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In order to evaluate mercury (Hg) accumulation patterns in Southern Ocean penguins, we measured Hg concentrations and carbon (δ 13 C) and nitrogen (δ 15 N) stable isotope ratios in body feathers of adult Adélie (Pygoscelis adeliae), gentoo (Pygoscelis papua), and chinstrap (Pygoscelis antarctica) penguins living near Anvers Island, West Antarctic Peninsula (WAP) collected in the 2010/2011 austral summer. With these and data from Pygoscelis and other penguin genera (Eudyptes and Aptenodytes) throughout the Southern Ocean, we modelled Hg variation using δ 13 C and δ 15 N values. Mean concentrations of Hg in feathers of Adélie (0.09 ± 0.05 μg g -1 ) and gentoo (0.16 ± 0.08 μg g -1 ) penguins from Anvers Island were among the lowest ever reported for the Southern Ocean. However, Hg concentrations in chinstrap penguins (0.80 ± 0.20 μg g -1 ), which undertake relatively broad longitudinal winter migrations north of expanding sea ice, were significantly higher (P < 0.001) than those in gentoo or Adélie penguins. δ 13 C and δ 15 N values for feathers from all three Anvers Island populations were also the lowest among those previously reported for Southern Ocean penguins foraging within Antarctic and subantarctic waters. These observations, along with size distributions of WAP krill, suggest foraging during non-breeding seasons as a primary contributor to higher Hg accumulation in chinstraps relative to other sympatric Pygoscelis along the WAP. δ 13 C values for all Southern Ocean penguin populations, alone best explained feather Hg concentrations among possible generalized linear models (GLMs) for populations grouped by either breeding site (AICc = 36.9, w i  = 0.0590) or Antarctic Frontal Zone (AICc = 36.9, w i  = 0.0537). Although Hg feather concentrations can vary locally by species, there was an insignificant species-level effect (w i  < 0.001) across the full latitudinal range examined. Therefore, feeding ecology at breeding locations, as tracked by δ 13 C, control Hg accumulation in penguin populations across the Southern Ocean., Competing Interests: Declaration of competing interest The authors declare no competing financial interests., (Copyright © 2024 Elsevier B.V. All rights reserved.)

Grass shrimp ( Palaemonetes pugio ) represent a potential link in the transfer of methylmercury (MeHg) from salt marsh sediments to transient young-of-the-year (YOY) fish. Across six salt marshes subject to varying degrees of Hg contamination, MeHg concentration in grass shrimp was significantly correlated with MeHg in sediment ( p < 0.05, R 2 = 0.81). Bioenergetic models show that grass shrimp alone account for 12-90% of MeHg observed in YOY striped bass and 6-22% of MeHg in YOY summer flounder. Direct accumulation of MeHg from grass shrimp to YOY fish increased with MeHg levels in grass shrimp and sediment. However, in the most contaminated salt marshes with the highest levels of MeHg in grass shrimp and sediment, indirect accumulation of MeHg from grass shrimp by YOY summer flounder, whose diet is dominated by benthic forage fish (mummichog), is predicted to plateau because higher concentrations of MeHg in grass shrimp are offset by a lower proportion of grass shrimp in the mummichog diet. Our results demonstrate that grass shrimp are an important trophic link in the bioaccumulation of MeHg in salt marsh food webs and that MeHg accumulation in YOY fish varies with both the concentration of MeHg in salt marsh sediments and benthic food web structure.

