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Algae are an entry point for mercury (Hg) into the food web. Bioconcentration of Hg by algae is crucial for its biogeochemical cycling and environmental risk. Herein, considering the cell heterogeneity, we investigated the bioconcentration of coexisting isotope-labeled inorganic ( 199 IHg) and methyl Hg ( 201 MeHg) by six typical freshwater and marine algae using dual-mass single-cell inductively coupled plasma mass spectrometry (scICP-MS). First, a universal pretreatment procedure for the scICP-MS analysis of algae was developed. Using the proposed method, the intra- and interspecies heterogeneities and the kinetics of Hg bioconcentration by algae were revealed at the single-cell level. The heterogeneity in the cellular Hg contents is largely related to cell size. The bioconcentration process reached a dynamic equilibrium involving influx/adsorption and efflux/desorption within hours. Algal density is a key factor affecting the distribution of Hg between algae and ambient water. Cellular Hg contents were negatively correlated with algal density, whereas the volume concentration factors almost remained constant. Accordingly, we developed a model based on single-cell analysis that well describes the density-driven effects of Hg bioconcentration by algae. From a novel single-cell perspective, the findings improve our understanding of algal bioconcentration governed by various biological and environmental factors.

Hg(I) may control Hg redox kinetics; however, its metastable nature hinders analysis. Herein, the stability of Hg(I) during standard preparation and analysis was studied. Gravimetric analysis showed that Hg(I) was stable in its stock solution (1000 mg L -1 ), yet completely disproportionated when its dilute solution (10 μg L -1 ) was analyzed using liquid chromatography (LC)-ICPMS. The Hg(I) dimer can form through an energetically favorable comproportionation between Hg(0) and Hg(II), as supported by density functional theory calculation and traced by the rapid isotope exchange between 199 Hg(0) aq and 202 Hg(II). However, the separation of Hg(0) and Hg(II) (e.g., LC process) triggered its further disproportionation. Polypropylene container, increasing headspace, decreasing pH, and increasing dissolved oxygen significantly enhanced the disproportionation or redox transformations of Hg(I). Thus, using a glass container without headspace and maintaining a slightly alkaline solution are recommended for the dilute Hg(I) stabilization. Notably, we detected elevated concentrations of Hg(I) (4.4-6.1 μg L -1 ) in creek waters from a heavily Hg-polluted area, accounting for 54-70% of total dissolved Hg. We also verified the reductive formation of Hg(I) in Hg(II)-spiked environmental water samples, where Hg(I) can stably exist in aquatic environments for at least 24 h, especially in seawater. These findings provide mechanistic insights into the transformation of Hg(I), which are indicative of its further environmental identification.

Microbial reduction plays a crucial role in Hg redox and the global cycle. Although intracellular Hg(II) reduction mediated by MerA protein is well documented, it is still unclear whether or how bacteria reduce Hg(II) extracellularly without its internalization. Herein, for the first time, we discovered the extracellular reduction of Hg(II) by a widely distributed aerobic marine bacterium Alteromonas sp. KD01 through a superoxide-dependent mechanism. The generation of superoxide by Alteromonas sp. KD01 was determined using 3-bis(2-methoxy-4-nitro-5-sulfophenyl)-2H-tetrazolium-5-carboxanilide and methyl cypridina luciferin analogue as probes via UV-vis and chemiluminescence detection, respectively. The results demonstrated that Hg(II) reduction was inhibited by superoxide scavengers (superoxide dismutase (SOD) and Cu(NO 3 ) 2 ) or inhibitors of reduced nicotinamide adenine dinucleotide (NADH) oxidoreductases. In contrast, the addition of NADH significantly improved superoxide generation and, in turn, Hg(II) reduction. Direct evidence of superoxide-mediated Hg(II) reduction was provided by the addition of superoxide using KO 2 in deionized water and seawater. Moreover, we observed that even superoxide at an environmental concentration of 9.6 ± 0.5 nM from Alteromonas sp. KD01 (5.4 × 10 6 cells mL -1 ) was capable of significantly reducing Hg(II). Our findings provide a greater understanding of Hg(II) reduction by superoxide from heterotrophic bacteria and eukaryotic phytoplankton in diverse aerobic environments, including surface water, sediment, and soil.

