Mercury methylation and bacterial activity associated to tropical phytoplankton

A Tropical Estuary Where Mercury Does Not Biomagnify and its Adjacent Waters that Render Extremely High Mercury Concentrations in Top Predators

In general, mercury (Hg) undergoes biomagnification in aquatic systems. The absence of Hg biomagnification in a certain aquatic environment constitutes an exceptional finding and this seems to be the case for Sepetiba Bay, in Rio de Janeiro state (RJ), Brazil. There are three distinct ecological populations of Guiana dolphins in the Sepetiba Bay (SB)-Ilha Grande Bay (IGB) Complex, inhabiting: (1) the inner part of SB; (2) SB entrance; and (3) IGB. In addition, there are two other delphinid species, rough-toothed dolphin and Atlantic spotted dolphin, that feed on the SB-IGB Complex. Considering the widely employed use of cetaceans as sentinels of environmental contamination by bioaccumulative toxicants, we have biopsy sampled individuals of the abovementioned ecological populations/species for measuring skin Hg concentrations. Two Bryde's whales and one humpback whale were biopsied in the SB-IGB Complex as well. Skin Hg concentrations [μg g-1 dry weight (dw)] of Guiana dolphins were the highest in IGB, followed by SB entrance and the inner part of SB (0.99-5.47; 0.09-6.00; 0.08-2.22). Considering all species investigated in the present study, skin Hg concentrations were found in the following order: humpback whale < Bryde's whale < Guiana dolphins from SB inner part < Guiana dolphins from SB entrance < Guiana dolphins IGB = Atlantic spotted dolphins < rough-toothed dolphins. The skin Hg concentrations found in Guiana dolphins from the inner part of Sepetiba Bay (0.08-2.22) and rough-toothed dolphins from the SB-IGB Complex (1.26-20.0) are among the lowest and highest ever reported for dolphins worldwide, respectively.

The relationship between zooplankton vertical distribution and the concentration of aqueous Hg in boreal lakes: A comparative field study

The production of the highly toxic monomethylmercury (MeHg) is heterogenous throughout the water column. Multiple factors have been identified to significantly affect this process, such as an extended anoxic water layer and a deep-water phytoplankton maximum. However, the role of water column heterogeneity on mercury (Hg) cycling is still poorly known, especially concerning the role of zooplankton grazers. Here, four boreal lakes with contrasting characteristics were sampled (i.e., transparency and the presence/absence of fish) at both day and night in order to maximize the heterogeneity in zooplankton abundance both among and within lakes, and to investigate their potential links with Hg vertical heterogeneity. Diel variation of the concentrations of both dissolved total Hg (DTHg) and total Hg (THg) were observed, with night samples significantly higher than day samples. Although this pattern was not related to diel changes in the vertical distribution of zooplankton, results showed that the presence of large copepods (>1.2 mm) and medium-sized (0.6 to 1.2 mm) cladocerans was significantly associated with lower concentrations of DTHg in the water at a given depth, whereas the presence of medium-sized copepods was significantly associated with the concentration of THg. The presence of cladocerans was significantly associated with the ratio between the dissolved MeHg and DTHg (conventionally used as a proxy of methylation potential). Phytoplankton biomass was directly correlated with the concentration of both dissolved and total MeHg and the methylation potential. At the same time, phytoplankton biomass was inversely related to the fraction of DTHg. These results suggest a potential key role of the heterogeneity of biotic factors in the water column, especially of phytoplankton and zooplankton, in the cycling of total Hg and MeHg in boreal lakes.

Metal-Enhanced Hg2+-Responsive Fluorescent Nanoprobes: From Morphological Design to Application to Natural Waters