Soot and mercury (Hg) are two notorious air pollutants, and the fate and transport of Hg may be affected by soot at various scales in the environment as soot may be both a carrier and a reactant for active Hg species. This study was designed to quantify photoreduction of Hg(II) in the presence of soot and the associated Hg isotope fractionation under both atmospheric aerosol and aqueous conditions (water-saturated). Photoreduction experiments were conducted with diesel soot particulate matter under controlled temperature and relative humidity (RH) conditions using a flow-through semibatch reactor system. Mass-dependent fractionation resulted in the enrichment of heavier Hg isotopes in the remaining Hg(II) with enrichment factors (ε 202 Hg) of 1.48 ± 0.02‰ (±2 standard deviation) to 1.75 ± 0.05‰ for aerosol-phase reactions (RH 28-68%) and from 1.26 ± 0.11 to 1.50 ± 0.04‰ for aqueous-phase reactions. Positive odd mass-independent fractionation (MIF) was observed in aqueous-phase reactions, resulting in Δ 199 Hg values for reactant Hg(II) as high as 5.29‰, but negative odd-MIF occurred in aerosol-phase reactions, in which Δ 199 Hg values of reactant Hg(II) varied from -1.02 to 0‰. The average ratio of Δ 199 Hg/Δ 201 Hg (1.1) indicated that under all conditions, MIF was dominated by magnetic isotope effects during photoreduction of Hg(II). Increasing RH resulted in higher reduction rates but lower extents of negative MIF in the aerosol-phase experiments, suggesting that the reduction of soot particle-bound Hg(II) was responsible for the observed negative odd-MIF. Our results suggest that mass-independent Hg isotope fractionation during Hg(II) photoreduction varies with soot aerosol water content and that Hg-stable isotope ratios may be used to understand the transformational histories of aerosol-bound Hg(II) in the environment.

Bluefin tuna (BFT), highly prized among consumers, accumulate high levels of mercury (Hg) as neurotoxic methylmercury (MeHg). However, how Hg bioaccumulation varies among globally distributed BFT populations is not understood. Here, we show mercury accumulation rates (MARs) in BFT are highest in the Mediterranean Sea and decrease as North Pacific Ocean > Indian Ocean > North Atlantic Ocean. Moreover, MARs increase in proportion to the concentrations of MeHg in regional seawater and zooplankton, linking MeHg accumulation in BFT to MeHg bioavailability at the base of each subbasin's food web. Observed global patterns correspond to levels of Hg in each ocean subbasin; the Mediterranean, North Pacific, and Indian Oceans are subject to geogenic enrichment and anthropogenic contamination, while the North Atlantic Ocean is less so. MAR in BFT as a global pollution index reflects natural and human sources and global thermohaline circulation., Competing Interests: The authors declare no competing interest.

Distinct spatiotemporal distributions of sea surface dissolved elemental mercury (DEM) and its air-sea exchange flux were observed in the river-dominated and monsoon-influenced East China Sea (ECS). Spatially, DEM concentrations were higher in the nearshore Changjiang diluted water (90 ± 20 to 260 ± 40 fM) than in the offshore Kuroshio water (60 ± 10 to 160 ± 40 fM) and correlated with salinity and total Hg concentrations, suggesting that the total Hg discharged from the Changjiang river is a controlling factor. In summer, monsoon-driven coastal upwelling formed a transient nearshore water mass with very elevated DEM concentrations (290 ± 20 to 320 ± 70 fM). Seasonally, DEM concentrations in all water masses were the highest in summer (120 ± 30 to 320 ± 70 fM). Estimated rate coefficients for DEM production varied seasonally and strongly correlated with sea surface temperature (SST). Hg 0 evasion fluxes also peaked in summer (670 ± 380 pmol m -2 day -1 ), while in winter, DEM was close to equilibrium with gaseous elemental mercury in the atmosphere. Based on the air-sea Hg fluxes for all four seasons from this study and regional atmospheric deposition fluxes from others, we conclude that the ECS is a net sink of Hg annually, but it is a source of Hg to the atmosphere in summer. Moreover, the contribution of the ECS to Hg evasion may increase as a result of flood-associated high Changjiang discharge and rising SST.

We examined mercury (Hg) accumulation in juvenile and adult subpopulations of Antarctic krill (Euphausia superba) collected west of the Antarctic Peninsula. Samples were collected along a northern cross-shelf transect beginning near Anvers Island and farther south near the sea ice edge in the austral summers of 2011, 2013, 2014, and 2015. Regardless of geographical position, mean concentrations of total Hg and methylmercury (MeHg), the form of Hg that biomagnifies in marine food webs, were significantly higher in juvenile than adult krill in all years. In 2013, juvenile Antarctic krill collected along the coast near Anvers Island had significantly higher MeHg concentrations than krill collected farther offshore, and in 2013 and 2014, coastal juvenile krill exhibited some of the highest MeHg concentrations of all subpopulations sampled. Across all sampling years, collection in northern (sea ice-free) or southern (sea ice edge) transects did not affect MeHg concentrations of juvenile or adult krill, suggesting similar levels and routes of MeHg exposure across the latitudes sampled. Developmental stage, feeding near the coast, and annual variations in sea ice-driven primary and export production were identified as potentially important factors leading to greater MeHg accumulation in juvenile than adult krill. Krill-dependent predators feeding primarily on juveniles may thus accumulate more MeHg than consumers foraging on older krill. These results report MeHg concentrations in Antarctic krill and will be useful for predicting Hg biomagnification in higher-level consumers in this productive Antarctic ecosystem., (Copyright © 2019. Published by Elsevier B.V.)

