.

Distinct spatiotemporal distributions of sea surface dissolved elemental mercury (DEM) and its air-sea exchange flux were observed in the river-dominated and monsoon-influenced East China Sea (ECS). Spatially, DEM concentrations were higher in the nearshore Changjiang diluted water (90 ± 20 to 260 ± 40 fM) than in the offshore Kuroshio water (60 ± 10 to 160 ± 40 fM) and correlated with salinity and total Hg concentrations, suggesting that the total Hg discharged from the Changjiang river is a controlling factor. In summer, monsoon-driven coastal upwelling formed a transient nearshore water mass with very elevated DEM concentrations (290 ± 20 to 320 ± 70 fM). Seasonally, DEM concentrations in all water masses were the highest in summer (120 ± 30 to 320 ± 70 fM). Estimated rate coefficients for DEM production varied seasonally and strongly correlated with sea surface temperature (SST). Hg0 evasion fluxes also peaked in summer (670 ± 380 pmol m-2 day-1), while in winter, DEM was close to equilibrium with gaseous elemental mercury in the atmosphere. Based on the air-sea Hg fluxes for all four seasons from this study and regional atmospheric deposition fluxes from others, we conclude that the ECS is a net sink of Hg annually, but it is a source of Hg to the atmosphere in summer. Moreover, the contribution of the ECS to Hg evasion may increase as a result of flood-associated high Changjiang discharge and rising SST.

Schwertmannite is an important sink for cadmium (Cd) in acid mine drainage (AMD) environments and is unstable when environmental conditions change. However, the release and redistribution of Cd during schwertmannite transformation with respect to pre-bound Cd are poorly understood. In this work, the transformation of cadmium-associated schwertmannite and subsequent Cd repartitioning behaviors were investigated. The way of schwertmannite associated with Cd was predominant by absorption, and the diffuse layer model (DLM) showed that Cd2+ existed as monodentate complexes ≡Fe(1)OCd+ and ≡Fe(2)OCd+ on schwertmannite surfaces. Kinetics of SO42− release and mineralogical characterization both showed that the mineral transformation rates decreased and more lepidocrocite aggregated with increasing adsorbed Cd levels. The shrinking core model revealed that Fe(II)-induced process would affect mineral dissolution by changing surface reaction-controlled step to internal diffusion-controlled step, and significantly promote the dissolution rate of Cd-adsorbed schwertmannite. Adsorbed Cd blocked the surface sites for later Fe(II) adsorption and the Fe(II)-Fe(III) electron transfer, then resulted in the decelerated transformation and the accumulation of intermediate phase lepidocrocite. The maximum release of aqueous Cd occurred after 1 mM Fe2+ addition, then over 69% of initial added Cd(aq) re-bound to solid-phase accompanying with mineral transformation, and finally, Cd was mainly associated with the secondary minerals by complexation with surficial OH groups. These findings are useful for developing the strategies for treating Cd contamination in AMD affected areas.

Understanding the mechanism of the microbial transformation of Fe(III)-oxyhydroxysulfate minerals is of considerable interest, because this transformation plays an important role in controlling the behaviour of toxic metals from acid mine drainage (AMD). In this study, we examined a sulfate reducing enrichment culture from AMD-contaminated sediments and predicted the possible pathway of electron transfer when incubated with schwertmannite, a common Fe(III)-oxyhydroxysulfate occurring in the AMD environment. Experiments were designed to distinguish the mechanisms by which bacteria facilitate direct (i.e., bacteria allowed to adhere to the mineral) or indirect (i.e., bacteria separated from the mineral by dialysis bag) electron transfer to reduce the mineral. The effects of adding anthraquinone-2,6-disulfonate (AQDS) as an exogenous electron shuttle were also investigated. Vivianite was detected as the main product of schwertmannite transformation. Reduction of sulfate and iron were more pronounced in direct treatments, while more non-reductive dissolution were observed in indirect treatments. The addition of AQDS lead to the production of more dissolved Fe2+ over 20 d than in the absence of AQDS. Microbial community composition differed in direct and indirect treatments, while the addition of AQDS did not significantly affect the community structure in each treatment. After incubation for 20 d, the growth of Desulfovibrio exceeded that of the originally dominant Citrobacter in direct treatments, while an unknown genus most closely related to Citrobacter within Enterobacteriaceae was predominant in indirect treatments. This monodominant community in indirect treatments was assumed not to transfer electron directly to schwertmannite but to rely on shuttling mechanism. PICRUSt results implied that bacteria in indirect treatment have potential to produce shuttling compounds or complexing agents. The absence of dsr genes and the putative fermentative process suggested that the Enterobacteriaceae might indirectly facilitate the dissolution and transformation of schwertmannite.

