1568

Environmental Toxicology and Chemistry, Vol. 16, No. 8, pp. 1568–1574, 1997q 1997 SETAC

Printed in the USA0730-7268/97 $6.00 1 .00

INHIBITION OF MERCURY METHYLATION IN ANOXIC FRESHWATERSEDIMENT BY GROUP VI ANIONS

YUAN CHEN,† JEAN-CLAUDE J. BONZONGO,‡ W. BERRY LYONS‡ and GLENN C. MILLER*††Department of Environmental and Resource Sciences, MS. 199, University of Nevada, Reno, Nevada 89557-0013, USA

‡Department of Geology, University of Alabama, Tuscaloosa, Alabama 35487-0338, USA

(Received 30 August 1996; Accepted 3 January 1997)

Abstract—The addition of group VI anions to sediment slurries resulted in the inhibition of the rate of mercury (Hg) methylation.The ranking of inhibition is as follows: tellurate ( ) . selenate ( ) . molybdate ( ) . tungstate ( ). In sediment22 22 22 22TeO SeO MoO WO4 4 4 4

slurries treated with and , methylmercury (MeHg) formation was significantly inhibited (p , 0.05) at the concentrations22 22TeO SeO4 4

.50 nM of and .270 nM of , while the significant inhibition (p , 0.05) of Hg-methylation by and22 22 22 22TeO SeO MoO WO4 4 4 4

was observed in slurries spiked at final concentrations $100 mM and $700 mM, respectively. Increasing the sulfate ( )22SO4

concentration while using fixed concentrations of inhibitors led to the partial reestablishment of some MeHg production in-treated slurries, whereas, no such significant change was noticed in sediment slurries treated with and . These22 22 22WO MoO TeO4 4 4

observations suggested that inhibits Hg methylation by a competitive mechanism, while and are noncompetitive22 22 22WO MoO TeO4 4 4

inhibitors. Selenate and showed a qualitatively similar effect on Hg methylation at concentrations tested, in that each showed22SO4

stimulation at low concentrations and inhibition at high concentrations. The depression of MeHg formation by group VI anionswas not accompanied by an inhibition of general microbial activity, suggesting that only particular microorganisms, such as sulfate-reducing bacteria, are responsible for Hg methylation. Finally, in the concentration ranges encountered in most natural aquaticenvironments, the inhibition of MeHg production by group VI anions is unlikely, except in systems where those elements are foundin anomalously high concentrations.

Keywords—Sediment Mercury Group VI anions MeHg production/inhibition

INTRODUCTION

Mercury methylation in sediment is favored under anoxicconditions and is linked to microbial sulfate reduction [1,2].The role of sulfate-reducing bacteria (SRB) in Hg methylationhas been extensively investigated in recent years [1–3]. Theresults suggest that SRB are important for Hg methylation,and their methylating activity is influenced by the concentra-tion of sulfate ( ) in the surrounding environment.22SO4

Microbial sulfate reduction, however, can also be inhibitedby certain anions, such as , , and [4,5].22 22 22MoO WO SeO4 4 4

These anions closely resemble in terms of size, charge,22SO4

and stereochemistry. Consequently, they can pass through cellwalls along the same pathways as and interfere with22SO4

metabolic processes involving sulfate reduction [6]. These in-terferences could result in a depletion of cellular ATP, mostlikely due to the inhibition of energy production associatedwith sulfate reduction.

In natural aquatic systems, the above oxyanions occur inconcentrations many orders of magnitude lower than ,22SO4

and their interference with sulfate reduction would not be ex-pected in environments such as coastal waters, where the ratioof to group VI anions is large. Banat and Nedwell [5]22SO4

observed a significant inhibition of microbial sulfate reductionfor to group VI anion ratios #3 when the final concen-22SO4

trations of tested anions were 10 and 20 mM. Although theseconcentrations are many orders of magnitude higher than thoseencountered in natural aquatic systems, the results suggest thatthese elements could act as inhibitors of sulfate reduction and

* To whom correspondence may be addressed.

probably affect other biological processes associated with sul-fate reduction.

