Production of methanethiol from dimethylsulfoniopropionate in marine surface waters

ELSEVIER

Marine Chemistry 54 (1996) 69-83

Production of methanethiol from dimethylsulfoniopropionate marine surface waters

in

Ronald P. Kiene apb a Department ofMarine Sciences, University of South Alabama, LSCB 25, Mobile, AL 36688-0002. USA b Dauphin Island Sea Lab, Dauphin Island, AL 36528, USA

Received 3 1 August 1995;accepted2 1December 1995

Abstract Degradation of n M levels of dissolved dimethylsulfoniopropionate [DMSP(d)] m . surface water samples from the Gulf of Mexico and Gulf of Maine was accompanied by the accumulation of both dimethylsulfide (DMS) and methanethiol (MeSH). The mean net yields for DMS and MeSH, in terms of sulfur from DMSP, were 32% (range 12-66%) and 22% (range 3-64%), respectively. In six out of seventeen experiments, maximum net accumulations of MeSH were equivalent to, or greater than, those obtained for DMS. No relationship between net DMS and MeSH accumulations was found when all seventeen experiments were considered. Inhibition of DMSP(d) degradation with 50 PM glycine betaine substantially lowered production of both MeSH and DMS, indicating that degradation of DMSP was required to produce these sulfur gases. The most likely route for MeSH formation is from demethiolation of 3-methiolpropionate (MMPA), a product of DMSP demethylation. Experimental additions of MMPA confirmed that MeSH could be produced from this compound. The MeSH produced from DMSP was rapidly lost in all water samples tested, much more rapidly than DMS. Direct determinations of MeSH loss rate constants showed these to fall in the range of 0.14-1.4 h-’ in different water samples. Filtration of water through 0.2~km membrane filters resulted in a 1.3-4.5-fold decrease in the whole water loss rate constants, suggesting biological or particle sinks for MeSH. Addition of Suwannee River humic acid accelerated the loss of MeSH from filtered water, suggesting a possible interaction between MeSH and DOM. The results of this study indicate that a substantial fraction of the DMSP(d) degraded in aerobic seawater is converted to MeSH. The diversion of DMSP-sulfur to MeSH represents an important biogeochemical control on the production of climatically active DMS. In addition, the production of highly reactive MeSH suggests that the degradation of DMSP may have a more important impact on the chemistry of marine surface waters than previously recognized.

1. Introduction

The distribution and biogeochemistry of volatile reduced sulfur compounds such as dimethylsulfide (DMS), sulfide (COS), carbon disulfide (CS,) and hydrogen sulfide (H,S) in the surface ocean has received considerable attention during the past several decades (Andreae and Raemdonck, 1983; Turner and Liss, 1985; Kim and Andreae, 1987; Cutter and Krahforst, 1988; Luther and Tsamakis, 1989; Cutter and Radford-Knoery, 1993; Walsh et al., 1994). Interest in these reduced sulfur compounds stems from the fact that they are chemically and biologically reactive, and also because they are exchanged from the oceans to the atmosphere carbonyl