Schwertmannite, a Fe(III)-oxyhydroxysulfate mineral formed in acidic (pH 3~4), iron- and sulfate-rich acid mine drainage (AMD) environments, tends to undergo phase transformations with changes in pH and redox condition, which may depend on the presence of various trace anions and cations. In the present study, the effects of Cu(II) on the stability of arsenate-, chromate-, and molybdate-substituted schwertmannite were investigated. The release of Fe(III) and sulfate from schwertmannite seems to be accelerated in the presence of Cu(II) at pH ~ 3, while Cu(II) retarded the dissolution of schwertmannite at pH ~ 5. XRD and SEM results showed that pure schwertmannite and chromate-substituted schwertmannite underwent transformation to goethite over a 2-month period, the presence of Cu(II) enhanced the stability of the mineral’s structure at both pH 3 and 5. However, the structures of arsenate- and molybdate-substituted schwertmannites showed no significant changes in the presence or absence of Cu(II) at both pH 3 and 5. During the phase transformation process, the amount of released oxyanions followed the sequence of chromate > molybdate > arsenate. Moreover, the release of arsenate and chromate from schwertmannite was retarded in the presence of Cu(II) at pH 5, whereas the release of molybdate was promoted. These results have important environmental implications for the stability of schwertmannite and its potential to immobilize contaminant trace elements under AMD conditions.

The cycling of Fe and Cr is intimately linked to the fate of schwertmannite in acid mine drainage (AMD) and acid sulfate soil (ASS) environments. Dissolved organic matter (DOM) can affect the stability of minerals and speciation, mobility and toxicity of heavy metals via redox reactions and complexation. However, knowledge about the fate of Fe and Cr upon reduction and complexation of CrO42 −-substituted schwertmannite by fulvic acid (FA) is poorly understood. In this study, experiments were devised to investigate the interaction between FA and schwertmannite, with major degeneration of the schwertmannite structure as well as the formation of secondary minerals. For CrO42 −-substituted schwertmannite, results indicated that the concentrations of total Fe and Cr in the solution were determined to be the maximum values at 360 h and 96 h, and then decreasing over the reaction time when FA was 10 mg/L at pH of 6.5. The characterization on the solid phase by X-ray diffraction, scanning electron microscopy and X-ray photo electron spectroscopy technologies revealed that goethite was the dominant newly secondary mineral phases which played a vital role in controlling the fate and transport of Fe and Cr. The possible mechanism was proposed to be synergistic effect including ligand-promoted and reduction. This study provides new insights into the understanding of trace element behavior in environments affected by DOM and also has guiding significance for Cr-contaminant remediation.

Methylmercury (MeHg), a highly neurotoxic substance, accumulates in aquatic food webs, and is enriched in odd isotopes (i.e., 199Hg and 201Hg), purportedly as a result of abiotic photodegradation in surface waters. Here, we highlight the potential role of phytoplankton in the mass independent fractionation (MIF) of MeHg in marine food-webs by providing evidence of (1) degradation of intracellular MeHg and reduction of intracellular inorganic mercury (Hg(II)) in the marine microalga, Isochrysis galbana; (2) a large, positive MIF (Δ199Hgreactant – Δ199Hgproduct ∼ 5–10‰) during intracellular degradation of MeHg in cells exposed to visible light with no UVB, consistent with the accumulation of odd isotope-enriched MeHg in marine food-webs; and (3) a negative MIF (−1‰) during intracellular reduction of Hg(II) in the presence of UVB light. If representative of the photochemical reactivity of MeHg in marine phytoplankton, our results indicate that algal cell-mediated demethylation of MeHg by visible light could ...