Metal-enhanced fluorescence (MEF) is a powerful tool in the design of sensitive chemical sensors by improving brightness and photostability of target-responsive fluorophores. Compounding these advantages with the modest hardware requirements of fluorescence sensing compared to that of centralized elemental analysis instruments, thus expanding the use of MEF to the detection of low-level inorganic pollutants, is a compelling aspiration. Among the latter, monitoring mercury in the environment, where some of its species disseminate through the food chain and, in time, to humans, has elicited a broad research effort toward the development of Hg²⁺-responsive fluorescent sensors. Herein, a Hg²⁺-sensitive MEF-enabled probe was conceived by grafting a Hg²⁺-responsive fluorescein derivative to concentric Ag@SiO₂ NPs, where the metallic core enhances fluorescence emission of molecular probes embedded in a surrounding silica shell. Time-resolved fluorescence measurements showed that the fluorophore's excited-state lifetime decreases from 3.9 ns in a solid, coreless silica sphere to 0.4 ns in the core-shell nanoprobe, granting the dye a better resistance to photobleaching. The Ag-core system showed a sizable improvement in the limit of detection at 2 nM (0.4 ppb) compared to 50 nM (10 ppb) in silica-only colloids, and its effectiveness for natural water analysis was demonstrated. Overall, the reported nanoarchitecture hints at the potential of MEF for heavy metal detection by fluorescence detection.

Microbial mercury methylation in the cryosphere: Progress and prospects

Mercury (Hg) is one of the most toxic heavy metals, and its cycle is mainly controlled by oxidation-reduction reactions carried out by photochemical or microbial process under suitable conditions. The deposition and accumulation of methylmercury (MeHg) in various ecosystems, including the cryospheric components such as snow, meltwater, glaciers, and ice sheet, and subsequently in the food chain pose serious health concerns for living beings. Unlike the abundance of knowledge about the processes of MeHg production over land and oceans, little is known about the sources and production/degradation rate of MeHg in cryosphere systems. In addition, processes controlling the concentration of Hg and MeHg in the cryosphere remains poorly understood, and filling this scientific gap has been challenging. Therefore, it is essential to study and review the deposition and accumulation by biological, physical, and chemical mechanisms involved in Hg methylation in the cryosphere. This review attempts to address knowledge gaps in understanding processes, especially biotic and abiotic, applicable for Hg methylation in the cryosphere. First, we focus on the variability in Hg concentration and mechanisms of Hg methylation, including physical, chemical, microbial, and biological processes, and transportation in the cryosphere. Then, we elaborate on the mechanism of redox reactions and biotic and abiotic factors controlling Hg methylation and biogeochemistry of Hg in the cryosphere. We also present possible mechanisms of Hg methylation with an emphasis on microbial transformation and molecular function to understand variability in Hg concentration in the cryosphere. Recent advancements in the genetic and physicochemical mechanisms of Hg methylation are also presented. Finally, we summarize and propose a method to study the unsolved issues of Hg methylation in the cryosphere.

Role of the floodplain lakes in the methylmercury distribution and exchanges with the Amazon River, Brazil

Seasonal variability of dissolved and particulate methylmercury (F-MeHg, P-MeHg) concentrations was studied in the waters of the Amazon River and its associated Curuai floodplain during hydrological year 2005-2006, to understand the MeHg exchanges between these aquatic systems. In the oxic white water lakes, with neutral pH, high F-MeHg and P-MeHg concentrations were measured during the rising water stage (0.70±0.37pmol/L, n=26) and flood peak (14.19±9.32pmol/g, n=7) respectively, when the Amazon River water discharge into the lakes was at its maximum. The lowest mean values were reported during the dry season (0.18±0.07pmol/L F-MeHg, n=10 and 1.35±1.24pmol/g P-MeHg, n=8), when water and suspended sediments were outflowing from the lakes into the River. In these lakes, the MeHg concentrations were associated to the aluminium and organic carbon/nitrogen changes. In the black water lakes, with acidic pH and reducing conditions, elevated MeHg concentrations were recorded (0.58±0.32pmol/L F-MeHg, n=16 and 19.82±15.13pmol/g P-MeHg, n=6), and correlated with the organic carbon and manganese concentrations. Elevated values of MeHg partition coefficient (4.87<K_d<5.08log (L/kg) indicate that MeHg is mainly transported associated with the particulate phase. The P-MeHg enrichment detected in all lakes suggests autochthonous MeHg inputs from the sediments into the water column. The MeHg mass balance showed that the Curuai floodplain is not the source of P-MeHg for the Amazon River.