Recently, relatively high concentrations of dissolved groupVI and other oxyanion-forming elements have been reportedfor waters in terminal basins of rivers draining from the eastfront of the Sierra Nevada, which include the Carson River[7,8]. For example, values as high as 0.1 mM and 0.7 mM forMo and W, respectively, have been found in terminal lakes ofthese aquatic systems. Our previous investigations of concen-trations and fate of Hg in the Carson River–Lahontan Reservoirsystem showed that, although the level of total-Hg (HgT) ishigh, concentrations of MeHg in both water and sediment arerelatively low, particularly in the artificial terminal basin (La-hontan Reservoir) where large values would be expected [9–11].

The present work was undertaken in order to (1) estimatethe effect of , , , and on rates of Hg22 22 22 22MoO WO SeO TeO4 4 4 4

methylation, especially at concentration levels similar to thosefound in most aquatic systems; (2) examine the impact of groupVI anions on microbial activity in sediment; and (3) evaluatethe effect of varying sulfate concentration on the inhibition ofHg methylation by group VI anions.

MATERIALS AND METHODS

Sample collection

Surface sediment samples (0–5 cm) were collected up-stream of ‘‘Table Mountain,’’ a sampling site in the CarsonRiver–Lahontan Reservoir system, located about 44 km down-stream of Carson City. The area under study and the samplinglocation have been described in detail elsewhere [9–11]. Sam-ples were collected using clean polyethylene shovels and put

Inhibition of mercury methylation by group VI anions Environ. Toxicol. Chem. 16, 1997 1569

in new Whirl-packt bags (Fisher Scientific, Fairlawn, NJ,USA). During the sampling and transportation period, sampleswere stored in the dark on ice in a cooler. In the laboratory,samples were refrigerated at 48C until processed.

Analyses

Methylmercury determinations were performed using amethod adapted from Horvat et al. [12]. Briefly, MeHg wasdetermined using a Hewlett-Packard 5890 gas chromatographwith electron capture detection (GC-ECD) and an FFAP cap-illary column (15-m long 3 0.53-mm i.d. 3 1.2- mm filmthickness). The detection limit was ;2 ng/g dry weight sed-iment, and the precision of the method was determined to be6.7% (n 5 6) (variation coefficient of six analyses of thesample collected from Table Mountain). Recovery of MeHgfrom spiked sediment samples averaged 93 6 3% (n 5 5).HgT in sediment samples was determined as described by Bon-zongo et al. [10]. Sediment water-extractable sulfate was de-termined by ion chromatography. The dry weight (dw) of sed-iment samples was determined by oven-drying preweighedsediment aliquots (n 5 5) at 608C until constant weight wasachieved.

Effect of , , , and on MeHg22 22 22 22MoO WO SeO TeO4 4 4 4

production

The effect of different concentrations of the above-men-tioned anions on Hg methylation was tested on homogenizedsediment samples, slurried in 1:10 ratios (equivalent of 1 gdw in 10 ml of double-distilled water). Background concen-trations of , HgT, and MeHg were 76 mM, 15 mg/g, and22SO4

23.4 ng/g, respectively. Slurries were used without addition ofbut spiked with inorganic Hg added as HgCl2 to a final22SO4