Consistent with the observation that ammonia-oxidizing bacteria (AOB) outnumber ammonia-oxidizing archaea (AOA) in many eutrophic ecosystems globally, AOB typically dominate activated sludge aeration basins from municipal wastewater treatment plants (WWTPs). In this study, we demonstrate that the growth of AOA strains inoculated into sterile-filtered wastewater was inhibited significantly, in contrast to uninhibited growth of a reference AOB strain. In order to identify possible mechanisms underlying AOA-specific inhibition, we show that complex mixtures of organic compounds, such as yeast extract, were highly inhibitory to all AOA strains but not to the AOB strain. By testing individual organic compounds, we reveal strong inhibitory effects of organic compounds with high metal complexation potentials implying that the inhibitory mechanism for AOA can be explained by the reduced bioavailability of an essential metal. Our results further demonstrate that the inhibitory effect on AOA can be alleviated by copper supplementation, which we observed for pure AOA cultures in a defined medium and for AOA inoculated into nitrifying sludge. Our study offers a novel mechanistic explanation for the relatively low abundance of AOA in most WWTPs and provides a basis for modulating the composition of nitrifying communities in both engineered systems and naturally occurring environments.

Jarosite is an iron-hydroxysulfate mineral commonly found in acid mine drainage (AMD). Given its strong adsorption capacity and its ability to co-precipitation with heavy metals, jarosite is considered a potent scavenger of contaminants in AMD-impacted environments. Sulfate-reducing bacteria (SRB) play an important role in the reductive dissolution of jarosite; however, the mechanism involved has yet to be elucidated. In this study, an indigenous SRB community enriched from the Dabaoshan mine area (Guangdong, China) was employed to explore the mechanism of the microbial reduction of jarosite. Different cultures, with or without dissolved sulfate and the physical separation of jarosite from bacteria by dialysis bags, were examined. Results indicate that the reduction of jarosite by SRB occurred via an indirect mechanism. In systems with dissolved sulfate, lactate was incompletely oxidized to acetate coupled with the reduction of SO42− to S2−, which subsequently reduced the Fe3+ in jarosite, forming secondary minerals including vivianite, mackinawite and pyrite. In systems without dissolved sulfate, jarosite dissolution occurred prior to reduction, and similar secondary minerals formed as well. Extracellular polymeric substances secreted by SRB appeared to facilitate the release of sulfate from jarosite. Structural sulfate in the solid phase of jarosite may not be available for SRB respiration. Although direct contact between SRB and jarosite is not necessary for mineral reduction, wrapping jarosite into dialysis bags suppressed the reduction to a certain extent. Microbial community composition differed in direct contact treatments and physical separation treatments. Physical separation of the SRB community from jarosite mineral supported the growth of Citrobacter, while Desulfosporosinus dominated in direct contact treatments.

We examined mercury (Hg) accumulation in juvenile and adult subpopulations of Antarctic krill (Euphausia superba) collected west of the Antarctic Peninsula. Samples were collected along a northern cross-shelf transect beginning near Anvers Island and farther south near the sea ice edge in the austral summers of 2011, 2013, 2014, and 2015. Regardless of geographical position, mean concentrations of total Hg and methylmercury (MeHg), the form of Hg that biomagnifies in marine food webs, were significantly higher in juvenile than adult krill in all years. In 2013, juvenile Antarctic krill collected along the coast near Anvers Island had significantly higher MeHg concentrations than krill collected farther offshore, and in 2013 and 2014, coastal juvenile krill exhibited some of the highest MeHg concentrations of all subpopulations sampled. Across all sampling years, collection in northern (sea ice-free) or southern (sea ice edge) transects did not affect MeHg concentrations of juvenile or adult krill, suggesting similar levels and routes of MeHg exposure across the latitudes sampled. Developmental stage, feeding near the coast, and annual variations in sea ice-driven primary and export production were identified as potentially important factors leading to greater MeHg accumulation in juvenile than adult krill. Krill-dependent predators feeding primarily on juveniles may thus accumulate more MeHg than consumers foraging on older krill. These results report MeHg concentrations in Antarctic krill and will be useful for predicting Hg biomagnification in higher-level consumers in this productive Antarctic ecosystem.

