Sulfate-Reducing Bacterium Desulfovibrio desulfuricans ND132 as a Model for Understanding Bacterial Mercury Methylation

Brown S.D., Gilmour C.C., Kucken A.M., Wall J.D., Elias D.A., Brandt C.C., Podar M., Chertkov O., Held B., Bruce D.C., Detter J.C., Tapia R., Han C.S., Goodwin L.A., Cheng J.-., et. al.

Bridou R., Monperrus M., Gonzalez P.R., Guyoneaud R., Amouroux D.

Zane G.M., Yen H.B., Wall J.D.

ABSTRACT The pathway of electrons required for the reduction of sulfate in sulfate-reducing bacteria (SRB) is not yet fully characterized. In order to determine the role of a transmembrane protein complex suggested to be involved in this process, a deletion in Desulfovibrio vulgaris Hildenborough was created by marker exchange mutagenesis that eliminated four genes putatively encoding the QmoABC complex and a hypothetical protein (DVU0851). The Qmo ( q uinone-interacting m embrane-bound o xidoreductase) complex is proposed to be responsible for transporting electrons to the dissimilatory adenosine-5′-phosphosulfate reductase in SRB. In support of the predicted role of this complex, the deletion mutant was unable to grow using sulfate as its sole electron acceptor with a range of electron donors. To explore a possible role for the hypothetical protein in sulfate reduction, a second mutant was constructed that had lost only the gene that codes for the DVU0851 protein. The second constructed mutant grew with sulfate as the sole electron acceptor; however, there was a lag that was not present with the wild-type or complemented strain. Neither deletion strain was significantly impaired for growth with sulfite or thiosulfate as the terminal electron acceptor. Complementation of the Δ( qmoABC -DVU0851) mutant with all four genes or only the qmoABC genes restored its ability to grow by sulfate respiration. These results confirmed the prediction that the Qmo complex is in the electron pathway for sulfate reduction and revealed that no other transmembrane complex could compensate when Qmo was lacking.

Rodríguez-González P., Epov V.N., Bridou R., Tessier E., Guyoneaud R., Monperrus M., Amouroux D.

This work reports the first results on the stable isotope fractionation of Hg during methylation by anaerobic bacteria under dark conditions. The GC-MC-ICPMS methodology employed is capable of simultaneously measuring the species-specific isotopic composition of different Hg species within the same sample. We have studied Hg isotopic fractionation caused by methylation of Hg(II) standard reference material NIST-3133 in the presence of the pure bacterial strain Desulfobulbus propionicus MUD10 (DSM 6523) under fermentative conditions. We have measured the isotopic composition of Hg(II) and monomethyl mercury (MMHg) in these cultures as a function of time and calculated delta-values for both species versus the starting material (NIST-3133) as a delta-zero standard. Two different strategies for the incubation were applied: single sampling cultures and a continuous sampling culture. The results obtained have shown that under the conditions employed in this work the methylation of Hg(II) causes mass-dependent fractionation of the Hg isotopes for both Hg(II) substrate and produced MMHg. Such a process occurred under the exponential growth of the bacteria which preferentially methylate the lighter isotopes of Hg. After 96 h for the continuous culture and 140 h for the single sampling cultures, we observed a change in the fractionation trend in the samples at a similar cell density value (ca. 6.0 x 10(7) cells mL(-1)) which suggests the increasing contribution of a simultaneous process balancing methylation extent such as demethylation. Assuming that Rayleigh type fractionation conditions are met before such suppression, we have obtained a alpha(202/198) fractionation factor of 1.0026 +/- 0.0004 for the single sampling cultures.

Dolor M.K., Gilmour C.C., Helz G.R.

