RESUMO
Photochemical reactions are major pathways for the removal of Hg species from aquatic ecosystems, lowering the concentration of monomethylmercury (MMHg) and its bioaccumulation in foodwebs. Here, we investigated the rates and environmental drivers of MMHg photodegradation and inorganic Hg (IHg) photoreduction in waters of two high-altitude lakes from the Bolivian Altiplano representing meso- to eutrophic conditions. We incubated three contrasting waters in situ at two depths after adding Hg-enriched isotopic species to derive rate constants. We found that transformations mostly occurred in subsurface waters exposed to UV radiation and were mainly modulated by the dissolved organic matter (DOM) level. In parallel, we incubated the same waters after the addition of low concentrations of natural MMHg and followed the stable isotope composition of the remaining Hg species by compound-specific isotope analysis allowing the determination of enrichment factors and mass-independent fractionation (MIF) slopes (Δ199Hg/Δ201Hg) during in situ MMHg photodegradation in natural waters. We found that MIF enrichment factors potentially range from -11 to -19 and average -14.3 ± 0.6 (1 SE). The MIF slope diverged depending on the DOM level, ranging from 1.24 ± 0.03 to 1.34 ± 0.02 for the low and high DOM waters, respectively, and matched the MMHg MIF slope recorded in fish from the same lake. Our in situ results thus reveal (i) a relatively similar extent of Hg isotopic fractionation during MMHg photodegradation among contrasted natural waters and compared to previous laboratory experiments and (ii) that the MMHg MIF recorded in fish is characteristic for the MMHg bonding environment. They will enable a better assessment of the extent and conditions conducive to MMHg photodegradation in aquatic ecosystems.
Assuntos
Mercúrio , Compostos de Metilmercúrio , Poluentes Químicos da Água , Altitude , Animais , Bolívia , Ecossistema , Monitoramento Ambiental , Peixes/metabolismo , Isótopos , Lagos/química , Mercúrio/análise , Isótopos de Mercúrio/análise , Compostos de Metilmercúrio/metabolismo , Água/metabolismo , Poluentes Químicos da Água/análiseRESUMO
Caiman yacare is considered one of the top predators in the Amazon basin, and understanding pollutant distribution within its tissues may help its sustainable management. As a top predator, C. yacare should have the highest mercury concentrations, but has lower Hg concentrations than carnivorous fish (Rivera et al. 2016), which are part of their diet. We compared total Hg among liver, kidney, fat, and muscle of C. yacare, and whether trends in the distribution of Hg among tissues were like other crocodilians, aquatic birds, omnivorous, and carnivorous fish. Fat had the lowest concentrations (0.025 ± 0.03 mg kg-1) followed by muscle (0.15 ± 0.06 mg kg-1), kidney (0.57 ± 0.30 mg kg-1) and liver (1.81 ± 0.80 mg kg-1). Such preferential accumulation makes C. yacare meat a safer alternative for human consumption than carnivorous fish. The relation between Hg accumulation in liver and muscle is highest in crocodilians, which has evolutive and environmental implications.
Assuntos
Jacarés e Crocodilos , Mercúrio , Poluentes Químicos da Água , Animais , Monitoramento Ambiental , Peixes , Humanos , Fígado/química , Mercúrio/análise , Músculos/química , Poluentes Químicos da Água/análiseRESUMO
Anthropogenic pressure in the high altitude lakes such as Titicaca and Uru (Bolivia) may favor the production of methylmercury (MeHg) known to accumulate in trophic chains. Periphyton associated with emerged aquatic plants (totoras) from the lake shores accumulates and demethylates MeHg providing a potential cost-effective water treatment technique. In this laboratory study, we measured the MeHg uptake kinetics of a consortium of green algae isolated from Lake Titicaca totora's periphyton. The most abundant algal consortium, composed of Oedogonium spp., Chlorella spp., Scenedesmus spp., was exposed to rising MeHg concentrations (from 5 to 200 ng·L-1) to assess their maximum potential capacity for MeHg accumulation. Various algal biomass concentrations were tested to choose the optimal one. Results provided a net MeHg uptake rate by this algal consortium of 2.38 amol ng-1·h-1·nM-1 (the total uptake was 2863 ng MeHg·g-1) for an initial concentration of 200 ng MeHg·L-1 with an algal biomass concentration of 0.02 g·L-1. This initial MeHg concentration is 1000 times higher than the one measured in the eutrophic Cohana Bay of Lake Titicaca, which shows the high accumulation potential of these green algae. Our data suggest that periphyton has a high potential for the treatment of Hg contaminated waters in constructing wetlands in the Andean Altiplano.
