temperature and the sulfur cycle control monomethylmercury cycling in high arctic coastal marine...

Temperature and the Sulfur Cycle Control MonomethylmercuryCycling in High Arctic Coastal Marine Sediments from Allen Bay,Nunavut, CanadaK. A. St. Pierre,† J. Chetelat,‡ E. Yumvihoze,† and A. J. Poulain†,*†Department of Biology, University of Ottawa, 30 Marie-Curie, Ottawa, Ontario, K1N 6N5, Canada‡Environment Canada, National Wildlife Research Centre, Ottawa, Ontario, Canada

*S Supporting Information

ABSTRACT: Monomethylmercury (MMHg) is a neurotoxinof concern in the Canadian Arctic due to its tendency tobioaccumulate and the importance of fish and wildlife in theInuit diet. In lakes and wetlands, microbial sedimentcommunities are integral to the cycling of MMHg; however,the role of Arctic marine sediments is poorly understood. Withprojected warming, the effect of temperature on theproduction and degradation of MMHg in Arctic environmentsalso remains unclear. We examined MMHg dynamics across atemperature gradient (4, 12, 24 °C) in marine sedimentscollected in Allen Bay, Nunavut. Slurries were spiked withstable mercury isotopes and amended with specific microbialstimulants and inhibitors, and subsampled over 12 days.Maximal methylation and demethylation potentials were low, ranging from below detection to 1.13 pmol g−1 h−1 and 0.02 pmolg−1 h−1, respectively, suggesting that sediments are likely not an important source of MMHg to overlying water. Our resultssuggest that warming may result in an increase in Hg methylation - controlled by temperature-dependent sulfate reduction,without a compensatory increase in demethylation. This study highlights the need for further research into the role of high Arcticmarine sediments and climate on the Arctic marine MMHg budget.

■ INTRODUCTION

Monomethylmercury (MMHg) is a potent neurotoxin found inaquatic ecosystems worldwide with a tendency to bioaccumu-late and biomagnify.1 In the Canadian Arctic, high MMHglevels pose a risk to the health of Inuit people whose traditionallocal diet is mostly comprised of marine mammals and fish.2

Despite the predominance of marine species in the Inuit diet,knowledge of MMHg dynamics in Arctic coastal systems issorely lacking. Our current understanding has been derivedfrom previous work focused on freshwater systems in the Arcticand/or coastal systems at temperate latitudes.3−5

Sources of MMHg in marine Arctic environments includeboth in situ production and external loading via atmosphericdeposition, snowmelt following atmospheric depletion eventsand river and ocean current transport.6−8 Analysis of lakesediment cores suggests that mercury (Hg) deposition in theArctic has increased during the last century and is beingretained within the environment.9

Both MMHg production and degradation can occur viaabiotic (mostly photochemical) and/or biotic pathways.10 Parkset. al (2013)11 recently identified two genes involved in Hgmethylation, proposing a corrinoid protein-mediated mecha-nism common to all methylating bacteria. While sulfate-reducing bacteria (SRB) are thought to be principally

responsible for methylation,12 recent work has shown thatiron-reducing microbes (FeRM) and methanogens contributeto these processes and to the net accumulation of MMHg inthe environment.13−15

Although both methylation and demethylation have beenshown to occur quickly in freshwaters, in marine environmentsthe rate of MMHg degradation is much reduced (<10−10 s−1 vs>10−6 s−1 in freshwater) as a result of increased stabilityconferred by elevated chloride concentrations.5 In freshwaterenvironments, sediments play an important role in the cyclingof MMHg, particularly in MMHg production, which neces-sitates reducing conditions. It has been hypothesized6,8 that thesedimentary contribution to the marine Arctic MMHg pool isnegligible, with the majority of methylation activity occurring inthe water column; however, little work has been done todetermine whether this is the case. Indeed, there exists noknown estimate of marine sediment Hg levels or methylationrates for the Arctic Archipelago.16

