Carbon Flow in Mercury Biomethylation by Desulfovibrio desulfuricans

Construction of protein-protein interaction network in sulfate-reducing bacteria: Unveiling of global response to Hg

Sulfate-reducing bacteria (SRB) play pivotal roles in the biotransformation of mercury (Hg). However, unrevealed global responses of SRB to Hg have restricted our understanding of details of Hg biotransformation processes. The absence of protein-protein interaction (PPI) network under Hg stimuli has been a bottleneck of proteomic analysis for molecular mechanisms of Hg transformation. This study constructed the first comprehensive PPI network of SRB in response to Hg, encompassing 67 connected nodes, 26 independent nodes, and 121 edges, covering 93% of differentially expressed proteins from both previous studies and this study. The network suggested that proteomic changes of SRB in response to Hg occurred globally, including microbial metabolism in diverse environments, carbon metabolism, nucleic acid metabolism and translation, nucleic acid repair, transport systems, nitrogen metabolism, and methyltransferase activity, partial of which could cover the known knowledge. Antibiotic resistance was the original response revealed by this network, providing insights into of Hg biotransformation mechanisms. This study firstly provided the foundational network for a comprehensive understanding of SRB's responses to Hg, convenient for exploration of potential targets for Hg biotransformation. Furthermore, the network indicated that Hg enhances the metabolic activities and modification pathways of SRB to maintain cellular activities, shedding light on the influences of Hg on the carbon, nitrogen, and sulfur cycles at the cellular level.

Mercury in wetlands over 60 years: Research progress and emerging trends

Wetlands are considered the hotspots for mercury (Hg) biogeochemistry, garnering global attention. Therefore, it is important to review the research progress in this field and predict future frontiers. To achieve that, we conducted a literature analysis by collecting 15,813 publications about Hg in wetlands from the Web of Science Core Collection. The focus of wetland Hg research has changed dramatically over time: 1) In the initial stage (i.e., 1959-1990), research mainly focused on investigating the sources and contents of Hg in wetland environments and fish. 2) For the next 20 years (i.e., 1991-2010), Hg transformation (e.g., Hg reduction and methylation) and environmental factors that affect Hg bioaccumulation have attracted extensive attention. 3) In the recent years of 2011-2022, hot topics in Hg study include microbial Hg methylators, Hg bioavailability, methylmercury (MeHg) demethylation, Hg stable isotope, and Hg cycling in paddy fields. Finally, we put forward future research priorities, i.e., 1) clarifying the primary factors controlling MeHg production, 2) uncovering the MeHg demethylation process, 3) elucidating MeHg bioaccumulation process to better predict its risk, and 4) recognizing the role of wetlands in Hg circulation. This research shows a comprehensive knowledge map for wetland Hg research and suggests avenues for future studies.

Microbial mercury transformations: Molecules, functions and organisms

Mercury (Hg) methylation, methylmercury (MeHg) demethylation, and inorganic redox transformations of Hg are microbe-mediating processes that determine the fate and cycling of Hg and MeHg in many environments, and by doing so influence the health of humans and wild life. The discovery of the Hg methylation genes, hgcAB, in the last decade together with advances in high throughput and genome sequencing methods, have resulted in an expanded appreciation of the diversity of Hg methylating microbes. This review aims to describe experimentally confirmed and recently discovered hgcAB gene-carrying Hg methylating microbes; phylogenetic and taxonomic analyses are presented. In addition, the current knowledge on transformation mechanisms, the organisms that carry them out, and the impact of environmental parameters on Hg methylation, MeHg demethylation, and inorganic Hg reduction and oxidation is summarized. This knowledge provides a foundation for future action toward mitigating the impact of environmental Hg pollution.

Microbial mercury methylation in the cryosphere: Progress and prospects

Mercury (Hg) is one of the most toxic heavy metals, and its cycle is mainly controlled by oxidation-reduction reactions carried out by photochemical or microbial process under suitable conditions. The deposition and accumulation of methylmercury (MeHg) in various ecosystems, including the cryospheric components such as snow, meltwater, glaciers, and ice sheet, and subsequently in the food chain pose serious health concerns for living beings. Unlike the abundance of knowledge about the processes of MeHg production over land and oceans, little is known about the sources and production/degradation rate of MeHg in cryosphere systems. In addition, processes controlling the concentration of Hg and MeHg in the cryosphere remains poorly understood, and filling this scientific gap has been challenging. Therefore, it is essential to study and review the deposition and accumulation by biological, physical, and chemical mechanisms involved in Hg methylation in the cryosphere. This review attempts to address knowledge gaps in understanding processes, especially biotic and abiotic, applicable for Hg methylation in the cryosphere. First, we focus on the variability in Hg concentration and mechanisms of Hg methylation, including physical, chemical, microbial, and biological processes, and transportation in the cryosphere. Then, we elaborate on the mechanism of redox reactions and biotic and abiotic factors controlling Hg methylation and biogeochemistry of Hg in the cryosphere. We also present possible mechanisms of Hg methylation with an emphasis on microbial transformation and molecular function to understand variability in Hg concentration in the cryosphere. Recent advancements in the genetic and physicochemical mechanisms of Hg methylation are also presented. Finally, we summarize and propose a method to study the unsolved issues of Hg methylation in the cryosphere.

