Mercury (Hg) is one of the major pollutants in the environment affecting soil microbial communities, and their life activities and community structures [1]. On the other hand, there is evidence that Se could protect against the toxic effects of Hg [2]. However, little is known about the protective role of Se against Hg toxicity within microbial communities. Unravelling the mechanisms by which those elements interact might be of high relevance to study the real impact of Hg pollution in soil ecosystems and how Se could mitigate the effect of this environmental contaminant. In this study, the protective role of Se against Hg toxicity has been evaluated in two widely employed bacterial model organism, Staphylococcus aureus and Escherichia coli, by combining ICP-MS in single particle mode (spICP-MS) along with electron microscopy (TEM) and X-ray diffraction (XRD).

For this purpose E. coli and S. aureus were exposed to 5 mg L^-1 of mercury chloride (Hg(II)) for 24 h after pre-incubation with 5 mg L^-1 of sodium selenite (Se(IV)) for 8 h. Bacteria cultures were also incubated independently with Se(IV) and Hg(II) at 5 mg L-1 for the same exposure times. Results from viability studies showed that Se might exhibit a protective effect of Se against Hg in S. aureus since its viability increased from 41±8% in the presence of Hg(II) to 93±10% after pre-exposure to Se(IV). However, in the case of E. coli, it was observed that once the bacteria had grown for 8 h in the presence or absence of Se(IV), Hg(II) did not affect their viability.

On the other hand, during the exposure the colour of bacterial pellets turned from nearly colourless to deep black or red depending on the element added to the culture media. Therefore, TEM analyses were performed to study the possible formation of Hg, Se or HgSe nanoparticles. TEM micrographs and EDX spectra revealed the presence of Hg or Se nanoparticles with a size range between 24 and 57 nm for S. aureus, and between 24 and 114 nm for E. coli when they were cultured independently with Hg or Se at 5 mg L^-1. After co-exposure nanoparticles of the same size were also observed and Hg and Se were also detected by EDXS. Those observations were confirmed by using ICP-MS in single particle mode. Spikes of Se and Hg were registered for both bacteria under all treatments to which they were exposed. However, in case of co-exposure experiments, these results did not allow to conclude whether the biogenic nanoparticles were of Se, Hg or HgSe, or even if there might be a coexistence of all of them. Thus, to confirm the real composition of the particles detected by TEM and spICP-MS, RX analyses were performed. Results confirmed the presence of HgSe in bacterial cultures treated with Hg and Se.

References

[1] X. Zheng , H. Cao, B. Liu, M. Zhang, C. Zhang, P. Chen and B. Yang. Microorganism, 2022, 10, 977.

[2] B. Gómez-Gómez, T. Fernández-Bautista, Y. Madrid. TrAC Trends in Analytical Chemistry., 2024, 174, 117661.