Received: August 30, 2013Revised: January 14, 2014Accepted: January 21, 2014Published: January 21, 2014

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© 2014 American Chemical Society 2680 dx.doi.org/10.1021/es405253g | Environ. Sci. Technol. 2014, 48, 2680−2687

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Potentially compounding the effect of increases in Hgdeposition is the effect of climate change, expected to beparticularly pronounced in these regions with even conservativemodels projecting continued warming for at least the nextcentury.17 While an increase in the rate of Hg methylation inpure cultures has been correlated with increased temper-atures,18 few studies have examined the direct effect, if any, oftemperature on the rates of Hg methylation and MMHgdegradation in marine coastal Arctic environments. Weinvestigrated the effects of temperature on mercury methylationand demethylation potentials in sediments collected from AllenBay, Nunavut, Canada. Sediment slurries were incubated fortwo weeks with isotope-enriched mercury tracers and specificmicrobial inhibitors and stimulants. The objectives of the studywere (a) to determine Hg methylation and demethylationpotentials in marine Arctic sediments; (b) to assess the relativecontribution of abiotic processes, SRB, FeRM, and metha-nogens to Hg methylation and demethylation; and (c) toestablish the effect of temperature on overall potentials of de/methylation in the sediments and the relative contributions ofthe microbial guilds of interest under different temperatureregimes.

■ MATERIALS AND METHODSSite Description and Sample Collection. Sediment

samples were collected from a tidal pool in Allen Bay(Cornwallis Island), Nunavut, Canada (74°47′N, 95°18′W)on August 12, 2011. The maximum depth of the pool sampledat low tide was around 1.0 m. At this depth, sediments wouldbe susceptible to natural seasonal changes in air temperatures.This study site is representative of organic-matter poor, ice-compacted coastal sediments of the Arctic.19,20 Over the last 10years, average summer (July−August) temperatures recorded atthe weather station in Resolute ranged, from 1 to 12.7 °C withextreme temperatures ranging from −3 to 20.1 °C. Sedimentswere collected using a long-bladed shovel, transferred to asterile (autoclaved) HDPE bottle in the field without aheadspace, and stored in a cooler. The same day, uponreturning to the research facilities in Resolute Bay, thesediments were frozen until further analysis. Frozen sedimentswere transported to facilities in Ottawa where they were kept at−20 °C until the incubation experiments were setup. Freezingthe sediments may alter the chemistry and the microbialcommunities present in the sediments; however, because of theshallow nature of the overlying water at this site, the sedimentslikely freeze naturally during the winter. Ambient dissolvedtotal mercury and MMHG in the overlying water were 0.43 ngL−1 and 0.05 ng L−1, respectively (Supporting Information (SI)Table S1).

Microcosm Preparations. In the laboratory, all sedimentmanipulations and incubations were conducted under anitrogen atmosphere in a Shel Lab Bactron anaerobic chamber.The sediments (0−2 cm depth) were thawed at 4 °C andhomogenized. Approximately 75 g of sediments were trans-ferred to 250 mL dark glass jars with Teflon-lined lids. 100 mLanoxic artificial seawater21 was then added and the resultingslurry shaken before spike, inhibitor and stimulant additions.Abiotic controls (A1) were prepared 48 h prior to the othermicrocosms by adding formaldehyde to a final concentration of1%.22 Being volatile, formaldehyde is easily removed from theslurry before the start of the incubations. Formaldehydeincubations did not alter the pH of the slurry. All treatments(Table 1) were conducted in duplicate, reflecting the high costof Arctic sample collection and the need to ensure that enoughsediments be used in preparing each microcosm for MMHgextraction from each subsample. The agreement betweenduplicates in treatments where the trends observed weresignificant (e.g., C1 and T2, see Results and Discussion below)lends credibility to the use of duplicates in this case. Themicrocosms were incubated at 4, 12, and 24 °C in parallel, butstaggered by 3 days to allow for immediate analysis of slurrychemistry following each subsampling. Temperatures werespecifically chosen to represent current temperatures (4 °C), apossible future water temperature scenario given currentclimate projections (12 °C) and a much higher temperature(24 °C) to ensure that a temperature effect (if present) wasdetectable.Specific inhibitors and stimulants were used to assess the