Microbial Biosynthesis of Thiol Compounds: Implications for Speciation, Cellular Uptake, and Methylation of Hg(II)

Cellular uptake of inorganic divalent mercury (Hg(II)) is a key step in microbial formation of neurotoxic methylmercury (MeHg), but the mechanisms remain largely unidentified. We show that the iron reducing bacterium Geobacter sulfurreducens produces and exports appreciable amounts of low molecular mass thiol (LMM-RSH) compounds reaching concentrations of about 100 nM in the assay medium. These compounds largely control the chemical speciation and bioavailability of Hg(II) by the formation of Hg(LMM-RS)₂ complexes (primarily with cysteine) in assays without added thiols. By characterizing these effects, we show that the thermodynamic stability of Hg(II)-complexes is a principal controlling factor for Hg(II) methylation by this bacterium such that less stable complexes with mixed ligation involving LMM-RSH, OH^-, and Cl^- are methylated at higher rates than the more stable Hg(LMM-RS)₂ complexes. The Hg(II) methylation rate across different Hg(LMM-RS)₂ compounds is also influenced by the chemical structure of the complexes. In contrast to the current perception of microbial uptake of Hg, our results adhere to generalized theories for metal biouptake based on metal complexation with cell surface ligands and refine the mechanistic understanding of Hg(II) availability for microbial methylation.

Kinetics of Enzymatic Mercury Methylation at Nanomolar Concentrations Catalyzed by HgcAB

Methylmercury (MeHg) is a potent bioaccumulative neurotoxin that is produced by certain anaerobic bacteria and archaea. Mercury (Hg) methylation has been linked to the gene pair hgcAB, which encodes a membrane-associated corrinoid protein and a ferredoxin. Although microbial Hg methylation has been characterized in vivo, the cellular biochemistry and the specific roles of the gene products HgcA and HgcB in Hg methylation are not well understood. Here, we report the kinetics of Hg methylation in cell lysates of Desulfovibrio desulfuricans ND132 at nanomolar Hg concentrations. The enzymatic Hg methylation mediated by HgcAB is highly oxygen sensitive, irreversible, and follows Michaelis-Menten kinetics, with an apparent K_m of 3.2 nM and V _max of 19.7 fmol · min^-1 · mg^-1 total protein for the substrate Hg(II). Although the abundance of HgcAB in the cell lysates is extremely low, Hg(II) was quantitatively converted to MeHg at subnanomolar substrate concentrations. Interestingly, increasing thiol/Hg(II) ratios did not impact Hg methylation rates, which suggests that HgcAB-mediated Hg methylation effectively competes with cellular thiols for Hg(II), consistent with the low apparent K_m Supplementation of 5-methyltetrahydrofolate or pyruvate did not enhance MeHg production, while both ATP and a nonhydrolyzable ATP analog decreased Hg methylation rates in cell lysates under the experimental conditions. These studies provide insights into the biomolecular processes associated with Hg methylation in anaerobic bacteria.IMPORTANCE The concentration of Hg in the biosphere has increased dramatically over the last century as a result of industrial activities. The microbial conversion of inorganic Hg to MeHg is a global public health concern due to bioaccumulation and biomagnification of MeHg in food webs. Exposure to neurotoxic MeHg through the consumption of fish represents a significant risk to human health and can result in neuropathies and developmental disorders. Anaerobic microbial communities in sediments and periphyton biofilms have been identified as sources of MeHg in aquatic systems, but the associated biomolecular mechanisms are not fully understood. In the present study, we investigate the biochemical mechanisms and kinetics of MeHg formation by HgcAB in sulfate-reducing bacteria. These findings advance our understanding of microbial MeHg production and may help inform strategies to limit the formation of MeHg in the environment.

Methylmercury's chemistry: From the environment to the mammalian brain

Methylmercury is a neurotoxicant that is found in fish and rice. MeHg's toxicity is mediated by blockage of -SH and -SeH groups of proteins. However, the identification of MeHg's targets is elusive. Here we focus on the chemistry of MeHg in the abiotic and biotic environment. The toxicological chemistry of MeHg is complex in metazoans, but at the atomic level it can be explained by exchange reactions of MeHg bound to -S(e)H with another free -S(e)H group (R¹S(e)-HgMe + R²-S(e)H ↔ R¹S(e)H + R²-S(e)-HgMe). This reaction was first studied by professor Rabenstein and here it is referred as the "Rabenstein's Reaction". The absorption, distribution, and excretion of MeHg in the environment and in the body of animals will be dictated by Rabenstein's reactions. The affinity of MeHg by thiol and selenol groups and the exchange of MeHg by Rabenstein's Reaction (which is a diffusion controlled reaction) dictates MeHg's neurotoxicity. However, it is important to emphasize that the MeHg exchange reaction velocity with different types of thiol- and selenol-containing proteins will also depend on protein-specific structural and thermodynamical factors. New experimental approaches and detailed studies about the Rabenstein's reaction between MeHg with low molecular mass thiol (LMM-SH) molecules (cysteine, GSH, acetyl-CoA, lipoate, homocysteine) with abundant high molecular mass thiol (HMM-SH) molecules (albumin, hemoglobin) and HMM-SeH (GPxs, Selenoprotein P, TrxR1-3) are needed. The study of MeHg migration from -S(e)-Hg- bonds to free -S(e)H groups (Rabenstein's Reaction) in pure chemical systems and neural cells (with special emphasis to the LMM-SH and HMM-S(e)H molecules cited above) will be critical to developing realistic constants to be used in silico models that will predict the distribution of MeHg in humans.

