Stability of Dimethyl Mercury in Seawater and Its Conversion to

Publication Date (Web): April 29, 2009. Copyright © 2009 American Chemical Society. * Corresponding author phone: (831) 459-5336; fax: (831) 459-3524...
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Environ. Sci. Technol. 2009, 43, 4056–4062

Stability of Dimethyl Mercury in Seawater and Its Conversion to Monomethyl Mercury F R A N K J . B L A C K , * ,†,‡ CHRISTOPHER H. CONAWAY,† AND A. RUSSELL FLEGAL† WIGS Laboratory, Department of Environmental Toxicology, University of California, Santa Cruz, California 95064, and Henry Oppenheimer Okavango Research Centre, University of Botswana, Private Bag 285, Maun, Botswana

Received January 17, 2009. Revised manuscript received March 19, 2009. Accepted March 23, 2009.

Dimethyl mercury (DMHg) is commonly detected in the world’s oceans, but little is known about the mechanisms responsible for DMHg degradation in natural waters or the products of this degradation. Similarly, the potential for the conversion of DMHg to monomethyl mercury (MMHg) under the acidic conditions commonly used to preserve samples for MMHg analysis has not been fully addressed. We provide evidence suggesting that DMHg in natural seawater is not readily photodegraded by sunlight as previously thought. Other experiments demonstrated that DMHg in seawater is, however, readily decomposed under acidic conditions, with MMHg as the predominant product. This facile conversion of DMHg to MMHg at low pH both necessitates an alternative preservation method to acidification for samples to be analyzed for MMHg when DMHg is present, and requires that data from previous studies of MMHg in seawater employing sample acidification be revisited in instances where appreciable DMHg concentrations were possible.

Introduction Dimethyl mercury (DMHg) is prevalent in the intermediate and deep waters throughout the oceans, where it is often the dominant methylated form of mercury (1-8). DMHg is typically found at depth in the oceanic water column at subpicomolar concentrations, but is generally not detectable in the mixed layer or coastal waters, with the exception of surface seawater in regions of the high Arctic (8), Antarctic, Atlantic (9), and upwelled waters in coastal California (10). Processes that might act to keep DMHg concentrations low in the upper waters of the ocean include evasion (degassing) of DMHg to the atmosphere, photodegradation, thermal decomposition, and/or biotic degradation pathways mediated by microbes, phytoplankton, or extracellular biomolecules originating from organisms in surface waters. Thermal instability and photodegradation are often cited to be among the more important of these processes based upon calculated diffusive flux rates, incubation experiments, and the reported instability of DMHg (1, 3, 5, 6, 11). However, many studies to date on the stability and degradation of DMHg employed Teflon containers, and * Corresponding author phone: (831) 459-5336; fax: (831) 4593524; e-mail: [email protected]. † University of California. ‡ University of Botswana. 4056

