Mercury bioavailability, transformations, and effects on freshwater biofilms

Matrix in River Water, Sediments, and Biofilms Mitigates Mercury Toxicity to Medaka (Oryzias Latipes)

Impacts of an environmental matrix on mercury (Hg) bioavailability and toxicity to medaka (Oryzias latipes) were investigated in matrix-free controls and treatments with a stepwise increased environmental matrix of river water, sediments, and biofilms. Generally, river water enhanced but the presence of sediments and biofilms reduced Hg bioavailability to medaka up to 105 times, so that Hgtotal concentrations/amounts among different environmental media cannot mirror Hg availability and toxicity to medaka. On average, 12.9 and 12.4% of Hg in medaka was, respectively, methylated to methylmercury (MeHg) in matrix-free and -containing treatments, indicating no influence of the environmental matrix on Hg methylation in medaka. All oxidative stress, inflammatory injury, and malformation parameters correlated strongly and significantly with Hgtotal and MeHg concentrations in medaka, notably with steeper slopes in matrix-free controls than in matrix-containing treatments, highlighting that the environmental matrix mitigated Hg and MeHg toxicity to medaka. Moreover, oxidative stress was more strongly mitigated than inflammatory injury according to the stronger decreases of the regression line slopes from matrix-free to -containing treatments. Here, we have newly identified that the potential of the environmental matrix to decrease Hg bioavailability and mitigate Hg toxicity to fish together could buffer Hg ecotoxicity in the aquatic environment.

Extracellular polymeric substances enhance dissolution and microbial methylation of mercury sulfide minerals

Due to the extremely low solubility, mercury sulfide minerals, as the major environmental mercury sinks, are generally considered to be inert mercury species with minimal bioavailability. Here, we demonstrate that extracellular polymeric substances (EPS), continuously secreted and released by anaerobic methylating bacteria, enhance the dissolution processes of cinnabar (α-HgS) minerals. The enhancing effects of EPS occur to a greater extent in the dissolution of nanoparticulate α-HgS compared to the bulk-scale counterpart. The released EPS-Hg(II) species are available for microbial methylation to produce bioaccumulative neurotoxin, methylmercury. This is probably due to the abundant aromatic proteins in EPS that strongly interact with surface Hg(II) via inner-sphere complexation as well as cation-π interaction. Our study discovers the potential environmental risks of "inert" mercury sulfide minerals in natural microbial habitats, particularly benthic biofilms with abundant microbial EPS, transformed to the severely toxic methylmercury. The mechanistic findings will facilitate an accurate understanding of the interactions between soft and transition metals and microorganism-derived organics, which may dictate the environmental fate and impact of these elements.

Arsenic and Mercury Distribution in an Aquatic Food Chain: Importance of Femtoplankton and Picoplankton Filtration Fractions

Arsenic (As) and mercury (Hg) were examined in the Yellowstone Lake food chain, focusing on two lake locations separated by approximately 20 km and differing in lake floor hydrothermal vent activity. Sampling spanned from femtoplankton to the main fish species, Yellowstone cutthroat trout and the apex predator lake trout. Mercury bioaccumulated in muscle and liver of both trout species, biomagnifying with age, whereas As decreased in older fish, which indicates differential exposure routes for these metal(loid)s. Mercury and As concentrations were higher in all food chain filter fractions (0.1-, 0.8-, and 3.0-μm filters) at the vent-associated Inflated Plain site, illustrating the impact of localized hydrothermal inputs. Femtoplankton and picoplankton size biomass (0.1- and 0.8-μm filters) accounted for 30%-70% of total Hg or As at both locations. By contrast, only approximately 4% of As and <1% of Hg were found in the 0.1-μm filtrate, indicating that comparatively little As or Hg actually exists as an ionic form or intercalated with humic compounds, a frequent assumption in freshwaters and marine waters. Ribosomal RNA (18S) gene sequencing of DNA derived from the 0.1-, 0.8-, and 3.0-μm filters showed significant eukaryote biomass in these fractions, providing a novel view of the femtoplankton and picoplankton size biomass, which assists in explaining why these fractions may contain such significant Hg and As. These results infer that femtoplankton and picoplankton metal(loid) loads represent aquatic food chain entry points that need to be accounted for and that are important for better understanding Hg and As biochemistry in aquatic systems. Environ Toxicol Chem 2023;42:225-241. © 2022 SETAC.