Assuntos
Chlorella , Monitoramento Ambiental , Compostos de Metilmercúrio/metabolismo , Poluentes Químicos da Água/metabolismo , Bolívia , Lagos/química , Mercúrio/análise , Perifíton , Poluentes Químicos da Água/análiseRESUMO
Latin America, like other areas in the world, is faced with the problem of high arsenic (As) background in surface and groundwater, with impacts on human health. We studied As biogeochemical cycling by periphyton in Lake Titicaca and the mine-impacted Lake Uru Uru. As concentration was measured in water, sediment, totora plants (Schoenoplectus californicus) and periphyton growing on stems, and As speciation was determined by X-ray absorption spectroscopy in bulk and EDTA-extracted periphyton. Dissolved arsenic was between 5.0 and 15 µg L-1 in Lake Titicaca and reached 78.5 µg L-1 in Lake Uru Uru. As accumulation in periphyton was highly variable. We report the highest As bioaccumulation factors ever measured (BAFsperiphyton up to 245,000) in one zone of Lake Titicaca, with As present as As(V) and monomethyl-As (MMA(V)). Non-accumulating periphyton found in the other sites presented BAFsperiphyton between 1281 and 11,962, with As present as As(III), As(V) and arsenosugars. DNA analysis evidenced several taxa possibly related to this phenomenon. Further screening of bacterial and algal isolates would be necessary to identify the organism(s) responsible for As hyperaccumulation. Impacts on the ecosystem and human health appear limited, but such organisms or consortia would be of great interest for the treatment of As contaminated water.
Assuntos
Arsênio/análise , Bioacumulação , Monitoramento Ambiental/métodos , Água Subterrânea/química , Lagos/química , Arseniatos/análise , Bolívia , Cyperaceae/química , Ecossistema , Sedimentos Geológicos/química , Humanos , Monossacarídeos/análise , Perifíton , Extratos Vegetais/química , Poluentes Químicos da Água/análise , Espectroscopia por Absorção de Raios XRESUMO
The sources and factors controlling concentrations of monomethylmercury (MMHg) in aquatic ecosystems need to be better understood. Here, we investigated Hg transformations in sediments, periphyton associated with green algae's or aquatic plants, and benthic biofilms from the Lake Titicaca hydrosystem and compared them to the occurrence of active methylating microorganisms and extracellular Hg ligands. Intense Hg methylation was found in benthic biofilms and green algae's periphyton, while it remained low in sediments and aquatic plants' periphyton. Demethylation varied between compartments but remained overall in the same range. Hg methylation was mainly carried out by sulfate reducers, although methanogens also played a role. Its variability between compartments was first explained by the presence or absence of the hgcAB genes. Next, both benthic biofilm and green algae's periphyton exhibited a great diversity of extracellular low-molecular-weight (LMW) thiols (13 or 14 compounds) present at a range of a few nmol L-1 or µmol L-1 but clearly dominated by cysteine and 3-mercaptopropionic acid. Hg methylation was overall positively correlated to the total thiol concentrations, albeit to different extents according to the compartment and conditions. This work is the first examining the interplay between active methylating bacterial communities and extracellular ligands in heterotrophic biofilms and supports the involvement of LMW thiols in Hg methylation in real aquatic systems.
Assuntos
Mercúrio , Compostos de Metilmercúrio , Perifíton , Poluentes Químicos da Água , Altitude , Biofilmes , Ecossistema , Lagos , Metilação , Compostos de SulfidrilaRESUMO
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.
Assuntos
Magnoliopsida/fisiologia , Compostos de Metilmercúrio/metabolismo , Microalgas/química , Poluentes Químicos da Água/metabolismo , Altitude , Biota , Bolívia , Clorófitas/química , Clorófitas/classificação , Cyperaceae/fisiologia , Diatomáceas/química , Diatomáceas/classificação , Dinoflagellida/química , Dinoflagellida/classificação , Monitoramento Ambiental , Lagos , Microalgas/classificaçãoRESUMO
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.
Assuntos
Eichhornia/microbiologia , Compostos de Metilmercúrio/metabolismo , Polygonum/microbiologia , Bactérias Redutoras de Enxofre/metabolismo , Poluentes Químicos da Água/metabolismo , Bolívia , Desulfurococcaceae/genética , Desulfurococcaceae/metabolismo , Eichhornia/metabolismo , Metilação , Raízes de Plantas/metabolismo , Raízes de Plantas/microbiologia , Polygonum/metabolismo , Rizosfera , Bactérias Redutoras de Enxofre/genéticaRESUMO
Five subgroups of sulfate-reducing bacteria (SRB) were detected by PCR in three macrophyte rhizospheres (Polygonum densiflorum, Hymenachne donacifolia, and Ludwigia helminthorriza) and three subgroups in Eichhornia crassipes from La Granja, a floodplain lake from the upper Madeira basin. The SRB community varied according to the macrophyte species but with different degrees of association with their roots. The rhizosphere of the C4 plant Polygonum densiflorum had higher frequencies of SRB subgroups as well as higher mercury methylation potentials (27.5 to 36.1%) and carbon (16.06 +/- 5.40%), nitrogen (2.03 +/- 0.64%), Hg (94.50 +/- 6.86 ng Hg g(-1)), and methylmercury (8.25 +/- 1.45 ng Hg g(-1)) contents than the rhizosphere of the C3 plant Eichhornia crassipes. Mercury methylation in Polygonum densiflorum and Eichhornia crassipes was reduced when SRB metabolism was inhibited by sodium molybdate.