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This study investigated the toxicity and bioaccumulation of monomethylmercury (MMHg) using two freshwater cyanobacteria— Oscillatoria tenuisa and Microcystis aeruginosa—that were isolated from a eutrophic reservoir. Tested concentrations were 1, 10, 50, and 250 μg L−1 of MMHg. The MMHg uptake depended on the phases of growth, period of exposure, and initial concentration of MMHg to which the cyanobacteria were exposed. The growth rate during the logarithmic phase was 0.17 day−1 for O. tenuisa, and 0.05 day−1 for M. aeruginosa in 250 μg L−1 of MMHg. These rates were a factor of 3.5–6 lower than that in the original medium without exposure to MMHg. The accumulation of MMHg by cyanobacteria increased rapidly within 4 weeks from the initial value of 1.2 × 10−4 – 1.6 × 10−4 ng cell−1 on sampling day 2 in the lag phase to 0.92 × 10−3 – 7.5 × 10−2 ng cell−1 on sampling day 28 during the stationary phase. The uptake rate of MMHg in the logarithmic phase cultures greatly exceeded that in the stationary cultures. The accumulation of MMHg by cyanobacteria depended on extracellular organic matter (EOM). Linear correlation analysis indicated a significant and negative correlation between the amount of bioaccumulation of MMHg in cyanobacterial cells and EOM concentration in suspension. Additionally, the effective concentrations (EC) of MMHg for inhibiting the growth of the two cyanobacterial species at 96 h (96 h EC50) were determined to be 340 μg L−1 for M. aeruginosa and 568 μg L−1 for O. tenuisa. The no observed effect concentrations (NOEC) for the two species were 64 μg L−1 and 115 μg L−1, respectively. Preliminary results that were based on SEM-map studies suggest that most of the MMHg accumulated in the cytoplasm (intercellular) of M. aeruginosa. [ABSTRACT FROM AUTHOR]

Surface sediments and sediment cores were collected in a mercury (Hg)-contaminated lagoon, namely Largo do Laranjo - Ria de Aveiro, Portugal and analysed in order to establish the monomethylmercury (MeHg) behaviour in this kind of environment. In surface sediments, this compound was only detected in one place (13.2 ng g-1 (dry weight)). In this site, it was determined one of the lowest redox potentials (22 mV), indicative of oxic/anoxic conditions, which favours Hg methylation by enhancing the sulphate-reducing bacteria activity. However, the MeHg percentage obtained was low, namely less than 0.1% of the total Hg. This is probably due to Hg deposition with organic matter and iron oxyhydroxides, decreasing Hg availability to methylation. At the deeper layers, MeHg was also determined, reaching 46.4 ng g-1 (dry weight) and representing less than 0.1% of the total Hg. The higher MeHg percentages were observed near the surface, where Hg seems to be faster methylated as a result of the lower sulphide concencentrations that render bioavailable the inorganic Hg. At depth the low MeHg percentages obtained are due to the formation of HgS and to the adsorption of Hg to iron monosulphides. [ABSTRACT FROM AUTHOR]

Monomethylmercury (CH3Hg+) is both the most ecologically significant and the least well characterized species of mercury in environmental settings. Our understanding of the environmental speciation behavior of this compound is limited both as the result of lesser available laboratory data (when compared to inorganic mercury) as well as the uncertainties associated with our understanding of the properties of environmental ligands. A careful examination and synthesis of data reported in the technical literature led to the following findings: (1) a 25°C, zero ionic strength bicarbonate ion complexation constant estimate is remarkably close to an earlier reported value at 0.4M: CH3Hg+ + HCO3- ...CH3HgHCO3, log10K = 2.6 (+0.22, 1 SD), (2) three 25°C zero ionic strength reaction constants reported by DeRobertis et al. (1998) were confirmed to within ~ +0.1 log10K units: CH3Hg+ +OH-...CH3HgOH, log10K = 9.47; 2CH3Hg+ +H2O...(CH3Hg)2OH+ +H+, log10K = -2.15; CH3Hg+ +Cl-...CH3HgCl, log10K = 5.45, (3) "best estimate" literature complexation constants corrected to zero ionic strength include: CH3Hg+ +F-...CH3HgF, log10K = 1.75 (20°C corr. Schwartzenbach and Schellenberg, 1965); CH3Hg+ +Br -...CH3HgBr, log10K = 6.87 (20°C corr. Schwartzenbach and Schellenberg, 1965); CH3Hg+ +l -...CH3Hgl, log10K = 8.85 (20°C corr. Schwartzenbach and Schellenberg, 1965); and CH3Hg+ +SO4 ²_...CH3HgSO4-, log10K = 2.64 (25°C, DeRobertis et al., 1998), (4) literature reported values for simulating monomethylmercury complexation with the carbonate ion may be too low: CH3Hg+ +CO3 ²-...CH3HgCO3-, log10K = 6.1 (Rabenstein et al., 1976; Erni, 1981), and (5) "best estimate" constants for simulating methyl mercury complexation with reduced environmental sulfur species include: CH3Hg+ +S² -...CH3HgS-, log10K = 21.1; CH3Hg+ +SH-...CH3HgSH, log10K = 14.5 (H+ +SH-..H2S, log10K = 6.88; Dyrssen and Wedborg, 1991); CH3Hg+ +RS-...CH3HgSR, log10K = 16.5 (H+ +RS-..RSH, log10K = 9.96; Qian et al., 2002); and CH3Hg+ +CH3HgS¹ -..(CH3Hg)2S, log10K = 16.32 (Schwartzenbach and Schellenberg, 1965; Rabenstein et al., 1978; and Erni, 1981). [ABSTRACT FROM AUTHOR]

The sequestering capacity of synthetic and naturally occurring polycarboxylate ligands towards monomethylmercury( II) was evaluated by stability quantitative data on the interaction of CH3Hg+ with different molecular weight synthetic polyacrylates (2 and 20 kDa average M.wt) and alginate (70-100 kDa) extracted from brown algae Macrocystis pyrifera. The influence of ionic medium was evaluated by measurements on the CH3Hg+-polyacrylate systems in NaNO3 medium at different ionic strengths (0.10, 0.25, 0.50 and 0.75 mol -1), and a Debye-Hückel type equation was used for the dependence of complex formation constants on ionic strength. Measurements on the CH3Hg+-alginate system were carried out at l=0.10 mol L-1 in NaNO3 medium. By using the stability data, the sequestering capacity of both ligands towards monomethylmercury(II) was determined at different pH values. Results obtained show that the binding ability of polyacrylic ligands (PAA) is stronger than the alginate (AA), following the trend PAA (20 kDa) > PAA (2 kDa) > AA. [ABSTRACT FROM AUTHOR]