Seasonal Variation of Mercury Methylation Potential in Pristine Coastal Marine Sediment from the Gulf of Trieste (Northern Adriatic Sea)

The Gulf of Trieste (northern Adriatic Sea) is considered as an important “natural bioreactor” that can be stimulated to increase sedimentary methylmercury (MeHg) production by mobilizing its extensive mercury (Hg) pool. Sediment samples, collected in the southern pristine, less contaminated, part of the gulf, were amended with radioactive 197Hg in incubation experiments to determine the seasonal variation of Hg methylation potential at two depth layers (surface 0–2 cm and subsurface 2–4 cm). In parallel, Hg and MeHg were determined in the sediment solid phase and pore water. Total Hg and MeHg in the sediment solid phase ranged between 151 and 223 ng g −1 and between 0.17 and 0.68 ng g −1, and in pore water between 3.1 and 25.6 ng L −1 and between 0.03 and 6.16 ng L −1. Distribution coefficients (K d) suggested that Hg species are strongly bound to the sediment (log K d 3.86–4.87 and 1.67–3.39 for Hg and MeHg, respectively). Hg methylation potentials showed that MeHg is continuously produced, albeit not at a constant rate. The methylation potential in the sediment surface layer was higher and more variable in autumn, in parallel to higher bottom water temperature and O 2 content and sediment sulfate reduction, than in late spring and summer except in July in the presence of higher labile sedimentary organic matter (OM) level. Surface sediment showed rapid methylation of added traced Hg (0–0.66% day −1 Hg methylated). However, a significant fraction of the radiolabeled Hg remained available. In the subsurface sediment, methylation was lower (0–0.22% day −1 Hg methylated). Hg methylation potentials suggest that MeHg is continuously produced, albeit not at a constant rate, in the studied sediment of the Gulf of Trieste.