concentration of 46 mg/g dw (or 8.13 mg/ml slurry). Duplicateflasks were prepared with the following treatments: (1) acid-killed control, (2) nonkilled-control without addition of groupVI anions, and (3) slurries treated with increasing concentra-tions of each oxyanion. Molybdate (as Na2MoO4) and tungstate(as Na2WO4) were added separately to slurries to a final con-centration of 0.05, 10, 50, 500, 2,000, and 20,000 mM. Selenate(as Na2SeO4) was added to a final concentration of 0.0027,0.014, 0.054, 0.27, 1, and 20,000 mM, while tellurate (asNa2TeO4) was tested at final concentrations of 0.005, 0.05,0.25, 1, and 20,000 mM. All bottles were deoxygenated for 10min with purified N2 to establish an anaerobic condition toenhance the rate of sulfate reduction. Following an incubationperiod of 5 d in the dark at room temperature (;228C), wenoted that the sediment slurries gave off a characteristic odorof H2S, which is consistent with anaerobic conditions. Con-centrations of MeHg in each bottle were determined and ratesof Hg methylation expressed as amount of MeHg producedper dw of sediment per day:

ng/g/d 5 (M5 2 M0, ng MeHg)/(g dw sed.)/(5 d)(M5 2 M0) 5 mass of MeHg produced after 5 d

The effect of different anions was determined as percentinhibition of methylation rates with regard to unamended con-trols.

The probit analysis procedure was used to calculate theEC50 value (the concentration of anion that inhibits the Hgmethylation rate 50% after a specific incubation time).

Effect of , , , and on microbial22 22 22 22MoO WO SeO TeO4 4 4 4

electron transport system (ETS) activity

MeHg production in freshwater sediment is primarily at-tributed to SRB, while its degradation involves SRB, methane-producing bacteria (MPB), and other anaerobes [13]. If so, aninhibitory effect on SRB by the tested anions is expected toreduce MeHg production and eliminate the competition be-tween SRB and other bacteria. Accordingly, the microbial ac-tivity of non-SRB may increase, unless the group VI anionsalso affect other bacteria due to their toxicity. To examine howgroup VI anions affected the microbial populations of the sed-iment, we measured the general ETS activity according to themethod proposed by Trevors [14], in which an alternative elec-tron acceptor, 12-(p-iodophenyl)-3-(p-nitrophenyl)-5-phenyl tet-razolium chloride (INT), is added to sediment slurries (thetreatment and incubation are conducted in the same way asdescribed in the first experiment). After incubation, the productof its reduction (INT-formazan in this case) is extracted intomethanol and measured spectrophotometrically at 480 nm.

The selective inhibitor of MPB, 2-bromoethane sulfonicacid (BESA) was also used, as outlined in Gunsalus et al. [15],to assess the contribution of MPB to the overall ETS activityin sediment slurries treated with different concentrations ofeach anion. These experiments were performed the same wayas those without BESA.

Effect of different ratios of to group VI anion on Hg22SO4

methylation

The following experiment was carried out in order to de-termine whether , , , and were com-22 22 22 22MoO WO SeO TeO4 4 4 4

petitive or noncompetitive inhibitors of sulfate reduction andHg methylation. First, the relationship between concen-22SO4

tration and Hg methylation rates was examined in anaerobicsediment slurries. Inorganic Hg-spiked sediment slurries (spikeconcentration: 46 mg/g dw) were treated with (added as22SO4

Na2SO4) to the following final concentrations: 76 (without ad-dition), 150, 230, 300, 600, and 1,200 mM. Second, in orderto determine the effect of different ratios of to group VI22SO4

anions, sediment slurries containing 46 mg Hg/g dw and fixedconcentrations of tested anions (2,000 mM for and22MoO4

, and 1 mM for and ) were spiked with22 22 22WO SeO TeO4 4 4

increasing concentrations of . In both cases, sediment22SO4

slurries were deoxygenated with purified N2 and then incubatedin the dark at room temperature (;228C) for 5 d. MeHg pro-duction was then determined.

For all of the experiments described above, the incubationswere run in duplicate and the experiments were repeated atleast once. The statistical analyses were conducted using bothanalysis of variance (ANOVA) and the Student t-test to eval-uate the difference between treatments. The differences amongtreatments were determined at a significance level of p , 0.05.

