biogeochemistry: mercury methylation made easy

92 nature geoscience | VOL 2 | FEBRUARY 2009 | www.nature.com/naturegeoscience

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during the last ice age relative to today8,10. Decreased deep-ocean ventilation rules out North Pacific deep-water formation during the last ice age, and also suggests that Antarctic deep-water formation was reduced.

Looking further back in time, we find that the onset of major ice-age cycles approximately 2.7 million years ago coincided with a sharp drop in the flux of biogenic detritus in both the North Pacific (Fig. 1) and the Antarctic3,4. Thus, the apparent link between cooling and stratification indicated by data from the most recent glacial cycles, also applies to other climate transitions.

Early work in the North Pacific indicated that the biogenic flux during interglacials, although higher than in the ice ages, did not reach pre-2.7 million year levels3 (Fig. 1). However, North Pacific sediment records show an enigmatic peak in biogenic flux at each of the large deglaciations during the past 2.7 million years (Fig. 1, inset). In their detailed study of these events, Gebhardt and colleagues1 argue that the peaks represent a brief reversion to the conditions before the past 2.7 million years of extremely weak stratification and high biogenic flux. If this interpretation of the early deglacial intervals is correct, then the North Pacific not only resembled its more potent state three million years ago, but also

came closer to the modern behaviour of its more muscular southern twin.

The past changes in the polar twins may have much to tell us about their future. Climate models of anthropogenic warming predict an increase in the stratification of these regions, with consequences for the productivity and fluxes of energy and carbon dioxide11. Yet the accumulated palaeoclimate data from the polar twins seems to argue for the opposite response, with stronger stratification in cold climates, not warm ones.

One would tend to put more faith in the model results, as they are the outcome of much work and thought. Moreover, the sensitivities of the models can be rationalized in terms of relatively simple dynamics: for example, warming tends to strengthen the poleward transport of water vapour through the atmosphere, which should work to stratify the two polar twins11. In contrast, the postulated tendency for polar-ocean stratification to increase as global temperatures fall is much harder to explain, and has never been adequately simulated.

However, ongoing changes in the Southern Ocean seem to contradict the anthropogenic warming simulations, and instead fit with the responses found in the palaeoclimate data12. Accumulating palaeoceanographic evidence, including this latest work by Gebhardt

and colleagues, indicate that, in the past, the polar twins have responded similarly to climate change. Is it just a matter of time before the North Pacific follows its braver twin and contradicts our model-based expectations? ❐

Gerald H. Haug1,2 and Daniel M. Sigman3 are at the 1Geological Institute, Department of Earth Sciences, ETH-Zentrum, 8092 Zürich, Switzerland; 2Leibniz Center for Earth Surface and Climate Studies, Institute for Geosciences Potsdam University, 14476 Potsdam, Germany; 3Department of Geosciences, Princeton University, Princeton, New Jersey, 08544, USA. e-mail: [email protected]

references1. Gebhardt, H. et al. Paleoceanography 23,

doi: 10.1029/2007PA001513 (2008).2. Francois, R. et al. Nature 389, 929–935 (1997).3. Haug, G. H., Sigman, D. M., Tiedemann, R., Pedersen, T. F. &

Sarnthein, M. Nature 401, 779–782 (1999).4. Sigman, D. M., Jaccard, S. L. & Haug, G. H. Nature

428, 59–63 (2004).5. Jaccard, S. L. et al. Science 308, 1003–1006 (2005).6. Jaccard, S. L. et al. Earth Planet. Sci. Lett.

277, 156–165 (2009).7. Brunelle, B. G. et al. Paleoceanography 22,

doi: 10.1029/2005PA001205 (2007).8. Galbraith, E. D. et al. Nature 449, 890–894 (2007).9. Keigwin, L. D. Paleoceanography 13, 323–339 (1998).10. Sarnthein, M., Grootes, P. M., Kennett, J. P. & Nadeau, M. J.