Effects of forest management on mercury bioaccumulation and biomagnification along the river continuum

Forest management can alter the mobilization of mercury (Hg) into headwater streams and its conversion to methylmercury (MeHg), the form that bioaccumulates in aquatic biota and biomagnifies through food webs. As headwater streams are important sources of organic materials and nutrients to larger systems, this connectivity may also increase MeHg in downstream biota through direct or indirect effects of forestry on water quality or food web structure. In this study, we collected water, seston, food sources (biofilm, leaves, organic matter), five macroinvertebrate taxa and fish (slimy sculpin; Cottus cognata) at 6 sites representing different stream orders (1-5) within three river basins with different total disturbances from forestry (both harvesting and silviculture). Methylmercury levels were highest in water and some food sources from the basin with moderate disturbance (greater clearcutting but less silviculture). Water, leaves, stoneflies and fish increased in MeHg or total Hg along the river continuum in the least disturbed basin, and there were some dissipative effects of forest management on these spatial patterns. Trophic level (δ¹⁵N) was a significant predictor of MeHg (and total Hg in fish) within food webs across all 18 sites, and biomagnification slopes were significantly lower in the basin with moderate total disturbance but not different in the other two basins. The elevated MeHg in lower trophic levels but its reduced trophic transfer in the basin with moderate disturbance was likely due to greater inputs of sediments and of dissolved organic carbon that is more humic, as these factors are known to both increase transport of Hg to streams and its uptake in primary producers but to also decrease MeHg bioaccumulation in consumers. Overall, these results suggest that the type of disturbance from forestry affects MeHg bioaccumulation and trophic transfer in stream food webs and some longitudinal patterns along a river continuum.

Binding characteristics of Hg(II) with extracellular polymeric substances: implications for Hg(II) reactivity within periphyton

Periphyton contains extracellular polymeric substances (EPS), yet little is known about how periphyton EPS affect the speciation and mobility of mercury (Hg(II)) in aquatic systems. This study extracted and characterized EPS from periphyton in Florida Everglades, and explored its role in Hg(II) binding and speciation using multiple approaches. Results from Fourier transform infrared spectroscopy (FTIR) revealed that colloidal and capsular EPS were primarily comprised of proteins, polysaccharides, phospholipids, and nucleic acids. Ultrafiltration experiments demonstrated that 77 ± 7.7% and 65 ± 5.5% of Hg(II) in EPS solution could be transformed into colloidal and capsular EPS-bound forms. Three-dimensional excitation emission fluorescence spectra (3D-EEMs) showed that the binding constants (K_b) between colloidal/capsular EPS and Hg(II) were 3.47×10³ and 2.62×10³ L·mol^-1. Together with 3D-EEMs and FTIR, it was found that the protein-like and polysaccharide-like substances in EPS contributed to Hg(II) binding. For colloidal EPS, COO- was the most preferred Hg(II) binding group, while C-N, C-O-C, and C-OH were the most preferred ones in capsular EPS. Using the stannous-reducible Hg approach, it was found that EPS significantly decreased the reactive Hg(II). Overall, this study demonstrated that EPS from periphyton are important organic ligands for Hg(II) complexation, which may further affect the migration and reactivity of Hg(II) in aquatic environment. These observations could improve our understanding of Hg(II) methylation and accumulation within periphyton in aquatic systems.