relative contributions of the microbial groups shown to play amajor role in mercury methylation and demethylation: sulfate-reducing bacteria (SRB), iron-reducing microbes (FeRM), andmethanogens (MPA).12,13,15,23 Sodium molybdate (Na2MoO4)was used as an inhibitor and sodium sulfate (Na2SO4) as astimulant of SRB.12 Although Na2MoO4 has been shown toinhibit some methanogens,24 its use is still widely accepted inSRB inhibition experiments.25,26 MPA were inhibited usingsodium-2-bromoethane sulfonate (BES). Both MPA and SRBrequire hydrogen (H2), so inhibitors are required to definitivelyattribute methylation and demethylation activity in the systemto either group. Despite there being no known inhibitor ofmicrobial iron reduction, the simultaneous use of Na2MoO4,BES, and iron(III) oxyhydroxide theoretically promotes theiron reduction pathway.12,13 In freshwater sediments, amor-phous iron(III) oxyhydroxide has been shown to be the mostbioavailable form of iron for FeRM.27

Stock solutions of Na2MoO4 (1 M), Na2SO4 (1 M), and BES(0.5 M) were prepared in Milli-Q water. Amorphousferrihydrite (Fe(OH)3) was synthesized according to Schwert-

Table 1. Description of Microcosms and Experimental Set-Upa

Hg Spikes, Stimulants and Inhibitors

ID description Hg spikes Na2MoO4 BES Fe(III) Na2SO4

N1 natural, unspikedC1 natural, spiked yesA1 abiotic control, spiked yesT1 SRB inhibited yes yesT2 MPA inhibited yes yesT3 SRB and MPA inhibited, FeRM stimulated yes yes yes yesT4 MPA inhibited, SRB stimulated yes yes yes

aAll microcosms were inoculated in duplicate. Na2MoO4, sodium molybdate; BES, sodium-2-bromoethane sulfonate; Fe(III), ferrihydrite −Fe(OH)3; Na2SO4, sodium sulfate; FeRM, iron-reducing prokaryotes; MPA, methanogens; SRB, sulfate-reducing bacteria.

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dx.doi.org/10.1021/es405253g | Environ. Sci. Technol. 2014, 48, 2680−26872681

mann and Cornell (2000).28 Inhibitors and stimulants wereadded prior to the Hg-tracers.25 With the exception ofNa2MoO4 and Na2SO4 additions (final concentrationsfollowing Compeau and Bartha 12), amendments were madebased on ambient concentrations of the compounds of interestwithin the sediments (Table 2).

MMHg enriched in Hg isotope 198 (CH3198HgCl) was

prepared by the methylcobalamin method29 and purified usinganhydrous sodium sulfate. Fresh inorganic Hg (199HgCl2)solutions were prepared at the beginning of each incubation.Hintelmann et al. (2000)30 suggested Hg additions between 13and 70% of ambient total mercury (THg) and MMHg;however, because ambient Hg concentrations were so low inAllen Bay sediments (1.34 ng THg g−1dry (6.7 pmol g−1) and0.15 ng MMHg g−1dry (0.6 pmol.g−1); Table 2), isotope-enriched MMHg yields obtained during preliminary experi-ments (results not shown) using the proposed upper range ofamendments were too low to be reliably detected by GC-ICPMS. Consequently, THg and MMHg concentrations wereincreased to 50 ng g−1 (240 pmol·g−1) and 7.5 ng g−1 (33.5pmol g−1), respectively, yielding Hg concentrations still wellwithin the natural levels found in the Arctic Ocean basin.31