Geophys. Monograph Series 173, 175–196 (2007).11. Sarmiento, J. L., Hughes, T. M. C., Stouffer, R. J. & Manabe, S.

Nature 393, 245 (1998).12. Toggweiler, J. R. & Russell, J. Nature 451, 286 (2008).

the cycling of mercury in aquatic environments is important because it is extremely toxic in organic

form. The most potent organic mercury compound is methyl mercury — a neurotoxin that accumulates in aquatic food chains. The transformation of inorganic mercury to methyl mercury is primarily mediated by aquatic microorganisms. How these microorganisms take up mercury, and how they convert it to methyl mercury — a process known as mercury methylation — is one of the last remaining uncertainties in the biogeochemical mercury cycle. On page 123 of this issue, Schaefer and Morel1 show that complexation of inorganic

mercury with the amino acid cysteine significantly enhances mercury uptake and the rate of methylation in the iron-reducing bacterium Geobacter sulfurreducens (Fig. 1).

It has been known for a long time that microorganisms are crucial for the production of methyl mercury, and several strains of bacteria have been isolated that are capable of methylating mercury when exposed to environmentally relevant inorganic mercury concentrations2. Most of these organisms fall into two distinct, but related groups: sulphate reducing bacteria3 and iron-reducing bacteria4. It was originally thought that inorganic mercury entered these bacteria in the form

of an uncharged, hydrophobic molecule (such as mercury dichloride or mercury sulphide) that passively diffused across the cell membrane2. However, since then it has been shown that microbial mercury uptake is enhanced at low pH and in the presence of certain amino acids; conditions in which passive diffusion is less probable as uncharged molecules containing mercury are not expected to dominate5,6. In addition, the influence of growth conditions on uptake rates in these experiments indicates that microbial physiology (also influenced by the growth environment) may exert more control over uptake than previously thought.

BIOgEOchEmIstry

mercury methylation made easyThe exact mechanism used by microorganisms to produce the neurotoxin methyl mercury is unclear. The latest laboratory studies point to the amino acid cysteine as an important aid for the uptake of inorganic mercury and its transformation to methyl mercury in Geobacter sulfurreducens.

richard sparling

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However, earlier experiments designed to investigate the mechanisms of microbial mercury uptake were performed using non-methylating microorganisms (Vibrio anguillarum and Escherichia coli)5,6, and therefore may or may not provide insight into mercury uptake in mercury-methylating organisms. Experiments with the sulphate-reducing bacterium Desulfobulbus propionicus (a methylating microorganism) have shown that mercury methylation is enhanced in the presence of mercury sulphide7, indicating that mercury uptake occurs passively in this organism. Thus, the jury is still out on the precise and potentially species-specific mechanism of mercury uptake.

Now, in a series of laboratory experiments, Schaefer and Morel1 investigate mercury uptake and methylation in the iron-reducing, mercury-methylating bacterium G. sulfurreducens. They show that the amount of inorganic mercury taken up by these bacteria, together with their rate of mercury methylation, is higher in the presence of the amino acid cysteine, and suggest that the formation of a mercury–cysteine complex (a hydrophilic organothiol complex) facilitates mercury uptake in these methylators. They go on to show that methylation is not enhanced in the presence of other, similar-sized hydrophilic organothiols, such as glutathione, dithioerythritol or penicillanime, suggesting that other hydrophilic mercury–organothiol complexes do not have the same effect.

Such a compound-specific response is characteristic of a protein-mediated uptake system, as they are expected to be highly substrate-specific. Thus, the results indicate that G. sulfurreducens may be actively taking up the mercury–cysteine complex. In contrast to the results obtained with D. propionicus7, mercury methylation in G. sulfurreducens was not enhanced in the presence of mercury sulphide, indicating that the principal mechanism of mercury uptake differs in these two organisms.