Recent advances in the study of mercury methylation in aquatic systems

With increasing input of neurotoxic mercury to environments as a result of anthropogenic activity, it has become imperative to examine how mercury may enter biotic systems through its methylation to bioavailable forms in aquatic environments. Recent development of stable isotope-based methods in methylation studies has enabled a better understanding of the factors controlling methylation in aquatic systems. In addition, the identification and tracking of the hgcAB gene cluster, which is necessary for methylation, has broadened the range of known methylators and methylation-conducive environments. Study of abiotic factors in methylation with new molecular methods (the use of stable isotopes and genomic methods) has helped elucidate the confounding influences of many environmental factors, as these methods enable the examination of their direct effects instead of merely correlative observations. Such developments will be helpful in the finer characterization of mercury biogeochemical cycles, which will enable better predictions of the potential effects of climate change on mercury methylation in aquatic systems and, by extension, the threat this may pose to biota.

Association of a Specific Algal Group with Methylmercury Accumulation in Periphyton of a Tropical High-Altitude Andean Lake

Periphyton relevance for methylmercury (MeHg) production and accumulation are now well known in aquatic ecosystems. Sulfate-reducing bacteria and other microbial groups were identified as the main MeHg producers, but the effect of periphyton algae on the accumulation and transfer of MeHg to the food web remains little studied. Here we investigated the role of specific groups of algae on MeHg accumulation in the periphyton of Schoenoplectus californicus ssp. (Totora) and Myriophyllum sp. in Uru Uru, a tropical high-altitude Bolivian lake with substantial fishing and mining activities accruing around it. MeHg concentrations were most strongly related to the cell abundance of the Chlorophyte genus Oedogonium (r ² = 0.783, p = 0.0126) and to no other specific genus despite the presence of other 34 genera identified. MeHg was also related to total chlorophyll-a (total algae) (r ² = 0.675, p = 0.0459), but relations were more significant with chlorophyte cell numbers, chlorophyll-b (chlorophytes), and chlorophyll-c (diatoms and dinoflagellates) (r ² = 0.72, p = 0.028, r ² = 0.744, p = 0.0214, and r ² = 0.766, p = 0.0161 respectively). However, Oedogonium explains most variability of chlorophytes and chlorophyll-c (r ² = 0.856, p = < 0.001 and r ² = 0.619, p = 0.002, respectively), suggesting it is the most influential group for MeHg accumulation and periphyton algae composition at this particular location and given time.

Multiple regression analysis to assess the role of plankton on the distribution and speciation of mercury in water of a contaminated lagoon

Spatial and seasonal variation of mercury species aqueous concentrations and distributions was carried out during six sampling campaigns at four locations within Laranjo Bay, the most mercury-contaminated area of the Aveiro Lagoon (Portugal). Inorganic mercury (IHg(II)) and methylmercury (MeHg) were determined in filter-retained (IHgPART, MeHgPART) and filtered (<0.45μm) fractions (IHg(II)DISS, MeHgDISS). The concentrations of IHgPART depended on site and on dilution with downstream particles. Similar processes were evidenced for MeHgPART, however, its concentrations increased for particles rich in phaeophytin (Pha). The concentrations of MeHgDISS, and especially those of IHg(II)DISS, increased with Pha concentrations in the water. Multiple regression models are able to depict MeHgPART, IHg(II)DISS and MeHgDISS concentrations with salinity and Pha concentrations exhibiting additive statistical effects and allowing separation of possible addition and removal processes. A link between phytoplankton/algae and consumers' grazing pressure in the contaminated area can be involved to increase concentrations of IHg(II)DISS and MeHgPART. These processes could lead to suspended particles enriched with MeHg and to the enhancement of IHg(II) and MeHg availability in surface waters and higher transfer to the food web.