Rhenium is enriched in suboxic and anoxic sediments relative to oxic sediments, a characteristic that is being exploited in its use as a paleoredox indicator. Rhenium is fixed at sediment depths where iron reduction and sulfate reduction are the dominant microbial terminal electron-accepting processes. In order to explore mechanisms of its fixation, we investigated perrhenate behavior in pure, batch cultures of two dissimilatory sulfate-reducing strains (Desulfovibrio desulfuricans subsp. desulfuricans and Desulfovibrio desulfuricans ND132) and two iron-reducing strains (Geobacter metallireducens GS-15 and Shewanella oneidensis MR-1). Perrhenate concentrations tested ranged from 0.04 to 12 μM, roughly 4 to 7 orders of magnitude larger than seawater Re concentrations. Within this broad concentration range, none of the organisms tested actively removed Re from solution during one week's growth to stationary phase. Despite these results, the sulfate-reducing cultures appeared to have reached supersaturation ...

Hollweg T.A., Gilmour C.C., Mason R.P.

Methylmercury (MeHg) concentration and production rates were studied in bottom sediments along the mainstem of Chesapeake Bay and on the adjoining continental shelf and slope. Our objectives were to 1) observe spatial and temporal changes in total mercury (HgT) and MeHg concentrations in the mid-Atlantic coastal region, 2) investigate biogeochemical factors that affect MeHg production, and 3) examine the potential of these sediments as sources of MeHg to coastal and open waters. Estuarine, shelf and slope sediments contained on average 0.5 to 1.5% Hg as MeHg (% MeHg), which increased significantly with salinity across our study site, with weak seasonal trends. Methylation rate constants ( k meth ), estimated using enriched stable mercury isotope spikes to intact cores, showed a similar, but weaker, salinity trend, but strong seasonality, and was highly correlated with % MeHg. Together, these patterns suggest that some fraction of MeHg is preserved thru seasons, as found by others [Orihel, D.M., Paterson, M.J., Blanchfield, P.J., Bodaly, R.A., Gilmour, C.C., Hintelmann, H., 2008. Temporal changes in the distribution, methylation, and bioaccumulation of newly deposited mercury in an aquatic ecosystem. Environmental Pollution 154, 77] Similar to other ecosystems, methylation was most favored in sediment depth horizons where sulfate was available, but sulfide concentrations were low (between 0.1 and 10 μM). MeHg production was maximal at the sediment surface in the organic sediments of the upper and mid Bay where oxygen penetration was small, but was found at increasingly deeper depths, and across a wider vertical range, as salinity increased, where oxygen penetration was deeper. Vertical trends in MeHg production mirrored the deeper, vertically expanded redox boundary layers in these offshore sediments. The organic content of the sediments had a strong impact on the sediment:water partitioning of Hg, and therefore, on methylation rates. However, the HgT distribution coefficient ( K D ) normalized to organic matter varied by more than an order of magnitude across the study area, suggesting an important role of organic matter quality in Hg sequestration. We hypothesize that the lower sulfur content organic matter of shelf and slope sediments has a lower binding capacity for Hg resulting in higher MeHg production, relative to sediments in the estuary. Substantially higher MeHg concentrations in pore water relative to the water column indicate all sites are sources of MeHg to the water column throughout the seasons studied. Calculated diffusional fluxes for MeHg averaged ∼ 1 pmol m − 2 day − 1 . It is likely that the total MeHg flux in sediments of the lower Bay and continental margin are significantly higher than their estimated diffusive fluxes due to enhanced MeHg mobilization by biological and/or physical processes. Our flux estimates across the full salinity gradient of Chesapeake Bay and its adjacent slope and shelf strongly suggest that the flux from coastal sediments is of the same order as other sources and contributes substantially to the coastal MeHg budget.

Marietou A., Griffiths L., Cole J.

ABSTRACT Desulfovibrio desulfuricans strain 27774 is one of a relative small group of sulfate-reducing bacteria that can also grow with nitrate as an alternative electron acceptor, but how nitrate reduction is regulated in any sulfate-reducing bacterium is controversial. Strain 27774 grew more rapidly and to higher yields of biomass with nitrate than with sulfate or nitrite as the only electron acceptor. In the presence of both sulfate and nitrate, sulfate was used preferentially, even when cultures were continuously gassed with nitrogen and carbon dioxide to prevent sulfide inhibition of nitrate reduction. The napC transcription start site was identified 112 bases upstream of the first base of the translation start codon. Transcripts initiated at the napC promoter that were extended across the napM-napA boundary were detected by reverse transcription-PCR, confirming that the six nap genes can be cotranscribed as a single operon. Real-time PCR experiments confirmed that nap operon expression is regulated at the level of mRNA transcription by at least two mechanisms: nitrate induction and sulfate repression. We speculate that three almost perfect inverted-repeat sequences located upstream of the transcription start site might be binding sites for one or more proteins of the CRP/FNR family of transcription factors that mediate nitrate induction and sulfate repression of nitrate reduction by D. desulfuricans .

