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1. Low mercury concentrations in a Greenland glacial fjord attributed to oceanic sources

   https://doi.org/10.1038/s43247-024-01474-9  

   Lindeman, M_R ; Straneo, F. ; Adams, H_M ; Nelson, M_J_S ; Schartup, A_T ( June 2024 , Communications Earth & Environment)

   As the role of the Greenland Ice Sheet in the Arctic mercury (Hg) budget draws scrutiny, it is crucial to understand mercury cycling in glacial fjords, which control exchanges with the ocean. We present full water column measurements of total mercury (THg) and methylmercury (MeHg) in Sermilik Fjord, a large fjord in southeast Greenland fed by multiple marine-terminating glaciers, whose circulation and water mass transformations have been extensively studied. We show that THg (0.23-1.1 pM) and MeHg (0.02-0.17 pM) concentrations are similar to those in nearby coastal waters, while the exported glacially-modified waters are relatively depleted in inorganic mercury (Hg(II)), suggesting that inflowing ocean waters from the continental shelf are the dominant source of mercury species to the fjord. We propose that sediments initially suspended in glacier meltwaters scavenge particle-reactive Hg(II) and are subsequently buried, making the fjord a net sink of oceanic mercury.

    

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2. 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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3. Distribution of mercury‐cycling genes in the Arctic and equatorial Pacific Oceans and their relationship to mercury speciation

   https://doi.org/10.1002/lno.11310  

   Bowman, Katlin L. ; Collins, R. Eric ; Agather, Alison M. ; Lamborg, Carl H. ; Hammerschmidt, Chad R. ; Kaul, Drishti ; Dupont, Christopher L. ; Christensen, Geoff A. ; Elias, Dwayne A. ( August 2019 , Limnology and Oceanography)

   Humans are exposed to potentially harmful amounts of the neurotoxin monomethylmercury (MMHg) through consumption of marine fish and mammals. However, the pathways of MMHg production and bioaccumulation in the ocean remain elusive. In anaerobic environments, inorganic mercury (Hg) can be methylated to MMHg through an enzymatic pathway involving thehgcABgene cluster. Recently,hgcA‐like genes have been discovered in oxygenated marine water, suggesting thehgcABmethylation pathway, or a close analog, may also be relevant in the ocean. Using polymerase chain reaction amplification and shotgun metagenomics, we searched for but did not find thehgcABgene cluster in Arctic Ocean seawater. However, we detected Hg‐cycling genes from themeroperon (including organomercury lyase,merB), andhgcA‐like paralogs (i.e.,cdhD) in Arctic Ocean metagenomes. Our analysis of Hg biogeochemistry and marine microbial genomics suggests that various microorganisms and metabolisms, and not just thehgcABpathway, are important for Hg methylation in the ocean.

    

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4. 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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5. Bluefin tuna reveal global patterns of mercury pollution and bioavailability in the world's oceans

   https://doi.org/10.1073/pnas.2111205118  

   Tseng, Chun-Mao ; Ang, Shin-Jing ; Chen, Yi-Sheng ; Shiao, Jen-Chieh ; Lamborg, Carl H. ; He, Xiaoshuai ; Reinfelder, John R. ( September 2021 , Proceedings of the National Academy of Sciences)

   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. 

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