Cyanobacteria as regulators of methylmercury production in periphyton

A new strategy of using periphyton to simultaneously promote remediation of PAHs-contaminated soil and production of safer crops

Contaminated farmland leads to serious problems for human health through biomagnification in the soil-crop-human chain. In this paper, we have established a new soil remediation strategy using periphyton for the production of safer rice. Four representative polycyclic aromatic hydrocarbons (PAHs), including phenanthrene (Phe), pyrene (Pyr), benzo[b]fluoranthene (BbF), and benzo[a]pyrene (BaP), were chosen to generate artificially contaminated soil. Pot experiments demonstrated that in comparison with rice cultivation in polluted soil with ΣPAHs (50 mg kg-1) but without periphyton, adding periphyton decreased ΣPAHs contents in both rice roots and shoots by 98.98% and 99.76%, respectively, and soil ΣPAHs removal reached 94.19%. Subsequently, risk assessment of ΣPAHs based on toxic equivalent concentration (TEQ), pollution load index (PLI), hazard index (HI), toxic unit for PAHs mixture (TUm), and incremental lifetime cancer risk (ILCR) indicated that periphyton lowered the ecological and carcinogenicity risks of PAHs. Besides, the role of periphyton in enhancing the rice productivity was revealed. The results indicated that periphyton alleviated the oxidative stress of PAHs on rice by reducing malondialdehyde (MDA) content and increasing total antioxidant capacity (T-AOC). Periphyton reduced the toxic stress of PAHs on the soil by promoting soil carbon cycling and metabolic activities as well. Periphyton also improved the soil's physicochemical properties, such as the percentage of soil aggregate, the contents of humic substances (HSs) and nutrients, which increased rice biomass. These findings confirmed that periphyton could improve rice productivity by enhancing soil quality and health. This study provides a new eco-friendly strategy for soil remediation and simultaneously enables the production of safe crops on contaminated land.

Mercury and methylmercury concentration in the feathers of two species of Kingfishers Megaceryle torquata and Chloroceryle amazona in the Upper Paraguay Basin and Amazon Basin

Mercury (Hg) contamination remains a significant environmental concern. In aquatic ecosystems, Hg can undergo methylation, forming its organic form, methylmercury (MeHg), which bioaccumulates and biomagnifies in the food chain, ultimately reaching the top predators, including waterfowl. The objective of this study was to investigate the distribution and levels of Hg in wing feathers, with a specific focus on evaluating heterogeneity in the primary feathers of two kingfisher species (Megaceryle torquata and Chloroceryle amazona). The concentrations of total Hg (THg) in the primary feathers of C. amazona individuals from the Juruena, Teles Pires, and Paraguay rivers were 4.724 ± 1.600, 4.003 ± 1.532, and 2.800 ± 1.475 µg/kg, respectively. The THg concentrations in the secondary feathers were 4.624 ± 1.718, 3.531 ± 1.361, and 2.779 ± 1.699 µg/kg, respectively. For M. torquata, the THg concentrations in the primary feathers from the Juruena, Teles Pires, and Paraguay rivers were 7.937 ± 3.830, 6.081 ± 2.598, and 4.697 ± 2.585 µg/kg, respectively. The THg concentrations in the secondary feathers were 7.891 ± 3.869, 5.124 ± 2.420, and 4.201 ± 2.176 µg/kg, respectively. The percentage of MeHg in the samples increased during THg recovery, with an average of 95% in primary feathers and 80% in secondary feathers. It is crucial to comprehend the current Hg concentrations in Neotropical birds to mitigate potential toxic effects on these species. Exposure to Hg can lead to reduced reproductive rates and behavioral changes, such as motor incoordination and impaired flight ability, ultimately resulting in population decline among bird populations.

Consortia cultivation of the Desulfobacterota from macrophyte periphyton: tool for increasing the cultivation of microorganisms involved in mercury methylation

Invasive macrophytes are a persistent environmental problem in aquatic ecosystems. They also cause potential health issues, since periphyton colonizing their aquatic roots are hot spot of mercury methylation. Because periphytons are at the base of the trophic chain, the produced methylmercury is bioamplified through the food webs. In this work, a consortia cultivation approach was applied in order to investigate methylators in the periphyton of Ludwigia sp., an invasive macrophyte. Five growth conditions were used in order to favor the growth of different sulfate reducers, the major mercury methylators in this periphyton. A total of 33 consortia containing putative Hg methylators were obtained. Based on the amino acid sequences of HgcA (essential enzyme for Hg methylation), the obtained consortia could be subdivided into five main clusters, affiliated with Desulfovibrionaceae, Desulfobulbaceae and Syntrophobacteraceae. The main cluster, related to Desulfovibrionaceae, showed the highest sequence diversity; notwithstanding most of the sequences of this cluster showed no close representatives. Through the consortia approach, species thus far uncultivated were cultivated. The successful cultivation of these species was probably possible through the metabolites produced by other members of the consortium. The analysis of the microbial composition of the consortia uncover certain microbial interactions that may exist within this complex environment.

