Methylmercury produced in upper oceans accumulates in deep Mariana Trench fauna

Ruoyu Sun,Jingjing Yuan,Yanxu Zhang,Yi Liu,Shun Chen,Jiubin Chen,Jeroen E Sonke,Congqiang Liu,Xiaotong Peng,Mei Meng,Tong Zhang,Wang Zheng

doi:10.1038/s41467-020-17045-3

Abstract

Monomethylmercury (MMHg) is a potent toxin that bioaccumulates and magnifies in marine food webs. Recent studies show abundant methylated Hg in deep oceans (>1000 m), yet its origin remains uncertain. Here we measured Hg isotope compositions in fauna and surface sediments from the Mariana Trench. The trench fauna at 7000–11000 m depth all have substantially positive mass-independent fractionation of odd Hg isotopes (odd-MIF), which can be generated only in the photic zone via MMHg photo-degradation. Given the identical odd-MIF in trench fauna and North Pacific upper ocean (<1000 m) biota MMHg, we suggest that the accumulated Hg in trench fauna originates exclusively from MMHg produced in upper oceans, which penetrates to depth by sorption to sinking particles. Our findings reveal little in-situ MMHg production in deep oceans and imply that anthropogenic Hg released at the Earth’s surface is much more pervasive across deep oceans than was previously thought.

Highlights

Monomethylmercury (MMHg) is a potent toxin that bioaccumulates and magnifies in marine food webs
MMHg is thought to be produced in situ in the oxygen-depleted zone of mesopelagic waters during microbial remineralization of sinking Particulate organic matter (POM) derived from phytoplankton primary production in the photic zone[9,12,14,15,16]
Mesopelagic MMHg can penetrate into deep oceans via downwelling or via complexation to or incorporation in sinking POM

Summary

Introduction

Monomethylmercury (MMHg) is a potent toxin that bioaccumulates and magnifies in marine food webs. Particulate organic matter (POM) scavenges IHg(II) in surface oceans and releases it at depth[6,7] This so-called biological pump results in a macronutrient-like vertical distribution of total Hg concentrations, with low values in epipelagic waters of the surface ocean (0–100 m), increasing through oxygen-depleted mesopelagic waters of the intermediate ocean (100–1000 m), and remaining high in deep ocean waters (>1000 m)[2,8,9,10,11]. MMHg is thought to be produced in situ in the oxygen-depleted zone of mesopelagic waters during microbial remineralization of sinking POM derived from phytoplankton primary production in the photic zone[9,12,14,15,16]. Due to lack of significant Hg isotope fractionation during accumulation and trophic transfer of MMHg in aquatic food chains[29,30], Hg isotope compositions of marine biota have demonstrated great potential in tracing the sources, production and degradation of MMHg in oceans[16,31,32]

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Results

Conclusion