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

The emergence and spread of antibiotic-resistant pathogenic bacteria is currently one of the most serious challenges to human health. To combat this problem, it is critical to understand the processes and pathways that result in the creation of antibiotic resistance gene pools in the environment. In this study, we examined the effects of mercury (Hg) exposure on the co-selection of Hg and antibiotic-resistant bacteria that colonize the gastrointestinal tract of the mummichog (Fundulus heteroclitus), a small, estuarine fish. We examined this connection in two experimental systems: (i) a short-term laboratory exposure study where fish were fed Hg-laced food for 15 days and (ii) an examination of environmental populations from two sites with very different levels of Hg contamination. In the lab exposure study, fish muscle tissue accumulation of Hg was proportional to food Hg concentration (R 2  = 0.99; P < 0.0001). In the environmental study, fish from the contaminated site contained threefold more Hg compared to fish from the reference site (P < 0.05). Further, abundance of the Hg resistance gene mercuric reductase was more than eightfold higher (P < 0.0001) in DNA extracts of ingesta of fish from the contaminated site, suggesting adaptation to Hg. Finally, resistance to three or more antibiotics was more common in Hg-resistant as compared to Hg-sensitive bacterial colonies that were isolated from fish ingesta (P < 0.001) demonstrating co-selection of Hg and antibiotic resistances. Together, our results highlight the possibility for the creation of antibiotic resistance gene pools as a result of exposure to Hg in contaminated environments.

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

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

As mercury (Hg) biosensors are sensitive to only intracellular Hg, they are useful in the investigation of Hg uptake mechanisms and the effects of speciation on Hg bioavailability to microbes. In this study, bacterial biosensors were used to evaluate the roles that several transporters such as the glutathione, cystine/cysteine, and Mer transporters play in the uptake of Hg from Hg-thiol complexes by comparing uptake rates in strains with functioning transport systems to strains where these transporters had been knocked out by deletion of key genes. The Hg uptake into the biosensors was quantified based on the intracellular conversion of inorganic mercury (Hg(II)) to elemental mercury (Hg(0)) by the enzyme MerA. It was found that uptake of Hg from Hg-cysteine (Hg(CYS)2) and Hg-glutathione (Hg(GSH)2) complexes occurred at the same rate as that of inorganic complexes of Hg(II) into Escherichia coli strains with and without intact Mer transport systems. However, higher rates of Hg uptake were observed in the strain with a functioning Mer transport system. These results demonstrate that thiol-bound Hg is bioavailable to E. coli and that this bioavailability is higher in Hg-resistant bacteria with a complete Mer system than in non-resistant strains. No difference in the uptake rate of Hg from Hg(GSH)2 was observed in E. coli strains with or without functioning glutathione transport systems. There was also no difference in uptake rates between a wildtype Bacillus subtilis strain with a functioning cystine/cysteine transport system, and a mutant strain where this transport system had been knocked out. These results cast doubt on the viability of the hypothesis that the entire Hg-thiol complex is taken up into the cell by a thiol transporter. It is more likely that the Hg in the Hg-thiol complex is transferred to a transport protein on the cell membrane and is subsequently internalized.