Ranchou-Peyruse M., Monperrus M., Bridou R., Duran R., Amouroux D., Salvado J.C., Guyoneaud R.

Mercury methylation has been extensively reported in the literature among “Firmicutes” and “Proteobacteria.” Nevertheless, results are hardly comparable because of differences in initial inorganic mercury concentrations used. The use of stable isotopic tracers now permits to study mercury transformations at concentrations close to environmental levels. Here, several strains, including strict fermentative and sulphate-reducing bacteria, were tested for their mercury methylation capacities and the results were compared with data available to date. Under such conditions, mercury methylation only occurs among the delta-Proteobacteria. The absence of relation between taxonomic/phylogenetic affiliation and mercury methylation capacities was pointed out and discussed for environmental studies.

Schaefer J.K., Morel F.M.

Methylmercury bioaccumulates in aquatic food chains and can cross the blood–brain barrier, making this organometallic compound a much more worrisome pollutant than inorganic mercury. Experimental evidence now indicates that mercury methylation by the bacterium Geobacter sulfurreducens can be greatly enhanced in the presence of the amino-acid cysteine. Methylmercury bioaccumulates in aquatic food chains and is able to cross the blood–brain barrier, making this organometallic compound a much more worrisome pollutant than inorganic mercury. We know that methylation of inorganic mercury is carried out by microbes in the anoxic layers of sediments and water columns, but the factors that control the extent of this methylation are poorly known. Mercury methylation is generally thought to be catalysed accidentally by some methylating enzyme1,2, and it has been suggested that cellular mercury uptake results from passive diffusion of neutral mercury complexes3. Here, we show that mercury methylation by the bacterium Geobacter sulfurreducens is greatly enhanced in the presence of low concentrations of the amino acid cysteine. The formation of a mercury–cysteine complex promotes both the uptake of inorganic mercury by the bacteria and the enzymatic formation of methylmercury, which is subsequently released to the external medium. Our results suggest that mercury uptake and methylation by microbes are controlled more tightly by biological mechanisms than previously thought, and that the formation of specific mercury complexes in anoxic waters modulates the efficiency of the microbial methylation of mercury.

Cole J.R., Wang Q., Cardenas E., Fish J., Chai B., Farris R.J., Kulam-Syed-Mohideen A.S., McGarrell D.M., Marsh T., Garrity G.M., Tiedje J.M.

The Ribosomal Database Project (RDP) provides researchers with quality-controlled bacterial and archaeal small subunit rRNA alignments and analysis tools. An improved alignment strategy uses the Infernal secondary structure aware aligner to provide a more consistent higher quality alignment and faster processing of user sequences. Substantial new analysis features include a new Pyrosequencing Pipeline that provides tools to support analysis of ultra high-throughput rRNA sequencing data. This pipeline offers a collection of tools that automate the data processing and simplify the computationally intensive analysis of large sequencing libraries. In addition, a new Taxomatic visualization tool allows rapid visualization of taxonomic inconsistencies and suggests corrections, and a new class Assignment Generator provides instructors with a lesson plan and individualized teaching materials. Details about RDP data and analytical functions can be found at http://rdp.cme.msu.edu/.

Mitchell C.P., Gilmour C.C.