Site-directed mutagenesis of HgcA and HgcB reveals amino acid residues important for mercury methylation

Methylmercury is a potent neurotoxin that is produced by anaerobic microorganisms from inorganic mercury by a recently discovered pathway. A two-gene cluster, consisting of hgcA and hgcB, encodes two of the proteins essential for this activity. hgcA encodes a corrinoid protein with a strictly conserved cysteine proposed to be the ligand for cobalt in the corrinoid cofactor, whereas hgcB encodes a ferredoxin-like protein thought to be an electron donor to HgcA. Deletion of either gene eliminates mercury methylation by the methylator Desulfovibrio desulfuricans ND132. Here, site-directed mutants of HgcA and HgcB were constructed to determine amino acid residues essential for mercury methylation. Mutations of the strictly conserved residue Cys93 in HgcA, the proposed ligand for the corrinoid cobalt, to Ala or Thr completely abolished the methylation capacity, but a His substitution produced measurable methylmercury. Mutations of conserved amino acids near Cys93 had various impacts on the methylation capacity but showed that the structure of the putative "cap helix" region harboring Cys93 is crucial for methylation function. In the ferredoxin-like protein HgcB, only one of two conserved cysteines found at the C terminus was necessary for methylation, but either cysteine sufficed. An additional, strictly conserved cysteine, Cys73, was also determined to be essential for methylation. This study supports the previously predicted importance of Cys93 in HgcA for methylation of mercury and reveals additional residues in HgcA and HgcB that facilitate the production of this neurotoxin.

Investigation of mercury methylation pathways in biofilm versus planktonic cultures of Desulfovibrio desulfuricans

Biofilms can methylate mercury (Hg) at higher rates than unattached bacteria and are increasingly recognized as important Hg methylation sites in the environment. Our previous study showed that methylation rates in biofilm cultures were up to 1 order of magnitude greater than those in planktonic cultures of a sulfate-reducing bacterium. To probe whether the differential Hg methylation rates resulted from metabolic differences between these two cultures, Hg methylation assays following molybdate or chloroform inhibition (a specific inhibitor of the acetyl-CoA pathway) were conducted on biofilm and planktonic cultures of Desulfovibrio desulfuricans strains M8 and ND132. Molybdate was as effective in inhibiting Hg methylation as well as growth in both planktonic and biofilm cultures. The addition of chloroform only impacted Hg methylation in biofilm cultures, suggesting that different pathways are used for methylation in biofilm compared to planktonic cultures. To investigate this further, expression of the cooS gene, which encodes for carbon monoxide dehydrogenase, a key enzyme in the acetyl-CoA pathway, was compared in biofilm and planktonic cultures of ND132. Biofilm cultures showed up to 4 times higher expression of cooS than planktonic cultures. On the basis of these results, the acetyl-CoA pathway appears to play an important role in methylation in biofilm cultures of this organism, possibly by supplying the methyl group to Hg methylating enzymes; methylation in planktonic cultures appears to be independent of this pathway. This observation has important implications, particularly in developing reliable models to predict Hg methylation rates in different environments and perhaps eventually in being able to control this undesirable chemical transformation.

Mechanisms regulating mercury bioavailability for methylating microorganisms in the aquatic environment: a critical review

Mercury is a potent neurotoxin for humans, particularly if the metal is in the form of methylmercury. Mercury is widely distributed in aquatic ecosystems as a result of anthropogenic activities and natural earth processes. A first step toward bioaccumulation of methylmercury in aquatic food webs is the methylation of inorganic forms of the metal, a process that is primarily mediated by anaerobic bacteria. In this Review, we evaluate the current state of knowledge regarding the mechanisms regulating microbial mercury methylation, including the speciation of mercury in environments where methylation occurs and the processes that control mercury bioavailability to these organisms. Methylmercury production rates are generally related to the presence and productivity of methylating bacteria and also the uptake of inorganic mercury to these microorganisms. Our understanding of the mechanisms behind methylation is limited due to fundamental questions related to the geochemical forms of mercury that persist in anoxic settings, the mode of uptake by methylating bacteria, and the biochemical pathway by which these microorganisms produce and degrade methylmercury. In anoxic sediments and water, the geochemical forms of mercury (and subsequent bioavailability) are largely governed by reactions between Hg(II), inorganic sulfides, and natural organic matter. These interactions result in a mixture of dissolved, nanoparticulate, and larger crystalline particles that cannot be adequately represented by conventional chemical equilibrium models for Hg bioavailability. We discuss recent advances in nanogeochemistry and environmental microbiology that can provide new tools and unique perspectives to help us solve the question of how microorganisms methylate mercury. An understanding of the factors that cause the production and degradation of methylmercury in the environment is ultimately needed to inform policy makers and develop long-term strategies for controlling mercury contamination.