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recent work by Parker and Bloom (12) reported that Teflon bottles are unsuitable for storing DMHg over time scales of even hours. As a result, the losses of DMHg observed during bottle incubation experiments in previous studies using Teflon or other plastic bottles were likely due, at least in part, to absorptive losses or diffusion of DMHg out of the bottles rather than degradation, and thus the stability of DMHg in seawater warrants revisiting. Although the source of DMHg in the marine environment is currently unknown, the degradation of oceanic DMHg may represent a source of monomethyl mercury (MMHg) to the marine environment (1, 3, 6). Indeed, it has been suggested that the degradation of DMHg alone might be able to account for the concentrations of MMHg measured in the ocean (1), although the importance of coastal/shelf derived MMHg has more recently been emphasized (13). MMHg is the form of mercury of most concern for ecological and human health because of its ability to be biomagnified in aquatic food chains to potentially toxic levels in fish and piscivores that are consumed by humans and wildlife (13, 14). If the dominant mechanism responsible for the decomposition of DMHg in the oceans at large, and for maintaining the low concentrations of DMHg in surface waters in particular, was photodegradation, this process could represent an important source of MMHg to surface waters, if MMHg was the major product (15, 16). MMHg is itself photodegraded (17, 18), and thus the potential for DMHg degradation in the photic zone to be a source of MMHg in surface waters of the ocean would depend upon the ratio of the rate constants for the photodegradation of DMHg and MMHg, as well as those processes responsible for the production or transport of DMHg to surface waters. The stability of DMHg and its potential conversion to MMHg are as poorly understood under laboratory conditions as they are in natural waters. While the artifactual formation of MMHg as a result of inorganic Hg(II) being methylated by dissolved organic matter during sample distillation is now widely acknowledged (19, 20), the artifactual formation of MMHg due to the decomposition of DMHg under acidic conditions has been largely overlooked, despite reports dating to the early 1990s. (11, 21). One of the most commonly used preservation techniques employed for MMHg samples is acidification to ∼0.05 M HCl for freshwaters or ∼0.02 M H2SO4 for saline waters (12, 22). Because MMHg is the predominant methylated form of mercury in freshwaters (where DMHg is generally absent, albeit rarely measured), the acidification of freshwater samples to be analyzed for MMHg likely poses no methodological problem. However, in seawater MMHg concentrations are lower (below the ∼0.05 pM detection limit for most studies) than those in freshwater, and DMHg concentrations are considerably higher (subpicomolar concentrations) (1-7). As a consequence, the acidification of seawater samples could lead to the measurement of erroneously high MMHg concentrations, if this acidification resulted in substantial conversion of ambient DMHg to MMHg prior to analysis. To address these unknowns we conducted experiments to (1) assess rates of DMHg photodegradation in seawater, (2) assess the stability of DMHg in seawater under the acidic conditions commonly used for the preservation of MMHg samples, and (3) determine if MMHg is a major product of DMHg degradation under acidic conditions or as the result of photodemethylation. 10.1021/es9001218 CCC: $40.75

 2009 American Chemical Society

Published on Web 04/29/2009

Experimental Section Sample Collection. Unfiltered seawater samples were collected from three locations in Monterey Bay, CA (Figure SI1 of the Supporting Information) on May 27, July 7, and July 29, 2008 from the Monterey Bay Aquarium Research Institute vessel R/V Point Lobos. Waters from Monterey Bay were used for these DMHg experiments because of their sufficiently high concentrations of naturally occurring DMHg (∼0.5 pM), which made the addition and handling of concentrated solutions of the highly toxic DMHg unnecessary. Sample casts were made between the surface and 200 m using a Sea-Bird 911plus CTD mounted in a SBE-32 Sea-Bird Carousel Water Sampler, with a 12 place rosette with Ocean Test Equipment 10 L Niskin bottles (Niskin bottle cleaning and tests for contamination are described in the Supporting Information). Water from the Niskin bottles was transferred on deck into acid-cleaned 1 or 2.5 L amber glass bottles with Teflon-lined caps (I-CHEM) and then transported in the dark on ice to the laboratory, where they were stored in the dark at 4 °C. Methods used for measuring other water quality parameters at these sites are given by Pennington and Chavez (23), and the relevant data are available in the Supporting Information. DMHg Photodegradation Experiments. Samples for the photodegradation experiments were carefully poured into either clear 260 mL borosilicate glass bottles (I-CHEM) with Teflon-lined caps (June 2, 2008) or 300 mL Pyrex glass bottles (July 10, 2008), all of which had been acid cleaned. Samples used to assess the initial concentration of DMHg or MMHg were also poured into these sample bottles before analysis to standardize sample handling across all treatment groups. Bottles were filled completely to avoid any headspace. Dark treatments were covered in two layers of 0.024 mm thick aluminum foil. Bottles for both light and dark treatments were randomly interspersed in shallow water baths (3 cm deep DI water) in fiberglass trays placed in sunlight in Santa Cruz, CA (36°59′56′′ N, 122°03′42′′ W) (Figure SI1 of the Supporting Information). The water baths were positioned away from any shading objects, and the bottles were placed at sufficient distances to prevent shading each other throughout the day. The temperatures of the baths were maintained between 9 and 14 °C during June 2, 2008, experiments to approximate surface water conditions in Monterey Bay, and between 14 and 19 °C during the July 10 experiments in order to test if higher, but still environmentally relevant, temperatures would facilitate the decomposition of DMHg (11). On June 2, 2008, samples were left in the sun for 10.5 h, while on July 10, 2008, samples were exposed to sunlight for 8.5 h. Both June 2 and July 10, 2008 were clear, near cloudless days. Solar irradiance was monitored and recorded for individual wavelengths with scans from 320 to 750 nm at 0.5 nm intervals every thirty minutes during the experiments using a calibrated HydroRad hyperspectral radiometer (HOBI Laboratories). All experiments were ended after the desired exposure time by sparging and measuring DMHg immediately using the methods described below. Measurements of light transmittance by the bottles and light absorption by the seawater used were made for 280-700 nm at 1 nm intervals using a spectrophotometer (Beckman DU530). Decomposition of DMHg to MMHg due to Acidification. Water samples were collected in triplicate from 200 m depths from three Monterey Bay sites (C1, M1, and M2) on July 8, 2008, and from 150 m depths at M1 and M2 on July 29, 2008. All sample containers were briefly opened in the laboratory (less than 5 s), and half of the replicates from each site were acidified to 18 mM H2SO4 (trace metal grade) as per standard preservation recommendations for MMHg (12, 22). These samples were stored in the original amber glass bottles with Teflon-lined caps in the dark at 4 °C until DMHg analysis 4 days later and MMHg analysis within 3 months. The other

