Bacterial removal of mercury from sewage

Mercury-resistant bacteria, which are able to reduce mercuric ion (Hg(2+)) to metallic mercury (Hg(0)), were examined for their ability to remove mercury from waste-water aerobically. Growth studies in artificial medium indicated that mercury increases the lag phase, but does not effect the growth rate of these bacteria. Further studies demonstrated that growth was minimal during a phase of rapid mercury removal, after which growth resumed. Small but significant amounts of carbohydrates are required for the mercuric ion reduction. Prolonged periods of bacterial growth under nonsterile conditions was accomplished without the loss of the mercuric reducing ability of the culture. A continuous culture of the resistant organism was maintained on raw sewage for two weeks, during which time relatively high concentrations of mercury (70 mg/L) were removed from the sewage at a rate of 2.5 mg/L h and at efficiencies exceeding 98%.     

Voltammetry on mercury and carbon electrodes as a tool for studies of metallothionein interactions with metal ions.

Rabbit liver Cd-metallothionein (CdMT) and Cd-complex of synthetically prepared pentapeptide (gamma-Glu-Cys)2-Gly were studied as examples of animal and plant metallothioneins. Using hanging mercury electrode, cathodic stripping voltammetry after adsorptive accumulation of the Cd(II)-SR complex at different potentials, is suitable for estimating changes occurring in metal coordination due to the presence of metal ions such as Zn2+, Cu2+, Hg2+ or excessive Cd2+. Conditions under which similar behaviour can be observed for both CdMT and Cd-pentapeptide complex are specified. On carbon electrodes, detailed study of reduction processes of Cd(II)-SR complexes is prevented by occurrence of a large catalytic current; oxidation processes are more suitable for study at these electrodes. Carbon composite paste electrode (10% SiO2) allows deposition of Cd(II)-SR complex during its reduction, as was demonstrated with Cd-cysteine, CdMT or Cd-pentapeptide complex. After deposition, oxidation peak of the uncomplexed Cd2+ ions and one or two oxidation peaks corresponding to a formation of the RS-Cd(II) complex are observed. Also, similarly as on Hg electrode, it was observed that excessive Cd2+ or Zn2+ ions influence oxidation peaks of the RS-Cd(II) complex formation. Combination of measurements on mercury electrode and composite paste electrode is recommended for studies of metallothionein interactions with metal ions or other metal complexes.     

Adsorption of bacteriophage on bacterial surface and on the surface of Hg dropping electrode

Summary The influence of the ionic strength of the medium on the adsorption of bacteriophage T 2 to the surfaces of a mercury dropping electrode on one hand and ofbacteria E. coli B on the other hand was studied. The adsorption on the mercury surface was determined by measurement of the differential capacity of the electrode double layer, the adsorption to bacteria was estimated from the decrease of free phage particles in a bacterial suspension with time. The adsorption to the mercury electrode increases with increasing ionic strength of the medium, but adsorption to the surface of bacteria increases at first, has a maximum at concentrations between 0,1 to 0,5 M and decreases with further increase of ionic strength. The decrease of adsorption of phage to the bacterial surface is assumed to be caused by the blocking of specific sites on the bacterial surface by adsorbed ions which sterically prevent the adsorption of the phage. Such specific sites are not present on the electrode surface, therefore adsorption increases further with increasing ionic strength probably due to the neutralization of surface charges of the phage and of the electrode. The saturated surface-concentration of the phage _s was calculated from the dependence of the differential capacity on the concentration. It is concluded from _s value obtained that the phage particles are scattered with wide intervals on the electrode surface with a degree of coverage of approximately 140.Abbreviations used DNA deoxyribonucleic acid - N Avogadro number The authors wishes to express their gratitude to the late Prof.Ferdinand Hercík, director of the Institute of Biophysics, for the initiation of this work and stimulating interest. The authors are also indebted to Dr. J.Koudelka for his kind gift of phage T 2 sample and to Dr. M.Vízdalová for her valuable comments during preparation of this article.     