Detailed assessment of the kinetics of Hg-cell association, Hg methylation, and methylmercury degradation in several Desulfovibrio species

The kinetics of inorganic Hg [Hg(II)(i)] association, methylation, and methylmercury (MeHg) demethylation were examined for a group of Desulfovibrio species with and without MeHg production capability. We employed a detailed method for assessing MeHg production in cultures, including careful control of medium chemistry, cell density, and growth phase, plus mass balance of Hg(II)(i) and MeHg during the assays. We tested the hypothesis that differences in Hg(II)(i) sorption and/or uptake rates drive observed differences in methylation rates among Desulfovibrio species. Hg(II)(i) associated rapidly and with high affinity to both methylating and nonmethylating species. MeHg production by Hg-methylating strains was rapid, plateauing after ∼3 h. All MeHg produced was rapidly exported. We also tested the idea that all Desulfovibrio species are capable of Hg(II)(i) methylation but that rapid demethylation masks its production, but we found this was not the case. Therefore, the underlying reason why MeHg production capability is not universal in the Desulfovibrio is not differences in Hg affinity for cells nor differences in the ability of strains to degrade MeHg. However, Hg methylation rates varied substantially between Hg-methylating Desulfovibrio species even in these controlled experiments and after normalization to cell density. Thus, biological differences may drive cross-species differences in Hg methylation rates. As part of this study, we identified four new Hg methylators (Desulfovibrio aespoeensis, D. alkalitolerans, D. psychrotolerans, and D. sulfodismutans) and four nonmethylating species (Desulfovibrio alcoholivorans, D. tunisiensis, D. carbinoliphilus, and D. piger) in our ongoing effort to generate a library of strains for Hg methylation genomics.

Genome sequence of the mercury-methylating and pleomorphic Desulfovibrio africanus Strain Walvis Bay

Desulfovibrio africanus strain Walvis Bay is an anaerobic sulfate-reducing bacterium capable of producing methylmercury (MeHg), a potent human neurotoxin. The mechanism of methylation by this and other organisms is unknown. We present the 4.2-Mb genome sequence to provide further insight into microbial mercury methylation and sulfate-reducing bacteria.

Sulfate-reducing bacterium Desulfovibrio desulfuricans ND132 as a model for understanding bacterial mercury methylation

We propose the use of Desulfovibrio desulfuricans ND132 as a model species for understanding the mechanism of microbial Hg methylation. Strain ND132 is an anaerobic dissimilatory sulfate-reducing bacterium (DSRB), isolated from estuarine mid-Chesapeake Bay sediments. It was chosen for study because of its exceptionally high rates of Hg methylation in culture and its metabolic similarity to the lost strain D. desulfuricans LS, the only organism for which methylation pathways have been partially defined. Strain ND132 is an incomplete oxidizer of short-chain fatty acids. It is capable of respiratory growth using fumarate as an electron acceptor, supporting growth without sulfide production. We used enriched stable Hg isotopes to show that ND132 simultaneously produces and degrades methylmercury (MeHg) during growth but does not produce elemental Hg. MeHg produced by cells is mainly excreted, and no MeHg is produced in spent medium. Mass balances for Hg and MeHg during the growth of cultures, including the distribution between filterable and particulate phases, illustrate how medium chemistry and growth phase dramatically affect Hg solubility and availability for methylation. The available information on Hg methylation among strains in the genus Desulfovibrio is summarized, and we present methylation rates for several previously untested species. About 50% of Desulfovibrio strains tested to date have the ability to produce MeHg. Importantly, the ability to produce MeHg is constitutive and does not confer Hg resistance. A 16S rRNA-based alignment of the genus Desulfovibrio allows the very preliminary assessment that there may be some evolutionary basis for the ability to produce MeHg within this genus.

Genome sequence of the mercury-methylating strain Desulfovibrio desulfuricans ND132

Desulfovibrio desulfuricans strain ND132 is an anaerobic sulfate-reducing bacterium (SRB) capable of producing methylmercury (MeHg), a potent human neurotoxin. The mechanism of methylation by this and other organisms is unknown. We present the 3.8-Mb genome sequence to provide further insight into microbial mercury methylation.

Mercury and other heavy metals influence bacterial community structure in contaminated Tennessee streams

High concentrations of uranium, inorganic mercury [Hg(II)], and methylmercury (MeHg) have been detected in streams located in the Department of Energy reservation in Oak Ridge, TN. To determine the potential effects of the surface water contamination on the microbial community composition, surface stream sediments were collected 7 times during the year, from 5 contaminated locations and 1 control stream. Fifty-nine samples were analyzed for bacterial community composition and geochemistry. Community characterization was based on GS 454 FLX pyrosequencing with 235 Mb of 16S rRNA gene sequence targeting the V4 region. Sorting and filtering of the raw reads resulted in 588,699 high-quality sequences with lengths of >200 bp. The bacterial community consisted of 23 phyla, including Proteobacteria (ranging from 22.9 to 58.5% per sample), Cyanobacteria (0.2 to 32.0%), Acidobacteria (1.6 to 30.6%), Verrucomicrobia (3.4 to 31.0%), and unclassified bacteria. Redundancy analysis indicated no significant differences in the bacterial community structure between midchannel and near-bank samples. Significant correlations were found between the bacterial community and seasonal as well as geochemical factors. Furthermore, several community members within the Proteobacteria group that includes sulfate-reducing bacteria and within the Verrucomicrobia group appeared to be associated positively with Hg and MeHg. This study is the first to indicate an influence of MeHg on the in situ microbial community and suggests possible roles of these bacteria in the Hg/MeHg cycle.

