Virtually all biotic, dark abiotic, and photochemical transformations of mercury (Hg) produce Hg isotope fractionation, which can be either mass dependent (MDF) or mass independent (MIF). The largest range in MDF is observed among geological materials and rainfall impacted by anthropogenic sources. The largest positive MIF of Hg isotopes (odd-mass excess) is caused by photochemical degradation of methylmercury in water. This signature is retained through the food web and measured in all freshwater and marine fish. The largest negative MIF of Hg isotopes (odd-mass deficit) is caused by photochemical reduction of inorganic Hg and has been observed in Arctic snow and plant foliage. Ratios of MDF to MIF and ratios of 199Hg MIF to 201Hg MIF are often diagnostic of biogeochemical reaction pathways. More than a decade of research demonstrates that Hg isotopes can be used to trace sources, biogeochemical cycling, and reactions involving Hg in the environment.

Mercury enters marine food webs in the form of microbially generated monomethylmercury. Microbial methylation of inorganic mercury, generating monomethylmercury, is widespread in low-oxygen coastal sediments. The degree to which microbes also methylate mercury in the open ocean has remained uncertain, however. Here, we present measurements of the stable isotopic composition of mercury in nine species of marine fish that feed at different depths in the central North Pacific Subtropical Gyre. We document a systematic decline in δ 202 Hg, 199 Hg and 201 Hg values with the depth at which fish feed. We show that these mercury isotope trends can be explained only if monomethylmercury is produced below the surface mixed layer, including in the underlying oxygen minimum zone, that is, between 50 and more than 400 m depth. Specifically, we estimate that about 20-40% of the monomethylmercury detected below the surface mixed layer originates from the surface and enters deeper waters either attached to sinking particles, or in zooplankton and micronekton that migrate to depth. We suggest that the remaining monomethylmercury found at depth is produced below the surface mixed layer by methylating microbes that live on sinking particles. We suggest that microbial production of monomethylmercury below the surface mixed later contributes significantly to anthropogenic mercury uptake into marine food webs.M ercury (Hg) is a globally distributed atmospheric pollutant that can form monomethyl-Hg (MMHg), which is neurotoxic and bioaccumulative in aquatic food webs. The main pathway for human exposure to MMHg is through consumption of piscivorous marine fish 1 . In the ocean Hg primarily exists as inorganic Hg(ii) or Hg (0) [5][6][7] . In agreement with these vertical profiles, total Hg (THg) levels in commercially important predatory pelagic fish and their prey increase with median depth of occurrence 8,9 , which is indicative of both foraging depth and habitat utilization. However, the biogeochemical factors controlling the distribution and speciation of Hg in the ocean and the bioaccumulation of MMHg in marine food webs are still not well understood 10 .Microbial activity and both dark and photochemical inorganic reactions ultimately control the chemical speciation and subsequent bioavailability of Hg (ref. 2), thus an understanding of the competing processes that produce and degrade MMHg at different depths in the ocean is critical to tracing its bioaccumulation in fish and biomagnification in marine food webs. MMHg typically comprises only a small fraction of THg in North Pacific sea water (<15%; ref. 4) but nearly all of the Hg (>95%) in large marine fish 2,11 and most of the Hg (>80%) in small fish with trophic position (TP) as low as 2.5 (ref. 12). DMHg concentrations are generally small relative to MMHg (refs 3,4,13), although at some (refs 13,14). As DMHg is not bioaccumulated and because its Hg isotopic composition has not been measured, it is not treated in detail here. MMHg production in the oxygen minimum zone (OMZ) of...

Mercury (Hg) undergoes systematic stable isotopic fractionation; therefore, isotopic signatures of Hg may provide a new tool to track sources, sinks, and dominant chemical transformation pathways of Hg in the environment. We investigated the isotopic fractionation of Hg by Hg(II) resistant (HgR) bacteria expressing the mercuric reductase (MerA) enzyme. The isotopic composition of both the reactant Hg(II) added to the growth medium and volatilized product (Hg(0)) was measured using cold vapor generation and multiple collector inductively coupled plasma mass spectrometry. We found that exponentially dividing pure cultures of a gram negative strain Escherichia coli JM109/pPB117 grown with abundant electron donor and high Hg(II) concentrations at 37, 30, and 22 degrees C, and a natural microbial consortium incubated in natural site water at 30 degrees C after enrichment of HgR microbes, preferentially reduced the lighter isotopes of Hg. In all cases, Hg underwent Rayleigh fractionation with the best estimates of alpha202/198 values ranging from 1.0013 to 1.0020. In the cultures grown at 37 degrees C, below a certain threshold Hg(II) concentration, the extent of fractionation decreased progressively. This study demonstrates mass-dependent kinetic fractionation of Hg and could lead to development of a new stable isotopic approach to the study of Hg biogeochemical cycling in the environment.

No abstract