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(Andreae, 1986; Andreae, 1990). By far the most abundant volatile sulfur compound in surface waters is DMS. DMS is derived primarily from the degradation of dimethylsulfoniopropionate ((CH,),S+CH,CH,COO-; DMSP), an osmotic solute produced by a wide variety of marine phytoplankton (Dacey and Wakeham, 1986; Keller et al., 1989; Andreae, 1990; Malin et al., 1993). Biogenic production of DMS sustains a net sea-to-air flux which contributes _ 50% of the global biogenic sulfur flux to the atmosphere (Andreae, 1990). Photochemical oxidation of DMS in marine air leads to production of sulfate and methanesulfonate, both of which contribute to the pool of sub+m sized aerosol particles and cloud condensation nuclei in the atmosphere (Andreae et al., 1995). It has been suggested by Charlson et al. (1987) that oceanic DMS emissions may modulate global climate by affecting cloud albedo, which in turn would affect the biogenesis of DMS in the oceans (Malin et al., 1992, 1994). Factors which control the concentrations of DMS and its precursor, DMSP, in surface waters have therefore attracted wide interest. DMSP is one of the most abundant forms of reduced sulfur found in the euphotic zone of the ocean, with concentrations of total DMSP (dissolved plus particulate) typically ranging from 5 to 50 nM. Even higher concentrations (> 100 nM) may be found in blooms of DMSP-producing algae (Malin et al., 1993). Though phytoplankton appear to be the source of DMSP in the water column, studies have shown that complex food web processes are involved in the degradation of DMSP. Degradation of DMSP via lyase enzymes yields DMS and acrylic acid (Cantoni and Anderson, 1956; de Souza and Yoch, 19951, and this reaction occurs universally in ocean surface waters, either by the actions of phytoplankton (Vairavamurthy et al., 1985; Stefels and VanBoekel, 1993), zooplankton (Dacey and Wakeham, 1986; Wolfe et al., 1994) or bacteria (Kiene, 1990; Ledyard and Dacey, 1994). Recent experimental evidence, however, suggests that only a relatively small portion (< 30%) of the DMSP undergoing degradation in seawater is converted to DMS (Belviso et al., 1990; Kiene and Service, 1991; Kiene, 1992). The relatively low yields of DMS have led Kiene and others to speculate that the major fraction of the DMSP was demethylated to 3-methiolpropionate (MMPA) and further degraded to methanethiol (MeSH) and or 3-mercaptopropionate (MPA), as has been observed with anoxic sediments and aerobic bacterial cultures (Mopper and Taylor, 1986; Kiene and Taylor, 1988a,b; Taylor and Gilchrist, 1991; Visscher et al., 1994). However, little evidence existed to support the production of sulfur compounds other than DMS from DMSP in oxygenated seawater. In the present study, data are presented which show that MeSH is a major sulfur product arising from DMSP degradation in surface seawater; its production being comparable to DMS in some instances. Evidence is also provided to illustrate that MeSH has a short residence time owing to its reactivity with particles and dissolved organic matter (DOM).

2. Materials and methods 2.1. Sample collection and processing Surface water samples used during this study were collected from coastal and shelf sites in two main geographic regions, the northern Gulf of Mexico and western Gulf of Maine. Most water samples from the Gulf of Mexico were collected from a pier on the east end of Dauphin Island, Alabama, U.S.A., located near the mouth of Mobile Bay (30“20’N, 88”lO’W). In several cases, water was collected from a boat N 10 km south of Dauphin Island at a site termed the Sea Buoy. Additional shore collection sites on the Gulf Coast included Santa Rosa Sound, Florida, and Fort Morgan Beach, Alabama. Salinities (as measured by refractometer) ranged from 17 to 33 and temperatures from 14“ to 28°C for the Gulf of Mexico water samples collected over the year-long period of the study. In all these cases, samples were collected with an acid-cleaned bucket or carboy and dispensed immediately into l-l or 250-ml Teflon bottles. Water samples were held in the dark and returned to the Dauphin Island Sea Lab where they were used immediately for experimental incubations. Transit times were 5 min for the Dauphin Island pier samples and I-2 h from the other sites. Temperate water samples were collected in the summer of 1994 on a flood tide from a shore site on the Piscataqua River, Great Bay, New