half of the replicates (those not acidified) from each site were stored in the dark at 4 °C until DMHg analysis 4 days later, with the analyzed water (now DMHg free) then transferred to acid-cleaned Teflon (PFA) or amber glass bottles and preserved by acidification for later MMHg analysis as above. MMHg and DMHg Measurements. For DMHg analysis, samples were transferred to acid-cleaned 1 L glass bubblers and gas purged (sparged) with high purity nitrogen gas for 20 min at 560 mL min-1 onto Carbotrap. Determinations of DMHg on Carbotraps were performed by isothermal gas chromatography cold vapor atomic fluorescence spectrometry (GC-CVAFS) using established methods (24). Sparging of samples to remove all DMHg prior to acidification and subsequent analysis for MMHg when desired was done by purging with high purity nitrogen in the same manner as during DMHg analyses. Because of the acute toxicity of DMHg, MMHg standards were used for producing calibration curves for DMHg quantification with the assumption that peak area response would be the same in the GC-CVAFS method for the two mercury species, which is appropriate given the quantitative nature of the analyses and because both MMHg and DMHg are thermally degraded to Hg0 prior to detection. Carbotraps loaded with DMHg provided by Nicolas Bloom (Studio Geochimica, Seattle, WA) were analyzed to identify the retention time of DMHg in our chromatograms and confirm that the peak response of DMHg and MMHg was indeed comparable. It was feared that sparging seawater acidified with H2SO4 through Carbotraps might result in acid fumes damaging the Carbotrap material. Therefore, the analysis of DMHg in samples that had been previously acidified was done by connecting the outlet of a first bubbler containing the sample to the inlet of a second bubbler filled with 150 mL Milli-Q water, with a Carbotrap placed on the outlet of this second bubbler. This setup allowed for the water in the second bubbler to act as an acid fume trap, while still allowing for the quantitative recovery of DMHg, as demonstrated in prior tests demonstrating no difference in the DMHg concentration of samples when analyzed using the single or double bubbler setup (data not shown). There were no detectable DMHg blanks associated with Niskin bottles, glass sampling or purging bottles, Carbotraps, or instrumental analysis using our methods. Breakthrough of DMHg during purging, tested by using additional Carbotraps in series, was not observed. Sparging efficiency was measured by purging samples for an additional 20 min, which yielded no additional DMHg. The DMHg detection limit, determined as 3 times the standard deviation of multiple analyses of 20 pg MMHg standards, was 2 pg, giving a volume specific detection limit of 0.01 pM for the 1 L seawater samples. Precision on sample triplicates (n ) 6) and duplicates (n ) 11) for DMHg was better than 10%. All MMHg concentration measurements were carried out within three months of the experiments and were made on 45 or 80 mL aliquots by distillation, aqueous phase ethylation, separation by gas chromatography, thermal decomposition, and quantification by cold vapor atomic fluorescence spectrometry (CVAFS), using established methods (22, 24). Each set of up to 20 samples distilled was accompanied by at least two distillation blanks (Milli-Q water amended to 0.1 M KCl and 18 mM H2SO4) and two MMHg matrix spikes. The average daily MMHg detection limit was 0.04 pM, calculated as either 3 times the standard deviation of distillation blanks or, when distillation blanks had no quantifiable MMHg peak, as 3 times the standard deviation of multiple ethylations and analyses of a 1.0 pg MMHg standard, and then converted to a volume-based detection limit. MMHg blanks associated with Niskin bottles, sample bottles, and glass bubblers were all were within the range of MMHg distillation blanks. MMHg VOL. 43, NO. 11, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 1. Results from the DMHg photodecomposition experiment conducted June 2, 2008, on a clear cloudless day in Santa Cruz, CA, using unfiltered Monterey Bay seawater from 150 m depth. There was no significant change (p ) 0.79, ANOVA) in the DMHg concentration in either dark controls or in samples exposed to sunlight in borosilicate bottles for 10.5 h (61.9 E m-2 for 320-700 nm). Error bars represent the standard deviation about the mean of replicates (n ) 4-8). matrix spike recoveries (n ) 9) ranged from 85% to 107%, with an average (mean ( standard deviation) of 95 ( 8%. Experiments demonstrating no artifactual MMHg formation during distillation and verifying concentrations of MMHg stock solutions are described in the Supporting Information.