FT-IR and synchronous fluorescence two-dimensional correlation spectroscopic analysis on the binding properties of mercury onto humic acids as influenced by pH modification and sulfide addition

Mercuric Hg²⁺ ion forms strong complexes with dissolved organic matter (DOM) in natural waters. The complexation of Hg²⁺ by sulfhydryl groups of DOM was regarded as the main mechanism for Hg²⁺-DOM interactions, particularly in anoxic sulfur and DOM-rich environments. In the present study, the influences of pH and sulfide addition on the molecular structure of Hg²⁺-DOM complexes and the characteristics of Hg²⁺ binding to DOM were investigated using FT-IR and synchronous fluorescence two-dimensional correlation spectroscopic analysis. Results showed that, during the Hg²⁺ binding process, the aromatic hydrogen CH in humic acids (HA) gave the fastest responses to pH perturbation and the S-reacted HA (S-HA) exhibited different reaction patterns from the unreacted HA. In S-HA, the esters/alcohols CO and carboxyl CO gave the fastest responses to Hg²⁺ binding. In the process of S-HA binding to Hg²⁺, the protein-like fractions including proteins, amino acids or monoaromatics played the leading role. Sulfide addition of HA enhanced the reactivity of small molecular weight compounds with low aromaticity and improved the binding ability of protein-like fractions to Hg²⁺. These findings provide a better understanding of the interaction mechanisms between Hg²⁺ and DOM at a molecular level and have important environmental implications in Hg²⁺ biogeochemical transformation, transport and cycling.

EL LEGADO DEL ARSÉNICO Y MERCURIO EN EL COMPLEJO CENAGOSO RAMSAR DE AYAPEL, (CÓRDOBA, COLOMBIA): APROXIMACIÓN A LA MACROCUENCA MAGDALENA-CAUCA

Colombia, país localizado al norte de Suramérica, afronta diferentes conflictos socioambientales y es uno de los más contaminados por mercurio en el mundo, situación que amenaza la riqueza natural y la biodiversidad del neotrópico, reconocida por sus altos niveles de endemismos. El complejo de humedales de Ayapel (Sitio Ramsar) ubicado al norte del país, reporta el desembarco del 23 % de la producción pesquera de la macrocuenca Magdalena-Cauca y a su vez sufre alta presión por minería de aluvión. Este trabajo evaluó la presencia de dos disruptores endocrinos, mercurio (Hg) y arsénico (As), en peces de consumo humano, agua y sedimentos, colectados en las ciénagas central de Ayapel, Escobillas y Paticos. En el sedimento y en el agua se registraron concentraciones de arsénico no reportadas previamente, y al ser un cancerígeno, es un hallazgo relevante ya que también puede estar presente en los cultivos de arroz, lo que aumenta el riesgo toxicológico para los pobladores de la región. Adicionalmente, el 96 % de los peces presentaron simultáneamente Hg y As con diferencias estadísticamente significativas entre las especies. El blanquillo (Sorubim cuspicaudus) fue el más contaminado por ambos polutantes y el bocachico (Prochilodus magdalenae) registró el nivel más alto de Hg en los últimos 15 años para la zona. Los niveles de Hg en la ictiofauna de la macrocuenca (1993-2020) alertan sobre los impactos no monitoreados en las redes tróficas y la urgencia de acciones intersectoriales para proteger la biodiversidad y la salud humana.

