Il mercurio nell'interfaccia acqua sedimento

Mercury (Hg) mobility at the sediment–water interface was investigated during a laboratory incubation experiment conducted with highly contaminated sediments (13 μg g-1) of the Gulf of Trieste. Undisturbed sediment was collected in front of the Isonzo River mouth, which inflows Hg-rich suspended material originating from the Idrija (NW Slovenia) mining district. Since hypoxic and anoxic conditions at the bottom are frequently observed and can influence the Hg biogeochemical behavior, a redox oscillation was simulated in the laboratory, at in situ temperature, using a dark flux chamber. Temporal variations of several parameters were monitored simultaneously: dissolved Hg (DHg) and methylmercury (MeHg), O2, NH4+, NO3- + NO2-, PO4 -3, H2S, dissolved Mn2+, dissolved inorganic and organic carbon (DIC and DOC). Under anoxic conditions, both Hg (665 ng m2 day-1) and MeHg (550 ng m2 day-1) fluxed from sediments into the water column, whereas re-oxygenation caused concentrations of MeHg and Hg to rapidly drop, probably due to re-adsorption onto Fe/Mn-oxyhydroxides and enhanced demethylation processes. Hence, during anoxic events, sediments of the Gulf of Trieste may be considered as an important source of DHg species for the water column. On the contrary, reoxygenation of the bottom compartment mitigates Hg and MeHg release from the sediment, thus acting as a natural “defence” from possible interaction between the metal and the aquatic organisms.

Mercury (Hg) transformation activities and sulfate (SO4 2
) reduction were studied in sediments of the Marano and Grado Lagoons in the Northern Adriatic Sea region as part of the “MIRACLE” project. The lagoons, which are sites of clam (Tapes philippinarum) farming, have been receiving excess Hg from the Isonzo River for centuries. Marano Lagoon is also contaminated from a chlor-alkali plant. Radiotracer methods were used to measure mercury methylation (230Hg, 197Hg), methylmercury (MeHg) demethylation (14C-MeHg) and SO4 2
reduction (35S) in sediment cores collected in autumn, winter and summer. Mercury methylation rate constants ranged from near zero to 0.054 day
1, generally decreased with depth, and were highest in summer. Demethylation rate constants were much higher than methylation reaching values of w0.6 day
1 in summer. Demethylation occurred via the oxidative pathway, except in winter when the reductive pathway increased in importance in surficial sediments. Sulfate reductionwas also most active in summer (up to 1600 nmol mL
1 day
1) and depth profiles reflected seasonally changing redox conditions near the surface. Methylation and demethylation rate constants correlated positively with SO4 2
reduction and pore-water Hg concentrations, and inversely with Hg sediment ewater partition coefficients indicating the importance of SO4 2
reduction and Hg dissolution on Hg cycling. Hg transformation rates were calculated using rate constants and concentrations of Hg species. In laboratory experiments, methylation was inhibited by amendments of the SO4 2
-reduction inhibitor molybdate and by nitrate. Lagoon sediments displayed a dynamic seasonal cycle in which Hg dissolution in spring/summer stimulated Hg methylation, which was followed by a net loss of MeHg in autumn from demethylation. Sulfate-reducing bacteria (SRB) tended to be responsible for methylation of Hg and the oxidative demethylation of MeHg. However, during winter in surficial sediments, iron-reducing bacteria seemed to contribute to methylation and Hg-resistant bacteria increased in importance in the reductive demethylation of MeHg. The high rates of MeHg demethylation in lagoon sediments may diminish the accumulation of MeHg.

The “MIRACLE” Project was aimed at two specific issues: understanding Hg biogeochemical cycling in the Marano and Grado Lagoon and testing the coexistence of clam farming with Hg contamination in the sediments. Mercury contamination was measured in several matrices (water, sediment, biota) and its mobility was tested along with its speciation in relation to biogeochemical processes occurring in the lagoon environment, where bacterial communities have a primary role in converting Hg to its more toxic form, methylmercury (MeHg). Bioaccumulation of the Hg species was investigated on natural and seeded clams (Ruditapes philippinarum), the most important commercial bivalves in the Lagoon. The Editorial summarizes the main results obtained from this multidisciplinary study and reported in the Special Issue.

As part of the “MIRACLE” project, the biogeochemical cycling of mercury (Hg) at the sedimentewater interface was studied in the field in the Marano and Grado Lagoon (Northern Adriatic Sea). Seasonal investigations were conducted at selected experimental sites, where Manila Clams (Tapes philippinarum) were previously seeded. Measurements were performed seasonally during three campaigns, using two benthic chambers, one transparent and one dark, to evaluate the effect of light on Hg cycling. Total dissolved Hg (THg), methylmercury (MeHg), and dissolved gaseous Hg (DGM) species were considered. Diurnal benthic fluxes were found to significantly exceed the diffusive fluxes at all stations. The assessment of the annual recycling of Hg species from sediments to the water column showed that up to 99% of MeHg is recycled annually to the water column, while Hg recycling ranges from 30 to 60%. MeHg poses the higher risk for potential bioaccumulation in clams, but it is partially mitigated by Hg reduction, which seems to be an important process leading to evasion losses of Hg from these environments. Estimated benthic fluxes suggest that Hg recycling at the sedimentewater interface is more active in the Grado sector. Hence, based on the estimated release of MeHg from sediments, it is suggested that the western sector seems to be more suitable for clam farming and the extension of rearing activities.