Diurnal variability and biogeochemical reactivity of mercury species in an extreme high-altitude lake ecosystem of the Bolivian Altiplano

Methylation and demethylation represent major transformation pathways regulating the net production of methylmercury (MMHg). Very few studies have documented Hg reactivity and transformation in extreme high-altitude lake ecosystems. Mercury (Hg) species concentrations (IHg, MMHg, Hg°, and DMHg) and in situ Hg methylation (M) and MMHg demethylation (D) potentials were determined in water, sediment, floating organic aggregates, and periphyton compartments of a shallow productive Lake of the Bolivian Altiplano (Uru Uru Lake, 3686 m). Samples were collected during late dry season (October 2010) and late wet season (May 2011) at a north (NS) and a south (SS) site of the lake, respectively. Mercury species concentrations exhibited significant diurnal variability as influenced by the strong diurnal biogeochemical gradients. Particularly high methylated mercury concentrations (0.2 to 4.5 ng L(-1) for MMHgT) were determined in the water column evidencing important Hg methylation in this ecosystem. Methylation and D potentials range were, respectively, <0.1-16.5 and <0.2-68.3 % day(-1) and were highly variable among compartments of the lake, but always higher during the dry season. Net Hg M indicates that the influence of urban and mining effluent (NS) promotes MMHg production in both water (up to 0.45 ng MMHg L(-1) day(-1)) and sediment compartments (2.0 to 19.7 ng MMHg g(-1) day(-1)). While the sediment compartment appears to represent a major source of MMHg in this shallow ecosystem, floating organic aggregates (dry season, SS) and Totora's periphyton (wet season, NS) were found to act as a significant source (5.8 ng MMHg g(-1) day(-1)) and a sink (-2.1 ng MMHg g(-1) day(-1)) of MMHg, respectively. This work demonstrates that high-altitude productive lake ecosystems can promote MMHg formation in various compartments supporting recent observations of high Hg contents in fish and water birds.

Yearly variation of bacterial production in the Arraial do Cabo protection area (Cabo Frio upwelling region): an evidence of anthropogenic pressure

Arraial do Cabo is where upwelling occurs more intensively on the Brazilian coast. Although it is a protection area it suffers anthropogenic pressure such as harbor activities and sporadic sewage emissions. Short-time studies showed a high variability of bacterial production (BP) in this region but none of them evaluated BP during long periods in a large spatial scale including stations under different natural (upwelling and cold fronts) and anthropogenic pressures. During 2006, we sampled surface waters 10 times (5 in upwelling and 5 in subsidence periods) in 8 stations and we measured BP, temperature as well as the concentrations of inorganic nutrients, pigments and particulate organic matter (POM). BP was up to 400 times higher when sewage emissions were observed visually and it had a positive correlation with ammonia concentrations. Therefore, in 2007, we did two samples (each during upwelling and subsidence periods) during sewage emissions in five stations under different anthropogenic pressure and we also measured particles abundance by flow cytometry. The 12 samples in the most impacted area confirmed that BP was highest when ammonia was higher than 2 μM, also reporting the highest concentrations of chlorophyll a and suspended particles. However, considering all measured variables, upwelling was the main disturbing factor but the pressure of fronts should not be neglected since it had consequences in the auto-heterotrophic coupling, increasing the concentrations of non fluorescent particles and POM. Stations clustered in function of natural and anthropogenic pressures degrees and both determined the temporal-spatial variability.

Trophic structure and mercury biomagnification in tropical fish assemblages, Iténez River, Bolivia

We examined mercury concentrations in three fish assemblages to estimate biomagnification rates in the Iténez main river, affected by anthropogenic activities, and two unperturbed rivers from the Iténez basin, Bolivian Amazon. Rivers presented low to moderate water mercury concentrations (from 1.25 ng L⁻¹ to 2.96 ng L⁻¹) and natural differences in terms of sediment load. Mercury biomagnification rates were confronted to trophic structure depicted by carbon and nitrogen stable isotopes composition (δ¹⁵N; δ¹³C) of primary trophic sources, invertebrates and fishes. Results showed a slight fish contamination in the Iténez River compared to the unperturbed rivers, with higher mercury concentrations in piscivore species (0.15 µg g⁻¹ vs. 0.11 µg g⁻¹ in the unperturbed rivers) and a higher biomagnification rate. Trophic structure analysis showed that the higher biomagnification rate in the Iténez River could not be attributed to a longer food chain. Nevertheless, it revealed for the Iténez River a higher contribution of periphyton to the diet of the primary consumers fish species; and more negative δ¹³C values for primary trophic sources, invertebrates and fishes that could indicate a higher contribution of methanotrophic bacteria. These two factors may enhance methylation and methyl mercury transfer in the food web and thus, alternatively or complementarily to the impact of the anthropogenic activities, may explain mercury differences observed in fishes from the Iténez River in comparison to the two other rivers.