Results and Discussion DMHg Photodegradation Experiments. We found no evidence for either the photoproduction or dark production of DMHg, as evidenced by the lack of detectable DMHg (detection limit 0.01 pM) in any samples that were sparged, then either exposed to sunlight for 10.5 h (n ) 8) or covered with aluminum foil during the experiment (n ) 6), and then analyzed for DMHg. Although a zero net change in DMHg concentration could have resulted from the simultaneous production and decomposition of DMHg at similar rates, the negligible or nondetectable ambient concentrations of MMHg in the marine waters used (300 nm) over the course of a cloudless solar day when compared to dark controls (p ) 0.54, ANOVA). Measurements of solar irradiance (Figure SI2 of the Supporting Information) demonstrated that UV levels were appreciable during these photoexperiments, and irradiance for 320-700 nm was 61.9 E m-2 during the 10.5 h experiment on June 2, 2008, and 52.6 E m-2 during the 8.5 h experiment on July 10, 2008. The observation that DMHg was not photodegraded in seawater in any of our experiments is in contrast to the results of previous studies that reported a decrease in DMHg concentration following exposure of seawater to sunlight (6, 21). It is possible that the absence of measurable DMHg photodegradation during our experiments was due to differences in some chemical and/or biological factor(s) between the seawater used in the different studies. It may also be that the light responsible for the decomposition of DMHg consists of wavelengths between 290 and 300 nm, which are not transmitted as effectively by the Pyrex bottles used in our study as the Teflon bottles used in previous studies (Figure 2). While we cannot preclude the possibility that DMHg was photodegraded during previous studies (6, 21), it is quite possible that the decrease in DMHg concentration measured in those studies was due, at least in part, to the diffusion and loss of gaseous DMHg out of the Teflon containers used or

