Methylmercury produced in upper oceans accumulates in deep Mariana Trench fauna

Abstract

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.

Mercury isotopes identify near-surface marine mercury in deep-sea trench biota

“…Hg isotope ratios of snailfish and amphipods from 6,000 to 10,250-m depth in the Mariana and Kermadec hadal trenches were measured and compared with Hg isotope measurements of a wide range of fish species in the central Pacific that feed at depths of 10–1,000 m. Based on odd-MIF and even-MIF isotopic signatures we conclude that the majority of Hg in the tissues of animals living in the hadal zone originates from the atmosphere, is methylated in the epipelagic (upper 200 m) and mesopelagic (200–1,000-m) zones, and is then delivered to the hadal zone (6,000–11,000 m) by the carcasses of fish, and to a lesser extent by particles ( 33 ). A near-surface source of Hg to hadal biota is consistent with recent documentation of “bomb 14 C” ( 59 ), lead ( 60 ), and persistent organic pollutants ( 61 ) in hadal biota.…”

“…They found that 2–59% of the Hg in amphipods (and 83% in the single snailfish) was in the form of MMHg, but they argued that the isotopic composition of THg in each sample was representative of the isotopic composition of MMHg in that sample because the inorganic Hg (IHg) in the biota was produced by in vivo degradation of MMHg rather than by assimilation of IHg. Sun et al ( 33 ) provided four lines of evidence that make a strong case for the idea that IHg formed by degradation of MMHg. Our study and the study of Sun et al ( 33 ) each measured THg and Hg isotopic composition of amphipods from the Mariana Trench.…”

“…Sun et al ( 33 ) provided four lines of evidence that make a strong case for the idea that IHg formed by degradation of MMHg. Our study and the study of Sun et al ( 33 ) each measured THg and Hg isotopic composition of amphipods from the Mariana Trench. We have additionally measured Hg isotope ratios in amphipods from the Kermadec Trench and snailfish from both the Kermadec and Mariana trenches.…”

Mercury in the tissues of five cephalopods species: First data on the nervous system

“…glutathione, GSH) or proteins as a dicysteinate complex, which was modeled using the Hg(Cys) 2 reference. This suggests that 1) iHg may come from the environment (Sun et al, 2020) through both dissolved and trophic pathways. Indeed, it has been demonstrated that cuttlefish efficiently bioconcentrate dissolved i 203 Hg, which is then distributed throughout the digestive gland (Lacoue-Labarthe et al, 2009).…”

“…In addition, ~90% of the i 203 Hg from prey is assimilated and then mostly (~70%) found in the digestive gland. Alternatively, 2) iHg may result from in vivo MeHg demethylation (Sun et al, 2020), which likely occurs in the digestive gland, facilitating the rapid elimination of produced iHg, based on experimental findings in which a short biological half-life (i.e. 4 days) of dietary 203 iHg was reported (Lacoue-Labarthe et al, 2009).…”