Economic Impacts on Human Health Resulting from the Use of Mercury in the Illegal Gold Mining in the Brazilian Amazon: A Methodological Assessment

Artisanal small-scale gold mining (ASGM) in the Amazon results in the dumping of tons of mercury into the environment annually. Despite consensus on the impacts of mercury on human health, there are still unknowns regarding: (i) the extent to which mercury from ASGM can be dispersed in the environment until it becomes toxic to humans; and (ii) the economic value of losses caused by contamination becomes evident. The main objective of this study is to propose a methodology to evaluate the impacts of ASGM on human health in different contexts in the Brazilian Amazon. We connect several points in the literature based on hypotheses regarding mercury dispersion in water, its transformation into methylmercury, and absorption by fish and humans. This methodology can be used as a tool to estimate the extent of environmental damage caused by artisanal gold mining, the severity of damage to the health of individuals contaminated by mercury and, consequently, can contribute to the application of fines to environmental violators. The consequences of contamination are evaluated by dose-response functions relating to mercury concentrations in hair and the development of the following health outcomes: (i) mild mental retardation, (ii) acute myocardial infarction, and (iii) hypertension. From disability-adjusted life years and statistical life value, we found that the economic losses range from 100,000 to 400,000 USD per kilogram of gold extracted. A case study of the Yanomami indigenous land shows that the impacts of mercury from illegal gold mining in 2020 totaled 69 million USD, which could be used by local authorities to compensate the Yanomami people.

The silver linings of mercury: Reconsideration of its impacts on living organisms from a multi-timescale perspective

Research on mercury (Hg), a naturally occurring element in Earth's lithosphere, has been extremely hot in the past few decades due to the outbreak of a series of disastrous poisoning incidents. However, such studies might provide us a biased view towards Hg if no thorough review about its long-term effects on living organisms from a multi-timescale perspective was performed. Hg might have played a mysterious role in critical intervals (e.g., mass extinction and oceanic anoxia events) in several geologic periods due to the elevated Hg levels induced by volcanism whereas it has long been used for various purposes in human history. Therefore, it is necessary to go through previous studies and historical records of different timescales (10⁰ to 10⁶ yr). In this work, we conducted a thorough review of Hg knowledge at three different timescales, i.e., geologic periods (10⁶ yr), human history (10³ yr), and contemporary history (10⁰ yr), summarizing recent advances and indicated potential research gaps. By doing so, we demonstrated that it is possible to achieve safe and sustainable Hg applications despite the current Hg crisis. However, such silver linings depend on a better understanding of ecosystem dynamics. Besides, considering the possible dire consequences of erupted Hg levels as suggested in geological periods, swift actions to mitigate the impacts of anthropogenic activities on the Hg cycle will be another key point. Overall, this review presented a unique perspective of Hg combining different timescales, shedding light on the importance of a better understanding of the global ecosystem as a whole and maintaining the sustainability of planet Earth in the future.

Algal Organic Matter Drives Methanogen-Mediated Methylmercury Production in Water from Eutrophic Shallow Lakes

Algal blooms bring massive amounts of algal organic matter (AOM) into eutrophic lakes, which influences microbial methylmercury (MeHg) production. However, because of the complexity of AOM and its dynamic changes during algal decomposition, the relationship between AOM and microbial Hg methylators remains poorly understood, which hinders predicting MeHg production and its bioaccumulation in eutrophic shallow lakes. To address that, we explored the impacts of AOM on microbial Hg methylators and MeHg production by characterizing dissolved organic matter with Fourier transform ion cyclotron resonance mass spectrometry (FTICR-MS) and three-dimensional excitation-emission matrix (3D-EEM) fluorescence spectroscopy and quantifying the microbial Hg methylation gene hgcA. We first reveal that the predominance of methanogens, facilitated by eutrophication-induced carbon input, could drive MeHg production in lake water. Specifically, bioavailable components of AOM (i.e., CHONs such as aromatic proteins and soluble microbial byproduct-like materials) increased the abundances (Archaea-hgcA gene: 438-2240% higher) and activities (net CH₄ production: 16.0-44.4% higher) of Archaea (e.g., methanogens). These in turn led to enhanced dissolved MeHg levels (24.3-15,918% higher) for three major eutrophic shallow lakes in China. Nevertheless, our model results indicate that AOM-facilitated MeHg production could be offset by AOM-induced MeHg biodilution under eutrophication. Our study would help reduce uncertainties in predicting MeHg production, providing a basis for mitigating the MeHg risk in eutrophic lakes.