Phytoplankton biomass in estuaries is controlled by complex biological and chemical processes that control growth and mortality, and physical processes that control transport and dilution. The effects of these processes on phytoplankton blooms were systematically analyzed, focusing on identifying the dominant controlling factors out of river-induced flushing, tidal dispersion, nutrient limitation, and light limitation. To capture the physical processes related to flow and sediment dynamics, we used the idealized width-averaged iFlow model. The model was extended with a nutrient-phytoplankton module to capture the essential biological-chemical processes. The model was applied to the Delaware River Estuary for the productive months of March to November. Model results were compared with field observations. It was found that phytoplankton blooms cannot form in the lower bay due to tidal dispersion, as water from the estuary and coastal ocean mix in early spring, and due to local effects of nitrogen limitation in summer. In the middle to upper bay, sediment-induced deterioration of the light climate limits the growth but allows for blooms in the mid bay, while no blooms can form in the turbidity maximum zone in the upper estuary. Further upstream in the tidal river, the effects of river-induced flushing dominate in early spring and prevent bloom formation. In the summer and fall, lower river discharges and higher growth rates at higher temperatures allow blooms to form and persist. Analysis of the connectivity between mid bay and tidal river blooms showed that coastal ocean phytoplankton may contribute to mid bay blooms, but do not penetrate beyond the turbidity maximum zone.

In paddy soils polluted by acid mine drainage (AMD), heavy metals contamination has been studied intensively, but little information has been gathered regarding the associated speciation of sulfur. Therefore, the concentrations of various sulfur species in paddy soil profiles were investigated at sites downstream of the Dabaoshan sulfide mining area, South China. No significant differences in water-soluble sulfate (WS), adsorbed sulfate (AS), or total sulfur (TS) were observed between upstream and downstream paddy soil profiles. A surprising finding was that seasonal change of WS in deeper soil may have contributed to the gradual decline in TS over the past 10 years. The largest concentrations of AS was found in 20 to 30 cm soil horizons, where a strong influence of large surface area soil particles, and abundant amorphous iron hydroxides and fine clays likely enhanced sulfate adsorption. Reduced inorganic sulfur (RIS) was found in highest concentrations at depths of 0 to 20 cm and was significantly positively correlated with total soil C and N contents. Furthermore, ester sulfate (ES) was the dominating sulfur species in every layer, which may result from (i) abundant WS and AS promoted ES formation, and (ii) the presence of active Al and Fe oxides inhibited ES mineralization. Concentrations of sulfur species and other soil properties were used in principle component analysis to identify three geochemically distinct layers in these paddy soils (0 to 20 cm, 20 to 30 cm and 30 to 80 cm). Due to AMD irrigation, differences in the speciation of sulfur among these layers illustrates how the chemical imprint of AMD irrigation has influenced and been affected by the background biogeochemistry of these soils. The present examination of the speciation and vertical and horizontal distribution of sulfur species provides valuable information for the continued management and remediation of these and other AMD-contaminated paddy soils.

The emergence and spread of antibiotic-resistant pathogenic bacteria is currently one of the most serious challenges to human health. To combat this problem, it is critical to understand the processes and pathways that result in the creation of antibiotic resistance gene pools in the environment. In this study, we examined the effects of mercury (Hg) exposure on the co-selection of Hg and antibiotic-resistant bacteria that colonize the gastrointestinal tract of the mummichog (Fundulus heteroclitus), a small, estuarine fish. We examined this connection in two experimental systems: (i) a short-term laboratory exposure study where fish were fed Hg-laced food for 15 days and (ii) an examination of environmental populations from two sites with very different levels of Hg contamination. In the lab exposure study, fish muscle tissue accumulation of Hg was proportional to food Hg concentration (R 2 = 0.99; P

The biological production of monomethylmercury (MeHg) in soils and sediments is an important factor controlling mercury (Hg) accumulation in aquatic and terrestrial food webs. In this study we examined the fractionation of Hg stable isotopes during Hg methylation in nongrowing cultures of the anaerobic bacteria Geobacter sulfurreducens PCA and Desulfovibrio desulfuricans ND132. Both organisms showed mass-dependent, but no mass-independent fractionation of Hg stable isotopes during Hg methylation. Despite differences in methylation rates, the two bacteria had similar Hg fractionation factors (αr/p = 1.0009 and 1.0011, respectively). Unexpectedly, δ(202)Hg values of MeHg for both organisms were 0.4‰ higher than the value of initial inorganic Hg after about 35% of inorganic Hg had been methylated. These results indicate that a (202)Hg-enriched pool of inorganic Hg was preferentially utilized as a substrate for methylation by these organisms, but that multiple intra- and/or extracellular pools supplied inorganic Hg for biological methylation. Understanding the controls of the Hg stable isotopic composition of microbially produced MeHg is important to identifying bioavailable Hg in natural systems and the interpretation of Hg stable isotopes in aquatic food webs.