RESULTS AND DISCUSSION

The effect of different concentrations of , ,22 22MoO WO4 4

, and on rates of Hg methylation in sediment22 22SeO TeO4 4

slurries are shown in Figure 1. The inhibition of Hg methyl-ation by and was found to occur at the nanomolar22 22TeO SeO4 4

level, while the inhibition by and occurs at the22 22MoO WO4 4

micromolar level. Overall, significant inhibition (p , 0.05) ofMeHg production was observed only in samples containing.50 nM of , .270 nM of , $100 mM of22 22TeO SeO4 4

, and $700 mM of , respectively.22 22MoO WO4 4

1570 Environ. Toxicol. Chem. 16, 1997 Y. Chen et al.

Fig. 1. Inhibitory effect of different concentrations of oxyanions on ratesof MeHg production. (a) ; (b) ; (c) ; (d) ; and22 22 22 22MoO WO SeO SeO4 4 4 3

(e) . Dashed lines show concentrations of oxyanion-forming ele-22TeO4

ments in natural aquatic environments. Vertical bars represent one stan-dard deviation and are not shown when smaller than the symbol.

Inhibition of mercury methylation by group VI anions Environ. Toxicol. Chem. 16, 1997 1571

Molybdate

The selective inhibitory effect of on SRB was first22MoO4

reported by Oremland and Taylor [4], who examined the eco-logical role of SRB in anoxic marine sediments. Because theaddition of 20 mM of per liter of slurry could com-22MoO4

pletely inhibit formation of sulfide, that concentration has beenused to examine the role of SRB in Hg methylation in bothestuarine and freshwater sediments [2,3]. On sediment col-lected from the Carson River, appeared to depress rates22MoO4

of MeHg formation at concentrations above 0.05 mM. How-ever, its inhibitory effect was statistically significant only atconcentrations higher than 100 mM (Fig. 1a). When 22MoO4

was added to sediment slurries at the highest concentrationused (20 mM), the rate of Hg methylation was 21% of the rateproduced in the control flask.

In oxic natural aquatic systems, molybdenum is thought tooccur primarily as the oxyanion , and its concentration22MoO4

rarely exceeds 0.1 mM [16,17]. Accordingly, would22MoO4

not be expected to inhibit significantly the production of MeHgin natural aquatic systems, except in contaminated environ-ments or in aquatic systems where geological materials areenriched in Mo and geochemical conditions are favorable toweathering reactions. In the Carson River–Lahontan Reservoirsystem, , which is found in concentrations up to 10022MoO4

mM, is unlikely to affect MeHg production significantly.

Tungstate

was found to be less inhibitory to MeHg production22WO4

than at high concentrations. At 20 mM, tungstate dem-22MoO4

onstrated the lowest inhibitory effect on Hg methylation(MeHg production was 39% of the control) as compared tomolybdate (21%), selenate (6%), and tellurate (,1%) (see Fig.1b). This observation may be related to several factors, in-cluding the fact that the recovery of SRB from inhibition22WO4

is more rapid than that of [5].22MoO4

However, as shown in Figure 1b, the concentration of tung-sten (700 mM) measured in the Carson River system is higherthan any of the other group VI anions examined and is suf-ficient to inhibit MeHg formation partially (p , 0.05), basedon these studies. Thus, although it is inherently less inhibitory,tungstate appears to be the only group VI anion found insufficient concentrations to inhibit Hg methylation in the Car-son River system.

Selenate

The effect of on Hg methylation rates is presented22SeO4

in Figure 1c. is a much stronger inhibitor in contrast to22SeO4

and and shows statistically significant inhibition22 22MoO WO4 4

at concentrations .270 nM. Alternatively, when was22SeO4

added to sediment slurries at a final concentration ,10 nM, aslight increase was observed in rates of MeHg production,which was not statistically significant (p . 0.05). Becauseselenium may also occur in the 14 oxidation state in naturalwaters, we tested the effect of selenite ( ) on Hg meth-22SeO3

ylation. The inhibitory effect of on Hg methylation was22SeO3

more consistent over the tested concentration range (0.65–1000nM), with about 40% inhibition at the highest concentration(l mM) (Fig. 1d). This difference may be related to the complexaqueous geochemistry of selenium as discussed below.