Mercury mobilization by chemical and microbial iron oxide reduction in soils of French Guyana

Iron oxy(hydr)oxides (oxides) are important mercury sinks in tropical oxisols and the geochemistry of these two elements are thus closely entwined. We hypothesized that bacterial Fe-oxide reduction in anoxic conditions could be a significant mechanism for mobilizing associated Hg. Iron oxide and mercury solubilisation in presence of two chemical reducers (ascorbate and dithionite, dissolving amorphous and amorphous plus well crystallized Fe-oxides, respectively) was compared to their solubilisation in presence of autochthonous ferri-reducing bacteria. This work was carried out on two soil profiles from a small catchment basin in French Guyana, an oxisol (O) from a well drained slope and a water-saturated hydromorphic soil (H). The chemical reductions showed that in the oxisol 20 and 48% of total Hg (Hg_T) was associated to amorphous and well crystallized iron oxides, respectively. However, in the hydromorphic soil, no Hg seemed to be associated to amorphous iron oxides while the well crystallized fraction contained less than 9% of Hg_T. Chemical Fe-oxide reduction showed that Hg solubility was correlated to Fe reduction in the oxisol, demonstrating a relationship between the geochemistry of these two metals. During bacterial growth, while bacterial iron reduction solubilised up to 3.2 mg Fe g^?1 soil in the oxisol sample, Hg_T remained unchanged. No mercury was detected in the culture medium either. However, chemical analysis showed a decrease of the amounts of Hg associated to amorphous and well crystallized Fe-oxides after 14 days of incubation, underlining the potential for iron-reducing bacteria to modify mercury distribution in soil.     

A soil microscale study to reveal the heterogeneity of Hg(II) impact on indigenous bacteria by quantification of adapted phenotypes and analysis of community DNA fingerprints

The short term impact of 50 μM Hg(II) on soil bacterial community structure was evaluated in different microenvironments of a silt loam soil in order to determine the contribution of bacteria located in these microenvironments to the overall bacterial response to mercury spiking. Microenvironments and associated bacteria, designated as bacterial pools, were obtained by successive soil washes to separate the outer fraction, containing loosely associated bacteria, and the inner fraction, containing bacteria retained into aggregates, followed by a physical fractionation of the inner fraction to separate aggregates according to their size (size fractions). Indirect enumerations of viable heterotrophic (VH) and resistant (Hg(R)) bacteria were performed before and 30 days after mercury spiking. A ribosomal intergenic spacer analysis (RISA), combined with multivariate analysis, was used to compare modifications at the community level in the unfractionated soil and in the microenvironments. The spatial heterogeneity of the mercury impact was revealed by a higher increase of Hg(R) numbers in the outer fraction and in the coarse size fractions. Furthermore, shifts in RISA patterns of total community DNA indicated changes in the composition of the dominant bacterial populations in response to Hg(II) stress in the outer and in the clay size fractions. The heterogeneity of metal impact on indigenous bacteria, observed at a microscale level, is related to both the physical and chemical characteristics of the soil microenvironments governing mercury bioavailability and to the bacterial composition present before spiking.     

Utilization of heat-dried Pseudomonas aeruginosa biomass for voltammetric determination of Pb(II)

In this research, thermally dried Pseudomonas aeruginosa cells were used as a biological material for the construction of a microbial biosensor. The preparation, optimization and application of the developed microbial biosensor, which analyzed Pb(II), are presented. The method was based on stripping of adsorbed metal ions from the modified electrode surface. Modified carbon paste electrodes were preconcentrated at open circuit, and then electrochemically measured by using cyclic voltammetry (CV) and differential pulse stripping voltammetry (DPSV) techniques. It was found that the thermally dried cells were capable of adsorbing Pb(II) ions from aqueous solutions and could determine the ions prominently at optimum experimental conditions. Many important parameters to acquire the best electrochemical response were carried out, including effect of different electrolyte solutions, pH, deposition potential, deposition time, ionic strength, preconcentration time, and effect of interference ions. Finally, a calibration graph was obtained with a linear range from 1.0×10(-6) to 2.0×10(-5) M Pb(II) (R(2)=0.9916) and detection limit was found as 6.0×10(-7) M Pb(II) by using 3×S(b)/m formula. Other analytical properties of the developed microbial biosensor were also investigated. The suggested usage format of P. aeruginosa for the determination of Pb(II) does not require complicated immobilization procedure, easy to handle, and not time consuming.     

