Proteomics of Desulfovibrio desulfuricans and X-ray absorption spectroscopy to investigate mercury methylation in the presence of selenium.

The effects of mercury added as Hg(2+) and selenium as selenite to cultures of the sulfate reducing bacterium Desulfovibrio desulfuricans were investigated under controlled laboratory conditions. There was no significant difference in the growth curves in comparison to control except in the 0.5 µM Hg-6.3 µM Se combined system in which Hg methylation was significantly reduced. A significant decrease in the production of methylmercury indicates a disruption of the methylation process due to the presence of the relatively high concentrations of Se in the system, suggesting a modification of the biological pathway. The results of detailed 2D gel electrophoresis in combination with mass spectrometry confirmed that the Hg methylation process should certainly be influenced when the protein Dde_1198 protein-glutamate O-methyltransferase was totally suppressed in a culture containing 0.5 µM Hg and 6.3 µM Se. Since this protein plays an important role in the methylation process, its suppression in the presence of Se brings a possible explanation for the antagonism between Se and Hg in natural systems. The experiment involving the determination of Hg and Se in membrane proteins separated by 1D gel thin-layer isoelectric focusing revealed that when both elements were present in a culture, the concentration of Hg in the separated proteins was significantly lower in comparison to those without added Se to the culture and vice versa. Finally, near-edge X-ray absorption spectroscopy and extended X-ray absorption fine structure were used to corroborate the presence of a very inert solid HgSe in the cell membrane obtained from the culture containing 0.5 µM Hg and 6.3 µM Se. This confirms the protective effect of Se against Hg assimilation at the molecular level and reinforces the findings of our research group in numerous field and laboratory studies.

Improvements of reliability for methylmercury determination in environmental samples.

The determination of methylmercury (MeHg) in environmental samples by ethylation derivation-gas chromatography-atomic fluorescence spectrometry (ED-GC-AFS) is associated with an intimate problem of water moisture accumulation introduced in the ethylation step, which enters the detection system and cause a spectroscopic interference. With a simple modification on the GC-AFS system, this problem was eliminated and the analytical quality of the measurements was significantly improved. The presence of dissolved sulfide in samples can also cause serious chemical interference in the ethylation step resulting in lower or total loss of the MeHg signal. It was found that a masking system of CuSO(4)-Na(2)C(2)O(4) was able to eliminate this interference. With this system, the accurate determination of trace amount of MeHg in high dissolved sulfide containing samples was achieved. Satisfactory analytical results were obtained with the certified reference sediment IAEA405, sulfate reducing bacteria culture and sulfide containing water samples. The limit of detection and quantitation of this masking system is 0.01 and 0.04ngL(-1) respectively. Other factors affecting ethylation are also discussed.

Abiotic formation of elemental selenium and role of iron oxide surfaces.

The possible abiotic reduction of selenite to form elemental Se was studied under controlled conditions in the presence of ferrous iron. The reduction of selenite and formation of Se (0) was found to be a surface-mediated reaction by iron oxides. Without the presence of the reactive surface of freshly precipitated iron oxides, the reduction reaction could not be detected, even under a well controlled low oxygen environment (30 ppmv). Our results clearly illustrate the crucial role of iron oxide in this redox process. In lake sediments, the complex sediment matrix seems to strongly adsorb SeO(2-) and make it less available for reduction into Se(0). When lake sediments were submitted to UV irradiation to eliminate bacteria, the percentage of iron oxides increased and it was found that higher levels of iron oxides, generated by the UV treatment, correlated with the higher formation of Se(0). Moreover, the results of our field studies on two distinctively different lake sediments also showed a strong influence of iron oxides on the formation of elemental Se, which agrees well with our laboratory simulations.