In addition to the toxicological and/or biological functionsof Se, it is also characterized by a complex cycling in theaquatic environment. Selenium may be present in natural wa-

ters in more than one oxidation state (22, 0, 14, and 16)[18]. When added as Se61 or Se41 in anoxic slurry, those oxy-anions may be biochemically reduced to elemental selenium(Se0) and ultimately to selenide (Se22) by anaerobic bacteria[19]. Despite the fact that Hg(II) was added to sediment slurriesin substantial quantities, its particle-reactive nature may sig-nificantly reduce its dissolved fraction, hence increasing theSe to Hg ratio. High concentrations of reduced Se may combinewith Hg(II) to form insoluble HgSe, which decreases the avail-ability of Hg(II). The observations from our experiments sug-gest that in addition to its toxic effect on bacteria at highconcentrations, selenium may negatively affect MeHg pro-duction by reducing the availability of Hg(II) to microbialprocesses due to the formation of insoluble HgSe. If so, thedifferences observed in trends obtained from selenate and sel-enite-treated slurries could, at least partly, be related to thedifference in rates of biochemical reduction of and22SeO4

to selenide.22SeO3

Relatively few data exist on the effect of Se on MeHgformation, and little is known about the influence of Se on Hgmethylation and other activities of natural microbial com-munities. Jackson [20] reported that the effects of Se on Hgmethylation were complex and inconsistent, as they may al-ternate between inhibition and stimulation over a wide rangeof Se concentrations. In those experiments, concentrations of50 mM Se or more were required to achieve total suppressionof Hg methylation. Our results were consistent with the pre-vious results in that 1 mM resulted in a Hg methylation22SeO4

rate of 66% of the controls and, at the highest concen-22SeO4

tration used (20 mM), only 6% of the control production ofHg methylation was observed. These observations suggest that

could behave just like at high concentration in22 22SeO SO4 4

that the reduction of this oxyanion to selenide would reducethe availability of Hg21 by forming insoluble HgSe, therebyinhibiting Hg methylation. If concentrations of Hg21 are notlimited for biomethylation, the effect of Se on Hg methylationmay depend on both the nature of microorganisms present inthe sediment and the length of the incubation period. Signif-icant inhibition of both sulfate reduction and Hg methylationby group VI anions was observed for incubation periods short-er than 5 d [5,10].

Tellurate

Tellurium and selenium are closely related chemically andusually associated with each other in the environment. In ourexperiments, the effect of mimics that of (Fig.22 22TeO SeO4 4

1c and e). However, within the concentration ranges tested,the inhibitory effect of was always more pronounced22TeO4

than that of . The similarity between trends of22 22SeO TeO4 4

and suggests the existence of similar mechanisms in22SeO4

their inhibitory effects on Hg methylation (i.e., reduction ofHg[II] availability by formation of nonsoluble HgTe).

Despite the fact that Te concentrations may be high in aquat-ic systems surrounded by Te-rich geologic materials affectedby geochemical weathering reactions or in systems impactedby anthropogenic activities, Te exists in most aquatic envi-ronments in concentrations below 0.6 nM [21]. Accordingly,a substantial inhibitory effect of tellurium on MeHg formationis unlikely in most natural systems.

Banat et al. [5], estimated the inhibitory effect of group VIanions on sulfate reduction in anoxic marine sediment andsuggested that the degree of inhibition was in the order

. . , at concentrations of 10 and 20 mM.22 22 22MoO SeO WO4 4 4

1572 Environ. Toxicol. Chem. 16, 1997 Y. Chen et al.

Table 1. EC50 values for the inhibition of methylation rates by groupVI anions in sediment slurries

Group VI anion Unit EC50 6 CVa (%)

TeO224

SeO224

MoO224

WO224

mMmMmMmM

1.99 6 15.715.9 6 15.31.51 6 19.819.9 6 14.6

a Mean 6 coefficient of variance.