Following Hg spikes, inhibitor and stimulant additions,approximately 1−2 mL of slurry was removed from each jar andtransferred to a 125 mL glass serum bottle to measure methaneproduction by methanogens in each treatment. Enough anoxicartificial seawater was then added to each serum bottle to fill totwo-thirds of its volume and the bottle was crimp-sealed. Serumbottles were incubated for the duration of the incubations. Atthe end of the experiment, methane concentrations weresubsequently determined using a SRI 8610C gas chromato-graph as a way to test for the efficacy of the methanogenesisinhibitor used.Subsamples for both slurry geochemistry and mercury

isotope analyses were removed at various times (0, 24, 48,96, 144, (170 at 12 °C), 192, 288 h), transferred to sterilecentrifuge tubes and frozen at −80 °C until further analysis.Sediment water content was measured by drying the

sediments for 24 h at 105 °C, while organic carbon content(%) was determined using loss on ignition (LOI) (400 °C for 8h).25

Aqueous Analyses. Subsamples were centrifuged, and theoverlying water was transferred to clean syringes fitted with 0.2μm filters (Sarstedt) under a nitrogen atmosphere. Ferrous iron(Fe(II)) and sulphide (S2‑) concentrations were determined

using the ferrozine32 and Cline’s methods,33,34 respectively. Allanalyses were conducted using a Varian Cary 300 spectropho-tometer.Due to the time sensitive nature of the analyses, the time

required for probe stabilization, and in an effort to limitmicrocosm exposure to light, pH and Eh were only measuredregularly in the microcosms incubated at 24 °C. pH wasmeasured in all microcosms with a Double Junction pHTestr30 (Oakton Instruments), while Eh was measured using a VWR(Eh) SP21 m in treatments N1, T1 and T3. pH was notmeasured during the final subsamplings (t = 192 and 288 h)due to insufficient slurry volume in which to immerse theprobe.

MMHg Isotope Analyses. Subsamples for MMHg analysiswere prepared according to the protocol outlined by Avramescuet al. (2010).35 Samples were acidified with 0.5 M nitric acidduring thawing and the solution was separated from thesediment by centrifugation at 3000 rpm for 15 min. Between1.2 and 4.5 g of sediments were used for analysis, depending onthe quantity of sediments remaining after slurry removal.MM201HgCl was added as an internal standard to correct forprocedural recovery during isotope dilution quantification.25

Concentrations of each of the isotope-enriched tracers weredetermined following gas chromatograph separation usinginductively coupled plasma mass spectrometry (GC-ICPMS;GC, Agilent Technologies 7890x; ICPMS, Agilent Technolo-gies 7700x).29 Four isotopes were measured: 198Hg (demethy-lation tracer), 199Hg (methylation tracer), 201Hg (internalstandard), and 202Hg (ambient). The limit of detection forquantifying excess MM199Hg and MM198Hg, calculatedaccording to Hintelmann and Evans,36 were 0.008 pmol.g−1

and 0.005 pmol.g−1, respectively.Statistical Analyses. All statistical analyses were completed

in R.37 Owing to the length of the study, concentrations of allcompounds of interest (S2−‑, Fe(II), MMHg) oscillated overtime, potentially reflecting the effect of opposing processes (i.e.,production/degradation; oxidation/reduction; etc.). In conse-quence, first order kinetics could not be assumed in all cases30

or corrected for6 and the rate constant approach was notrepresentative of the dynamics of the system (e.g., Figure 1 andSI Figures S2 and S3). Data were visually assessed for linearityand divided into linear segments based on the dynamics

Table 2. Ambient Sediment Characteristics and Changes to199Hg2+ and CH3

198HgCl Spikes and Specific InhibitorAdditions for Incubations

parameter Allen Bay

organic content 2.56 ± 0.31%water content 22.62 ± 1.68%ambient total mercury 1.04 ± 0.10 ng/gwet (1.34 ng/gdry)ambient methylmercury 0.12 ± 0.01 ng/gwet (0.15 ng/gdry)199HgII 50 ng/gdryCH3

198HgCl 7.5 ng/gdryNa2MoO4 amendment (SRB inhibitor) 20 mMa

Na2SO4 addition (SRB stimulant) 62.04 mMb

Na-BES amendment (MPA inhibitor) 30 mMa

Fe(OH)3 amendment (FeRM stimulant) 0.41 mMb

aCompeau and Bartha 1985. bYu et al. 2012.