We investigated the effect of the temporary ban of local industrial activities during the Asia-Pacific Economic Cooperation (APEC) summit (4th-14th Nov 2014) in Beijing, China on total suspended particulate mercury (HgTSP) concentrations and isotope compositions. We measured Hg concentrations and isotope ratios in 33 TSP samples from central Beijing, including 21 samples collected from Jun 2012 to Apr 2014, and 12 samples collected from 14th Oct-19th Nov 2014. Volumetric concentrations of both TSP and HgTSP during the APEC summit were a factor of 2 lower than during the pre-APEC period, indicating substantial reductions in total particulate matter and HgTSP as a result of emissions controls. However, mass-normalized concentrations and mercury isotope ratios of HgTSP did not vary significantly between samples collected before, during, or after the APEC summit. These results show that local emissions are important sources of particle bound mercury (PBM) in Beijing and that their control can be used to immediately lower the volumetric concentration of HgTSP. They also indicate that a similarly complex mixture of sources contributed to PBM in Beijing before and during emissions controls were put in place and that PBM concentrations in Beijing are primarily controlled by emissions and secondarily by photoreduction.

Schwertmannite is an amorphous iron(III)-oxyhydroxysulfate that forms in acid mine drainage (AMD) environments. The characteristic of high heavy metal adsorption capability makes schwertmannite a potentially useful, environmentally friendly material in wastewater treatment. Unstable schwertmannite is prone to recrystallization. Understanding the mechanisms that induce schwertmannite labilization and affect its capacity to remove heavy metals are of great environmental and geochemical significance. Thiocyanate (SCN¯) is a hazardous pseudohalide that is also normally found in AMD. However, little is known about the impact of Fe(III)-binding ligand SCN¯ on schwertmannite stability and its subsequent capacity to bind trace elements. Here, we investigated the adsorption of SCN¯ on schwertmannite and subsequent mineral transformation to characterize this little-known process. The appearance of Fe2+ indicated that the interactions between schwertmannite and SCN¯ may involve complexation and reduction reactions. Results showed that the majority of the adsorbed-SCN¯ was immobilized on schwertmannite during the 60-days transformation. The transformation rates of schwertmannite increased with increasing concentrations of SCN¯. Goethite was detected as the dominant transformation product with or without SCN¯. The mechanisms of SCN¯-promoted dissolution of schwertmannite can be described as follows: (1) formation of Fe(III)-NCS complexes on the schwertmannite surface and in solution, a process which increases the reactivity of solid phase Fe(III); (2) the extraction of Fe(III) from schwertmannite by SCN¯ and subsequent schwertmannite dissolution; and (3) the formation of secondary minerals from extracted Fe(III). These findings may improve AMD treatment strategies and provide insight into the use and potential reuse of schwertmannite as a trace element sorbent.

