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1. Synergistic Effects of a Chalkophore, Methanobactin, on Microbial Methylation of Mercury

   https://doi.org/10.1128/AEM.00122-20  

   Yin, X ; Wang, L ; Zhang, L ; Chen, H ; Liang, X ; Lu, X ; DiSpirito, AA ; Semrau, JD ; Gu, B. ( January 2020 , Applied and environmental microbiology)

   Microbial production of the neurotoxin, methylmercury (MeHg), is a significant health and environmental concern as it can bioaccumulate and biomagnify in the food web. A chalkophore or a copper-binding compound, termed methanobactin (MB), has been shown to form strong complexes with mercury [as Hg(II)] and also enables some methanotrophs to degrade MeHg. It is unknown, however, if Hg(II) binding with MB can also impede Hg(II) methylation by other microbes. Contrary to expectations, MB produced by the methanotroph Methylosinus trichosporium OB3b (OB3b-MB) enhanced the rate and efficiency of Hg(II) methylation more than that observed with thiol compounds (such as cysteine) by the mercury-methylating bacteria, D. desulfuricans ND132 and G. sulfurreducens PCA. Compared to no-MB controls, OB3b-MB decreased the rates of Hg(II) sorption and internalization, but increased methylation by 5–7 fold, suggesting that Hg(II) complexation with OB3b-MB facilitated exchange and internal transfer of Hg(II) to the HgcAB proteins required for methylation. Conversely, addition of excess amounts of OB3b-MB or a different form of MB from Methylocystis strain SB2 (SB2-MB) inhibited Hg(II) methylation, likely due to greater binding of Hg(II). Collectively our results underscore complex roles of exogenous metal-scavenging compounds produced by microbes in controlling net production and bioaccumulation of MeHg in the environment. 

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2. Mercury methylation by metabolically versatile and cosmopolitan marine bacteria

   https://doi.org/10.1038/s41396-020-00889-4  

   Lin, Heyu ; Ascher, David B. ; Myung, Yoochan ; Lamborg, Carl H. ; Hallam, Steven J. ; Gionfriddo, Caitlin M. ; Holt, Kathryn E. ; Moreau, John W. ( June 2021 , The ISME Journal)

   Abstract Microbes transform aqueous mercury (Hg) into methylmercury (MeHg), a potent neurotoxin that accumulates in terrestrial and marine food webs, with potential impacts on human health. This process requires the gene pair hgcAB , which encodes for proteins that actuate Hg methylation, and has been well described for anoxic environments. However, recent studies report potential MeHg formation in suboxic seawater, although the microorganisms involved remain poorly understood. In this study, we conducted large-scale multi-omic analyses to search for putative microbial Hg methylators along defined redox gradients in Saanich Inlet, British Columbia, a model natural ecosystem with previously measured Hg and MeHg concentration profiles. Analysis of gene expression profiles along the redoxcline identified several putative Hg methylating microbial groups, including Calditrichaeota, SAR324 and Marinimicrobia, with the last the most active based on hgc transcription levels. Marinimicrobia hgc genes were identified from multiple publicly available marine metagenomes, consistent with a potential key role in marine Hg methylation. Computational homology modelling predicts that Marinimicrobia HgcAB proteins contain the highly conserved amino acid sites and folding structures required for functional Hg methylation. Furthermore, a number of terminal oxidases from aerobic respiratory chains were associated with several putative novel Hg methylators. Our findings thus reveal potential novel marine Hg-methylating microorganisms with a greater oxygen tolerance and broader habitat range than previously recognized. 

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3. Environmental formation of methylmercury is controlled by synergy of inorganic mercury bioavailability and microbial mercury‐methylation capacity

   https://doi.org/10.1111/1462-2920.16364  

   Peterson, Benjamin D. ; Krabbenhoft, David P. ; McMahon, Katherine D. ; Ogorek, Jacob M. ; Tate, Michael T. ; Orem, William H. ; Poulin, Brett A. ( March 2023 , Environmental Microbiology)

   Methylmercury (MeHg) production is controlled by the bioavailability of inorganic divalent mercury (Hg(II)_i) and Hg‐methylation capacity of the microbial community (conferred by thehgcABgene cluster). However, the relative importance of these factors and their interaction in the environment remain poorly understood. Here, metagenomic sequencing and a full‐factorial MeHg formation experiment were conducted across a wetland sulfate gradient with different microbial communities and pore water chemistries. From this experiment, the relative importance of each factor on MeHg formation was isolated. Hg(II)_ibioavailability correlated with the dissolved organic matter composition, while the microbial Hg‐methylation capacity correlated with the abundance ofhgcAgenes. MeHg formation responded synergistically to both factors. Notably,hgcAsequences were from diverse taxonomic groups, none of which contained genes for dissimilatory sulfate reduction. This work expands our understanding of the geochemical and microbial constraints on MeHg formation in situ and provides an experimental framework for further mechanistic studies.