Environmental assessment of trace element bioaccumulation in sipunculan from seagrass and wetland sediments

This study is the first measurement of trace elements in sipunculan and their surrounding sediments. The bioaccumulation characteristics of arsenic (As), cadmium (Cd), chromium (Cr), copper (Cu), mercury (Hg), and zinc (Zn) were analyzed and compared in two sipunculan species, Sipuncula nudus and Siphonosoma vastum, which were collected from seagrass beds and wetlands in Taiwan. The sipunculan and sediment samples were analyzed using an inductively coupled plasma mass spectrometer. Both sipunculan in the wetlands and seagrass beds had a high Cu bioaccumulation mechanism. Multivariate analysis, principle component analysis, and partial least squares for discriminant analysis of trace element levels and bioaccumulation factors were used to distinguish the element distributions that corresponded to the two habitats (seagrass beds and wetlands). Different levels of certain trace elements in these two sipunculan species may result not only from the environmental factors of various habitats but also from the accumulation characteristics of various species. The As, Cd, Cr, Cu, Hg, and Zn concentrations were markedly lower in sipunculan than in other invertebrates from the adjacent polluted regions. The public health issues regarding the consumption of sipunculan are also discussed.

Ecological and biological determinants of methylmercury accumulation in tropical coastal fish

This research investigated whether environmental conditions, biological fish characteristics and anthropogenic impacts influenced mercury (Hg) assimilation into the muscle tissue of two fish species from two Brazilian bays, Ilha Grande Bay and Guanabara Bay. Fish and superficial water were collected in different periods. Hg was determined by CV-AAS. Methylmercury (MeHg) was identified and quantified by ECD-GC. Chlorophyll a concentrations in the water column indicated that Ilha Grande Bay and Guanabara Bay were oligotrophic and eutrophic, respectively. Hg in fish ranged from 2.10 to 870.17 μg kg(-1) dry wt. in Ilha Grande Bay and 40.90 to 809.24 μg kg(-1) dry wt. in Guanabara Bay. Slight differences were found between the length-normalized Hg concentrations and its percent of Hg in a voracious predator from the bays. In Guanabara Bay, where the presence of a chlor-alkali plant causes Hg input, the iliophagous fish species showed the highest length-normalized Hg concentrations and the voracious predator the lowest. Iliophagous fish is consumed by voracious predator and, consequently, acts as their MeHg food supply. Iliophagous fish from Ilha Grande Bay presented a higher percent of MeHg (80.0 %) than specimens from Guanabara Bay (54.5 %). This fact suggests that more MeHg was transferred from iliophagous fish to voracious predator in Ilha Grande Bay. At Guanabara Bay, the bioproduction is greater than that at Ilha Grande Bay, presenting the highest biomass in it ecosystem, which may subsequently dilute Hg and reduce its availability to the biota; i.e., influencing in Hg and MeHg availability throughout the food chain. Consequently, more MeHg is available in the aquatic environment of Ilha Grande Bay.

Adaptation of the 3H-leucine incorporation technique to measure heterotrophic activity associated with biofilm on the blades of the seaweed Sargassum spp

The ecological interaction between microorganisms and seaweeds depends on the production of secondary compounds that can influence microbial diversity in the water column and the composition of reef environments. We adapted the (3)H-leucine incorporation technique to measure bacterial activity in biofilms associated with the blades of the macroalgae Sargassum spp. We evaluated (1) if the epiphytic bacteria on the blades were more active in detritus or in the biofilm, (2) substrate saturation and linearity of (3)H-leucine incorporation, (3) the influence of specific metabolic inhibitors during (3)H-leucine incorporation under the presence or absence of natural and artificial light, and (4) the efficiency of radiolabeled protein extraction. Scanning electron microscopy showed heterogeneous distribution of bacteria, diatoms, and polymeric extracellular secretions. Active bacteria were present in both biofilm and detritus on the blades. The highest (3)H-leucine incorporation was obtained when incubating blades not colonized by macroepibionts. Incubations done under field conditions reported higher (3)H-leucine incorporation than in the laboratory. Light quality and sampling manipulation seemed to be the main factors behind this difference. The use of specific metabolic inhibitors confirmed that bacteria are the main group incorporating (3)H-leucine but their association with primary production suggested a symbiotic relationship between bacteria, diatoms, and the seaweed.