Mercury (Hg) bound to fine aerosols (PM2.5-Hg) may undergo photochemical reaction that causes isotopic fractionation and obscures the initial isotopic signatures. In this study, we quantified Hg isotopic compositions for 56 PM2.5 samples collected between Sept. 15th and Oct. 16th, 2015 from Beijing, China, among which 26 were collected during the daytime (between 8:00 a.m. and 6:30 p.m.) and 30 during night (between 7:00 p.m. and 7:30 a.m.). The results show that diel variation was statistically significant (p < 0.05) for Hg content, Δ199Hg and Δ200Hg, with Hg content during the daytime (0.32 ± 0.14 µg g−1) lower than at night (0.48 ± 0.24 µg g−1) and Δ199Hg and Δ200Hg values during the daytime (mean of 0.26 ‰ ± 0.40 ‰ and 0.09 ‰ ± 0.06 ‰, respectively) higher than during the nighttime (0.04 ‰ ± 0.22 ‰ and 0.06 ‰ ± 0.05 ‰, respectively), whereas PM2.5 concentrations and δ202Hg values showed insignificant (p > 0.05) diel variation. Geochemical characteristics of the samples and the air mass backward trajectories (PM2.5 source related) suggest that diel variation in Δ199Hg values resulted primarily from the photochemical reduction of divalent PM2.5-Hg, rather than variations in emission sources. The importance of photoreduction is supported by the strong correlations between Δ199Hg and: (i) Δ201Hg (positive, slope = 1.1), (ii) δ202Hg (positive, slope = 1.15), (iii) content of Hg in PM2.5 (negative), (iv) sunshine durations (positive), and (v) ozone concentration (positive) observed for consecutive day-night paired samples. Our results provide isotopic evidence that local, daily photochemical reduction of divalent Hg is of critical importance to the fate of PM2.5-Hg in urban atmospheres and that, in addition to variation in sources, photochemical reduction appears to be an important process that affects both the particle mass-specific abundance and isotopic composition of PM2.5-Hg.

Exposure to dietary sources of methylmercury (MeHg) is the focus of public health concerns with environmental mercury (Hg) contamination. MeHg is formed in anoxic environments by anaerobic microorganisms. This process has been studied mostly with single-species culture incubations, although the relevance of such studies to Hg(II)-methylation in situ is limited because microbial activities in the environment are critically modulated by interactions among microbial functional groups. Here we describe experiments in which Hg(II)-methylation was examined within the context of various microbial syntrophies. We show enhanced Hg(II)-methylation under conditions that established syntrophy by interspecies hydrogen and acetate transfer. Relative to activity of monocultures, interactions of Hg(II) methylating sulfate-reducing bacteria with a methanogen stimulated potential Hg(II)-methylation rates 2-fold to 9-fold, and with Syntrophobacter sp. 1.7-fold to 1.8-fold; those of a Hg(II) methylating Syntrophobacter sp. with a methanogen increased Hg(II)-methylation 2-fold. Under sulfate-depleted conditions, higher Hg(II)-methylation rates in the syntrophic incubations corresponded to higher free energy yields (ΔG°') than in the monocultures. Based on energetic considerations, we therefore propose that syntrophic microbial interactions are likely a major source of MeHg in sulfate- and iron-limited anoxic environments while in sulfate-replete environments, MeHg formation via sulfate reduction dominates.

Mercury resistant bacteria play a critical role in mercury biogeochemical cycling in that they convert methylmercury (MeHg) and inorganic mercury to elemental mercury, Hg(0). To date there are very few studies on the effects of speciation and bioavailability of MeHg in these organisms, and even fewer studies on the role that binding to cellular ligands plays on MeHg uptake. The objective of this study was to investigate the effects of thiol complexation on the uptake of MeHg by measuring the intracellular demethylation-reduction (transformation) of MeHg to Hg(0) in Hg-resistant bacteria. Short-term intracellular transformation of MeHg was quantified by monitoring the loss of volatile Hg(0) generated during incubations of bacteria containing the complete mer operon (including genes from putative mercury transporters) exposed to MeHg in minimal media compared to negative controls with non-mer or heat-killed cells. The results indicate that the complexes MeHgOH, MeHg-cysteine, and MeHg-glutathione are all bioavailable in these bacteria, and without the mer operon there is very little biological degradation of MeHg. In both Pseudomonas stutzeri and Escherichia coli, there was a pool of MeHg that was not transformed to elemental Hg(0), which was likely rendered unavailable to Mer enzymes by non-specific binding to cellular ligands. Since the rates of MeHg accumulation and transformation varied more between the two species of bacteria examined than among MeHg complexes, microbial bioavailability, and therefore microbial demethylation, of MeHg in aquatic systems likely depends more on the species of microorganism than on the types and relative concentrations of thiols or other MeHg ligands present.