Stable carbon and nitrogen isotopes explain methylmercury concentrations in stream food webs of Lake George, New York (USA)

Mercury has been studied extensively in lakes due to health risks associated with the consumption of contaminated fish, while stream ecosystems have received less attention. To better understand mercury bioavailability in the lower food web of streams, we collected macroinvertebrates (predators and detritivore) along with autochthonous (epilithic algae) and allochthonous (leaf litter) basal resources in eight streams entering Lake George. Samples were analyzed for methylmercury (MeHg), total mercury, and carbon and nitrogen isotopes (δ¹³C & δ¹⁵N) to determine how mercury concentrations in basal resources, biomagnification rates, and environmental factors (watershed characteristics and water chemistry) effected MeHg concentrations in predatory macroinvertebrates. While biomagnification rates, calculated as trophic magnification slope, explained between 68% and 98% of MeHg variability within a stream food web, the range was small (0.310-0.387) resulting in the biotic components following a consistent pattern of increasing MeHg among streams. The stream order was negatively related to basin slope for all biotic components and explained 70% of MeHg variability in predatory macroinvertebrates. Methylmercury concentrations were significantly and negatively related to δ¹³C in predators, epilithic algae, and leaf litter. We believe the biofilms on leaf litter utilized bacterial-respired carbon dioxide decreasing δ¹³C (<-28‰) and increasing MeHg while epilithic algal δ¹³C increased due to enhanced primary production resulting in biodilution of MeHg. Methylmercury in basal resources responded to δ¹³C similarly but through different processes. Our findings show shallow slopes elevate MeHg in basal resources and explain most of the predator MeHg variation among streams with little influence of biomagnification rates.

Transcriptomic evidence for versatile metabolic activities of mercury cycling microorganisms in brackish microbial mats

Methylmercury, biomagnifying through food chains, is highly toxic for aquatic life. Its production and degradation are largely driven by microbial transformations; however, diversity and metabolic activity of mercury transformers, resulting in methylmercury concentrations in environments, remain poorly understood. Microbial mats are thick biofilms where oxic and anoxic metabolisms cooccur, providing opportunities to investigate the complexity of the microbial mercury transformations over contrasted redox conditions. Here, we conducted a genome-resolved metagenomic and metatranscriptomic analysis to identify putative activity of mercury reducers, methylators and demethylators in microbial mats strongly contaminated by mercury. Our transcriptomic results revealed the major role of rare microorganisms in mercury cycling. Mercury methylators, mainly related to Desulfobacterota, expressed a large panel of metabolic activities in sulfur, iron, nitrogen, and halogen compound transformations, extending known activities of mercury methylators under suboxic to anoxic conditions. Methylmercury detoxification processes were dissociated in the microbial mats with methylmercury cleavage being carried out by sulfide-oxidizing Thiotrichaceae and Rhodobacteraceae populations, whereas mercury reducers included members of the Verrucomicrobia, Bacteroidetes, Gammaproteobacteria, and different populations of Rhodobacteraceae. However most of the mercury reduction was potentially carried out anaerobically by sulfur- and iron-reducing Desulfuromonadaceae, revising our understanding of mercury transformers ecophysiology.

Association between Chromosome 4 and mercury accumulation in muscle of the three-spined stickleback (Gasterosteus aculeatus)

Anthropogenic stressors, such as pollutants, act as selective factors that can leave measurable changes in allele frequencies in the genome. Metals are of particular concern among pollutants, because of interference with vital biological pathways. We use the three-spined stickleback as a model for adaptation to mercury pollution in natural populations. We collected sticklebacks from 21 locations in Flanders (Belgium), measured the accumulated levels of mercury in the skeletal muscle tissue, and genotyped the fish by sequencing (GBS). The spread of muscle mercury content across locations was considerable, ranging from 21.5 to 327 ng/g dry weight (DW). We then conducted a genome-wide association study (GWAS) between 28,450 single nucleotide polymorphisms (SNPs) and the accumulated levels of mercury, using different approaches. Based on a linear mixed model analysis, the GWAS yielded multiple hits with a single top hit on Chromosome 4, with eight more SNPs suggestive of association. A second approach, a latent factor mixed model analysis, highlighted one single SNP on Chromosome 11. Finally, an outlier test identified one additional SNP on Chromosome 4 that appeared under selection. Out of all ten SNPs we identified as associated with mercury in muscle, three SNPs all located on Chromosome 4 and positioned within a 2.5 kb distance of an annotated gene. Based on these results and the genome coverage of our SNPs, we conclude that the selective effect of mercury pollution in Flanders causes a significant association with at least one locus on Chromosome 4 in three-spined stickleback.