Anaerobic microflora of everglades sediments: effects of nutrients on population profiles and activities

Everglades sediments (wetland soils) near sources of agricultural runoff had low redox potentials, were blackened with sulfide, and displayed high porewater phosphorus (total) concentrations and high water column conductivities. These sediments yielded 10(sup3)- to 10(sup4)-fold-higher numbers of culturable anaerobes, including methanogens, sulfate reducers, and acetate producers, than did sediments from Everglades and Lake Okeechobee comparative control sites not as directly associated with agricultural runoff. These observations demonstrated that there was a general, rather than specific, enhancement of the anaerobic microflora in the sediments most likely influenced by agricultural runoff. Despite these differences in microfloral patterns, methylmercury and total mercury levels were similar among these contrasting sediments. Although available sulfate and phosphorus appeared to stimulate the productivity of sulfate reducers in Everglades sediments, the number of culturable sulfate reducers did not directly correspond to the concentration of sulfate and phosphorus in porewaters. Microcosms supplemented with sulfate, nitrate, and phosphate altered the initial capacities of the sediment microflora to produce acetate and methane from endogenous matter. For sediments nearest sources of agricultural runoff, phosphorus temporarily enhanced acetate formation and initially suppressed methane production, sulfate enhanced acetate formation but did not significantly alter the production of methane, and nitrate totally suppressed the initial production of both methane and acetate. In regards to the latter, microbes capable of dissimilating nitrate to ammonium were present in greater culturable numbers than denitrifiers. In microcosms, acetate was a major source of methane, and supplemental hydrogen was directed towards the synthesis of acetate via CO(inf2)-dependent acetogenesis. These findings demonstrate that Everglades sediments nearest agricultural runoff have enhanced anaerobic microbial profiles and that the anaerobic microflora are poised to respond rapidly to phosphate, sulfate, and nitrate input.

Methylmercury decomposition in sediments and bacterial cultures: involvement of methanogens and sulfate reducers in oxidative demethylation

Demethylation of monomethylmercury in freshwater and estuarine sediments and in bacterial cultures was investigated with CH(3)HgI. Under anaerobiosis, results with inhibitors indicated partial involvement of both sulfate reducers and methanogens, the former dominating estuarine sediments, while both were active in freshwaters. Aerobes were the most significant demethylators in estuarine sediments, but were unimportant in freshwater sediments. Products of anaerobic demethylation were mainly CO(2) as well as lesser amounts of CH(4). Acetogenic activity resulted in fixation of some CO(2) produced from CH(3)HgI into acetate. Aerobic demethylation in estuarine sediments produced only CH(4), while aerobic demethylation in freshwater sediments produced small amounts of both CH(4) and CO(2). Two species of Desulfovibrio produced only traces of CH(4) from CH(3)HgI, while a culture of a methylotrophic methanogen formed traces of CO(2) and CH(4) when grown on trimethylamine in the presence of the CH(3)HgI. These results indicate that both aerobes and anaerobes demethylate mercury in sediments, but that either group may dominate in a particular sediment type. Aerobic demethylation in the estuarine sediments appeared to proceed by the previously characterized organomercurial-lyase pathway, because methane was the sole product. However, aerobic demethylation in freshwater sediments as well as anaerobic demethylation in all sediments studied produced primarily carbon dioxide. This indicates the presence of an oxidative pathway, possibly one in which methylmercury serves as an analog of one-carbon substrates.

Methylmercury Resistance in Desulfovibrio desulfuricans Strains in Relation to Methylmercury Degradation

Two strains of Desulfovibrio desulfuricans, one known to synthesize monomethylmercury from ionic mercury, were grown to determine methylmercury toxicity and for comparison with an anaerobic strain of Clostridium pasteurianum, a H(2) producer, and with the broad-spectrum mercury-resistant Pseudomonas putida strain FB-1, capable of degrading 1 mug of methylmercury to methane and elemental mercury in 2 h. The CH(3)HgCl resistance of D. desulfuricans strains was 10 times that of P. putida FB-1 and 100 times that of C. pasteurianum. The methylmercury resistance of D. desulfuricans was related to the disappearance of methylmercury from cultures by transformation to dimethylmercury, metacinnabar, methane, and traces of ionic mercury. During a 15-day experiment the kinetics of the two volatile compounds dimethylmercury [(CH(3))(2)Hg] and methane were monitored in the liquid by a specific new technique with purge-and-trap gas chromatography in line with Fourier transform infrared spectroscopy and in the headspace by gas chromatography with flame ionization detection. Insoluble metacinnabar (cubic HgS) of biological origin was detected by X-ray diffractometry in the gray precipitate from the insoluble residue of the pellet of a 1-liter culture spiked with 100 mg of CH(3)HgCl. This was compared with a 1-liter culture of D. desulfuricans LS spiked with 100 mg of HgCl(2). In a further experiment, it was demonstrated that insoluble, decomposable, white dimethylmercury sulfide [(CH(3)Hg)(2)S] formed instantly in the reaction of methylmercury with hydrogen sulfide. This organomercurial was extracted with chloroform and identified by gas chromatography in line with mass spectrometry. The D. desulfuricans strains were resistant to high concentrations of methylmercury because they produced insoluble dimethylmercury sulfide, which slowly decomposed under anaerobic conditions to metacinnabar and volatilized to dimethylmercury and methane between pHs 6.2 and 6.5 for high (4.5-g . liter) or low (0.09-g . liter) sulfate contents. Methane was produced from CH(3)HgCl at a lower rate than by the broad-spectrum Hg-resistant P. putida strain FB-1.