Estuarine sediments support the production of monomethylmercury (MeHg) which accumulates in aquatic organisms. While natural variation in mercury stable isotope ratios can potentially be used to track sources and transformations of mercury in the environment, the isotopic signature of MeHg in sediments has not been measured directly. The isotopic composition of MeHg has been studied in laboratory experiments and fish using tandem gas chromatography-multicollector inductively coupled plasma mass spectrometry (MC–ICP-MS) systems; however, the precision and sensitivity of this method may be too low for the analysis of many environmental samples including sediments in which MeHg constitutes 1% or less of the total mercury. In this study, we developed an offline separation method for the precise measurement of the Hg isotopic composition of MeHg in estuarine sediments. Separation of MeHg from inorganic species was accomplished by distillation and chemical ethylation-GC, and was followed by gold amalgam trapping to collect and preconcentrate pyrolyzed MeHg, which was then released into an oxidizing solution. MeHg standards processed in this way were collected with an average yield of 97.5%. External precision for all replicate isotope analyses of MeHg process standards was ± 0.14‰ (2 SD, n = 8) for δ202Hg and no detectable fractionation of Hg stable isotopes occurred during the separation. δ202Hg values for MeHg separated from estuarine sediments using our approach varied from − 0.41 to + 0.41‰ and were generally higher, and spatially and temporally more variable, than those for total Hg (− 0.21 to − 0.48‰).

Lacking sufficient clean water, the paddy soils along the Hengshi River have suffered from long-term acid mine drainage (AMD) contamination. The impacted cropland is too heavily contaminated to grow food safely. The microbial communities inhabiting the environment play pivotal roles in the crop growth, health, and ecological services. In this study, the bacterial, archaeal, and fungal communities in the impacted paddy soil were examined using high-throughput Illumina MiSeq sequencing. The results showed that AMD irrigation considerably enriched the bacterial phylum Acidobacteria and the archaeal phylum Crenarchaeota, while the fungal community was more stable. The abundances of Acidobacteria and Crenarchaeota were significantly positively correlated with the AMD-related environmental factors of pH and heavy metals (Cu, Pb, and Zn). In the most contaminated samples, communities were dominated by the bacteria Candidatus Solibacter and Candidatus Koribacter from the Acidobacteria family. Functional gene profile analysis demonstrated that the energy metabolic processes of the microbial communities, especially C/N related pathways, have adjusted and are well-adapted to tolerating AMD contamination. The present study described the structural and functional differentiation of microbial communities in the rice paddy soil under AMD irrigation. The results are useful for the development of bioremediation strategies using native microbes in the cleanup and biorestoration of AMD-contaminated agriculture soil.

The mechanisms of carbon concentration in marine diatoms are controversial. At low CO2 , decreases in O2 evolution after inhibition of phosphoenolpyruvate carboxylases (PEPCs), and increases in PEPC transcript abundances, have been interpreted as evidence for a C4 mechanism in Thalassiosira pseudonana, but the ascertainment of which proteins are responsible for the subsequent decarboxylation and PEP regeneration steps has been elusive. We evaluated the responses of T. pseudonana to steady-state differences in CO2 availability, as well as to transient shifts to low CO2 , by integrated measurements of photosynthetic parameters, transcript abundances and quantitative proteomics. On shifts to low CO2 , two PEPC transcript abundances increased and then declined on timescales consistent with recoveries of Fv /Fm , non-photochemical quenching (NPQ) and maximum chlorophyll a-specific carbon fixation (Pmax ), but transcripts for archetypical decarboxylation enzymes phosphoenolpyruvate carboxykinase (PEPCK) and malic enzyme (ME) did not change. Of 3688 protein abundances measured, 39 were up-regulated under low CO2 , including both PEPCs and pyruvate carboxylase (PYC), whereas ME abundance did not change and PEPCK abundance declined. We propose a closed-loop biochemical model, whereby T. pseudonana produces and subsequently decarboxylates a C4 acid via PEPC2 and PYC, respectively, regenerates phosphoenolpyruvate (PEP) from pyruvate in a pyruvate phosphate dikinase-independent (but glycine decarboxylase (GDC)-dependent) manner, and recuperates photorespiratory CO2 as oxaloacetate (OAA).