Liquid elemental mercury droplet (Hg(0) l ) is an important species in heavy Hg-contaminated environments. The oxidation processes of Hg(0) l and its related mechanisms are still poorly understood. Herein, for the first time, it was verified that mercurous species [Hg(I)] was an important species in natural water contaminated by Hg(0) l as well as in the simulated dark oxidation of Hg(0) l . The formation and further transformation of Hg(I) controlled the overall oxidation process of Hg(0) l and were affected by different environmental factors. Through kinetic modeling using ACUCHEM program, oxidation of Hg(0) to Hg(I) (Hg(0) → Hg(I)) was determined to be the rate-limiting step in Hg(0) l oxidation because its k value ((8.7 ± 0.21) × 10 -11 s -1 ) is seven orders of magnitude lower than that of Hg(I) oxidation (Hg(I) → Hg(II), (4.7 ± 0.15) × 10 -4 s -1 ). Ligands like OH - , Cl - , and natural organic matter enhanced the formation of Hg(I) via promoting the constants of comproportionation (up to (9.5 ± 0.78) × 10 -4 s -1 ). These findings highlight the importance of Hg(I) in Hg(0) l oxidation process by controlling the transformation kinetics of Hg species, facilitating an improved understanding of the environmental redox cycles of Hg., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2023 Elsevier Ltd. All rights reserved.)

Dissolved organic matter (DOM)-mediated divalent mercury (Hg(II)) aging kinetics play a crucial role in controlling Hg(II) transformation and bioavailability in natural aquatic environments. However, the differential environmental behaviors of new and aged Hg(II) in a same reaction system remains unknown. In this study, multi-isotope tracing was used to investigate the impacts of binding site and sulfidation during DOM-mediated Hg(II) aging processes on Hg(II) reduction and microbial methylation in the same reaction system. Stepwise reduction approach and liquid chromatography-inductively coupled plasma mass spectrometry (LC-ICP-MS) demonstrate that DOM-mediated dark aging processes are mainly driven by the rearrangement of DOM binding sites with Hg(II), but not the formation of mercury sulfide nanoparticles (HgS NP ). The abundant but weaker RO/N (carboxyl and amino) Hg(II)-binding sites are replaced with stronger RSH (thiol) moieties towards Hg(II) binding with aging, resulting in a decrease in Hg(II) reduction. In contrast, besides reduction, DOM-mediated Hg(II) photoaging induces the formation of HgS NP , as revealed by LC-ICP-MS, which in turn decreases the microbial methylation potential of Hg(II). These findings help better understand and predict the kinetic characteristics of Hg(II) reactivity and its influence on Hg cycle within natural aquatic environments., Competing Interests: Declaration of Competing Interest The authors declare that they have no knowing competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2023 Elsevier Ltd. All rights reserved.)

Mercury (Hg) and its organic compounds, especially monomethylmercury (MeHg), cause major damage to the ecosystem and human health. In surface water or sediments, microorganisms play a crucial role in the methylation and demethylation of Hg. Given that Hg transformation processes are intracellular reactions, accurate assessment of the bioavailability of Hg(II)/MeHg in the environment, particularly for microorganisms, is of major importance. Compared with traditional analytical methods, bacterial whole-cell biosensors (BWCBs) provide a more accurate, convenient, and cost-effective strategy to assess the environmental risks of Hg(II)/MeHg. This Review summarizes recent progress in the application of BWCBs in the detection of bioavailable Hg(II)/MeHg, providing insight on current challenges and strategies. The principle and components of BWCBs for Hg(II)/MeHg bioavailability analysis are introduced. Furthermore, the impact of water chemical factors on the bioavailability of Hg is discussed as are future perspectives of BWCBs in bioavailable Hg analysis and optimization of BWCBs., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2023 Elsevier B.V. All rights reserved.)