[1] In a detailed study of the biogeochemical factors affecting the methylation of mercury in a Chesapeake Bay salt marsh, we examined relationships between mercury methylation and numerous variables, including sulfate reduction rates, organic carbon mineralization rates, iron and sulfur chemistry, and the character of dissolved organic matter (DOM). Our data show that salt marshes are important sites of de novo methylmercury (MeHg) production in coastal ecosystems. Some of the controls on MeHg production that have been well-described in other ecosystems also impacted MeHg production in this salt marsh, specifically the effect of sulfide accumulation on mercury bioavailability. We observed some novel biogeochemical relationships with Hg(II)-methylation and MeHg accumulation, particularly the positive association of Hg(II)-methylation with zones of microbial iron reduction. On the basis of this relationship, we suggest caution in wetland and groundwater remediation approaches involving iron additions. Aqueous phase Hg complexation appeared to be the dominant control on Hg bioavailability across the marsh sites examined, rather than Hg partitioning behavior. A detailed examination of DOM character in the marsh suggested a strong positive association between Hg(II)-methylation rate constants and increasing DOM molecular weight. Overall, our results indicate that net MeHg production is controlled by a balance between microbial activity and geochemical effects on mercury bioavailability, but that a significant zone of MeHg production can persist in near surface salt marsh soils. Production of MeHg in coastal marshes may negatively impact ecosystems via export to adjacent estuaries or through direct bioaccumulation in birds, fish and amphibians that feed in these highly productive ecosystems.

Creswell J.E., Kerr S.C., Meyer M.H., Babiarz C.L., Shafer M.M., Armstrong D.E., Roden E.E.

[1] Hyporheic pore water samples were collected from two sites within the Allequash Creek wetland, in Vilas County, northern Wisconsin, from August 2003 to October 2004. Samples were collected simultaneously at the surface and at 2, 5, 7, 10, and 15 cm below the sediment-water interface. Concentration ranges were 3.7 to 58 pM for inorganic mercury,

Klonowska A., Clark M.E., Thieman S.B., Giles B.J., Wall J.D., Fields M.W.

Desulfovibrio vulgaris Hildenborough is a well-studied sulfate reducer that can reduce heavy metals and radionuclides [e.g., Cr(VI) and U(VI)]. Cultures grown in a defined medium had a lag period of approximately 30 h when exposed to 0.05 mM Cr(VI). Substrate analyses revealed that although Cr(VI) was reduced within the first 5 h, growth was not observed for an additional 20 h. The growth lag could be explained by a decline in cell viability; however, during this time small amounts of lactate were still utilized without sulfate reduction or acetate formation. Approximately 40 h after Cr exposure (0.05 mM), sulfate reduction occurred concurrently with the accumulation of acetate. Similar amounts of hydrogen were produced by Cr-exposed cells compared to control cells, and lactate was not converted to glycogen during non-growth conditions. D. vulgaris cells treated with a reducing agent and then exposed to Cr(VI) still experienced a growth lag, but the addition of ascorbate at the time of Cr(VI) addition prevented the lag period. In addition, cells grown on pyruvate displayed more tolerance to Cr(VI) compared to lactate-grown cells. These results indicated that D. vulgaris utilized lactate during Cr(VI) exposure without the reduction of sulfate or production of acetate, and that ascorbate and pyruvate could protect D. vulgaris cells from Cr(VI)/Cr(III) toxicity.

Ekstrom E.B., Morel F.M.

Sulfate-reducing bacteria (SRB) have been identified as the primary organisms responsible for monomethylmercury (MeHg) production in aquatic environments, but little is known of the physiologyand biochemistry of mercury(Hg) methylation. Corrinoid compounds have been implicated in enzymatic Hg methylation, although recent experiments with a vitamin B12 inhibitor indicated that incomplete-oxidizing SRB likely do not use a corrinoid-enzyme for Hg methylation, whereas experiments with complete-oxidizing SRB were inconclusive due to overall growth limitation. Here we explore the role of corrinoid-containing methyltransferases, which contain a cobalt-reactive center, in Hg methylation. To this end, we performed cobalt-limitation experiments on two SRB strains: Desulfococcus multivorans, a complete-oxidizer that uses the acetyl-CoA pathway for major carbon metabolism, and Desulfovibrio africanus, an incomplete-oxidizer that does not contain the acetyl-CoA pathway. Cultures of D. multivorans grown with no direct addition of Co or B12 became cobalt-limited and produced 3 times less MeHg per cell than control cultures. Differences in growth rate and Hg bioavailability do not account for this large decrease in MeHg production upon Co limitation. In contrast, the growth and Hg methylation rates of D. africanus cultures remained nearly constant regardless of the inorganic cobalt and vitamin B12 concentrations in the medium. These results are consistent with mercury being methylated by different pathways in the two strains: catalyzed by a B12-containing methyltransferase in D. multivorans and a B12-independent methyltransferase in D. africanus. If complete-oxidizing SRB like D. multivorans account for the bulk of MeHg production in coastal sediments as reported, the ambient Co concentration and speciation may control the rate of Hg methylation.