Redox cycling between elemental [Hg(0)] and divalent [Hg(II)] mercury is a key control on the fate and transport of Hg in groundwater systems. In this study, we tested the ability of anaerobic bacteria to oxidize dissolved Hg(0) to Hg(II). Controlled laboratory experiments were carried out with the obligate anaerobic bacterium Geothrix fermentans H5, and the facultative anaerobic bacteria Shewanella oneidensis MR-1 and Cupriavidus metallidurans AE104. Under anoxic conditions, all three bacterial strains reacted with dissolved gaseous Hg(0) to form non-purgeable Hg. In mass balance experiments, the formation of non-purgeable Hg corresponded to the loss of volatile Hg. To determine if the non-purgeable Hg was oxidized, we performed ethylation experiments on Hg(0)-reacted cell suspensions and X-ray absorption near edge structure (XANES) spectroscopy on Hg(0)-reacted cells. Derivatization of non-purgeable Hg to diethylmercury and the Hg L III -edge position of the XANES spectra demonstrated that the reacted bacterial samples contained Hg(II). XANES analysis also revealed that cell-associated Hg(II) was covalently bound to bacterial functional groups, most likely to thiol moieties. Finally, experiments with metabolically active and heat-inactivated cells indicated that both live and dead cells oxidized Hg(0) to Hg(II). Hg(0) oxidation rates for metabolically active cultures increase in the order S. oneidensis MR-1 (1.6 × 10 − 4 fg/cell/min), C. metallidurans AE104 (2.5 × 10 − 4 fg/cell/min), and G. fermentans H5 (23.1 × 10 − 4 fg/cell/min). The results of this study suggest that reactivity towards Hg(0) is widespread among diverse anaerobic bacteria, and passive microbial oxidation of Hg(0) may play an important role in the redox transformation of mercury contaminants in subsurface environments.

Methylmercury (MeHg), a neurotoxic substance that accumulates in aquatic food chains and poses a risk to human health, is synthesized by anaerobic microorganisms in the environment. To date, mercury (Hg) methylation has been attributed to sulfate- and iron-reducing bacteria (SRB and IRB, respectively). Here we report that a methanogen, Methanospirillum hungatei JF-1, methylated Hg in a sulfide-free medium at comparable rates, but with higher yields, than those observed for some SRB and IRB. Phylogenetic analyses showed that the concatenated orthologs of the Hg methylation proteins HgcA and HgcB from M. hungatei are closely related to those from known SRB and IRB methylators and that they cluster together with proteins from eight other methanogens, suggesting that these methanogens may also methylate Hg. Because all nine methanogens with HgcA and HgcB orthologs belong to the class Methanomicrobia , constituting the late-evolving methanogenic lineage, methanogenic Hg methylation could not be considered an ancient metabolic trait. Our results identify methanogens as a new guild of Hg-methylating microbes with a potentially important role in mineral-poor (sulfate- and iron-limited) anoxic freshwater environments.

The transformation of inorganic mercury (Hg) to methylmercury (MeHg) plays a key role in determining the amount of Hg that is bioaccumulated in aquatic food chains. An accurate knowledge of Hg methylation mechanisms is required to predict the conditions that promote MeHg production in aquatic environments. In this study, we conducted experiments to examine the oxidation and methylation of dissolved elemental mercury [Hg(0)] by the anaerobic bacterium Desulfovibrio desulfuricans ND132. Anoxic cultures of D. desulfuricans ND132 were exposed to Hg(0) in the dark, and samples were collected and analyzed for the loss of Hg(0), formation of non-purgeable Hg, and formation of MeHg over time. We found that D. desulfuricans ND132 rapidly transformed dissolved gaseous mercury into non-purgeable Hg, with bacterial cultures producing approximately 40 μg/L of non-purgeable Hg within 30 min, and as much as 800 μg/L of non-purgeable Hg after 36 h. Derivatization of the non-purgeable Hg in the cell suspensions to diethylmercury and analysis of Hg(0)-reacted D. desulfuricans ND132 cells using X-ray absorption near edge structure (XANES) spectroscopy demonstrated that cell-associated Hg was dominantly in the oxidized Hg(II) form. Spectral comparisons and linear combination fitting of the XANES spectra indicated that the oxidized Hg(II) was covalently bonded to cellular thiol functional groups. MeHg analyses revealed that D. desulfuricans ND132 produced up to 118 μg/L of methylmercury after 36 h of incubation. We found that a significant fraction of the methylated Hg was exported out of the cell and released into the culture medium. The results of this work demonstrate a previously unrecognized pathway in the mercury cycle, whereby anaerobic bacteria produce MeHg when provided with dissolved Hg(0) as their sole Hg source.