Periphyton enhances arsenic release and methylation at the soil-water interface of paddy soils

Periphyton is ubiquitous in rice paddy fields, however its role in paddy soil arsenic (As) biogeochemistry remains unexplored. In this study, microcosm incubations and extensive field sampling were used to better understand the roles of periphyton on As mobility and transformation at the soil-water interface. Microcosm incubations revealed that periphyton on the paddy soil surface enhanced As release to water and increased methylated As contents at the soil-water interface. Experimental additions of dissolved phosphate did not significantly affect these processes. The presence of periphyton increased the dissolved organic carbon (DOC) content of the surface soil which may have played a role in the increased As mobility. However, the increase in methylated As species at the soil-water interface is indicative of detoxification processes of As by periphyton. The results from the field study revealed a high abundance and diversity of As biotransformation and detoxification genes in periphyton. Genera of Kineosporia, Limisphaera, Ornatilinea, Ktedonosporobacter and Anaerolinea played key roles in shaping arsM harboring microbe communities in field periphyton. These results highlight the importance of periphyton in the behavior of As in paddy soils and can potentially facilitate improved management of As contamination in paddy soils.

Nutrient Exposure Alters Microbial Composition, Structure, and Mercury Methylating Activity in Periphyton in a Contaminated Watershed

The conversion of mercury (Hg) to monomethylmercury (MMHg) is a critical area of concern in global Hg cycling. Periphyton biofilms may harbor significant amounts of MMHg but little is known about the Hg-methylating potential of the periphyton microbiome. Therefore, we used high-throughput amplicon sequencing of the 16S rRNA gene, ITS2 region, and Hg methylation gene pair (hgcAB) to characterize the archaea/bacteria, fungi, and Hg-methylating microorganisms in periphyton communities grown in a contaminated watershed in East Tennessee (United States). Furthermore, we examined how nutrient amendments (nitrate and/or phosphate) altered periphyton community structure and function. We found that bacterial/archaeal richness in experimental conditions decreased in summer and increased in autumn relative to control treatments, while fungal diversity generally increased in summer and decreased in autumn relative to control treatments. Interestingly, the Hg-methylating communities were dominated by Proteobacteria followed by Candidatus Atribacteria across both seasons. Surprisingly, Hg methylation potential correlated with numerous bacterial families that do not contain hgcAB, suggesting that the overall microbiome structure of periphyton communities influences rates of Hg transformation within these microbial mats. To further explore these complex community interactions, we performed a microbial network analysis and found that the nitrate-amended treatment resulted in the highest number of hub taxa that also corresponded with enhanced Hg methylation potential. This work provides insight into community interactions within the periphyton microbiome that may contribute to Hg cycling and will inform future research that will focus on establishing mixed microbial consortia to uncover mechanisms driving shifts in Hg cycling within periphyton habitats.

Microbial Diversity and Mercury Methylation Activity in Periphytic Biofilms at a Run-of-River Hydroelectric Dam and Constructed Wetlands

Periphytic biofilms have the potential to greatly influence the microbial production of the neurotoxicant monomethylmercury in freshwaters although few studies have simultaneously assessed periphyton mercury methylation and demethylation rates and the microbial communities associated with these transformations. We performed a field study on periphyton from a river affected by run-of-river power plants and artificial wetlands in a boreal landscape (Québec, Canada). In situ incubations were performed on three sites using environmental concentrations of isotopically enriched monomethylmercury (MM198Hg) and inorganic mercury (200Hg) for demethylation and methylation rate measurements. Periphytic microbial communities were investigated through 16S rRNA gene analyses and metagenomic screenings for the hgcA gene, involved in mercury methylation. Positive mercury methylation rates ([5.9 ± 3.4] × 10-3 day-1) were observed only in the wetlands, and demethylation rates averaged 1.78 ± 0.21 day-1 for the three studied sites. The 16S rRNA gene analyses revealed Proteobacteria as the most abundant phylum across all sites (36.3% ± 1.4%), from which families associated with mercury methylation were mostly found in the wetland site. Metagenome screening for HgcA identified 24 different hgcA sequences in the constructed wetland site only, associated with 8 known families, where the iron-reducing Geobacteraceae were the most abundant. This work brings new information on mercury methylation in periphyton from habitats of impacted rivers, associating it mostly with putative iron-reducing bacteria.IMPORTANCE Monomethylmercury (MMHg) is a biomagnifiable neurotoxin of global concern with risks to human health mostly associated with fish consumption. Hydroelectric reservoirs are known to be sources of MMHg many years after their impoundment. Little is known, however, on run-of-river dams flooding smaller terrestrial areas, although their numbers are expected to increase considerably worldwide in decades to come. Production of MMHg is associated mostly with anaerobic processes, but Hg methylation has been shown to occur in periphytic biofilms located in oxic zones of the water column. Therefore, in this study, we investigated in situ production of MMHg by periphytic communities in habitats impacted by the construction of a run-of-river dam by combining transformation rate measurements with genomic approaches targeting hgcAB genes, responsible for mercury methylation. These results provide extended knowledge on mercury methylators in river ecosystems impacted by run-of-river dams in temperate habitats.