Fig. 2. Effect of different concentrations of group VI anions on mi-crobial electron transport system (ETS) activity. (a) ; (b)22TeO4

; (c) ; and (d) . Vertical bars represent one standard22 22 22SeO MoO WO4 4 4

deviation and are not shown when smaller than the symbol.

The EC50 value of various anions for methylation rates esti-mated in our investigation shows that the degree of inhibitionon Hg methylation is in the order . .22 22 22TeO SeO MoO4 4 4

. within the concentration range tested (Table 1). Over-22WO4

all, although and are the most inhibitory group22 22TeO SeO4 4

VI anions (based on EC50 values), they were not found in theCarson River at sufficient concentrations to expect significantinhibition of Hg methylation. may marginally affect22MoO4

Hg methylation at the concentrations observed in the CarsonRiver, and the concentration of appears to be sufficiently22WO4

high that it is likely to exert a small inhibitory effect on MeHgproduction. These data were obtained from laboratory studiesthat examined individual group VI anions, and additive andinteractive effects of these substances have not been examined.If, for example, and act in an additive manner,22 22WO MoO4 4

due to potentially similar mechanisms of action, the overallinhibitory effect may be larger than predicted, based on as-sessments of the individual substances. However, in general,our results suggest that the tested oxyanions could negativelyaffect MeHg production only in aquatic systems with anom-alously high concentrations of these oxyanions.

Effect of , , , and on microbial22 22 22 22MoO WO SeO TeO4 4 4 4

ETS activity

The effect of the group VI anions on ETS activity wasdetermined (Fig. 2). Activities were determined on sedimentslurries incubated either with or without the addition of a meth-anogen inhibitor, BESA. The microbial ETS activity in sedi-ment slurries treated with , , and appeared22 22 22MoO SeO TeO4 4 4

to decrease with increasing concentrations of anions. The low-est concentrations of these three anions were previously shownto inhibit methylation (Fig. 1). However, the lack of significantdifferences in microbial ETS activity between BESA-treatedand nontreated slurries when , , and were22 22 22TeO SeO MoO4 4 4

present suggests that a general microbial toxicity may be in-volved (Fig. 2a–c), although this effect is probably minor.

Samples treated with showed a different trend (Fig.22WO4

2d). Tungstate alone had the lowest inhibitory effect on mi-crobial ETS activity, and it was significantly different than thecombination of and BESA (p , 0.05). The absence of22WO4

inhibition on the microbial ETS activity by may be22WO4

partly attributable to the fact that microorganisms are able todiscriminate between the lower weight group IV anions and

, which is the heaviest of the oxyanion-forming elements22WO4

tested. This same effect was previously observed in the relativeranking of Hg methylation activity; was the least in-22WO4

hibitory of the four anions examined (Table 1).Because sulfate reducers and methanogens in sediment may

compete for the same carbon sources and electron donors [4],the inhibition of sulfate reducers by group IV anions maychannel the flow of electrons toward the methanogens. In thiscase, the total microbial ETS activity of sediment would not

significantly decrease, except by the addition of an inhibitorof methanogens. However, except for , the addition of22WO4

BESA only slightly increases the inhibition of microbial ETSactivity, compared to the addition of the group IV anions alone.This result may reflect a lack of significant increase in substrateavailability to methanogens when sulfate reduction is inhib-ited, although it does not exclude the possibility of a smallgeneral toxicity effect. Consequently, our studies suggest thatthe effects of group VI anions will likely occur to particularmicrobial species, such as sulfate-reducing bacteria, or to par-ticular types of biochemical activity rather than generally causean effect on the entire microbial community. These results areconsistent with previous studies [3–5].