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.

Pollutants affect algae-bacteria interactions: A critical review

With increasing concerns on the ecological risks of pollutants, many efforts have been devoted to revealing the toxic effects of pollutants on algae or bacteria in their monocultures. However, how pollutants affect algae and bacteria in their cocultures is still elusive but crucial due to its more environmental relevance. The present review outlines the interactions between algae and bacteria, reveals the influential mechanisms of pollutants (including pesticides, metals, engineered nanomaterials, pharmaceutical and personal care products, and aromatic pollutants) to algae and bacteria in their coexisted systems, and puts forward prospects for further advancing toxic studies in algal-bacterial systems. Pollutants affect the physiological and ecological functions of bacteria and algae by interfering with their relationships. Cell-to-cell adhesion, substrate exchange and biodegradation of organic pollutants, enhancement of signal transduction, and horizontal transfer of tolerance genes are important defense strategies in algal-bacterial systems to cope with pollution stress. Developing suitable algal-bacterial models, identifying cross-kingdom signaling molecules, and deciphering the horizontal transfer of pollutant resistant genes between algae and bacteria under pollution stress are the way forward to fully exploit the risks of pollutants in natural aquatic environments.

Proton Competition and Free Ion Activities Drive Cadmium, Copper, and Nickel Accumulation in River Biofilms in a Nordic Ecosystem

Biofilms can be used as a biomonitoring tool to determine metal bioavailability in streams affected by mining and other anthropogenic activities. Surface water and biofilm were sampled over two years from rivers located in the vicinity of a mine located in a Nordic ecosystem (Nunavik, Quebec). Biofilm metal content (Cd, Cu, and Ni) as well as a variety of physicochemical properties were determined to examine relationships between metal accumulation and water quality. Among the three metals of interest, copper and nickel had the highest levels of accumulation and cadmium had the lowest. When considering the exposure levels, nickel was the most abundant metal in our sampling sites. Both exposure and accumulation levels were consistent over time. Biofilm metal content was highly correlated to the ambient free metal ion concentration for sites of circumneutral pHs for all three metals. When the surface water pH was below 6, biofilm metal content was much lower than at other sites with similar aqueous metal concentrations of exposure. This apparent protective effect of decreasing pH can be explained by proton competition with dissolved metals for uptake binding sites at the surface of the organisms within the biofilm as described by the Biotic Ligand Model principles. The relationships obtained for Cd and Cu were overlapping those observed in previous publications, indicating strong similarities in metal accumulation processes in biofilms over very large geographical areas. Although more data are needed for Ni, our results show that biofilms represent a promising metal biomonitoring tool.

Effects of Extracellular Polymeric Substances on the Formation and Methylation of Mercury Sulfide Nanoparticles

Growing evidence has suggested that microbial biofilms are potential environmental "hotspots" for the production and accumulation of a bioaccumulative neurotoxin, methylmercury. Here, we demonstrate that extracellular polymeric substances (EPS), the main components of biofilm matrices, significantly interfere with mercury sulfide precipitation and lead to the formation of nanoparticulate metacinnabar available for microbial methylation, a natural process predominantly responsible for the environmental occurrence of methylmercury. EPS derived from mercury methylating bacteria, particularly Desulfovibrio desulfuricans ND132, substantially increase the methylation potential of nanoparticulate mercury. This is likely due to the abundant aromatic biomolecules in EPS that strongly interact with mercury sulfide via inner-sphere complexation and consequently enhance the short-range structural disorder while mitigating the aggregation of nanoparticulate mercury. The EPS-elevated bioavailability of nanoparticulate mercury to D. desulfuricans ND132 is not induced by dissolution of these nanoparticles in aqueous phase, and may be dictated by cell-nanoparticle interfacial reactions. Our discovery is the first step of mechanistically understanding methylmercury production in biofilms. These new mechanistic insights will help incorporate microbial EPS and particulate-phase mercury into mercury methylation models, and may facilitate the assessment of biogeochemical cycling of other nutrient or toxic elements driven by EPS-producing microorganisms that are prevalent in nature.

