Enhanced availability of mercury bound to dissolved organic matter for methylation in marine sediments

Abstract

The forms of inorganic mercury (HgII) taken up and methylated by bacteria in sediments still remain largely unknown. From pure cultures studies, it has been suggested that dissolved organic matter (DOM) may facilitate the uptake either by acting as a shuttle molecule, transporting the HgII atom to divalent metal transporters, or by binding HgII and then being transported into the cell as a carbon source. Enhanced availability of Hg complexed to DOM has however not yet been demonstrated in natural systems. Here, we show that HgII complexed with DOM of marine origin was up to 2.7 times more available for methylation in sediments than HgII added as a dissolved inorganic complex (HgII(aq)). We argue that the DOM used to complex HgII directly facilitated the bacterial uptake of HgII whereas the inorganic dissolved HgII-complex adsorbed to the sediment matrix before forming bioavailable dissolved HgII complexes. We further demonstrate that differences in net methylation in sediments with high and low organic carbon content may be explained by differences in the availability of carbon to stimulate the activity of Hg methylating bacteria rather than, as previously proposed, be due to differences in HgII binding capacities between sediments.

The interplay between total mercury, methylmercury and dissolved organic matter in fluvial systems: A latitudinal study across Europe

“…In contrast, the negative correlation between %MeHg and HIX suggests that high concentrations of highly humified terrestrial DOM hampered Hg methylation processes most likely by decreasing Hg bioavailability (Chiasson-Gould et al, 2014;Soerensen et al, 2017) and/or bacterial activity (Bravo et al, 2017). High Hg methylation rates have been associated with algal derived OM (Bravo et al, 2017;Gascon Diez et al, 2016;Herrero Ortega et al, 2018;Mazrui et al, 2016). Our study provides further evidence of the potential role of algal and/or microbial DOM in the in situ formation of MeHg in streams.…”

“…2and Fe 3+ ) (Bravo et al, 2015) and the molecular composition of OM, which influences IHg-speciation (Graham et al, 2013;Jonsson et al, 2014), IHg-availability (Chiasson-Gould et al, 2014;Mazrui et al, 2016;Schaefer and Morel, 2009;Schartup et al, 2015b) and microbial activity (Bravo et al, 2017). Specifically, IHg methylation processes are enhanced by the presence of algal OM (Bravo et al, 2017;Schartup et al, 2013) and fresh humic OM leached from organic rich soils (Herrero Ortega et al, 2018).…”

Biotic formation of methylmercury: A bio–physico–chemical conundrum

“…In the context of the biological and chemical interplays controlling Hg II methylation, sulfur plays a central role by directly affecting Hg II speciation and solubility (Jonsson et al ; Graham et al ; Hsu‐Kim et al ; Liem‐Nguyen et al ) and consequently its bioavailability (Schaefer and Morel ; Chiasson‐Gould et al ; Schartup et al ; Mazrui et al ). Reactions between Hg II and sulfide control the formation of the solid phase metacinnabar, β‐HgS(s) but also aqueous complexes such as Hg(SH) 2 0 , HgS 2 H − , and HgS 2 2− or polysulfides HgS n SH−(aq) ( n = 4–6) (Liem‐Nguyen et al ).…”

“…Natural OM, besides its strong capacity to bind Hg II and influence Hg II speciation, controls microbial activity and Hg II methylation (Graham et al ; Hsu‐Kim et al ). The molecular composition of OM shows a central role in controlling Hg II methylation (Bravo et al ), notably phytoplankton derived OM and fresh humic matter were associated with both high bacterial activity and Hg II methylation rates in sediments (Graham et al ; Schartup et al ; Mazrui et al ; Bravo et al ; Christensen et al ; Herrero Ortega et al ). Also, an increase in nutrients, associated to an enhanced algae biomass production, increased MMHg formation in sediments (Bravo et al ; Herrero Ortega et al ).…”

Recent advances in the study of mercury methylation in aquatic systems

“…In Louisiana wetlands within a salinity gradient, Hall et al (2008) observed that concentrations of MeHg were correlated with the fraction of highly reactive dissolved organic carbon (DOC). While concentrations of DOM in marine systems are lower than in freshwater systems, the availability of organic carbon has been correlated to MeHg distribution in marine surface and intermediate waters in the Arctic (Cossa et al 2009;Sunderland et al 2009;Kirk et al 2012), north Pacific (Sunderland et al 2009), and Mediterranean (Cossa et al 2009), as well as in estuarine (Lambertsson and Nilsson 2006) and marine (Mazrui et al 2016) sediment. In rice paddies, the organic matter from rice waste both acts a nutrient source for methylating microbes and promotes the partitioning of MeHg to the dissolved phase (Rothenberg et al 2011), thereby potentially increasing its bioavailability (Fleck et al 2014).…”