Mercury in tropical and subtropical coastal environments

Anthropogenic activities influence the biogeochemical cycles of mercury, both qualitatively and quantitatively, on a global scale from sources to sinks. Anthropogenic processes that alter the temporal and spatial patterns of sources and cycling processes are changing the impacts of mercury contamination on aquatic biota and humans. Human exposure to mercury is dominated by the consumption of fish and products from aquaculture operations. The risk to society and to ecosystems from mercury contamination is growing, and it is important to monitor these expanding risks. However, the extent and manner to which anthropogenic activities will alter mercury sources and biogeochemical cycling in tropical and sub-tropical coastal environments is poorly understood. Factors as (1) lack of reliable local/regional data; (2) rapidly changing environmental conditions; (3) governmental priorities and; (4) technical actions from supra-national institutions, are some of the obstacles to overcome in mercury cycling research and policy formulation. In the tropics and sub-tropics, research on mercury in the environment is moving from an exploratory "inventory" phase towards more process-oriented studies. Addressing biodiversity conservation and human health issues related to mercury contamination of river basins and tropical coastal environments are an integral part of paragraph 221 of the United Nations document "The Future We Want" issued in Rio de Janeiro in June 2012.

Mercury and flooding cycles in the Tapajós River basin, Brazilian Amazon: the role of periphyton of a floating macrophyte (Paspalum repens)

Methylmercury (MeHg) increases mercury (Hg) toxicity and is biomagnified in the trophic chain contaminating riverine Amazon populations. Freshwater macrophyte roots are a main site of Hg methylation in different Brazilian environments. Paspalum repens periphyton was sampled in four floodplain lakes during the dry, rainy and wet seasons for measurement of total Hg (THg), MeHg, Hg methylation potentials, %C, %N, δ(13)C, δ(15)N and bacterial heterotrophic production as (3)H-leucine incorporation rate. THg concentration varied from 67 to 198 ng/g and the potential of Me(203)Hg formation was expressive (1-23%) showing that periphyton is an important matrix both in the accumulation of Hg and in MeHg production. The concentration of MeHg varied from 1 to 6 ng/g DW and was positively correlated with Me(203)Hg formation. Though methylmercury formation is mainly a bacterial process, no significant correlation was observed between the methylation potentials and bacterial production. The multiple regressions analyses suggested a negative correlation between THg and %C and %N and between methylation potential and δ(13)C. The discriminant analysis showed a significant difference in periphyton δ(15)N, δ(13)C and THg between seasons, where the rainy season presented higher δ(15)N and the wet period lighter δ(13)C, lower THg values and higher Me(203)Hg formation. This exploratory study indicates that the flooding cycle could influence the periphyton composition, mercury accumulation and methylmercury production.

Importance of sulfate reducing bacteria in mercury methylation and demethylation in periphyton from Bolivian Amazon region

Sulfate reducing bacteria (SRB) are important mercury methylators in sediments, but information on mercury methylators in other compartments is ambiguous. To investigate SRB involvement in methylation in Amazonian periphyton, the relationship between Hg methylation potential and SRB (Desulfobacteraceae, Desulfobulbaceae and Desulfovibrionaceae) abundance in Eichhornia crassipes and Polygonum densiflorum root associated periphyton was examined. Periphyton subsamples of each macrophyte were amended with electron donors (lactate, acetate and propionate) or inhibitors (molybdate) of sulfate reduction to create differences in SRB subgroup abundance, which was measured by quantitative real-time PCR with primers specific for the 16S rRNA gene. Mercury methylation and demethylation potentials were determined by a stable isotope tracer technique using 200HgCl and CH3(202)HgCl, respectively. Relative abundance of Desulfobacteraceae (<0.01-12.5%) and Desulfovibrionaceae (0.01-6.8%) were both highly variable among samples and subsamples, but a significant linear relationship (p<0.05) was found between Desulfobacteraceae abundance and net methylmercury formation among treatments of the same macrophyte periphyton and among all P. densiflorum samples, suggesting that Desulfobacteraceae bacteria are the most important mercury methylators among SRB families. Yet, molybdate only partially inhibited mercury methylation potentials, suggesting the involvement of other microorganisms as well. The response of net methylmercury production to the different electron donors and molybdate was highly variable (3-1104 pg g(-1) in 12 h) among samples, as was the net formation in control samples (17-164 pg g(-1) in 12 h). This demonstrates the importance of community variability and complexity of microbial interactions for the overall methylmercury production in periphyton and their response to external stimulus.