Mercury sulfide nanoparticles (HgS NP ), as natural metal-containing nanoparticles, are the dominant Hg species in anoxic zones. Although the microbial Hg methylation of HgS NP has been previously reported, the importance of this process in Hg methylation has yet to be clarified due to the lack of knowledge on the internalization and transformation of HgS NP . Here, we investigated the internalization and transformation of HgS NP in microbial methylator Geobacter sulfurreducens PCA through total Hg analysis and different Hg species quantification in medium and cytoplasm. We found that the microbial uptake of HgS NP , via a passive diffusion pathway, was significantly higher than that of the Hg 2+ -dissolved organic matter (Hg 2+ -DOM) complex. Internalized HgS NP were dissolved to Hg 2+ in cytoplasm with a maximal dissolution of 41%, suggesting a "Trojan horse" mechanism. The intracellular Hg 2+ from HgS NP exposure at the initial stage (8 h) was higher than that in Hg 2+ -DOM group, which led to higher methylation of HgS NP . Furthermore, no differences in methylmercury (MeHg) production from HgS NP were observed between the hgcAB gene knockout (Δ hgcAB ) and wild-type strains, suggesting that HgS NP methylation may occur through HgcAB-independent pathways. Considering the possibility of a broad range of hgcAB -lacking microbes serving as methylators for HgS NP and the ubiquity of HgS NP in anoxic environments, this study highlights the importance of HgS NP internalization and methylation in MeHg production and demonstrates the necessity of understanding the assimilation and transformation of nutrient and toxic metal nanoparticles in general.

Sediment is the primary hotspot for microbial production of toxic and bio-accumulative methylmercury (MeHg). Common remediation strategies such as sediment dredging and capping can be too expensive and cannot degrade MeHg efficiently. Here, we constructed an Escherichia coli strain overexpressing merB gene (DH5α J23106) and assessed the effectiveness of this recombinant strain in degradation of MeHg in culture medium and sediment. DH5α J23106 can efficiently degrade MeHg (with initial concentration from 0.01 to 50 ng/mL) to more than 81.6% in a culture medium under anoxic and oxic conditions. Enriched isotope addition ( 199 HgCl 2 ) revealed that this recombinant strain can degrade 78.6% of newly produced Me 199 Hg in actual sediment, however the biodegradation decreased to 36.3% for intrinsic MeHg. Degradation of spiked MeHg after aging in anoxic and oxic sediments further demonstrated DH5α J23106 can efficiently degrade newly produced MeHg and the degradation decreased with aging significantly, especially for oxic sediment. Eight sediments were further assessed for the biodegradation of aged MeHg by DH5α J23106 under oxic conditions, with degradation ratios ranging from 9.0% to 66.9%. When combined with (NH 4 ) 2 S 2 O 3 leaching, the degradation of MeHg increased by 15.8-38.8% in on-site and off-site modes through enhanced MeHg bioavailability in some of these sediments. Thus, this recombinant strain DH5α J23106 can degrade MeHg efficiently and have the potential for remediating bioavailable MeHg in contaminated sediments., Competing Interests: Declaration of Competing Interest The authors declare that they have no knowing competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2022 Elsevier Ltd. All rights reserved.)