Lin C., Jay J.A.

While biofilms are now known to be the predominant form of microbial growth in nature, very little is yet known about their role in environmental mercury (Hg) methylation. Findings of Hg methylation in periphyton communities have indicated the importance of investigating how environmental biofilms affect Hg methylation, as periphyton can be the base of the food webs in aquatic ecosystems. Chemical speciation influences the microbial uptake and methylation of inorganic Hg by planktonic cultures of sulfate-reducing bacteria; however, the effect of speciation on Hg methylation by biofilm cultures of these organisms has previously not been studied. In the present study, Hg methylation rates in biofilm and planktonic cultures of two isolates of Desulfovibrio desulfuricans from a coastal wetland were compared. Notably, the specific Hg methylation rate found was approximately an order of magnitude higher (0.0018 vs. 0.0002 attomol cell(-1) day(-1)) in biofilm cells than in planktonic cells, suggesting an important role for environmental biofilms in Hg methylation. To investigate the role of chemical speciation of Hg, experiments were conducted at two levels of sulfide. Both biofilm and planktonic cultures produced methylmercury at roughly twice the rate at low sulfide, when HgS(0)(aq), rather than HgHS2-, was the dominant Hg species. This indicates that the presence of a biofilm does not alter the relative availability of the dominant Hg species in sulfidic medium, in accordance with our previous studies of Hg uptake by Escherichia coli along a chloride gradient.

Ma P., Wang D., Xu D., Lovley D.R.

Twining C.W., Blanco A., Dutton C., Kainz M.J., Harvey E., Kowarik C., Kraus J.M., Martin‐Creuzburg D., Parmar T.P., Razavi N.R., Richoux N., Saboret G., Sarran C., Schmidt T.S., Shipley J.R., et. al.

ABSTRACTAquatic and terrestrial ecosystems are linked through the reciprocal exchange of materials and organisms. Aquatic‐to‐terrestrial subsidies are relatively small in most terrestrial ecosystems, but they can provide high contents of limiting resources that increase consumer fitness and ecosystem production. However, they also may carry significant contaminant loads, particularly in anthropogenically impacted watersheds. Global change processes, including land use change, climate change and biodiversity declines, are altering the quantity and quality of aquatic subsidies, potentially shifting the balance of costs and benefits of aquatic subsidies for terrestrial consumers. Many global change processes interact and impact both the bright and dark sides of aquatic subsidies simultaneously, highlighting the need for future integrative research that bridges ecosystem as well as disciplinary boundaries. We identify key research priorities, including increased quantification of the spatiotemporal variability in aquatic subsidies across a range of ecosystems, greater understanding of the landscape‐scale extent of aquatic subsidy impacts and deeper exploration of the relative costs and benefits of aquatic subsidies for consumers.

Fang J., Yin B., Wang X., Pan K., Wang W.

Peterson B.D., Janssen S.E., Poulin B.A., Ogorek J.M., White A.M., McDaniel E.A., Marick R.A., Armstrong G.J., Scheel N.D., Tate M.T., Krabbenhoft D.P., McMahon K.D.

Bakour I., Isaure M., Barrouilhet S., Goñi-Urriza M., Monperrus M.