Determination of mercury in hair by square-wave anodic stripping voltammetry at a rotating gold disk electrode after microwave digestion

A simple and reliable method for the determination of mercury in hair on a rotating gold disk electrode using subtractive anodic stripping voltammetry without removal of oxygen is reported. Voltammetric and microwave parameters were optimized to obtain the best analytical results. Parameters such as supporting electrolyte concentration, influence of chloride in the Hg peak, deposition potential, scan rate, accumulation time, rotation rate, square-wave amplitude, and electrode conditioning were studied. Pressurized microwave-assisted digestion of hair, suitable for the accurate voltammetric determination of Hg, was evaluated using six acid mixtures and several time-power programs. Under the optimized conditions, no interference by copper, cadmium, lead, nickel, manganese, iron, or zinc was found at concentrations corresponding to their occurrence in normal hair. A calibration plot between 6,67 and 46,69 μg/L was linear, with r ² better than 0.999. The detection limit for a deposition time of 60 s at 25g, was calculated as 1.92 nM (3σ). Validation of the method was demonstrated with the use of a certified reference sample of hair. Eight real samples of hair (four unexposed children and four exposed persons) were also analyzed.     

Adsorptive transfer stripping voltammetry: Determination of nanogram quantities of DNA immobilized at the electrode surface

In adsorptive transfer stripping voltammetry (AdTSV), DNA is first adsorbed at the electrode, the electrode is washed and transferred (with the adsorbed layer) in the medium not containing DNA, and voltammetric analysis is performed in this medium. Adsorption can be performed from a drop of DNA solution, which makes it possible to reduce the volume of the analyzed sample by two orders of magnitude as compared to that of conventional voltammetry. With the hanging mercury drop electrode the limit of detection of single-stranded DNA is below 0.1 micrograms/ml; thus if the adsorption is performed from a 10-microliter drop of DNA solution subnanogram quantities of single-stranded DNA are sufficient for the analysis. In AdTSV the behavior of single- and double-stranded DNAs markedly differ from each other in a manner similar to that in the conventional voltammetric or polarographic analysis; AdTSV can thus be used in DNA structure analysis. In AdTSV the DNA transport and its adsorption at the electrode are separated from the electrode process; due to this fact it is possible (a) to perform the voltammetric analysis of DNA from media not suitable for voltammetric analysis of the conventional type, (b) to study the interaction of immobilized DNA with other substances in solution without the results of the voltammetric analysis being influenced by DNA interactions in the bulk of solution, and (c) to exploit the differences of adsorbability of DNA and other substances in order to separate them on the electrode.     

Detection of viable Salmonella typhimurium by impedance measurement of electrode capacitance and medium resistance

Three-electrode electrochemical impedance technique was investigated for detection of Salmonella typhimurium by monitoring the growth of bacteria in selenite cystine (SC) broth supplemented with trimethylamine oxide hydrochloride (TMAO.HCl) and mannitol (M). The change in the system impedance during the growth of bacteria was studied using frequency spectral scanning. It was found that the impedance at low frequencies (<10 kHz) mainly came from the double-charged layer capacitance, reflecting the changes at the electrode interface and the adsorption on the electrode surface. While at high frequencies (>10 kHz), the system impedance mainly depended on the medium resistance. The adsorption of bacteria on the electrode surface was detected by measuring low frequency impedance, and verified with Faradic impedance spectroscopy. Enumeration of S. typhimurium using a low frequency (1 Hz) capacitance measurement and a high frequency (1 MHz) resistance measurement were compared. The detection times were determined for quantitative analysis based on the growth curves of bacteria referring to either the medium resistance or electrode capacitance. The regression equations for the detection times (t(d), h) and the initial cell number (N, cells.ml(-1)) were t(d)=-1.24logN+13.4 with R(2)=0.98 and t(d)=-1.40logN+14.46 with R(2)=0.97 for the medium resistance and electrode capacitance methods, respectively.     

An electrochemical sensor for determination of calcium dobesilate based on PoPD/MWNTs composite film modified glassy carbon electrode

A poly-o-phenylenediamine and multi-wall carbon nanotubes composite (PoPD/MWNTs) modified glassy carbon electrode (GCE) was prepared by in situ electropolymerization using an ionic surfactant as the supporting electrolyte. The morphology of the resulting PoPD/MWNTs composite was characterized by TEM and the electrochemical properties of the modified electrode were characterized by cyclic voltammetry. The electrochemical behavior of calcium dobesilate on PoPD/MWNTs modified electrode was also investigated. The large current response of calcium dobesilate on PoPD/MWNTs modified electrode is probably caused by the synergistic effect of the electrocatalytic property of PoPD and MWNTs. The reductive peak current increased linearly with the concentration of calcium dobesilate in the range of 0.1–1.0 μmol/L and 4.0–400 μmol/L by square wave adsorptive stripping voltammetry, respectively. The detection limit (three times the signal blank/slope) was 0.035 μmol/L. The modified electrode could eliminate the interference of dopamine, norepinephrine and epinephrine at 100-, 90- and 70-fold concentration of 1.0 μmol/L calcium dobesilate, respectively. The proposed modified electrode provides a new promising and alternative way to detect calcium dobesilate.     

