Correspondence/Rebuttal pubs.acs.org/est
Comment on “Anaerobic Mercury Methylation and Demethylation by Geobacter bemidjiensis Bem”
S
ince the discovery in the midsixties that inorganic Hg is converted to highly toxic CH3Hg (MeHg) in lake sediment,1 thousands of papers have been published on microbial Hg methylation. Yet, this process is far from completely understood. The results presented by Lu et al.2 could help reveal important aspects of the Hg methylation process. This comment concerns interpretations of the methylation/demethylation assays. An iron reducer, Geobacter bemidjienis Bem, was shown to both methylate Hg (added as HgCl2) and demethylate MeHg. Hg0 was identified as a product of demethylation.2 The observation that more Hg0 than MeHg was produced from HgCl2 suggests that the rate of demethylation (Kdemeth) was higher than the rate of methylation (Kmeth) (Figure 12), unless Hg0 was a result mainly of direct reduction of the added Hg. It is common to determine Kmeth and Kdemeth from the equation3 d[MeHg]/dt = K meth[Hg] − Kdemeth[MeHg]
In methylation assays in which [MeHg]t=0 = 0, the time to reach equilibrium is a function of Kmeth + Kdemeth (eq 2), and the level at which [MeHg] stabilizes is simply determined by Kmeth/Kdemeth and [Hg]t=0 (eq 3). Assuming Kdemeth to be zero, Lu et al.2 determined Kmeth to be 0.9 ± 0.2 h−1 for the first 8 h of the methylation assay (Figure 1a2). However, fitting eq 2 to the 0−8 h [MeHg] data of Lu et al.2 (extracted from Figure 1a2) one gets Kmeth ∼ 0.03 h−1 and Kdemeth ∼ 0.6 h−1. A Kdemeth/Kmeth ratio of 20 agrees well with what others have found for sulfate reducing bacteria.3 Then assuming a linear decrease in Kmeth from 0.03 to 0.015 h−1 for the period 8−120 h, a curve closely resembling the one in Figure 1a2 is generated (Figure 1a). A decrease in Kmeth could result from a negative feed-back when MeHg builds up in the cells (Figure 3a2), and/or from decreased availability of Hg for methylation. Linear decreases in both Kmeth and Kdemeth, for example, because of lowered microbial activity, would also fit the 8−120 h [MeHg] data, provided that Kmeth/Kdemeth decreases to 50% of its initial value and that Kdemeth does not decrease too much. In the actual case, the values and the stability of Kmeth and Kdemeth partly depends on whether Hg0 is included in [Hg] of eq 1. Here, for simplicity, Hg0 was included in [Hg] without accounting for any decrease in Kmeth once Hg0 forms.4 Hence, Kmeth = 0.03 h−1 is an integrated mean of all Hg species present. It is unclear how Lu et al.2 handled this issue. In any case, Kmeth 0.9 ± 0.2 h−1 and Kdemeth = 0 would not fit the 0−8 h [MeHg] data. Lu et al.2 explained the linear decline in [MeHg] after 8 h by a zero-order demethylation reaction. It must then have been assumed that Kmeth dropped precipitously from 0.9 h−1, i.e. from the value determined for the first 8 h. Also, the analysis performed here suggests that the demethylation reaction must
(1)
([Hg] = concentration of Hg available for methylation, [MeHg] = concentration of MeHg available for demethylation) Provided that [MeHg]t=0 = 0, the analytical solution to eq 1 is [MeHg](t ) = [Hg]t = 0 K meth(1 − e−(K meth + Kdemeth)t) /(K meth + Kdemeth)
(2)
([Hg]t=0 = the starting concentration of Hg available for methylation) At equilibrium, that is, when d[MeHg]/dt =0, eq 1 can be written as [MeHg]/[Hg] = K meth /Kdemeth
(3)
Figure 1. A model-based curve (see text) that closely fits the results obtained by Lu et al.2 in the methylation assay (a). A model-based curve (see text) that closely fits the results obtained by Lu et al.2 in the demethylation assay (b). The conversion of HgII/MeHg to Hg0 was not accounted for when the methylation assay was modeled (see text). Published: August 12, 2016 © 2016 American Chemical Society
9798
DOI: 10.1021/acs.est.6b02097 Environ. Sci. Technol. 2016, 50, 9798−9799
Environmental Science & Technology
Correspondence/Rebuttal
have changed from being a rapid first-order reaction to being a slow zero-order reaction. In a separate demethylation assay, [MeHg]t=0 was 5 nM. A similar linear decline in [MeHg] over time was seen as that after 8 h in the methylation assay (Figure 1a2 and Figure 2a from Lu et al.2), except that approximately five times as much MeHg was demethylated per unit of time. Modifying eq 2 to fit a case when all mercury is MeHg at t = 0, we get [MeHg](t ) = [MeHg]t = 0 (K meth + Kdemeth e−(K meth + Kdemeth)t ) /(K meth + Kdemeth)
(4)
Assuming that methylation of demethylated MeHg was insignificant (Kmeth = 0) and applying a Kdemeth of 0.008 h−1 results in a demethylation curve (Figure 1b) that closely resembles the one experimentally determined (Figure 2a2). First-order reactions therefore seem possible for demethylation both in the methylation and the demethylation assays. However, the demethylation rate would have to be lower for MeHg added externally than for MeHg produced internally.
Olof Regnell*
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Department of Biology/Aquatic Ecology, Lund University, SE-223 62 Lund, Sweden
AUTHOR INFORMATION
Corresponding Author
*Phone: +46 46 2223781; e-mail:
[email protected]. Notes
The author declares no competing financial interest.
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REFERENCES
(1) Jensen, S.; Jernelöv, A. Biological methylation of mercury in aquatic organisms. Nature 1969, 223, 753−754. (2) Lu, X.; Liu, Y.; Johs, A.; Zhao, L.; Wang, T.; Yang, Z.; Lin, H.; Elias, D. A.; Pierce, E. M.; Liang, L.; Barkay, T.; Gu, B. Anaerobic mercury methylation and demethylation by Geobacter bemidjiensis Bem. Environ. Sci. Technol. 2016, 50, 4366−4373. (3) Bridou, R. C.; Monperrus, M.; Gonzales, P. R.; Guyoneaud, R.; Amouroux, D. Simultaneous determination of mercury methylation and demethylation capacities of various sulfate-reducing bacteria using species-specific isotopic tracers. Environ. Toxicol. Chem. 2011, 30, 337−344. (4) Hu, H.; Lin, H.; Zheng, W.; Tomanicek, J.; Johs, A.; Feng, X.; Elias, D. A.; Liang, L.; Gu, B. Oxidation and methylation of dissolved elemental mercury by anaerobic bacteria. Nat. Geosci. 2013, 6, 751− 754.
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DOI: 10.1021/acs.est.6b02097 Environ. Sci. Technol. 2016, 50, 9798−9799