Inorganic and methylmercury: do they transfer along a tropical coastal food web?

Methylmercury (MeHg) and inorganic mercury (Hg(inorg)) were evaluated in the water of a Brazilian estuary, with two size classes of plankton and seven fish species of different feeding habits. Water partition coefficients (PCs) in microplankton were fourfold higher for MeHg than for Hg(inorg); and water PCs in mesoplankton were 26 times higher for MeHg than Hg(inorg). Difference between microplankton and mesoplankton MeHg bioaccumulation factor (BAF) was higher (0.60 log units) than Hg(inorg) BAF (0.24 log units), indicating that trophic transfer of MeHg between planktonic organisms is more efficient than Hg(inorg) transference. MeHg concentrations, proportion of mercury as MeHg and its biotransference factors (BTFs) in the microplankton, mesoplankton and fish increased with increasing trophic level while biotic concentrations of Hg(inorg) and proportion of mercury as Hg(inorg) decreased thus indicating that MeHg was indeed the biomagnified species of mercury. MeHg reflected the vertical trophic guilds distribution, due to the fact that the top predator fish presented the highest concentration (0.77 μg g(-1)d.w.), followed by the less voracious species (0.43 μg g(-1)d.w.); while planktivorous fish presented the lowest concentrations (0.044 μg g(-1)d.w.). Hg(inorg) did not present the same behavior. Results suggest that feeding habits and trophic guild are important parameters, influencing biotransference and biomagnification processes.

Mercury methylation rates of biofilm and plankton microorganisms from a hydroelectric reservoir in French Guiana

The Petit-Saut ecosystem is a hydroelectric reservoir covering 365km(2) of flooded tropical forest. This reservoir and the Sinnamary Estuary downstream of the dam are subject to significant mercury methylation. The mercury methylation potential of plankton and biofilm microorganisms/components from different depths in the anoxic reservoir water column and from two different sites along the estuary was assessed. For this, reservoir water and samples of epiphytic biofilms from the trunk of a submerged tree in the anoxic water column and from submerged branches in the estuary were batch-incubated from 1h to 3 months with a nominal 1000ng/L spike of Hg(II) chloride enriched in (199)Hg. Methylation rates were determined for different reservoir and estuarine communities under natural nutrient (reservoir water, estuary freshwater) and artificial nutrient (culture medium) conditions. Methylation rates in reservoir water incubations were the highest with plankton microorganisms sampled at -9.5m depth (0.5%/d) without addition of biofilm components. Mercury methylation rates of incubated biofilm components were strongly enhanced by nutrient addition. The results suggested that plankton microorganisms strongly contribute to the total Hg methylation in the Petit-Saut reservoir and in the Sinnamary Estuary. Moreover, specific methylation efficiencies (%Me(199)Hg(net)/cell) suggested that plankton microorganisms could be more efficient methylating actors than biofilm consortia and that their methylation efficiency may be reduced in the presence of biofilm components. Extrapolation to the reservoir scale of the experimentally determined preliminary methylation efficiencies suggested that plankton microorganisms in the anoxic water column could produce up to 27mol MeHg/year. Taking into account that (i) demethylation probably occurs in the reservoir and (ii) that the presence of biofilm components may limit the methylation efficiency of plankton microorganisms, this result is highly consistent with the annual net MeHg production estimated from mass balances (8.1mol MeHg/year, Muresan et al., 2008a).