Mercury and Atherosclerosis: Cell Biology, Pathophysiology, and Epidemiological Studies

Today atherosclerosis is considered as a main cause of death in the worldwide. There is a significant association between heavy metal exposure and atherosclerosis. In this study, we discussed the scientific literature about the effect of mercury on the pathogenesis of atherosclerosis. We also considered the epidemiological studies on mercury as a risk factor for atherosclerosis. Web of Science, Google Scholar, Medline, PubMed, and Scopus were searched by using the following keywords to 2019: (cardiovascular diseases OR atherosclerosis OR endothelial dysfunction) AND (mercury). Mercury has the potential to act as one of the novel risk factors for atherosclerosis development. The findings have indicated the role of mercury in the pathogenesis of atherosclerosis, vascular endothelial dysfunction, oxidative stress, inflammation, and dyslipidemia. Mercury can induce atherosclerosis indirectly via increasing the total cholesterol, triglycerides, and LDL-C levels as well as decreasing the HDL-C level. Mercury can be considered as a risk factor in the atherosclerosis progression. However, more studies are required to find the exact mechanisms involved in the pathogenesis of atherosclerosis induced by mercury.

Biotic formation of methylmercury: A bio-physico-chemical conundrum

Mercury (Hg) is a natural and widespread trace metal, but is considered a priority pollutant, particularly its organic form methylmercury (MMHg), because of human's exposure to MMHg through fish consumption. Pioneering studies showed the methylation of divalent Hg (Hg^II) to MMHg to occur under oxygen-limited conditions and to depend on the activity of anaerobic microorganisms. Recent studies identified the hgcAB gene cluster in microorganisms with the capacity to methylate Hg^II and unveiled a much wider range of species and environmental conditions producing MMHg than previously expected. Here, we review the recent knowledge and approaches used to understand Hg^II-methylation, microbial biodiversity and activity involved in these processes, and we highlight the current limits for predicting MMHg concentrations in the environment. The available data unveil the fact that Hg^II methylation is a bio-physico-chemical conundrum in which the efficiency of biological Hg^II methylation appears to depend chiefly on Hg^II and nutrients availability, the abundance of electron acceptors such as sulfate or iron, the abundance and composition of organic matter as well as the activity and structure of the microbial community. An increased knowledge of the relationship between microbial community composition, physico-chemical conditions, MMHg production, and demethylation is necessary to predict variability in MMHg concentrations across environments.

Biofilms grown in aquatic microcosms affect mercury and selenium accumulation in Daphnia

Experiments examining mercury (Hg) toxicity in Daphnia are usually conducted in highly standardized conditions that prevent the formation of biofilm. Although such standardization has many advantages, extrapolation of results to natural conditions and inference of ecological effects is challenging. This is especially true since biofilms can accumulate metals/metalloids and play a key role in their transfer to higher trophic level organisms. In this study, we experimentally tested the effects of spontaneously appearing biofilm in Daphnia cultures on accumulation of Hg and its natural antagonist selenium (Se) in Daphnia magna. We added Hg (in the form of mercury (II) chloride) at two concentrations (0.2 µg/L and 2 µg/L) to experimental microcosms and measured the uptake of Hg and Se by D. magna in the presence and absence of biofilm. To test for consistent and replicable results, we ran two identical experimental sets one week apart. Biofilm presence significantly reduced the accumulation of Hg, while increasing the tissue Se content in D. magna, and these findings were reproducible across experimental sets. These findings indicate that highly standardized tests may not be adequate to predict the bioaccumulation and potential toxicity of metals/metalloids under natural conditions.