Abiotic methylation of mercury in the aquatic environment

Methylation of inorganic mercury in the aquatic environment has been considered to be largely the result of biological processes, primarily involving sulfate-reducing bacteria. However, these processes cannot account for all of the methylmercury that is formed naturally. A growing body of evidence suggests that chemical reactions represent another possible pathway for mercury methylation in the aquatic environment. In order to assess the abiotic contribution to mercury methylation in the water column, and specifically the conditions under which this contribution may be significant, the current state of knowledge about environmentally significant methylation reactions is reviewed. Results of our laboratory-based investigations of aqueous mercury reactions with some potential methyl donors, including MeCo(dmg)(2)(H2O), a simple model for methylcobalamin, various methyltin compounds and methyl iodide, are presented. In each reaction, the yield of methylmercury and the rate of methylation depend strongly on environmental factors such as pH, temperature, and the presence of complexing agents, especially chloride.

Mercury methylation from unexpected sources: molybdate-inhibited freshwater sediments and an iron-reducing bacterium

Methylmercury has been thought to be produced predominantly by sulfate-reducing bacteria in anoxic sediments. Here we show that in circumneutral pH sediments (Clear Lake, CA) application of a specific inhibitor of sulfate-reducing bacteria at appropriate concentrations typically inhibited less than one-half of all anaerobic methylation of added divalent mercury. This suggests that one or more additional groups of microbes are active methylators in these sediments impacted by a nearby abandoned mercury mine. From Clear Lake sediments, we isolated the iron-reducing bacterium Geobacter sp. strain CLFeRB, which can methylate mercury at a rate comparable to Desulfobulbus propionicus strain 1pr3, a sulfate-reducing bacterium known to be an active methylator. This is the first time that an iron-reducing bacterium has been shown to methylate mercury at environmentally significant rates. We suggest that mercury methylation by iron-reducing bacteria represents a previously unidentified and potentially significant source of this environmental toxin in iron-rich freshwater sediments.

Mercury methylation from unexpected sources: molybdate-inhibited freshwater sediments and an iron-reducing bacterium

Methylmercury has been thought to be produced predominantly by sulfate-reducing bacteria in anoxic sediments. Here we show that in circumneutral pH sediments (Clear Lake, CA) application of a specific inhibitor of sulfate-reducing bacteria at appropriate concentrations typically inhibited less than one-half of all anaerobic methylation of added divalent mercury. This suggests that one or more additional groups of microbes are active methylators in these sediments impacted by a nearby abandoned mercury mine. From Clear Lake sediments, we isolated the iron-reducing bacterium Geobacter sp. strain CLFeRB, which can methylate mercury at a rate comparable to Desulfobulbus propionicus strain 1pr3, a sulfate-reducing bacterium known to be an active methylator. This is the first time that an iron-reducing bacterium has been shown to methylate mercury at environmentally significant rates. We suggest that mercury methylation by iron-reducing bacteria represents a previously unidentified and potentially significant source of this environmental toxin in iron-rich freshwater sediments.

Mercury body burdens in Gambusia holbrooki and Erimyzon sucetta in a wetland mesocosm amended with sulfate

This study used an experimental model of a constructed wetland to evaluate the risk of mercury methylation when the soil is amended with sulfate. The model was planted with Schoenoplectus californicus and designed to reduce copper, mercury, and metal-related toxicity in a wastestream. The sediments of the model were varied during construction to provide a control and two levels of sulfate treatment, thus allowing characterization of sulfate's effect on mercury methylation and bioaccumulation in periphyton and two species of fish--eastern mosquitofish (Gambusia holbrooki) and lake chubsucker (Erimyzon sucetta). After one year in the experimental model, mean dry-weight normalized total mercury concentrations in mosquitofish from the non-sulfate treated controls (374+/-77 ng/g) and the reference location (233+/-17 ng/g) were significantly lower than those from the low and high sulfate treatments (520+/-73 and 613+/-80 ng/g, respectively). For lake chubsucker, mean total mercury concentration in fish from the high sulfate treatment (276+/-63 ng/g) was significantly elevated over that observed in the control (109+/-47 ng/g), the low sulfate treatment (122+/-42 ng/g), and the reference population (41+/-2 ng/g). Mercury in periphyton was mostly inorganic as methylmercury ranged from 6.6 ng/g (dry weight) in the control to 9.8 ng/g in the high sulfate treatment, while total mercury concentrations ranged from 1147 ng/g in the control to a high of 1297 ng/g in the low sulfate treatment. Fish methylmercury bioaccumulation factors from sediment ranged from 52 to 390 and from 495 to 3059 for water. These results suggest that sulfate treatments add a factor of risk due to elevated production of methylmercury in sediment and porewater which biomagnified into small fish, and may potentially increase through the food web.