The genome of G. sulfurreducens has been sequenced8 and is amenable to genetic manipulation9. As a next step, one may be able to confirm genetically whether specific transport proteins are indeed important for the uptake of inorganic mercury under specific growth conditions in this organism.

For now, the results of Schaefer and Morel1 suggest that the mechanisms of mercury uptake are highly diverse and under tighter biological control than previously thought. A more precise understanding of the specific mechanisms involved in mercury uptake may lead to novel approaches when it comes to the mitigation of mercury methylation in the environment. ❐

Richard Sparling is at the Department of Microbiology, University of Manitoba, Winnipeg, Manitoba R3T 2N2, Canada.e-mail: [email protected]

references1. Schaefer, J. K. & Morel, F. M. M. Nature Geosci. 2, 123–126 (2009).2. Morel, F. M. M., Kraepiel, A. M. L. & Amyot, M.

Ann. Rev. Ecol. Syst. 29, 543–566 (1998).3. Compeau, G. C. & Bartha, R. Appl. Env. Microbiol.

50, 498–502 (1985).4. Kerin, E. J. et al. Appl. Environ. Microbiol. 72, 7919–7921 (2006).5. Golding G. R. et al. Environ. Sci. Technol.

41, 5685–5692 (2007).6. Golding, G. R., Sparling, R. & Kelly, C. A. Appl. Environ. Microbiol.

74, 667–675 (2008).7. Benoit, J. M., Gilmour, C. C. & Mason, R. P.

Appl. Environ. Microbiol. 67, 51–58 (2001).8. Mathé, B. A. et al. Science 302, 1967–1969 (2003).9. Coppi, M. D. et al. Appl. Environ. Microbiol. 67, 3180–3187 (2001).

For the past decade, many outlet glaciers in Greenland that terminate in the ocean have accelerated, thinned and

retreated. To explain these dynamic changes, two hypotheses have been discussed. Atmospheric warming has increased surface melting and may have also increased the amount of meltwater reaching the glacier bed, increasing lubrication at the base and hence the rate of glacier sliding1. Alternatively, a change in the delicate balance of forces where the glacier fronts meet the ocean could trigger the changes2-4. On page 110 of this issue, Faezeh Nick and

colleagues5 present ice-sheet modelling experiments that mimic the observations on Helheim glacier, East Greenland, indicating that the dynamic behaviour of outlet glaciers follows from perturbations at their marine fronts.

Greenland’s ice sheet loses mass partly through surface melting and partly through fast-flowing outlet glaciers that connect the vast plateau of inland ice with the ocean. As the outlet glaciers flow into the sea, icebergs calve from their fronts. As highlighted in the fourth assessment report of the Intergovernmental Panel on Climate

Change6, earlier ice-sheet models have failed to reproduce the dynamic variability shown by ice sheets over time. It has therefore not been possible to distinguish with confidence between basal lubrication from surface meltwater and changes at the glaciers’ marine fronts as causes for the observed changes on Greenland’s outlet glaciers.

The distinction bears directly on sea- level rise — the motivation for much of modern-day glaciology. If the recent dynamic mass loss from Greenland’s outlet glaciers is linked to changing atmospheric temperatures, it may persist for as long as

glacIOlOgy

From the frontThe causes of recent dynamic thinning of Greenland’s outlet glaciers have been debated. Realistic simulations suggest that changes at the marine fronts of these glaciers are to blame, implying that dynamic thinning will cease once the glaciers retreat to higher ground.

stephen price

Figure 1 | Geobacter sulfurreducens; mercury uptake and methylation by this bacterium are significantly increased in the presence of the amino acid cysteine, according to Schaefer and Morel1.

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Mercury methylation made easy

Richard Sparling

Nature Geoscience 2, 92–93 (2009); published online: 30 January 2009; corrected after print: 30 January 2009.

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In the print version of this News & Views the page numbers given in the text and reference 1 were incorrect; the correct range is 123–126. These errors have been corrected in the HTML and PDF versions.


biogeochemistry: mercury methylation made easy

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