Ecosystem Controls on Methylmercury Production by Periphyton Biofilms in a Contaminated Stream: Implications for Predictive Modeling

Periphyton biofilms produce a substantial fraction of the overall monomethylmercury (MMHg) flux in East Fork Poplar Creek, an industrially contaminated, freshwater creek in Oak Ridge, Tennessee. We examined periphyton MMHg production across seasons, locations, and light conditions using mercury stable isotopes. Methylation and demethylation rate potentials (k_{m, trans av} and k_{d, trans av} , respectively) were calculated using a transient availability kinetic model. Light exposure and season were significant predictors of k_{m, trans av} , with greater values in full light exposure and in the summer. Season, light exposure, and location were significant predictors of k_{d, trans av} , which was highest in dark conditions, in the spring, and at the upstream location. Light exposure was the controlling factor for net MMHg production, with positive production for periphyton grown under full light exposure and net demethylation for periphyton grown in the dark. Ambient MMHg and k_{m, trans av} were significantly correlated. Transient availability rate potentials were 15 times higher for k_m and 9 times higher for k_d compared to full availability rate potentials (k_{m, full av} and k_{d, full av} ) calculated at 1 d. No significant model for the prediction of k_{m, full av} or k_{d, full av} could be constructed using light, season, and location. In addition, there were no significant differences among treatments for the full availability k_{m, full av} , k_{d, full av} , or net MMHg calculated using the full availability rate potentials. k_{m, full av} was not correlated with ambient MMHg concentrations. The present results underscore the importance of applying transient availability kinetics to MMHg production data when estimating MMHg production potential and flux. Environ Toxicol Chem 2019;38:2426-2435. © 2019 SETAC.

Impact of biochar on mobilization, methylation, and ethylation of mercury under dynamic redox conditions in a contaminated floodplain soil

Mercury (Hg) is a highly toxic element, which is frequently enriched in flooded soils due to its anthropogenic release. The mobilization of Hg and its species is of ultimate importance since it controls the transfer into the groundwater and plants and finally ends in the food chain, which has large implications on human health. Therefore, the remediation of those contaminated sites is an urgent need to protect humans and the environment. Often, the stabilization of Hg using amendments is a reliable option and biochar is considered a candidate to fulfill this purpose. We tested two different pine cone biochars pyrolyzed at 200 °C or 500 °C, respectively, with a view to decrease the mobilization of total Hg (Hg_t), methylmercury (MeHg), and ethylmercury (EtHg) and/or the formation of MeHg and EtHg in a contaminated floodplain soil (Hg_t: 41 mg/kg). We used a highly sophisticated automated biogeochemical microcosm setup to systematically alter the redox conditions from ~-150 to 300 mV. We continuously monitored the redox potential (E_H) along with pH and determined dissolved organic carbon (DOC), SUVA₂₅₄, chloride (Cl^-), sulfate (SO₄^2-), iron (Fe), and manganese (Mn) to be able to explain the mobilization of Hg and its species. However, the impact of biochar addition on Hg mobilization was limited. We did not observe a significant decrease of Hg_t, MeHg, and EtHg concentrations after treating the soil with the different biochars, presumably because potential binding sites for Hg were occupied by other ions and/or blocked by biofilm. Solubilization of Hg bound to DOC upon flooding of the soils might have occurred which could be an indirect impact of E_H on Hg mobilization. Nevertheless, Hg_t, MeHg, and EtHg in the slurry fluctuated between 0.9 and 52.0 μg/l, 11.1 to 406.0 ng/l, and 2.3 to 20.8 ng/l, respectively, under dynamic redox conditions. Total Hg concentrations were inversely related to the E_H; however, ethylation of Hg was favored at an E_H around 0 mV while methylation was enhanced between -50 and 100 mV. Phospholipid fatty acid profiles suggest that sulfate-reducing bacteria may have been the principal methylators in our experiment. In future, various biochars should be tested to evaluate their potential in decreasing the mobilization of Hg and to impede the formation of MeHg and EtHg under dynamic redox conditions in frequently flooded soils.