Figure 1. Production of MM199Hg+ (pmol g−1) over time in themethanogenesis-inhibited treatment (T2) at 4, 12, 24 °C.



(increasing or decreasing concentrations) observed. Simplelinear regression was used to calculate maximum sulphideproduction, iron reduction, Hg methylation and demethylationpotentials for each microcosm on each linear segment.Additional information on linear segment division can befound in the SI. The maximum slope among the segments for agiven microcosm was then chosen to represent the potential ofthe microcosm with respect to each of the biogeochemicalprocesses of interest.Two-way analyses of variance (ANOVA) with Tukey

multiple comparisons were used to evaluate differences insulphide and iron production rates, and Hg methylation anddemethylation potentials between microcosms and acrosstemperature treatments.

■ RESULTS AND DISCUSSIONHg Methylation and MMHg Demethylation Poten-

tials. Both methylation and demethylation activities were lowwithin the sediments (SI Table S2). Methylation potentialsranged from the detection limit (0.008 pmol.g−1) to 1.12 pmolMM199Hg formed g−1 h−1. Demethylation potentials rangedfrom the detection limit (0.005 pmol.g−1) to 26.3 fmolMM198Hg lost g−1 h−1, 2 orders of magnitude less than theupper range of observed methylation potentials. The overalllow methylation potentials from sediment slurries supportprevious studies6,38 which suggested that water columnmethylation was responsible for the greatest proportion ofMMHg in polar marine waters.Concentrations of MMHg were variable over time, likely

reflecting the length of the study and presumably time for bothMMHg production and degradation. In the absence ofsignificant sulfate reduction (i.e., all treatments except C1 andT2), variability in the potentials measured was associated withthe uncertainty of the method at concentrations close to orbelow our detection limit. Note that all experiments performedat 4 °C yielded de/methylation potentials below our detectionlimits (0.008 pmol.g−1 for methylation and 0.005 pmol.g−1 fordemethylation) (SI Table S2).Maximum methylation potentials differed significantly by

temperature (F = 54.13, p ≪ 0.05) and treatment (F = 22.48, p≪ 0.05) (Figure 2). Methylation potentials were highest in C1(natural control) and T2 (MPA inhibited; SRB dominated) at

12 °C between 170 and 192 h (Figure 1). This finding is incontrast to previous studies, during which the majority ofmethylation activities occurred within the first 48 h of isotopeadditions.30 These systems were not monitored for as long aswas done here, so it is unclear whether this difference in thetiming of maximum methylation is a characteristic of the AllenBay system, simply a reflection of the differences in monitoringtimes between studies, or the time needed for the systems toequilibrate following stimulant and inhibitor additions. Thedelay in methylation relative to previous studies may furtherreflect a dependence on other biogeochemical processes, suchas sulfate reduction (discussed below).Maximum demethylation potentials were not affected by

temperature (F = 1.49, p≫0.05), but were dependent ontreatment (F = 4.66, p ≪ 0.05) (Figure 3). Tukey multiple

comparisons revealed that the only significant difference wasbetween the unspiked control (N1) and the iron-amendedmicrocosm (T3), the microcosm with the greatest demethyla-tion. Most demethylation occurred between 24 and 48 h afterspike additions.