its absorption to the Teflon, as reported for Hg(0) (28). A recent study of methods and materials for the collection, preservation, and analysis of different species of mercury in aqueous media (12) concluded that Teflon bottles are unsuitable for storing DMHg over time scales of multiple hours or longer (unfortunately, the type of Teflon used was not reported in that study). This is in contrast to evidence from our own laboratory (data not shown), in which DMHg concentrations in seawater samples are stable for many days to weeks when stored in the dark in amber glass bottles at 4 °C. Thus, while most types of Teflon bottles exhibit greater transmission of light below 300 nm than most glass bottles (Figure 2), they both transmit less UVA, and their absorption or permeability to DMHg makes them unsuitable for DMHg photodegradation or other experiments involving storage times of multiple hours or longer. The loss of DMHg from the Teflon bottles used by Mason and Sullivan (6) (the type of Teflon used was not reported in this study either) could account for the “bottle effect” they described, the decrease in DMHg concentration reported even in deep water samples stored in the dark at 4 °C during their experiments, and why the highest rates of DMHg loss were not measured in their treatment group exposed to the most sunlight. Using the published results of the (poor) suitability of Teflon for DMHg experiments (12) to re-evaluate or correct the results of previous experiments on DMHg photodegradation (6) is problematic because the experiments are not directly comparable because their experimental conditions (type of Teflon, size of bottles used, bottle wall thickness, temperature, and other experimental variables that would play a role in the absorption or diffusion of DMHg out of bottles) were generally not reported. In addition, it was not reported if treatments were performed in replicate, nor was any measurement of the variance or error structure reported. As a result, the rate constants reported for the various groups can not be compared in a meaningful way to identify statistically significant differences and exclude the possibility that any differences were within experimental error. If one overlooked the methodological concerns of using Teflon bottles and assumed that DMHg can be photodegraded in seawater as reported in previous studies but is not degraded by wavelengths >300 nm as indicated by our experiments, then the photodegradation of DMHg would have to be limited to radiation with wavelengths of less than 300 nm. Such an assertion would seem unlikely, however. Although DMHg in the gas phase absorbs light and is photodegraded by wavelengths below 270 nm (29, 30), it does not absorb light between 270 and 350 nm in either the gas phase or in an aqueous solution (31, 32). This suggests that direct photolysis of DMHg is unlikely at sea level, where incident sunlight is essentially limited to wavelengths >290 nm (33). If DMHg photodegradation was limited to wavelengths between 290 and 300 nm, it would also suggest that the secondary oxidants most often implicated in the photodegradation of other organic compounds in natural waters are not involved in the photodegradation of DMHg. The quantum yield of such secondary reactive photointermediates is often higher for wavelengths in the range 290-300 nm, but wavelengths >300 nm are still capable of generating appreciable levels of hydroxyl radicals, singlet oxygen, super oxide, hydrogen peroxide, alkylperoxyl radicals, hydrated electrons, photosensitized DOM species, and other reactive photointermediates in natural waters (e.g., 34-38). Consequently, the possibility that DMHg is photodegraded in seawater by natural sunlight limited to wavelengths of 290-300 nm by direct or indirect photochemical mechanisms appears improbable. The observed stability of DMHg in the presence of sunlight over the course of a day in our experiments suggests that the mechanisms responsible for its loss or decomposition in the

FIGURE 3. Unfiltered Monterey Bay seawater (collected July 8, 2008) sample subsets sparged of DMHg before being acidified prior to MMHg analysis had significantly lower MMHg concentrations (p , 0.001, paired t test) than corresponding sample subsets that were not sparged prior to acidification and MMHg analysis. Error bars represent the standard deviation about the mean of replicates (n ) 3). The dashed horizontal line denotes the MMHg detection limit of 0.04 pM. ocean above the thermocline warrant revisiting. If DMHg is not easily photodegraded as previously believed, other processes must play a role in maintaining the low DMHg concentrations found in the mixed layer of the world’s oceans compared to intermediate and deep waters. Given the low solubility of DMHg in water and its high Henry’s law constant [0.31 for (DMHg)(g)/(DMHg)(w) at 25 °C (39)], a likely loss mechanism for DMHg from oceanic surface waters is the evasion of gaseous DMHg to the atmosphere. The estimation of DMHg fluxes across the water-air interface is hampered by a lack of information on the concentration of DMHg in marine air. For example, DMHg concentrations in air measured in Santa Cruz, CA, adjacent to Monterey Bay were below the limit of detection (∼0.01 ng m-3) on multiple occasions using methods described elsewhere (40). Nonetheless, it is clear that the evasion of DMHg from the ocean to the atmosphere is very favorable. At a DMHg concentration of 0.10 pM for surface waters in Monterey Bay during the upwelling season (10), the direction of DMHg flux would still be from the sea to the atmosphere even at concentrations of DMHg in marine air as high as 7 ng m-3. This value is nearly 3 orders of magnitude greater than both our detection limit (0.01 ng m-3) and atmospheric DMHg concentrations reported elsewhere in the marine environment (41). Instantaneous diffusive fluxes of DMHg from upwelling waters in Monterey Bay in early summer were calculated (see the Supporting Information for a full description) after Wanninkof and McGillis (42) and St. Louis et al. (8) assuming an atmospheric DMHg concentration of 0.01 ng m-3 (41) and a surface seawater DMHg concentration of 0.10 pM (10). Calculated instantaneous DMHg fluxes to the atmosphere are highly dependent upon wind speed (42) and vary from 0.035 ng m-2 hr-1 for a short-term wind speed of 4 m s-1 to 0.95 ng m-2 hr-1 at wind speeds of 12 m s-1. These DMHg fluxes are similar to those estimated from seawater in the high Arctic (8) as well as the Antarctic and Atlantic Oceans (9). Comparison of DMHg Evasion to Photodecomposition in Oceanic Surface Waters. To evaluate the relative importance of evasion versus photodegradation as loss mechanisms for DMHg in the upper waters of the ocean, we present the following hypothetical situation in which a parcel of subsurface Monterey Bay seawater from 150 m (representing the water used in our photodegradation experiments) is displaced to the upper 40 m of the surface water zone (roughly VOL. 43, NO. 11, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 4. Decrease in DMHg concentration due to acidification and decomposition of DMHg (A) was proportional to the increase in MMHg concentration measured in the same samples when not sparged before acidification (B) due to the conversion of DMHg to MMHg at low pH. The same unfiltered Monterey Bay seawater samples from 150 m collected on July 29, 2008, were used in both experiments. (A) DMHg concentrations measured in nonacidified samples were significantly higher (p , 0.001, paired t test) than in the same samples when acidified prior to DMHg analysis. Acidified samples had no detectable DMHg (detection limit 0.01 pM) after a 4 day storage period, so the open bars cannot be seen. (B) Concentrations of MMHg measured in samples not sparged of DMHg prior to acidification were significantly higher (p , 0.001, paired t test) than in the same samples when sparged of DMHg prior to acidification. Error bars represent the standard deviation about the mean of replicates (n ) 3). Dashed horizontal lines denote detection limits for DMHg (0.01 pM) and MMHg (0.04 pM).