Aerobic and Anaerobic Bacterial Mercury Uptake is Driven by Algal Organic Matter Composition and Molecular Weight

The biological mobilization of mercury (Hg) into microbes capable of Hg methylation is one of the limiting steps in the formation of the neurotoxin methylmercury (MeHg). Although algal dissolved organic matter (DOM) has been associated with increased MeHg production, the relationship between bacterial Hg uptake and algal DOM remains unexplored. In this study, we aimed to address how the quantity and quality of DOM, freshly harvested from several algae, affected the bacterial uptake of Hg with the use of a biosensor capable of functioning both aerobically and anaerobically. We combined biosensor measurements with high-resolution mass spectrometry and field-flow fractionation to elucidate how DOM composition and molecular weight influenced microbial Hg uptake. We showed that freshly harvested DOM from Chlorophyte and Euglena mutabilis strongly inhibited aerobic and anaerobic Hg uptake, whereas DOM harvested from Euglena gracilis did not exhibit this same pronounced effect. Once fractionated, we found that amino acids and polyamines, most abundant in Euglena gracilis DOM, were positively correlated to increase Hg uptake, suggesting that these molecules are potentially underappreciated ligands affecting Hg bioavailability. As water quality is affected by eutrophication, algal community assemblages will change, leading to variations in the nature of autochthonous DOM released in aquatic systems. Our results highlight that variations in the emergent properties of DOM originating from varying algal species can have a profound effect on bacterial Hg uptake and thus methylation.

Major and minor elemental compositions of streambed biofilms and its implications of riverine biogeochemical cycles

Chemical compositions of streambed biofilms from a major river of central Japan (the Kushida River) were obtained, with data of associated sediments (fine-grained fractions < 63 μm) and dissolved components of waters, in order to provide preliminary information about biogeochemical significance of streambed biofilms. During the sampling period (July 31st to August 3rd, 2013), dissolved components of the river waters were influenced by the dam reservoir. Concentrations of NO₃^-, silica (as Si), SO₄^2-, PO₄^3- and Ca²⁺ decreased across the dam, whereas Fe and Mn increased across the dam, and then decreased downstream rapidly. Streambed biofilms contain significant amount of non-nutrient elements such as Al (up to 21% as Al₂O₃ on water and others-free basis), indicating that they are contaminated as siliciclatic (silt and clay) materials. Siliciclastic materials in the biofilms are basically compositionally similar to fine-grained (<63 μm) fractions of streambed sediments. However, some elements such as Ca, P, Mn, and Zn are markedly enriched in the biofilms. Particularly, Mn concentrations in the biofilm samples collected just below the dam reservoir are very high (∼4.0 wt %), probably due to accumulation from the discharged water. Concentrations of trace elements such as P, Cr, Cu, Zn and V appear to be controlled by amounts of Fe-oxides and/or Mn-oxides in biofilms. Numbers of factors are involved in controlling chemical compositions of streambed biofilms, including amount of contaminated siliciclastics, authigenic mineral formation, adsorption of dissolved materials and microbial metabolisms. As demonstrated by this study, systematic analyses including major elements and comparison with associated sediments and waters could reveal biogeochemistry of this complex system.

Graphene oxide-silver nanocomposites modulate biofilm formation and extracellular polymeric substance (EPS) production

Biofilms with positive and negative actions ubiquitously affect medical infections, environmental remediation and industrial processes. However, it remains challenging to control the growth of harmful biofilms as well as to exploit the use of beneficial biofilms. Here we investigated the effect of an antibacterial graphene oxide-silver nanoparticles (GO-AgNPs) composite on Pseudomonas aeruginosa biofilm formation. We found that GO-AgNPs prevented biofilm formation in a dose-dependent manner, with a threshold of 15 μg mL^-1. Interestingly, the bacterial biomass significantly decreased, but extracellular polymeric substance (EPS) production remarkably increased in mature biofilms treated with GO-AgNPs of an appropriate concentration, suggesting that GO-AgNPs effectively modulate biofilm development and structure. Moreover, we established that GO-AgNPs caused bacterial death via both physical damage and oxidative stress, showing the synergic action of GO and AgNPs. These findings facilitate the use of graphene-based nanocomposites for greener antibiotic applications.