Controls on MMHg in Arctic Marine Sediments. In theabsence of significant organic matter (Table 2), sulfurchemistry appears to be the primary chemical control onMMHg concentrations in the Allen Bay sediments. Maximumsulphide production rates (SPR) ranged between 0.076 and26.74 μM.h−1. SPR varied by temperature (F = 43.90; p <0.05), treatment (F = 4.95; p < 0.05) and by treatment overtemperature (interaction; F = 5.58; p < 0.05) (Figure 4). Therewas no significant difference between SPR at 4 and 12 °C butboth differed significantly from 24 °C (p < 0.05), thetemperature at which highest sulphide concentrations wereobserved.At 24 °C in C1 and T2 (where SRB dominated and

MM199Hg production was highest at 12 °C), MM199Hgconcentrations were highly correlated (R2 = 0.97 and 0.87,respectively; p ≪ 0.05) with sulphide concentrations (SI FigureS2, panels A and B). Based purely on metabolism, we wouldexpect methylation rates to increase at higher temperatures,mirroring the increase in sulfate reduction; however, the highestmethylation potentials were observed at 12 °C, not 24 °C. Thecorrelation between sulphides and MMHg was not significant

Figure 2. Maximum methylation potentials in pmol g−1 h−1. Means (n= 2 replicated experiments) and error bars (±1 SD) are shown.Treatments are described in Table 1

Figure 3. Maximum demethylation potentials in pmol g−1 h−1. Means(n = 2 replicated experiments) and error bars (±1 SD) are shown.Treatments are described in Table 1



at 4 and 12 °C (p > 0.05, for example, SI Figure S2, panel C),suggesting that sulphide concentrations only become highenough to limit MMHg production at higher temperatures. Infact, the increase in MM199Hg observed in C1 and T2 clearlyprecedes the increase in sulphides (SI Figure S2, panels A andB). At low sulphide levels (<10 μM), inorganic Hg tends to befound principally in highly bioavailable, neutral species (e.g.,HgS0); while at higher concentrations, it is believed that lessbioavailable forms (e.g., HgSH2

−) dominate.39 In pure cultures,sulphides have been shown to inhibit sulfate reduction atconcentrations as low as 0.25 mM.39 In Allen Bay, however, at24 °C in T2, sulphide concentrations did not stabilize until aconcentration of approximately 1.5 mM, corresponding to a 6-fold increase over other pure culture-based estimates. In C1,sulphides reached a concentration of 3.4 mM, (ca. 14-foldincrease over pure cultures), and had not yet reached a plateauby the end of the experimental period. Continued methylationdespite millimolar level sulphides have previously been reportedin marine sediments from the Skidaway River by King et al.18,40

This suggests that in this arctic marine environment, sulphidealone may not be as strong a limitation on methylation. Indeed,Graham et al. (2012)41 recently showed that Hg methylationoccurs most rapidly in mildly sulfidic environments.Maximum ferrous iron production rates (Fe(II)PR) ranged

between 0.0008 and 0.0087 mM.h−1. Fe(II)PR differedsignificantly by treatment (F = 5.9122; p < 0.05), but not bytemperature (p ≫ 0.05). Although amorphous iron oxy-hydroxide (Fe(OH)3) has been shown to be the mostbioavailable form of ferric (III) iron in freshwater sediments,27

maximum Fe(II)PR were lowest in the iron-amended treatment(T3). Few studies have attempted to isolate the Fe (III)reduction pathway in the context of Hg transformations inmarine sediments.42 There exist several possible explanationsfor this: (1) Fe(OH)3 may not be the most bioavailable form tothe Fe(III) reducing microbes present in Allen Bay; (2) Fe(III)reducing microbes were outcompeted by another microbialguild not accounted for by the treatments; or, (3) othermicrobial guilds were directly or indirectly involved in thereduction of Fe(III). This latter explanation seems likely seeingas SRB seemed to predominate in the sediments. In marinesediments, SRB can initiate Fe(III) reduction indirectly,

through the production of sulphides which chemically reduceFe(III), sequestering Fe(II) in the form of iron sulphideminerals, or directly through enzymatic processes.43 Thisprocess can also affect Hg availability through the dissolutionof adsorbed Hg(II) or Hg-sulphide complexes, therebyincreasing the dissolved Hg(II) available for methylation.44