Benthic methylmercury production in lacustrine ecosystems of Nahuel Huapi National Park, Patagonia, Argentina

Seasonal trends of benthic methylmercury (methyl-Hg) production were examined in both littoral and open water sites of three lakes (Escondido, Moreno, and Morenito) in the North Andean Patagonia region of Argentina. Potentials of methyl-Hg production were measured by amending sediment samples with inorganic (197)Hg(II), incubating for either 24 and 32 h at room temperature, and subsequently assaying the radiolabelled organomercury produced. Seasonal variations of benthic methyl-Hg production were studied but no significant correlation was observed. Lake littoral sites exhibited up to two fold higher methyl-Hg production potentials in most cases. Sediment from lakes Moreno and Morenito generally exhibited much lower (up to 10 fold) methyl-Hg production potentials than those from Lake Escondido, possibly due to differences in particulate and dissolved organic matter quantity and quality, which is higher in Lake Escondido and primarily allochthonous, whereas in lakes Moreno and Morenito is primarily autochthonous. This study represents the first to directly examine benthic microbial Hg(II)-methylation in aquatic ecosystems of Patagonia.

Adsorption properties and gaseous mercury transformation rate of natural biofilm

Biofilms were developed on glass microscope slides in a natural aquatic environment and their mercury adsorption properties were evaluated. Results demonstrated that the biofilms contained a large number of bacterial cells and associated extracellular polymers. Mercury forms detected in the biofilms were mainly bound to residual matter and organic acids. The adsorption processes could be described by a Langmuir isotherm. The optimum conditions for adsorption of mercury to natural biofilm were an ionic strength of 0.1 mol/L, pH 6 and an optimum adsorption time of 40 min. The transformation rate was 0.79 microg gaseous mercury per gram of biofilm.

[3H]Leucine incorporation method as a tool to measure secondary production by periphytic bacteria associated to the roots of floating aquatic macrophyte

The present study assessed the application of [(3)H]Leucine incorporation into protein by periphytic bacteria associated with the roots of the floating aquatic macrophyte Eichornia crassipes. Basic assumptions underlying the method, such as linearity of leucine incorporation, saturation level of incorporation rates, incorporation into other macromolecules, specificity of incorporation for bacterial assemblages and [(3)H]Leucine degradation during samples storage were tested, and two procedures for extracting the incorporated leucine were compared. Both methods gave the same results, however, the hot TCA extraction method was less time consuming than the alkaline extraction method. Incorporation of [(3)H]Leucine was linear for up to 40 min. Saturation concentration of [(3)H]Leucine incorporation into protein was 1500 nM. An experiment with prokaryotic and eukaryotic inhibitors showed no significant [(3)H]Leucine incorporation into eukaryotes even in high leucine concentrations. No significant amounts of radiolabel were incorporated into other macromolecules. The maximum time of sample storage after the incubation is 15 days. The leucine incorporation method can be a reliable tool to measure bacterial production in the periphyton root-associated bacteria.

Methodological considerations regarding the use of inorganic 197Hg(II) radiotracer to assess mercury methylation potential rates in lake sediment

Methodological considerations on the determination of benthic methyl-mercury (CH(3)Hg) production potentials were investigated on lake sediment, using (197)Hg radiotracer. Three methods to arrest bacterial activity were compared: flash freezing, thermal sterilization, and gamma-irradiation. Flash freezing showed similar CH(3)Hg recoveries as thermal sterilization, which was both 50% higher than the recoveries obtained with gamma-ray irradiation. No additional radiolabel was recovered in kill-control samples after an additional 24 or 65 h of incubation, suggesting that all treatments were effective at arresting Hg(II)-methylating bacterial activity, and that the initial recoveries are likely due to non-methylated (197)Hg(II) carry-over in the organic extraction and/or [(197)Hg]CH(3)Hg produced via abiotic reactions. Two CH(3)Hg extraction methods from sediment were compared: (a) direct extraction into toluene after sediment leaching with CuSO(4) and HCl and (b) the same extraction with an additional back-extraction step to thiosulphate. Similar information was obtained with both methods, but the low efficiency observed and the extra work associated with the back-extraction procedure represent significant disadvantages, even tough the direct extraction involves higher Hg(II) carry over.