the depth of the mixed layer and euphotic zone). Assuming a DMHg concentration in seawater of 0.15 pM (10) and a wind speed of 10 m s-1, the DMHg evasive flux would constitute a DMHg loss rate of 0.010 d-1 for this parcel of water. Because we did not detect any DMHg photodegradation, to estimate a photodegradation rate constant for our hypothetical situation we assumed a conservative rate constant of 4.8 × 10-4 E-1m2, which is half of the DMHg photodegradation rate at which we had a power of 0.8 to quantify a statistically significant loss of DMHg at the p ) 0.05 level. We used an incident sunlight irradiance of 65 E m-2 d-1 for 290-700 nm with a spectra similar to that shown in Figure SI2 of the Supporting Information, made the conservative assumption that all light of 290-700 nm is equally responsible for any DMHg photodegradation, and calculated the attenuation of light with depth in the water column only with respect to clear seawater (26). We calculated a DMHg loss rate due to potential photodecomposition of 0.011 d-1 for our hypothetical 40 m parcel of water. The favorable comparison between the rates of DMHg evasion (0.010 d-1) and potential photodegradation (0.011 d-1) supports our conclusion that photodegradation is not the dominant loss mechanism for DMHg in upper waters of the ocean, and that the evasion of DMHg to the atmosphere is at least as important. The fate of DMHg lost via gas exchange across the water-air interface has been little studied, but DMHg has been shown to be rapidly degraded in the gas phase by hydroxide radicals and chloride (15, 16), with MMHg being a major product. Thus, the evasion of volatile DMHg from marine surface waters and its subsequent decomposition to MMHg in the atmosphere could represent a source of MMHg to surface waters and adjacent terrestrial systems if deposited before the MMHg is itself decomposed. Such a process might also account for the MMHg found in rainwater, the source of which is currently unknown. However, there are reasons to doubt that the evasion of DMHg from the marine environment and its subsequent conversion to MMHg in the atmosphere could account for the majority of MMHg found in precipitation in marine or terrestrial systems, as discussed elsewhere (43). Instead, DMHg once released in the atmosphere may be rapidly degraded by nitrate radicals 4060