Although our results suggest the FeRM are not involved in thedirect methylation of Hg, highest methylation potentials weremeasured in those treatments where both SRB and FeRM wereactive (C1, T2). FeRM may indirectly influence methylation bySRB as Fe(III) reduction can oxidize excess S2− to sulfate,thereby prolonging sulfate reduction and methylation.45

pH remained circumneutral over the course of theincubations but increased over time from 6.6 ± 0.1 to 7.4 ±0.1 (not including the abiotic control, A1). The increase in pHlikely reflects the consumption of H+ during microbialmetabolism.46 The entire incubations were carried out underreducing conditions (−163.7 mV at t = 0 h). Eh decreased overtime in the three treatments monitored. The change washowever not uniform, with a much greater difference in thenatural unspiked control (Δ = 253.1 mV) when compared toT1 and T3 (mean Δ = 47.4 mV). This potentially reflects thefact that sulfate reduction, a proton consuming process, wasinhibited in both treatments, but not in the natural control.Both Eh and pH were well within the range over which sulfate-reducing bacteria are known to occur.40

Factors controlling biotic methylation and demethylationultimately reflect (1) the bioavailability of Hg, and/or (2) theactivity of the microbes involved.47 Recent work showed thatthe genetic basis for methylation is found sporadically among adiversity of microbial genera.11 Likewise, demethylation activityseems to be widespread among guilds. Although demethylationhas often been shown to occur in the presence ofmethanogens,48 methane production was low in Allen Bay(SI Figure S1), possibly reflecting an organic input limitation.In cases of methanogenesis limitation in marine sediments,FeRM may have the greatest potential for demethylation, asshown here, in the absence of other microbial guilds (discussedbelow).Photochemical demethylation has been identified as a crucial

abiotic sink of MMHg in marine Arctic waters.6 Because thesediment slurries were incubated in the dark, photodemethy-lation would not have occurred. Despite this, demethylationwas still observed (albeit to a lesser extent than in the biotictreatments) in the killed control suggesting the possibility of 1)an alternative abiotic pathway of demethylation, such asreaction with hydrogen sulphide49 or 2) the presence ofspore-forming bacteria or another microbial guild resistant toformaldehyde and uncontrolled for by the other treatments.While the use of inhibitors and stimulants allowed for the

determination of the methylation and demethylation potentialsby each microbial guild, it could not accurately reflect theinteractions that may occur between microbes in the naturalsample. By comparing C1 to T2, competition between SRBsand methanogens appears to limit the accumulation ofMM199Hg relative to when methanogens are inhibited.Likewise, ferrous iron (Fe(II)) production is reduced in thepresence of SRB, potentially suggesting competition betweenSRB and FeRM for carbon sources and/or electron donors.50

The similarity between treatments T2 (methanogens inhibited)and T4 (methanogens inhibited with addition of sulfate)suggests that sulfate does not limit SRB activity within thesystem. This was not surprising given that sulfate is naturally

Figure 4. Maximum rates of sulphide production (μM h−1) bysediment slurry treatment at three (4, 12, 24 °C) incubationtemperatures. Means (n = 2 replicated experiments) and error bars(±1 SD) are shown. Treatments are described in Table 1



abundant in seawater. Despite the fact that SRB were targetedin both T2 and T4, methylation potentials in T4 did not mirrorthose in C1 and T2: the addition of sulfate may have altered thenatural sulfur cycle in the T4 microcosms, preventingmethylation.Ecological Significance. We showed that methylation and