Mercury methylation by anaerobic microorganisms, including sulfate-reducing bacteria (SRB), is a key process in the production of neurotoxic methylmercury (MeHg). The chemical speciation of mercury (Hg) strongly influences its bioavailability as well as its potential for methylation and demethylation, with sulfur-containing ligands playing a critical role in these processes. In this study, we used isotopically enriched mercury species (199Hg(II), Me202Hg) to investigate how molecular speciation of mercury affects both methylation and demethylation processes by the sulfate-reducer Pseudodesulfovibrio hydrargyri BerOc1. Experimental assays were carried out: (i) without external addition of S-ligands, (ii) with the addition of increasing concentrations of exogenous cysteine (Cys) (0.01, 0.1, and 0.5 mM), or (iii) with the addition of exogenous sulfide (0.1 mM). We showed that the highest methylation rate (Kmeth) was obtained without the external addition of S-ligands, whereas the addition of Cys or sulfide decreased Hg methylation regardless of Cys concentration. By quantitatively determining Hg(II) speciation in extracellular fractions, we demonstrated that Hg(II) was mostly present in the form of Hg(Cys)2, when Cys was added. However, metabolically sulfide production from Cys degradation shifted the chemical speciation of Hg(II) from Hg(Cys)2 to a more insoluble fraction (HgS(S)). In the assay without externally added ligands (Cys or sulfide), speciation models were generated by taking in account the metabolically produced thiols. These models established the predominance of Hg(II) complexes with a mixed ligation involving biosynthesized thiols, OH−, and Cl− ions. Our results suggest that these complexes with lower thermodynamic stabilities enhance the MeHg formation rate compared to the more stable Hg(Cys)2 or HgS(s) species. Unlike Hg(II) methylation, the addition of S-ligands did not affect the rates of demethylation (Kdemeth) of MeHg, even though it caused a shift in the chemical speciation of MeHg (from MeHgCl to MeHgCys and MeHgSH). These findings contribute to our understanding of the potential role of specific S-ligands and chemical speciation in governing the environmental fate and toxicity of mercury.

da Silva Ribeiro L., de Souza Neta L.C., Pereira N.S., de Godoi Pereira M.

Le Bars M., Monperrus M., Barrouilhet S., Petrel M., Goñi-Urriza M., Isaure M.

Lin S., Zhao F., Chen Y., Wang M.

Regnell O., Tesson S.V.

There is an elevated presence of mercury (Hg) in the biosphere because of anthropogenic activities. The resulting damage to ecosystems and human health increases dramatically when microorganisms produce highly toxic methylmercury (MeHg). Total Hg (THg), MeHg and ancillary water chemistry were measured in two connected lakes, separated by a short stream stretch, before (1996, 1998 and 2003) and after (2007, 2009 and 2010) the removal of Hg-polluted pulp fiber sediment. Over the study period, there was a decrease in sulfate in the surface water of both lakes, presumably because of declining atmospheric sulfate deposition. Together, the reductions in OM, sulfate, and Hg, resulted in decreased MeHg concentrations as well as decreased MeHg:THg ratios in the bottom water overlying the sediment. There was also a reduction in zooplankton MeHg and fish total Hg in both lakes. Multiple regressions, using the bottom water data before and after remediation from both lakes, indicated that both the yearly maximum MeHg concentration [MeHg

Ding C., Ding Z., Liu Q., Liu W., Chai L.

Microbial transformation processes of heavy metals, including immobilization, oxidation or reduction, and (de)methylation, can provide various bioremediation strategies for heavy metals-contaminated enviroments.

