| Abstract
| - Biotic transformation of inorganic mercury, Hg(II), tomono methyl mercury (MeHg) is proposed to be largelycontrolled by passive uptake of neutral Hg complexes bysulfate reducing bacteria (SRB). In this study, the chemicalspeciation of Hg(II) in seven locally contaminatedsediments covering environments such as (i) brackishwater, (ii) low-productivity freshwater, and, (iii) high-productivity freshwater was related to potential Hg methylationrates, determined by incubation at 23 °C for 48 h underN2(g), and to total MeHg concentrations in sediments. Porewater speciation was modeled considering Hg complexeswith halides, organic thiols [Hg(SR)2(aq), associated todissolved organic matter], monosulfides, and bisulfides. Thesum of neutral mercury sulfides [Hg(SH)20(aq)] and[HgS0(aq)] was significantly, positively (p< 0.001, n = 20)correlated to the specific methylation rate constant (Km,day-1) at depths of 5−100 cm in two brackish water sediments.Total Hg, total mercury sulfides or Hg(SR)2(aq) in porewater gave no significant relationships with Km. In two sub-sets of freshwater sediments, neutral mercury sulfideswere positively correlated to total Hg in pore water, andtherefore, total Hg also gave significant relationships withKm. The sum of [Hg(SH)20(aq)] and [HgS0(aq)] wassignificantly, positively correlated to total sediment MeHg(μg kg-1) in brackish waters (p< 0.001, n = 23), insouthern, high-productivity freshwaters (p< 0.001, n =20), as well as in northern, low-productivity freshwater (p= 0.048, n = 6). The slopes (b, b‘) of the relationshipsKm (day-1) = a + b([Hg(SH)20(aq)] + [HgS0(aq)]) and MeHg(μg kg-1) = a‘ + b‘([Hg(SH)20(aq)] + [HgS0(aq)]) showedan inverse relationship with the C/N ratio, supposedly reflectingdifferences in primary production and energy-rich organicmatter availability among sites. We conclude thatconcentrations of neutral inorganic mercury sulfidespecies, together with the availability of energy-rich organicmatter, largely control Hg methylation rates in contaminatedsediments. Furthermore, Hg(SH)20(aq) is suggested tobe the dominant species taken up by MeHg producingbacteria in organic-rich sediments without formation ofHgS(s).
|