demethylation potentials were low in Allen Bay, which suggeststhat sediments are likely not an important source of MMHg tooverlying water. We used inhibitors and stimulants to identifykey microbial players involved in MMHg cycling in marinecoastal arctic sediments, about which there has beenspeculation, but no testing. Our data highlight the potentialof temperature as a driving factor of methylation at lowtemperatures (Figures 1 and 2), although it is difficult toextricate the effect of temperature on sulfate reduction fromthat on methylation in these sediments. The observeddominance of sulfate reducing bacteria (SRB) activity in theunamended sediments (C1) and the relatively little distinctionbetween demethylation by treatment or temperature suggestthat the activity of SRB (and by extension Hg methylation)may be disproportionately influenced by temperature relative tothe opposing process of demethylation. Under currentconditions, sediments from coastal areas of the high Arcticmay not be an important source of methylmercury to marinefood webs. Our study site is representative of organic-matterpoor, ice-compacted coastal sediments in the Arctic. Given theabsence of coastal sediment surveys for the Canadian Arctic, itis not possible to quantify how representative they are.However, studies show that organic-poor coastal sediments(%C <5%) are common in other Arctic regions (Greenland,Siberia)19,20 and therefore this type of environment is likely alsovery common in the Canadian high Arctic. Furthermore, thesesediments are not representative of all types of coastal sedimentin the Arctic and our results may in fact underestimate MMHgproduction in organic-rich sediments of more productive Arcticcoastlines where microbial activity would be greater. Never-theless, the conservative estimates from Allen Bay show thatMMHg production increases with higher temperatures, and thisnovel finding is likely applicable to more productive sediments.Additional work is needed to verify this relationship across theArctic Ocean and incorporate spatial heterogeneity of sedimentcharacteristics in the importance of sediments to the polarmarine MMHg pool.In addition to the direct influence of temperature on

microbial metabolism studied here, it is expected that a numberof factors influencing Hg inputs to the marine Arctic system willbe amplified or altered under a warmer climate scenario. In theArctic terrestrial environment, permafrost thaw in particular, aswell as boreal soils and peatlands represent important sourcesof Hg.51,52 As terrestrial inputs from rivers to the Arctic Oceanincrease, the amount of mercury present in the ocean basin andby extension, available for methylation is also expected toincrease.7 Rivers are also a source of organic matter, necessaryfor SRB and MPA metabolism. At only 2% in the Allen Baysediments, organic matter may be limiting for the sedimentmicrobial community at present. On the other hand, reductionsin ice cover53 may increase the importance of photo-demethylation in the system, reducing the MMHg pool.These projected changes and the uncertainty as to themagnitude of their effect on the natural cycling of the Hg inArctic marine systems highlight the need for strategies to dealwith the potential of increased MMHg production and itsconsequences for human health in the Canadian Arctic.

■ ASSOCIATED CONTENT*S Supporting InformationMore Information regarding sampling site characterization,methylation and demethylation rates is available. This materialis available free of charge via the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATIONCorresponding Author*Phone: 1-613-562-5800 ×2373; fax: 1-613-562-5486; e-mail:[email protected] authors declare no competing financial interest.

■ ACKNOWLEDGMENTSWe are grateful to Igor Lehnherr and Brian Dimock for helpwith isotopic calculations, Ian Clark and Yanping Xing for theuse of the gas chromatograph; Linda Kimpe for lab assistance;Catherine Girard and Dominic Belanger for providing the fieldmeasurements presented in SI Table S1; Pilipoosie Iqaluk forfield assistance; and, Vincent St. Louis for providing feedbackon the manuscript. We thank four anonymous reviewers fortheir constructive comments that improved the quality of ourmanuscript. This project was funded through the NorthernContaminant Program of Aboriginal Affairs and NorthernDevelopment Canada as well as through a discovery grant fromthe Natural Science and Engineering Research Council ofCanada (NSERC) to AJP and an Undergraduate StudentResearch Award from NSERC to KASP.

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temperature and the sulfur cycle control monomethylmercury cycling in high arctic coastal marine...

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