Aleku D.L., Olesya Lazareva, Thomas Pichler

Mercury (Hg) is one of the most toxic global pollutants of continuing concern, posing a severe threat to human health and wildlife. Due to its mobility, Hg is easily transported through the atmosphere and directly deposited onto water, sediments and soils or incorporated in biota. In groundwater, Hg concentrations can be influenced by either geogenic or anthropogenic sources, causing critical health effects such as damage to the respiratory and nervous systems. The geogenic sources of Hg include rocks and minerals containing Hg (cinnabar, organic-rich shales, and sulfide-rich volcanic) and geothermal fluids. The anthropogenic Hg sources include the combustion of fossil fuels, gold mining, chemical discharges from dental preparation, laboratory activities and legacy sites. In groundwater, the average background concentration of Hg is < 0.01 μg/L. Mercury can be mobilized into groundwater from geogenic or anthropogenic sources due to changes in redox potential (Eh), with concentrations reaching above the WHO drinking water standard of 1 μg/L. Under reducing conditions, microbial activity facilitates the reductive dissolution of FeOOH, causing the release of sorbed Hg2+ into groundwater. The released Hg2+ may be reduced to Hg0 by either dissolved organic matter or Fe2+. The stability of Hg species (Hg0, Hg22+, Hg2+, MeHg) in groundwater is controlled by Eh and pH. While high Eh and low pH conditions can mobilize Hg from the solid into aqueous phases, the soil binding ability can sequestrate the mobilized Hg via adsorption of Hg2+ by goethite, hematite, manganese oxides, hydrous ferric oxides, or organic matter restricting it from leaching into groundwater. During groundwater contamination, remediation using nanomaterials such as pumice-supported nanocomposite zero-valent iron, brass shavings, polyaniline-Fe3O4-silver diethyldithiocarbamate, and CoMoO/γ-Al2O3 has been documented. These promising emerging technologies utilize the principle of adsorption to remove up to 99.98 % of Hg from highly contaminated groundwater. This study presents an overview of groundwater contamination, remediation, complex biogeochemical processes, and a hydrogeochemical conceptual model concerning Hg's mobility, fate, and transport.

Gonzalez V., Abarca-Hurtado J., Arancibia A., Claverías F., Guevara M.R., Orellana R.

Some sulfate-reducing bacteria (SRB), mainly belonging to the Desulfovibrionaceae family, have evolved the capability to conserve energy through microbial extracellular electron transfer (EET), suggesting that this process may be more widespread than previously believed. While previous evidence has shown that mobile genetic elements drive the plasticity and evolution of SRB and iron-reducing bacteria (FeRB), few have investigated the shared molecular mechanisms related to EET. To address this, we analyzed the prevalence and abundance of EET elements and how they contributed to their differentiation among 42 members of the Desulfovibrionaceae family and 23 and 59 members of Geobacteraceae and Shewanellaceae, respectively. Proteins involved in EET, such as the cytochromes PpcA and CymA, the outer membrane protein OmpJ, and the iron–sulfur cluster-binding CbcT, exhibited widespread distribution within Desulfovibrionaceae. Some of these showed modular diversification. Additional evidence revealed that horizontal gene transfer was involved in the acquiring and losing of critical genes, increasing the diversification and plasticity between the three families. The results suggest that specific EET genes were widely disseminated through horizontal transfer, where some changes reflected environmental adaptations. These findings enhance our comprehension of the evolution and distribution of proteins involved in EET processes, shedding light on their role in iron and sulfur biogeochemical cycling.

Adams H.M., Cui X., Lamborg C.H., Schartup A.T.

Dong H., Wang Y., Zhi T., Guo H., Guo Y., Liu L., Yin Y., Shi J., He B., Hu L., Jiang G.

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.

Gionfriddo C.M., Lin H., Moreau J.W.

All forms of mercury (Hg) are toxic to the environment and to all organisms, particularly the organometallic neurotoxin methylmercury. Microbes play key roles in controlling the form and fate of mercury in the environment and therefore in mediating mercury toxicity. They are the sole natural source of methylmercury, forming this toxin from mostly anthropogenic mercury deposited in the atmosphere globally. Microbes can also detoxify and recycle mercury from sediments or waters into the atmosphere, thereby acting both to immobilize and mobilize this element in its biogeochemical cycle. Phylogenetic analyses support the interpretation that microorganisms have been interacting in such ways with the element mercury since very early in the geological evolution of the earth, where most of the mercury in the environment would have been emitted from volcanoes or seafloor hydrothermal vents. Recent advances in metagenomics, bioinformatics, and computational modeling, coupled with extensive cultivation-driven experimental research, have yielded important insights into which microorganisms are capable of transforming mercury species by changing their valence state and/or speciation, including the formation or destruction of organometallic mercury species. These studies have also generated new conceptual models of the proteins that mediate these transformations and the biochemical mechanisms involved. Still, many long-standing questions remain to be answered; this concise review attempts to illustrate recent progress and identify current questions in the study of the geomicrobiology of mercury.