Diversity and characterization of mercury-resistant bacteria in snow, freshwater and sea-ice brine from the High Arctic

It is well-established that atmospheric deposition transports mercury from lower latitudes to the Arctic. The role of bacteria in the dynamics of the deposited mercury, however, is unknown. We characterized mercury-resistant bacteria from High Arctic snow, freshwater and sea-ice brine. Bacterial densities were 9.4 × 10(5), 5 × 10(5) and 0.9-3.1 × 10(3) cells mL(-1) in freshwater, brine and snow, respectively. Highest cultivability was observed in snow (11.9%), followed by freshwater (0.3%) and brine (0.03%). In snow, the mercury-resistant bacteria accounted for up to 31% of the culturable bacteria, but <2% in freshwater and brine. The resistant bacteria belonged to the Alpha-, Beta- and Gammaproteobacteria, Firmicutes, Actinobacteria, and Bacteriodetes. Resistance levels of most isolates were not temperature dependent. Of the resistant isolates, 25% reduced Hg(II) to Hg(0). No relation between resistance level, ability to reduce Hg(II) and phylogenetic group was observed. An estimation of the potential bacterial reduction of Hg(II) in snow suggested that it was important in the deeper snow layers where light attenuation inhibited photoreduction. Thus, by reducing Hg(II) to Hg(0), mercury-resistant bacteria may limit the supply of substrate for methylation processes and, hence, contribute to lowering the risk that methylmercury is being incorporated into the Arctic food chains.     

Expression of mercuric ion reductase in Eastern cottonwood (Populus deltoides) confers mercuric ion reduction and resistance

Mercury is one of the most hazardous heavy metals and is a particular problem in aquatic ecosystems, where organic mercury is biomagnified in the food chain. Previous studies demonstrated that transgenic model plants expressing a modified mercuric ion reductase gene from bacteria could detoxify mercury by converting the more toxic and reductive ionic form [Hg(II)] to less toxic elemental mercury [Hg(0)]. To further investigate if a genetic engineering approach for mercury phytoremediation can be effective in trees with a greater potential in riparian ecosystems, we generated transgenic Eastern cottonwood (Populus deltoides) trees expressing modified merA9 and merA18 genes. Leaf sections from transgenic plantlets produced adventitious shoots in the presence of 50 microm Hg(II) supplied as HgCl2, which inhibited shoot induction from leaf explants of wild-type plantlets. Transgenic shoots cultured in a medium containing 25 microm Hg(II) showed normal growth and rooted, while wild-type shoots were killed. When the transgenic cottonwood plantlets were exposed to Hg(II), they evolved 2-4-fold the amount of Hg(0) relative to wild-type plantlets. Transgenic merA9 and merA18 plants accumulated significantly higher biomass than control plants on a Georgia Piedmont soil contaminated with 40 p.p.m. Hg(II). Our results indicate that Eastern cottonwood plants expressing the bacterial mercuric ion reductase gene have potential as candidates for in situ remediation of mercury-contaminated soils or wastewater.     

Interactions of surface-confined DNA with electroreduced mitomycin C comparison with acid-activated mitomycin C

The anticancer activity of the antineoplastic drug mitomycin C (MC) was investigated using transfer stripping cyclic voltammetry (TSCV) with single-stranded DNA-modified hanging mercury drop electrode (HMDE). Reductive activation of MC is necessary for drug covalent binding to DNA, and we have found that some potential-controlled interactions of MC with DNA occur at the electrode, i.e. MC can be activated by electroreduction. Acid and electroreductive MC activations were compared and different adducts were subsequently generated, suggesting that the drug can bind to DNA in more than one way. Under conditions of acid activated MC, a monofunctional adduct between C-1 of MC and N-7 of guanine was formed on the electrode surface, reduced at - 0.44 V (vs. SCE). However, when the DNA-modified electrode was immersed in a MC solution and potentials corresponding to the quinone moiety reduction (- 0.3 V or more negative vs. SCE) were applied, an intrastrand bifunctional adduct between C-1 and C-10 of MC and two N-7 of a pair of adjacent guanines in ssDNA were formed at the electrode, reduced at - 0.49 V, i.e. 50 mV more negative than the monoadduct. The results presented in this paper show for the first time electrochemical detection of DNA-MC adducts at the hanging mercury drop electrode.     