Methylmercury formation in a wetland mesocosm amended with sulfate

This study used an experimental model to evaluate methylmercury accumulation when the soil of a constructed wetland is amended with sulfate. The model was planted with Schoenoplectus californicus and designed to reduce wastestream metals and metal-related toxicity. The soil was varied during construction to provide a control and two sulfate treatments which were equally efficient at overall mercury and copper removal. After an initial stabilization period, methylmercury concentrations in porewater were up to three times higher in the sulfate-treated porewater (0.5-1.6 ng/L) than in the control (<0.02-0.5 ng/L). Mean percent methylmercury was 9.0% in the control with 18.5 and 16.6% in the low- and high-sulfate treatments, respectively. Methylmercury concentrations measured in mesocosm surface water did not reflect the differences between the control and the sulfate treatments that were noted in porewater. The mean bulk sediment methylmercury concentration in the top 6 cm of the low-sulfate treatment (2.33 ng/g) was significantly higher than other treatment means which ranged from 0.96 to 1.57 ng/g. Total mercury in sediment ranged from 20.8 to 33.4 ng/g, with no differences between treatments. Results suggest that the non-sulfate-amended control was equally effective in removing metals while keeping mercury methylation low.

Anaerobic benzene biodegradation--a new era

Benzene is biodegraded in the absence of oxygen under a variety of terminal electron-accepting conditions. However, the mechanism by which anaerobic benzene degradation occurs is unclear. Phenol and benzoate have been consistently detected as intermediates of anaerobic benzene degradation, suggesting that the hydroxylation of benzene to phenol is one of the initial steps in anaerobic benzene degradation. The conversion of phenol to benzoate could then occur by the carboxylation of phenol to form 4-hydroxybenzoate followed by the reductive removal of the hydroxyl group to form benzoate. 13C-Labeling studies suggest that the carboxyl carbon of benzoate is derived from one of the carbons of benzene. Although the fumarate addition reaction is commonly used to activate many hydrocarbons for anaerobic degradation, the large activation energy required to remove hydrogen from the benzene ring argues against such an approach for anaerobic benzene metabolism. The alkylation of benzene to toluene has been detected in several mammalian tissues, and offers an interesting alternate hypothesis for anaerobic benzene degradation in microbial systems. In support of this, anaerobic benzene degradation by Dechloromonas strain RCB, the only known species to degrade benzene in the absence of oxygen, is stimulated by the addition of vitamin B12 and inhibited by the addition of propyl iodide which is consistent with the involvement of a corrinoid enzymatic step. Alkylation of benzene to toluene is also consistent with labeling data that suggests that the carboxyl carbon of benzoate is derived from one of the benzene carbons. However, it is difficult to envision how phenol would be formed if benzene is alkylated to toluene. As such, it is possible that diverse mechanisms for anaerobic benzene degradation may be operative in different anaerobic microorganisms.

Validation of methylmercury determinations in aquatic systems by alkyl derivatization methods for GC analysis using ICP-IDMS

Isotope dilution mass spectrometry (IDMS), using an inductively coupled plasma quadrupole mass spectrometer (ICPMS) and a species-specific methylmercury spike was applied to validate the commonly used GC method for methylmercury (MeHg+) determination, which is based on the formation of volatile methylethylmercury by derivatizationwith NaBEt4. The spike compound, Me201Hg+, was synthesized by reaction of 201Hg-enriched mercury chloride with methylcobalamin. By analyzing different environmental aquatic samples, it was found that in most cases, transformation of MeHg+ into elemental mercury (Hg0) took place. From investigations of synthetic solutions, it could be followed that halide ions are responsible for this transformation process. Chloride and bromide converted MeHg+ into Hg0, whereas iodide caused transformation into Hg2+ and Hg0. It could also be shown that transformation of MeHg+ took place only during the derivatization step. In contrast to ethylation, propylation by NaBPr4 did not cause any transformation; however, accurate results of MeHg+ determinations could be obtained by propylation as well as by ethylation when GC/ ICP-IDMS was applied. This work demonstrates the great power of isotopically labeled element compounds for the validation of element speciation methods and for species-specific IDMS analyses.

Metabolic Pathways Leading to Mercury Methylation in Desulfovibrio desulfuricans LS

The synthesis of methylmercury by Desulfovibrio desulfuricans LS was investigated on the basis of 14 C incorporation from precursors and the measurement of relevant enzyme activities in cell extracts. The previously observed incorporation of C-3 from serine into methylmercury was confirmed by measurement of relatively high activities of serine hydroxymethyltransferase and other enzymes of this pathway. High rates of label incorporation into methylmercury from H 14 COO - and H 14 CO 3 - prompted the assay of enzymes of the acetyl coenzyme A (CoA) synthase pathway. These enzymes were found to be present but at activity levels much lower than those reported for acetogens. Propyl iodide inhibited methylmercury and acetyl-CoA syntheses to similar extents, and methylmercury synthesis was found to compete with acetyl-CoA synthesis for methyl groups. On the basis of these findings, we propose that in methylmercury synthesis by D. desulfuricans LS the methyl group is transferred from CH 3 -tetrahydrofolate via methylcobalamin. The methyl group may originate from C-3 of serine or from formate via the acetyl-CoA synthase pathway. These pathways are not unique to D. desulfuricans LS, and thus the ability of this bacterium to methylate mercury is most likely associated with the substrate specificity of its enzymes.