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(44) or atomic oxygen (45) with inorganic Hg(II) as the predominant product. Conversion of DMHg to MMHg due to Acidification. Concentrations of MMHg measured in seawater collected from 200 m depths from each of three locations (C1, M1, and M2) in Monterey Bay on July 8, 2008, were significantly lower (p , 0.001, paired t test) when analyzed for MMHg after sparging off any DMHg present prior to acidification, compared to when not sparged prior to acidification (Figure 3). Unsparged, acidified samples had an average MMHg concentration (mean ( standard deviation) of 0.53 ( 0.04 pM, while sample subsets thoroughly sparged prior to acidification had an average MMHg concentration of only 0.07 ( 0.07 pM. The higher concentration of MMHg measured in samples not sparged prior to acidification and analysis was not due to the loss of some volatile MMHg complex during sparging. This was demonstrated in experiments in which Monterey Bay seawater samples were sparged of DMHg, spiked to 10 pM MMHg, allowed to equilibrate for one hour, and then sparged a second time (30 min at 200 mL/min with high purity nitrogen) onto either Tenax (n ) 5) or Carbotrap (n ) 5) packing material. When analyzed, none of these traps had detectable levels of any form of mercury. Five of those samples spiked with MMHg and then sparged were subsequently distilled and analyzed for MMHg. MMHg spike recoveries were 94 ( 7%, which is within the range of spike recoveries for unsparged samples, demonstrating no measurable loss of MMHg upon sparging. The higher MMHg concentrations measured in unsparged sample subsets were the result of the conversion of DMHg to MMHg under acidic conditions. This conclusion was substantiated by separate experiments demonstrating a concurrent decrease in DMHg following acidification with H2SO4, which was proportional to the increase in MMHg concentration. Such experiments were initially conducted with Monterey Bay seawater collected on July 8, 2008, and then repeated using Monterey Bay seawater collected on July 29, 2008. When those unacidified samples (n ) 6) were stored in the dark at 4 °C for 4 days and analyzed for DMHg, the average DMHg concentrations (mean ( standard deviation) for sites M1 and M2 were 0.24 ( 0.04 pM and 0.28 ( 0.09 pM, respectively. These DMHg concentrations were significantly

greater (p < 0.001, paired t test) than for subsets of the same samples that were acidified prior to analysis (n ) 6), despite also being stored for 4 days in the dark at 4 °C (Figure 4). In fact, DMHg was not detectable in any of these acidified replicates (DMHg detection limit of 0.01 pM), demonstrating the facile decomposition of DMHg under acidic conditions (pH < 3) over a matter of days. The decrease in DMHg in these unfiltered seawater samples due to its degradation when acidified was proportional to and statistically indistinguishable (p ) 0.36, ANOVA) from the increase in MMHg concentration measured in the same samples, when comparing treatment groups that were either sparged or not sparged before acidification (Figure 4). Thus, the acidification of samples resulted in a decrease in DMHg that was accompanied by an increase in MMHg of roughly the same amount (Figure 4), with a mass balance revealing that 82 ( 35% of the decomposed DMHg was converted to MMHg. Taken together, we conclude that DMHg is degraded under acidic conditions, and that MMHg is the predominant product. Our finding that DMHg is degraded under acidic conditions is similar to previous reports (11, 12, 21, 32) describing the acidolysis of DMHg in seawater and freshwater at pH < 4. These results collectively demonstrate that the decomposition of DMHg observed was dependent upon pH, not the specific acid involved. The work by Mason (21) and Fitzgerald and Mason (11) identified MMHg as the predominant product of DMHg decomposition under acidic conditions, as was also true in our study. The observation that DMHg is decomposed to MMHg at low pH may necessitate revisiting data from previous studies of MMHg in marine waters when appreciable DMHg may have been present and when acidification was used as a preservation technique. This may well prove to be applicable to most locations of the ocean as MMHg concentrations are often less than detection limits, while DMHg is present at detectable levels in many marine waters (14). The analytical artifact resulting from the conversion of DMHg to MMHg when using acidification to preserve MMHg samples can be substantial (Figures 3 and 4), highlighting the need for the adoption of an alternative method of sample handling and preservation when analyzing seawater, marine hydrothermal vent fluid, and other samples for MMHg when DMHg potentially represents a sizable fraction of the organo-mercury pool. We propose that samples be sparged of DMHg at the time of collection and then either be analyzed immediately or preserved by acidification for later MMHg analysis. As described above, this additional sparging step still provides for the quantitative recovery of MMHg, and our MMHg bubbler blanks routinely have no quantifiable MMHg peak, demonstrating that the transfer and sparging of samples can be accomplished without contaminating samples with MMHg.

Acknowledgments We acknowledge Timothy Pennington, Francisco Chavez, and the crew of the MBARI research vessel R/V Point Lobos for assistance in sample collection. We acknowledge Lydia Jennings and Priya Ganguli for assistance with sampling and analysis and Nicholas Bloom for providing DMHg standards. This research was made possible by a UC Santa Cruz Chancellor’s dissertation year fellowship.

Supporting Information Available Analytical details, ancillary data, calculations of DMHg evasive fluxes, and figures showing sample locations and sunlight irradiance are available. This information is available free of charge via the Internet at http://pubs.acs.org.

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