Mercury(II) removal from aqueous solutions by nonviable Bacillus sp. from a tropical estuary

Use of microorganisms for removing mercury is an effective technology for the treatment of industrial wastewaters and can become an effective tool for the remediation of man-impacted coastal ecosystems with this metal. Nonviable biomass of an estuarine Bacillus sp. was employed for adsorbing Hg(II) ions from aqueous solutions at six different concentrations. It was observed that 0.2 g dry weight of nonviable biomass was found to remove from 0.023 mg (at 0.25 mg L(-1) of Hg(II)) to 0.681 mg (at 10.0 mg L(-1) of Hg(II)). Most of the mercury adsorption occurred during the first 20 min. It was found that changes in pH have a significant effect on the metal adsorption capacity of the bacteria, with the optimal pH value between 4.5 and 6.0 at 25 degrees C when solutions with 1.0, 5.0 and 10.0 mg L(-1) of Hg(II) were used.     

Adsorption of toxic mercury(II) by an extracellular biopolymer poly(gamma-glutamic acid)

Adsorption of mercury(II) by an extracellular biopolymer, poly(gamma-glutamic acid) (gamma-PGA), was studied as a function of pH, temperature, agitation time, ionic strength, light and heavy metal ions. An appreciable adsorption occurred at pH>3 and reached a maximum at pH 6. Isotherms were well predicted by Redlich-Peterson model with a dominating Freundlich behavior, implying the heterogeneous nature of mercury(II) adsorption. The adsorption followed an exothermic and spontaneous process with increased orderliness at solid/solution interface. The adsorption was rapid with 90% being attained within 5 min for a 80 mg/L mercury(II) solution, and the kinetic data were precisely described by pseudo second order model. Ionic strength due to added sodium salts reduced the mercury(II) binding with the coordinating ligands following the order: Cl(-) >SO(4)(2-) >NO(3)(-). Both light and heavy metal ions decreased mercury(II) binding by gamma-PGA, with calcium(II) ions showing a more pronounced effect than monovalent sodium and potassium ions, while the interfering heavy metal ions followed the order: Cu(2+) > Cd(2+) > Zn(2+). Distilled water adjusted to pH 2 using hydrochloric acid recovered 98.8% of mercury(II), and gamma-PGA reuse for five cycles of operation showed a loss of only 6.5%. IR spectra of gamma-PGA and Hg(II)-gamma-PGA revealed binding of mercury(II) with carboxylate and amide groups on gamma-PGA.     

Adsorption behavior of globular proteins at the water/mercury interface.

The adsorption of globular proteins at solid/liquid or liquid/liquid interfaces provides evidence of unfolded molecular conformation. Proteins with high apolar character are strongly unfolded, while those with high polar character are generally incompletely unfolded. Structural changes of globular proteins at adsorption on mercury electrodes were studied by ac polarography and capacity–time curves. The surface area per molecule of nine globular proteins was determined from the adsorption kinetics at the dropping mercury electrode. For all the proteins investigated, this value was greater than the maximal molecular cross section of the native proteins. The surface area was about 19 Ų per amino acid residue, which coincides with the value for unfolded proteins at the water/air interface. Differences between dropping mercury electrode and hanging drop mercury electrode occurred only with lysozyme and phosphorylase; for the other proteins, the structure of the adsorption layer was independent of the time of interaction at the electrode. Since not all of the reducible groups of the adsorbed proteins come into contact with the electrode, the flattening should be incomplete.     