Mercury cycling in freshwater systems - An updated conceptual model

The widely accepted conceptual model of mercury (Hg) cycling in freshwater lakes (atmospheric deposition and runoff of inorganic Hg, methylation in bottom sediments and subsequent bioaccumulation and biomagnification in biota) is practically accepted as common knowledge. There is mounting evidence that the dominant processes that regulate inputs, transformations, and bioavailability of Hg in many lakes may be missing from this picture, and the fixation on the temperate stratified lake archetype is impeding our exploration of understudied, but potentially important sources of methylmercury to freshwater lakes. In this review, the importance of understudied biogeochemical processes and sites of methylmercury production are highlighted, including the complexity of redox transformations of Hg within the lake system itself, the complex assemblage of microbes found in biofilms and periphyton (two vastly understudied important sources of methylmercury in many freshwater ecosystems), and the critical role of autochthonous and allochthonous dissolved organic matter which mediates the net supply of methylmercury from the cellular to catchment scale. A conceptual model of lake Hg in contrasting lakes and catchments is presented, highlighting the importance of the autochthonous and allochthonous supply of dissolved organic matter, bioavailable inorganic mercury and methylmercury and providing a framework for future convergent research at the lab and field scales to establish more mechanistic process-based relationships within and among critical compartments that regulate methylmercury concentrations in freshwater ecosystems.

Dynamics of mercury in the plankton of a hydroelectric reservoir, Western Amazon

The energy transfer in the aquatic food chain is an important way for mercury (Hg) to enter other trophic levels. The objective of this work was to evaluate the Hg concentrations in plankton upstream and downstream of the Samuel Hydroelectric Reservoir, Rondônia, Brazil. Phytoplankton and zooplankton samples were collected with 20-μm and 68-μm nylon nets. An aliquot was removed for taxonomic analysis and another for total mercury determination, performed by cold vapor atomic absorption spectroscopy. Water physical-chemical parameters were also measured. The Hg concentrations in total plankton (phytoplankton and zooplankton samples) obtained at the three sampling upstream stations showed the same behavior, with the highest values registered in June 2005 (232 μg kg^-1, 118 μg kg^-1, 128 μg kg^-1). The lowest values at stations J1 and M1 were recorded in November 2005 (4 μg kg^-1 and 22 μg kg^-1, respectively), while the lowest values at stations M4 and M8 were recorded in October 2005 (22 μg kg^-1 and 5 μg kg^-1, respectively). The Hg results found in the plankton in this study corroborate the results of other recent studies in the same region. The statistical analyses revealed that Hg concentrations in plankton do not explain the distribution of these organisms at the four sampling stations of Samuel Reservoir. Graphical Abstract.

Marine mercury-methylating microbial communities from coastal to Capbreton Canyon sediments (North Atlantic Ocean)

Microbial mercury (Hg) methylation transforms inorganic mercury to neurotoxic methylmercury (MeHg) mainly in aquatic anoxic environments. Sampling challenges in marine ecosystems, particularly in submarine canyons, leads to a lack of knowledge about the Hg methylating microbia in marine sediments. A previous study showed an enrichment of mercury species in sediments from the Capbreton Canyon where both geochemical parameters and microbial activities constrained the net MeHg production. In order to characterize Hg-methylating microbial communities from coastal to deeper sediments, we analysed the diversity of microorganisms' (16S rDNA-based sequencing) and Hg methylators (hgcA based cloning and sequencing). Both, 16S rDNA and hgcA gene analysis demonstrated that the putative Hg-methylating prokaryotes were likely within the Deltaproteobacteria, dominated by sulfur-compounds based reducing bacteria (mainly sulfate reducers). Additionally, others clades were also identified as carrying HgcA gene, such as, Chloroflexi, Spirochaetes, Elusimicrobia, PVC superphylum (Plantomycetes, Verrucomicrobia and Chlamydiae) and Euryarchaea. Nevertheless, 61% of the hgcA sequences were not assigned to specific clade, indicating that further studies are needed to understand the implication of new microorganisms carrying hgcA in the Hg methylation in marine environments. These first results suggest that sulfur cycle drives the Hg-methylation in marine ecosystem.