Cobalamin-mediated mercury methylation by Desulfovibrio desulfuricans LS

The prominence of sulfate reducers in mercury biomethylation prompted the examination of the methyl carrier and mercury methylation activity of Desulfovibrio desulfuricans LS. There was a low degree of mercury tolerance and a high degree of methylation during fermentative growth; the opposite was true during sulfate reduction. During 2 days of fermentative growth, up to 37% of HgCl2 was methylated at 0.1 micrograms/ml, but only 1.5% was methylated at 10.0 micrograms/ml. Less than 1% of the added HgCl2 was methylated under sulfate-reducing conditions. D. desulfuricans LS radioimmunoassay results were positive for cobalamin. The addition of CoCl2 and benzimidazole to fermentative cultures increased methylation activity. From D. desulfuricans LS grown in the presence of (57)CoCl2, a corrinoid was extracted and purified. High-performance liquid chromatography analysis of the purified extract yielded a single peak with the retention time of cobalamin, and 97% of the (57)Co radioactivity was associated with this peak. Fast atom bombardment and UV and visible spectra of the isolated corrinoid matched those of cobalamin. When methylated with (14)CH3I, the isolated corrinoid methylated Hg(2+) with a 93.9% preservation of (14)C specific activity. We conclude that D. desulfuricans LS methylates mercury via cobalamin (vitamin B12). Under physiological conditions, the enzymatic catalysis of this reaction is likely.

The relationships of Hg(II) volatilization from a freshwater pond to the abundance ofmer genes in the gene pool of the indigenous microbial community

The role of biological activities in the reduction and volatilization of Hg(II) from a polluted pond was investigated. Elemental mercury was evolved from pond water immediately following spiking with(203)Hg(NO3)2, whereas an acclimation period of 36 hours was required in control samples collected from a nearby, unpolluted river before onset of volatilization. Genes encoding the bacterial mercuric reductase enzyme (mer genes) were abundant in DNA fractions extracted from biomass of the pond microbial community, but not in samples extracted from control communities. Thus, evolution of Hg(0) was probably due to activities mediated by the bacterial mercuric reductase. Of four characterizedmer operons, the system encoded by transposon 501 (mer(Tn501)) dominated and likely contributed to the majority of the observed Hg(II) volatilization. Thus,mer-mediated reduction and volatilization could be used to reduce Hg(II) concentrations in polluted waters, in turn decreasing rates of methylmercury formation by limiting substrate availability.

Laboratory Study of Chemical Speciation of Mercury in Lake Sediment and Water under Aerobic and Anaerobic Conditions

Chemical speciation and partitioning of radiolabeled HgCl 2 were studied in model aquatic systems consisting of undisturbed eutrophic lake sediment and water in plastic cylinders. The cylinders were either gradually made anaerobic by a gentle flow of N 2 -CO 2 or kept aerobic by air flow. The proportion of methylated 203 Hg was significantly higher, in both water and sediment, in the anaerobic systems than in the aerobic systems. The composition and total concentration of fatty acids originating from bacterial phospholipids, as well as the concentration of vitamin B 12 , including related cobalamins, were similar in sediments from the anaerobic and aerobic systems. Bacterial cell numbers were, on average, 3.6 times higher in the anaerobic water columns than in the aerobic ones. Volatilization of 203 Hg occurred in all systems except in an autoclaved control and was of similar magnitudes in the anaerobic and aerobic systems. Incorporation of 203 Hg into the sediment was significantly faster in the aerobic systems than in the anaerobic systems. These results suggest that episodes of anoxia in bottom waters and sediment cause an increase in net mercury methylation and, hence, an increase in bioavailable mercury.

Mercury methylation in aquatic systems affected by acid deposition

Recently, it has been noted that fish in acidified lakes may contain elevated levels of mercury. While there is correlation among lakes between depressed pH and high mercury concentrations in fish, the cause of this problem is unknown. A number of hypotheses have been advanced in explanation, including increased mercury deposition, changes in mercury mobility due to acidification, pH dependent changes in mercury uptake by biota, and alterations in population size and/or structure which result in increased bioaccumulation in fish. Because fish accumulate mercury mainly in an organic form, methylmercury, changes in the biogeochemical cycling of this compound might account for elevated bioaccumulation. Mercury methylation is predominantly a microbial process which occurs in situ in lakes. This review focuses on microbiological and biogeochemical changes that may lead to increased levels of methylmercury in fresh waters impacted by acid-deposition. In particular, we focus on the hypothesis that sulfate-reducing bacteria are important mediators of metal methylation in aquatic systems and, moreover, that sulfate-deposition may stimulate methylmercury production by enhancing the activity of sulfate-reducing bacteria in sediments.