A closed chlor-alkali plant (CAP) discharged Hg for decades into the Aussa River, which flows into Marano Lagoon, resulting in the large-scale pollution of the lagoon. In order to get information on the role of bacteria as mercury detoxifying agents, analyses of anions in the superficial part (0e1 cm) of sediments were conducted at four stations in the Aussa River. In addition, measurements of biopolymeric carbon (BPC) as a sum of the carbon equivalent of proteins (PRT), lipids (LIP), and carbohydrates (CHO) were performed to correlate with bacterial biomass such as the number of aerobic heterotrophic cultivable bacteria and their percentage of Hg-resistant bacteria. All these parameters were used to assess the bioavailable Hg fraction in sediments and the potential detoxification activity of bacteria. In addition, fifteen isolates were characterized by a combination of molecular techniques, which permitted their assignment into six different genera. Four out of fifteen were Gram negative with two strains of Stenotrophomonas maltophilia, one Enterobacter sp., and one strain of Brevibacterium frigoritolerans. The remaining strains (11) were Gram positive belonging to the genera Bacillus and Staphylococcus.We found merA genes in only a few isolates. Mercury volatilization from added HgCl2 and the presence of plasmids with the merA gene were also used to confirm Hg reductase activity. We found the highest number of aerobic heterotrophic Hg-resistant bacteria (one order magnitude higher) and the highest number of Hgresistant species (11 species out of 15) at the confluence of the River Aussa and Banduzzi’s channel, which transport Hg from the CAP, suggesting that Hg is strongly detoxified [reduced to Hg(0)] at this location.

Coastal areas in the northernmost part of the Adriatic Sea (Gulf of Trieste and the adjacent Grado Lagoon) are characterized by high levels of mercury (Hg), both in sediments and in the water column, mainly originating from the suspended material inflowing through the Isonzo/Soca River system, draining the Idrija (NW Slovenia) mining district, into the Gulf of Trieste. Hypoxic and anoxic conditions at the sediment-water interface (SWI) are frequently observed in the Gulf of Trieste and in the lagoon, due to strong late summer water stratification and high organic matter input. Hg mobility at the SWI was investigated at three sampling points located in the Gulf of Trieste (AA1, CZ) and in the Grado Lagoon (BAR). Experiments were conducted under laboratory conditions at in situ temperature, using a dark flux chamber simulating an oxic-anoxic transition. Temporal variations of dissolved Hg and methylmercury (MeHg) as well as O2, NH4+, NO3- + NO2-, PO4 -3, H2S, dissolved Fe and Mn, dissolved inorganic carbon (DIC) and dissolved organic carbon (DOC) were monitored simultaneously. Benthic Hg fluxes were higher under anoxic conditions than in the oxic phase of the experiment. MeHg release was less noticeable (low or absent) in the oxic phase, probably due to similar methylation and demethylation rates, but high in the anoxic phase of the experiment. The MeHg flux was linked to sulphate reduction and dissolution of Fe (and Mn) oxyhydroxides, and formation of sulphides. Reoxygenation was studied at sampling point CZ, where concentrations of MeHg and Hg dropped rapidly probably due to re-adsorption onto Fe (Mn) oxyhydroxides and enhanced demethylation. Sediments, especially during anoxic events, should be, hence, considered as a primary source of MeHg for the water column in the northern Adriatic coastal areas.

Previous research recognized most of the Northern Adriatic coastal lagoon environments as contaminated by mercury (Hg) from multiple anthropogenic sources. Among them, the Pialassa Baiona (P.B.) Lagoon, located near the city of Ravenna (Italy), received between 100 and 200 tons of Hg, generated by an acetaldehyde factory in the period 1957-1977. Further east, the Grado Lagoon has been mainly affected by a long-term Hg input from the Idrija mine (western Slovenia) through the Isonzo River since the 16th century. Hg cycling at the sediment-water interface (SWI) of the two lagoons was investigated and compared by means of an in situ benthic chamber, estimating diffusive Hg and Methyl-Hg fluxes in the summer season. Major chemical features in porewaters (Fe, Mn, H2S, dissolved inorganic (DIC) and organic carbon (DOC), nutrients) and in the solid phase (Corg, N and S) were also explored to understand the general biogeochemical conditions of the system in response to benthic respiration. The daily integrated flux for the methylated Hg form was extremely low in P.B. Lagoon, accounting for only 7% of the corresponding flux calculated for the Grado Lagoon. Despite a higher sedimentary Hg content in the P.B. Lagoon (14.4-79.0 µg g-1) compared to the Grado Lagoon (10.7-12.5 µg g-1), the in situ fluxes of Hg in the two experimental sites appeared similar. A selective sequential extraction procedure was applied to the solid phase, showing that the stable crystalline mineral phase cinnabar (HgS) is the predominant Hg fraction (about 50 %) in the Grado Lagoon surface sediments. Conversely, Hg mobilization and sequestration in the P.B. Lagoon is related to the extremely anoxic redox conditions of the system where the intense sulfate reduction, by the release of sulfur and the formation of sulfides, limits the metal recycling at the SWI and its availability for methylation processes. Thus, the environmental conditions at the SWI in the P.B. Lagoon seem to represent a natural “barrier” for the potential risk of Hg transfer to the aquatic trophic chain.