Electrochemical monitoring of malondialdehyde biomarker in biological samples via electropolymerized amino acid/chitosan nanocomposite

This study reports on the electropolymerization of a low toxic and biocompatible nanopolymer with entitle poly arginine‐graphene quantum dots‐chitosan (PARG‐GQDs‐CS) as a novel strategy for surface modification of glassy carbon surface and preparation of a new interface for measurement of malondialdehyde (MDA) in exhaled breath condensate. Electrochemical deposition, as a well‐controlled synthesis procedure, has been used for subsequently layer‐by‐layer preparation of GQDs‐CS nanostructures on a PARG prepolymerized on the surface of glassy carbon electrode using cyclic voltammetry techniques in the regime of ?1.5 to 2 V. The modified electrode appeared as an effective electroactivity for detection of MDA by using cyclic voltammetry, linear sweep voltammetry, and differential pulse voltammetry. The prepared modified electrode demonstrated a noticeably good activity for electrooxidation of MDA than PARG. Enhancement of peak currents is ascribed to the fast heterogeneous electron transfer kinetics that arise from the synergistic coupling between the excellent properties of PARG and semiconducting polymer, GQDs as high density of edge plane sites and subtle electronic characteristics and unique properties of CS such as excellent film‐forming ability, high permeability, good adhesion, nontoxicity, cheapness, and a susceptibility to chemical modification. The prepared sensor showed 1 oxidation processes for MDA at potentials about 1 V with a low limit of quantification 5.94 nM. Finally, application of new sensor for determination of MDA in exhaled breath condensate was suited. In general, the simultaneous attachment of GQDs and CS to structure of poly amino acids provides new opportunities within the personal healthcare.     

Reaction of nucleic acid bases with the mercury electrode: Determination of purine derivatives at submicromolar concentrations by means of cathodic stripping voltammetry

It was found that adenine, guanine, hypoxanthine, 8-hydroxyadenine, and a number of further purine derivatives react in alkaline media with mercury of the electrode charged to potentials close to zero V (against the saturated calomel electrode) and form sparingly soluble compounds. Formation of these compounds with mercury is manifested on the polarographic (voltammetric) curves by characteristic anodic waves (peaks) which can be exploited for analytical purposes. Differential pulse polarography renders it possible to determine bases at concentrations of 10^?5–10^?6m. Substantially higher sensitivity can be reached by cathodic stripping voltammetry (CSV). This method is based on a slow accumulation of the sparingly soluble compound at the electrode surface and its subsequent rapid cathodic stripping. A number of purine derivatives can be determined by CSV at concentrations as low as 10^?8m (the limit of adenine detection is about 2 × 10^?9m). As compared with sulphur-containing substances CSV analysis of the purine derivatives is limited to a narrower range of deposition potentials. It was shown that the presence of an excess of proteins or DNA does not interfere with determination of purine bases.     

Copper-psychoactive drug complexes: a voltammetric approach to complexation by 1,4-benzodiazepines

Copper complexation by the 1,4-benzodiazepines medazepam, diazepam, flurazepam, nitrazepam, and clonazepam was investigated using differential pulse polarography and cyclic voltammetry at a mercury electrode in 0.10 M KNO3 and pH 7.0 +/- 0.1. Because the 1,4-benzodiazepines are easily reduced at a mercury electrode through the two-electron reduction of the 4,5-azomethine functional group, copper reduction, as well as that of the ligands, was analyzed under varying experimental conditions. In most situations adsorption phenomena occurred and their influence on voltammetric signals had to be carefully analyzed. The voltammetric behavior was then interpreted in terms of complex formation. The results showed that all benzodiazepines can act as ligands toward copper(II) ions, forming 1:1 and 1:2 complexes with similar stabilities. The stoichiometric acidity constants of the benzodiazepines under study were also determined by potentiometric titration in water-ethanol medium and 0.10 M KNO3 and then extrapolated to 0% concentration of ethanol.     

Selenium and mercury concentrations in some fish species of the Madeira River, Amazon Basin, Brazil

Samples of 7 species of piscivorous, omnivorous, and herbivorous fish caught at 12 different sites on the Madeira River, Amazon Basin, were analyzed for selenium and mercury. Selenium was determined by anodic stripping voltammetry and mercury by cold vapor atomic absorption spectrophotometry. The means for selenium concentrations ranged from 0.49 to 3.11 nmol/g and for mercury from 0.41 to 6.66 nmol/g depending on the fish species. The molar ratios of Hg:Se increased according to the fish trophic level. Piscivorous species had the highest mean ratio (4.0) and herbivorous species the lowest (0.9). There was a positive and statistically significant correlation between selenium and mercury concentrations for the herbivorous species (r = 0.716;p = 0.0088) not seen for omnivororus and piscivorous species (r = -0.2032;p = 0.3407). These findings are significant for the fish-eating population of the Madeira River because the ingestion of mercury would always be in excess of selenium.