Dynamic mercury methylation and demethylation in oligotrophic marine water

Abstract

Monomethylmercury bioaccumulation in open ocean food webs depends on the net rate of inorganic mercury conversion to monomethylmercury in the water column. We measured significant methylation rates across large gradients in oxygen utilization in the oligotrophic central Pacific Ocean. Overall, methylation rates over 24 hour incubation periods were comparable to those previously published 20 from Arctic and Mediterranean waters despite differences in productivity between these marine environments. In contrast to previous studies that have attributed Hg methylation to heterotrophic bacteria, we measured higher methylation rates in filtered water compared to unfiltered water. Furthermore, we observed enhanced demethylation of newly produced methylated mercury in incubations of unfiltered water relative to filtered water. The addition of station-specific bulk suspended particulate matter, a Biogeosciences Discuss., https://doi

Mercury Cycling in the North Pacific Subtropical Gyre as Revealed by Mercury Stable Isotope Ratios

“…Patterns in Δ 199 Hg values with depth for marine particles and zooplankton (Figure b) are consistent with active dark demethylation of Hg below the mixed layer, which was also suggested by Munson et al () based on an experimental study in the Pacific Ocean. The Δ 199 Hg values of THg within particles are relatively constant with depth as expected in the absence of light, with the exception of samples from 400 m and one sample from 690 m. At these depths the particles have significantly higher Δ 199 Hg values (by 0.26‰ in February, 0.37‰ in September, and 0.18‰ in May) compared to all the samples collected at 25–150 m from all three seasons (average = 0.12 ± 0.05‰, 1 SD, n = 8).…”

“…The specific mechanism for methylation of Hg (II) in the open ocean water column is uncertain but is likely microbially mediated, as it is in freshwater and terrestrial ecosystems (Cossa et al, , ; Morel et al, ; Sunderland et al, ). There is, however, also evidence from filtered seawater experiments (Munson et al, ) for noncellular or extracellular methylation in the water column in the oligotrophic Pacific. Methylation of Hg in the water column was also shown to be consistent with patterns of Hg isotope variation with the depth of marine fish feeding (Blum et al, ).…”

“…The major forms of Hg in aquatic ecosystems are Hg (II) complexed to organic ligands, chloride or sulfide, and organic mercury in the form of MMHg (Fitzgerald et al, ; Morel et al, ). Within the marine water column, Hg transformations include not only methylation of Hg (II) by abiotic and biotic processes (Munson et al, ; Sunderland et al, ) but also degradation of MMHg to Hg (II) and Hg(0) by photochemical and dark biotic processes (Fitzgerald et al, ; Sunderland et al, ). The limited understanding of the marine cycle of Hg is in part because of technical challenges associated with tracking and measuring different chemical forms of Hg in marine environments, especially in organisms like plankton at the base of the food web.…”

“…Elevated oceanic Hg concentrations have been attributed to evasion from sediments (Hammerschmidt & Fitzgerald, ; Monperrus et al, ), emission from seafloor hydrothermal vents (Lamborg et al, ), and horizontal advection from coastal areas (Hammerschmidt & Bowman, ). Elevated MMHg in the water column has been attributed to biotic methylation of Hg (II) on marine particles (Blum et al, ; Cossa et al, ; Sunderland et al, ) and abiotic methylation in the water column (Munson et al, ). Thus, the biogeochemical pathway(s) for MMHg production in the open ocean are complex and remain uncertain.…”

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

“…However, most organisms with the hgcAB gene cluster are from anoxic environments, which are rare in seawater samples from the open ocean (Bowman et al, ; Podar et al, ). Incubations of Hg stable isotopes have shown that active production of MeHg from Hg II occurs both in coastal and in shelf sediments (Hammerschmidt et al, ; Hollweg et al, ; Sunderland et al, ) and the marine water column (Lehnherr et al, ; Monperrus et al, ; Munson et al, ; Schartup et al, ). High concentrations of MeHg have been reported in subsurface marine waters (approximately 200–500 m) at different regions as well as near hydrothermal vents (Bowman et al, , ; Bratkič et al, ; Canário et al, ; Cossa et al, ; Hammerschmidt & Bowman , ; Kim et al, ; Lamborg et al, ; Munson et al, ; Sunderland et al, ).…”