The role of the major biogeochemical processes in the mercury (Hg) cycling at the sediment-water interface was investigated in the Grado lagoon (northern Adriatic Sea). This wetland system has been extensively contaminated from the Idrija Hg Mine (Slovenia) through the Isonzo River suspended load carried by tidal fluxes. Three approaches were used to study the sediment-water exchange of total Hg (THg), methylmercury (MeHg), reactive Hg (RHg) and dissolved gaseous Hg (DGHg): (1) estimation of diffusive fluxes from porewater and overlying water concentrations, (2) measurements of benthic fluxes using a deployed light benthic chamber in situ and (3) measurements of benthic fluxes during oxic-anoxic transition during a laboratory incubation experiment. The THg solid-phase, ranging between 9.5 and 14.4 µg g-1, showed slight variability with depth and time. Conversely, MeHg contents were the highest (up to 21.9 ng g-1) at the surface; they tended to decrease to nearly zero concentration with depth, thus suggesting that MeHg production and accumulation occur predominantly just below the sediment-water interface. Porewater MeHg concentrations (0.9-7.9 ng L-1, 0.15-15 % of THg) varied seasonally; higher contents were observed in the warmer period. The MeHg diffusive fluxes (up to 17 ng m-2 day-1) were similar to those in the nearby Gulf of Trieste (Covelli et al., 1999), although the lagoon sediments contained 4 folds higher THg concentrations. Conversely, the THg diffusive fluxes in the lagoon (up to 110 ng m-2 day-1) were once-twice higher than those previously estimated for the Gulf of Trieste. The diurnal MeHg benthic fluxes were the highest in summer at both sites (41000 and 33000 ng m-2 day-1 at the fishfarm and in the open lagoon, respectively), thus indicating the influence of temperature on microbial processes. The diurnal variations of dissolved THg and especially MeHg were positively correlated with O2 and inversely with DIC, suggesting an important influence of benthic photosynthetic activities on lagoon benthic Hg cycling, possibly through the production of organic matter promptly available for methylation. The results from the dark chamber incubated in the laboratory showed that the regeneration of dissolved THg was slightly affected by the oxic-anoxic transition. Conversely, the benthic flux of MeHg was up to 15 folds higher in sediments overlain by O2 depleted waters. In the anoxic phase, the MeHg fluxes proceeded in parallel with Fe fluxes and the methylated form reached approximately 100% of dissolved THg. The MeHg is mostly released into overlying water (mean recycling efficiency of 89%) until the occurrence of sulphide inhibition, due to scavenging of the available Hg substrate for methylation. Our results suggest that sediments in the Grado lagoon, especially during anoxic events, should be considered as a primary source of MeHg for the water column.

The Gulf of Trieste is one of the most mercury contaminated areas in the Mediterranean Sea. It is characterised by high mercury inputs from the Isonzo river, whose tributary, the Idrijca river, drains the mercury mining area of Idrija in Slovenia where the extraction activity was operative for nearly 500 years. This appears, therefore, to be one of the most suitable sites for studying processes which affect Hg cycling in the marine environment and for determining whether sediments might act as secondary sources of mercury species in the water column. Pore water seasonal distribution of total dissolved Hg (HgT) and methylmercury (MeHg) was investigated. Using in situ benthic chambers it was possible to determine benthic fluxes of HgT and MeHg at the water-sediment interface throughout the year. Benthic fluxes were also compared with diffusive fluxes calculated from pore water profiles. Our results indicate that, following hypoxic conditions which occurred in late summer in the sea-bottom layer, highest benthic effluxes and pore water concentrations of Hg and MeHg appeared during autumn and winter. This was probably due to the transition from rapid sulphate reduction in late summer to cooler temperatures, higher oxygenation of the bottom water layer, and lower microbial activity which is well suited for Hg transformations, accumulation and flux. A tentative budget based on benthic flux measurements indicates that 75 % of HgT is buried into the sediment whereas 25 % of HgT, approximately 23 % in methylated form, is annually recycled and released at the water-sediment interface.