Particle-bound mercury (Hg P ), ubiquitously present in aquatic environments, can be methylated into highly toxic methylmercury, but it remains challenging to assess its bioavailability. In this study, we developed an Escherichia coli -based whole-cell biosensor to probe the microbial uptake of inorganic Hg(II) and assess the bioavailability of Hg P sorbed on natural and model particles. This biosensor can quantitatively distinguish the contribution of dissolved Hg(II) and Hg P to intracellular Hg. Results showed that the microbial uptake of Hg P was ubiquitous in the environment, as evidenced by the bioavailability of sorbed-Hg(II) onto particulate matter and model particles (Fe 2 O 3 , Fe 3 O 4 , Al 2 O 3 , and SiO 2 ). In both oxic and anoxic environments, Hg P was an important Hg(II) source for microbial uptake, with enhanced bioavailability under anoxic conditions. The composition of particles significantly affected the microbial uptake of Hg P , with higher bioavailability being observed for Fe 2 O 3 and lower for Al 2 O 3 particles. The bioavailability of Hg P varied also with the size of particles. In addition, coating with humic substances and model organic compound (cysteine) on Fe 2 O 3 particles decreased the bioavailability of Hg P . Overall, our findings highlight the role of Hg P in Hg biogeochemical cycling and shed light on the enhanced Hg-methylation in settling particles and sediments in aquatic environments.

Redox transformation of mercury (Hg) is critical for Hg exchange at the air-sea interface and it can also affect the methylation of Hg in marine environments. However, the contributions of microalgae and aerobic bacteria in oxic seawater to Hg 2+ reduction are largely unknown. Here, we studied the reduction of Hg 2+ mediated by microalgae and aerobic bacteria in surface marine water and microalgae cultures under dark and sunlight conditions. The comparable reduction rates of Hg 2+ with and without light suggest that dark reduction by biological processes is as important as photochemical reduction in the tested surface marine water and microalgae cultures. The contributions of microalgae, associated free-living aerobic bacteria, and extracellular substances to dark reduction were distinguished and quantified in 7 model microalgae cultures, demonstrating that the associated aerobic bacteria are directly involved in dark Hg 2+ reduction. The aerobic bacteria in the microalgae cultures were isolated and a rapid dark reduction of Hg 2+ followed by a decrease of Hg 0 was observed. The reduction of Hg 2+ and re-oxidation of Hg 0 were demonstrated in aerobic bacteria Alteromonas spp. using double isotope tracing ( 199 Hg 2+ and 201 Hg 0 ). These findings highlight the importance of algae-associated aerobic bacteria in Hg transformation in oxic marine water.

Foliar accumulation of atmospheric mercury (Hg), particularly gaseous elemental mercury (Hg(0)), accounts for a significant amount of atmospheric Hg deposition and influences the Hg geochemical cycle. Foliar Hg accumulation occurs as the net result of uptake, adsorption, fixation, re-emission, and other processes. The combination of these processes influences foliar Hg speciation, translocation, toxicity, transport, and bioavailability, which eventually affects the environmental risk of Hg. Therefore, a systematic understanding of the various processes involved in foliar Hg accumulation is urgently needed. This review summarizes the current knowledge, research progress, and remaining knowledge gaps regarding foliar Hg accumulation processes and the biochemical mechanisms involved. Atmospheric Hg is biochemically fixed within foliage after stomatal and/or nonstomatal uptake, with foliar Hg then volatilized after reduction, washed off via throughfall, deposited on soil surfaces via litterfall or stored within vegetation over time. Mechanisms underlying these processes and future perspectives on foliar accumulation are hereby discussed with the aim of reducing knowledge gaps on foliar Hg accumulation. The combination of different methods, such as Hg speciation analysis, isotope tracer techniques, element imaging, biological technologies, and ecologically microcosmic methods, will help clarify Hg speciation, transformation, and subcellular distribution in foliage, and quantitate different Hg fluxes during foliar Hg accumulation and litterfall decomposition. Measurements and modeling on the foliar accumulation process of atmospheric Hg under natural and human perturbations are also essential. These future investigations will help comprehensively understand the processes influencing atmospheric Hg interactions with foliage and evaluate the environmental fates and risks of foliar Hg. [ABSTRACT FROM AUTHOR]