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

Speciation and reactivity of mercury were examined in Berry's Creek estuary downstream of a highly mercury-contaminated U.S. EPA Superfund site during the summers of 2002 and 2003. Surface water samples from Berry's Creek estuary, its confluence with the Hackensack River, and upstream of that confluence were analyzed for total (THg), particulate (PHgT), and dissolved (DHg) mercury, total and particulate monomethylmercury (MeHg), dissolved gaseous mercury (DGM), and bacterial merA gene and transcript abundances. Surface water concentrations of THg in Berry's Creek estuary (210-6800 pM) are among the highest in North America. A downstream gradient of Hg contamination is a permanent feature of Berry's Creek estuary, and the upper estuary appears to be a perennial source of Hg to the lower estuary and the Hackensack River. MeHg concentrations in Berry's Creek surface waters ranged from 2 to 14 pM, with the highest concentrations occurring at a midestuary site 2 km downstream of the tide gate. The suspended particle phase dominated Hg and MeHg speciation throughout this system, accounting for > 90% of THg in Berry's Creek estuary and 35-94% of THg in the Hackensack River. Concentrations of DGM in Berry's Creek estuary (0.1-1.0 pM) are similar to levels of DGM in other much less contaminated estuaries (0.04-0.75 pM). In addition, expression levels of the bacterial mercuric reductase gene, merA, a gene of the inorganic Hg(II)-regulated, mercury resistance (mer) operon, were low throughout Berry's Creek estuary. Thus, despite very high concentrations of mercury in Berry's Creek estuary, relatively low concentrations of DGM and merA gene expression levels indicate limited bioavailability of inorganic Hg in the estuary's surface waters. A system-wide limitation on the bioavailability of inorganic Hg, together with bacterial demethylation activity, may account for observed MeHg concentrations that, although elevated, are lower than expected given the concentrations of THg in this estuary.

Upland placement of dredged materials from navigation channels in the New York/New Jersey Harbor is currently being used to manage sediments deemed inappropriate for open water disposal. Although upland placement sites are equipped with engineering controls (leachate collection and/or barrier walls), little is known of the potential impacts of this approach to air quality. The aim of this study was to estimate the flux of mercury to the atmosphere from New York/New Jersey Harbor stabilized dredged material (SDM) that was used for land reclamation at a site in northeastern New Jersey. Total gaseous mercury (TGM) was measured at a site receiving SDM in August and October 2001 and May and November 2002. TGM was also monitored at an urban reference site 3.5 km west of the SDM site in September 2001 and from February 2002 to July 2002 and from October 2002 to February 2003. The concentration of TGM at the urban reference site averaged 2.2 +/- 1.1 ng m(-3), indicating some local contribution to the Northern Hemisphere background. TGM concentrations exhibited seasonality with the highest values in summer (3.3 +/- 2.1 ng m(-3) in June 2002) and the lowest in winter (1.7 +/- 0.6 ng m(-3) in January 2003). TGM concentrations at the SDM placement site ranged from 2 to 7 ng m(-3) and were significantly higher (p < 0.001) than those at the urban reference site. Sediment-air fluxes of Hg at the SDM placement site estimated by the micrometeorological technique ranged from -13 to 1040 ng m(-2) h(-1) (sediment to air fluxes being positive) and were significantly correlated to solar radiation (r2 = 0.81). The estimated contribution of Hg emissions from land-applied SDM to local TGM concentrations was found to be negligible (<4%). However, the estimated annual volatilization rate of TGM atthe SDM site (130 kg y(-1))was comparable to those of other industrial sources in New Jersey (140-450 kg y(-1)).

The curious phenomenon of similar levels of methylmercury (MeHg) accumulation in fish from contaminated and pristine environments may be explained by the observation that the proportion of total mercury (HgT) present as MeHg is inversely related to HgT in natural waters. We hypothesize that this "MeHg accumulation paradox" is explained by the quantitative induction of bacterial enzymes that are encoded by the mercury resistance (mer) operon, organomercury lyase (MerB), and mercuric reductase (MerA) by inorganic Hg (Hg[II]). We tested this hypothesis in two ecosystems in New Jersey: Berry's Creek in the Meadowlands (ML) and Pine Barren (PB) lakes. Across all sites, an inverse correlation (r2 = 0.80) between the concentration of HgT (ML, 113-4220 ng L(-1); PB, 0.3-5.4 ng L(-1)) and the proportion of HgT as MeHg (MeHg in ML and PB ranged from 0.08 to 1.6 and from 0.03 to 0.34 ng L(-1), respectively) was observed. The planktonic microbial community in Meadowlands surface waters exhibited adaptation to mercury, the presence of mer genes and mRNA transcripts, and high rates of reductive demethylation (k(deg) = 0.19 day(-1)). In contrast, the microbial community of PB was not adapted to mercury and demonstrated low rates of oxidative demethylation (k(deg) = 0.01 day(-1)). These results strongly support our hypothesis and show that the degradation of MeHg by mer-encoded enzymes by the water column microbiota of contaminated environments can significantly affect the amount of MeHg that is available for entry into the aquatic food web.