    

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4. Linked mercury methylation and nitrification across oxic subpolar regions

   https://doi.org/10.3389/fenvc.2023.1109537  

   Despins, Marissa C. ; Mason, Robert P. ; Aguilar-Islas, Ana M. ; Lamborg, Carl H. ; Hammerschmidt, Chad R. ; Newell, Silvia E. ( May 2023 , Frontiers in Environmental Chemistry)

   Methylmercury (MeHg) is a neurotoxin that bioaccumulates to potentially harmful concentrations in Arctic and Subarctic marine predators and those that consume them. Monitoring and modeling MeHg bioaccumulation and biogeochemical cycling in the ocean requires an understanding of the mechanisms behind net mercury (Hg) methylation. The key functional gene pair for Hg methylation,hgcAB, is widely distributed throughout ocean basins and spans multiple microbial phyla. While multiple microbially mediated anaerobic pathways for Hg methylation in the ocean are known, the majority ofhgcAhomologs have been found in oxic subsurface waters, in contrast to other ecosystems. In particular, microaerophilicNitrospina, a genera of nitrite-oxidizing bacteria containing ahgcA-like sequence, have been proposed as a potentially important Hg methylator in the upper ocean. The objective of this work was therefore to examine the potential of nitrifiers as Hg methylators and quantify total Hg and MeHg across three Arctic and Subarctic seas (the Gulf of Alaska, the Bering Sea and the Chukchi Sea) in regions whereNitrospinaare likely present. In Spring 2021, samples for Hg analysis were obtained with a trace metal clean rosette across these seas. Mercury methylation rates were quantified in concert with nitrification rates using onboard incubation experiments with additions of stable isotope-labeled Hg and NH₄⁺. A significant correlation between Hg methylation and nitrification was observed across all sites (R²= 0.34,p< 0.05), with the strongest correlation in the Chukchi Sea (R²= 0.99,p< 0.001).Nitrospina-specifichgcA-like genes were detected at all sites. This study, linking Hg methylation and nitrification in oxic seawater, furthers understanding of MeHg cycling in these high latitude waters, and the ocean in general. Furthermore, these studies inform predictions of how climate and human interactions could influence MeHg concentrations across the Arctic in the future.

    

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5. A Global Model for Methylmercury Formation and Uptake at the Base of Marine Food Webs

   https://doi.org/10.1029/2019GB006348  

   Zhang, Yanxu ; Soerensen, Anne L. ; Schartup, Amina T. ; Sunderland, Elsie M. ( February 2020 , Global Biogeochemical Cycles)

   Monomethylmercury (CH₃Hg) is the only form of mercury (Hg) known to biomagnify in food webs. Here we investigate factors driving methylated mercury [MeHg = CH₃Hg + (CH₃)₂Hg)] production and degradation across the global ocean and uptake and trophic transfer at the base of marine food webs. We develop a new global 3‐D simulation of MeHg in seawater and phyto/zooplankton within the Massachusetts Institute of Technology general circulation model. We find that high modeled MeHg concentrations in polar regions are driven by reduced demethylation due to lower solar radiation and colder temperatures. In the eastern tropical subsurface waters of the Atlantic and Pacific Oceans, the model results suggest that high MeHg concentrations are associated with enhanced microbial activity and atmospheric inputs of inorganic Hg. Global budget analysis indicates that upward advection/diffusion from subsurface ocean provides 17% of MeHg in the surface ocean. Modeled open ocean phytoplankton concentrations are relatively uniform because lowest modeled seawater MeHg concentrations occur in oligotrophic regions with the smallest size classes of phytoplankton, with relatively high uptake of MeHg and vice versa. Diatoms and synechococcus are the two most important phytoplankton categories for transferring MeHg from seawater to herbivorous zooplankton, contributing 35% and 25%, respectively. Modeled ratios of MeHg concentrations between herbivorous zooplankton and phytoplankton are 0.74–0.78 for picoplankton (i.e., no biomagnification) and 2.6–4.5 for eukaryotic phytoplankton. The spatial distribution of the trophic magnification factor is largely determined by the zooplankton concentrations. Changing ocean biogeochemistry resulting from climate change is expected to have a significant impact on marine MeHg formation and bioaccumulation.

    

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