Sulfate–group VI anion interaction

When increasing concentrations of were added to sed-22SO4

iment slurries spiked with fixed concentrations of each inhib-itor, the inhibition of methylation rate was decreased whentellurate, molybdate, and tungstate were present but increasedwhen selenate was present (Fig. 3).

Increasing sulfate concentration had the greatest effect onreduction of inhibition, and the highest concentration22WO4

of sulfate (1,200 mM) reduced the inhibition on Hg22WO4

methylation significantly (p , 0.05) from 20% to 5.6%. Thesedata suggested, therefore, that inhibition of Hg methylation bytungstate was a competitive process because the inhibition ofmethylation rate was reduced by a factor of 3.6 (Table 2).Banat and Nedwell [5] have also concluded that the inhibitionof sulfate reduction by tungstate was by a competitive mech-anism, and the ratio of rate of sulfate reduction at 60 mM tothat at 30 mM is 3.9. However, there was only a marginallysignificant decrease of percent inhibition of methylation rateat the highest sulfate concentration (1,200 mM) when mo-lybdate and tellurate were present, and the decrease in percentinhibition of methylation rate was only by a factor of 1.1 (Table

Inhibition of mercury methylation by group VI anions Environ. Toxicol. Chem. 16, 1997 1573

Fig. 3. Effect of increasing concentrations of on the inhibition22SO4

of Hg methylation by group VI anions. Vertical bars represent onestandard deviation and are not shown when smaller than the symbol.

Fig. 4. Effect of different concentrations of (a) and (b)22 22SO SeO4 4

on MeHg production. Vertical bars represent one standard deviationand are not shown when smaller than the symbol.

Table 2. Effect of varying sulfate concentration on the methylationrate inhibited by group VI anions

SO concn. (mM)224

Percent inhibition of Hg methylation rateby addition of group VI anions

TeO224

(1 mM)SeO22

4

(1 mM)

MoO224

(2,000mM)

WO224

(2,000mM)

761,200Ratio of inhibition

76 mM/1,200 mM SO224

32.729.1

1.12

32.340.7

0.79

56.047.4

1.18

20.25.62

3.59

2). These data suggested that molybdate and tellurate inhibitedHg methylation by a noncompetitive mechanism.

The increase of concentrations appeared to exacerbate22SO4

the inhibitory effect of on Hg methylation (Fig. 3 and22SeO4

Table 2). This latter observation suggests that may play22SeO4

the same role as . Addition of high concentrations of both22SO4

and may result in an additive inhibitory effect on22 22SeO SO4 4

Hg methylation due to formation of both HgSe and HgS, there-by, reducing Hg bioavailability.

The trend of the effect of on Hg methylation in sed-22SO4

iment collected from the Carson River was similar to predic-tions previously reported by Gilmour and Henry [1] (Fig. 4a).MeHg production from added HgCl2 increased with the ad-dition of up to a concentration of 225 mM. This trend22SO4

was reversed at concentrations higher than 225 mM.22SO4

MeHg formation may be inhibited by the precipitation of bio-available Hg21 as HgS [1]. Selenate, tested at a much lowerconcentration range (0–1 mM), showed a similar trend to thatof (Fig. 4b). Assuming that Hg bioavailability for meth-22SO4

ylation is in excess in the slurry, in comparison with addedSe, the observed inhibition may be partly attributable to tox-icity of Se on microorganisms [10]. This observation is sup-ported by the fact that no significant differences were observedon microbial ETS activity in sediment slurries treated or nottreated with BESA (Fig. 2d).

Acknowledgement—We are grateful to K. Heim for his help in samplecollection and to P. Lechler for the analysis of total-Hg. This workwas supported by National Science Foundation Grant-EAR-9528589and National Institute of Environmental Health and Sciences ContractP42ES05961

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