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Hampshire. These samples (17°C; salinity of 28) were collected directly into l-1 glass incubation bottles and returned to the laboratory at the University of New Hampshire where they were used for experiments within 1 h. In June 1995, samples were collected aboard the RV “Cape Hatteras” (cruise CH079.5) from several sites in the Gulf of Maine. Station locations will be given along with presentation of the data below. Water was collected from 5-m depth using a 5-l Niskin bottle attached to a CTD rosette. The water was dispensed directly into 250-ml or l-l Teflon bottles. During the incubations, all samples were maintained within + 1°C of in situ temperature, and most were kept in the dark, except during sub-sampling ( < 2-min duration) when they were exposed to room light. The Gulf of Maine samples used for DMSP degradation were incubated in natural sun light in a flow through deck incubator. In this case, two layers of window screening were used to reduce ambient light intensities to m 30% of the incident levels. The samples for MeSH loss determinations were incubated in the dark. The water samples were not shaken during the incubations, but were gently inverted several times before sub-samples were removed for sulfur compound determinations. 2.2. Incubations with additions of DMSP and other sulfur gas precursors To test the effects of DMSP on sulfur gas production, experimental additions of dissolved DMSP (hydrochloride salt; Research Plus Inc., Toms River, New Jersey) were made to whole water samples to increase the concentrations 30-50 nM above endogenous levels. Endogenous DMSP(d) concentrations were always < 8 nM. The concentrations of dissolved DMSP [DMSP(d)], particulate DMSP [DMsP(~)], DMS, and MeSH were monitored in the water samples during incubations which lasted up to 10 h. The first sub-sample for measurement of DMSP pools and sulfur gases was taken within 2 min of the addition and this time point was designated as time zero. In one experiment, glycine betaine * HCl (GBT; Sigma Chemical Corporation; 50 @4 final concentration) was added to water samples just prior to addition of 40 nM dissolved DMSP. GBT is a structural analog of DMSP and is a potent short-term inhibitor of DMSP degradation (Kiene and Gerard, 1995). In another experiment, the effects of other potential precursors of MeSH (each at 50 nM) were compared to additions of 50 nh4 DMSP. These precursors included L-methionine, 3-methiolpropionic acid (MMPA) and its methyl ester, methyl 3-methiolpropionic acid (methyl-MMPA). MMPA was obtained from alkaline hydrolysis of methyl-MMPA (Sigma Chemical Corporation). Also included in all experiments of this type, were control samples which received no additions. In most cases, unamended samples showed relatively steady concentrations of the sulfur compounds during the incubations. Because the consumption kinetics for DMSP(d) were fast (see Section 3.1), and a rapid sampling schedule needed to be maintained, most experiments included single bottles for each treatment. Duplicate treatments were occasionally run and replication for sulfur gas and DMSP(d) measurements was usually better than + 10%. 2.3. MeSH loss kinetics Experiments were carried out to determine the kinetics of MeSH loss and some of the factors which influence this loss. Water samples for these experiments were incubated in either 250-ml Teflon bottles or in sterile 130-ml glass serum bottles, which were filled to within a few ml of capacity. All incubations for MeSH loss determinations were carried out in the dark in order to minimize possible photochemical reactions. Samples were exposed to fluorescent room lighting only during sub-sample removal (- 2 min). MeSH gas was obtained from a permeation tube (VICI Metronics, Santa Clara, California) which was sealed in a lOO-ml, N,-flushed serum bottle. The gas phase within the bottle was enriched in MeSH and a small (lo-50 (~1) sub-sample was removed from the vial with a gas-tight syringe. This MeSH vapor was then introduced into the incubation bottle which was carefully tilted such that the injected gas bubbles rose to a trapped air bubble in the neck of the bottle. In this way the incubation bottle could be sealed without loosing any of the added MeSH. The bottle was then shaken for 2 min to mix the added MeSH into the water. This approach allowed MeSH to be added to final

12

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Chemisrry 54 (1996) 69-83

concentrations in the range of lo-20 nM with minimal variations between replicate bottles. In addition to MeSH, the gas phase from the permeation tube contained dimethyldisulfide (DMDS), an autooxidation product of MeSH, which could be detected during gas chromatographic runs. The final concentration of DMDS in the water samples was N l-2 n M or m 10% of the MeSH concentration. Storage of the permeation tube under N, minimized the accumulation of DMDS in the vial. The concentration of DMDS was monitored in MeSH loss experiments to check whether autooxidation occurred in the seawater. MeSH was added to either whole, unfiltered water or water which was filtered through 47-mm 0.2~pm membrane filters (Nuclepore). In the latter case, water was first vacuum filtered through Gelman AE or Whatman GF/F glass fiber filters to remove larger particles. The Gulf of Maine samples were gravity filtered through GF/F filters. Just prior to the MeSH addition, the pre-filtered water was vacuum filtered through the 0.2~km membranes using sterile glassware. Before dispensing from the vacuum flask to the incubation flask, the water was agitated vigorously to re-introduce oxygen which might have been lost due to the vacuum. These filtration steps were carried out in fluorescent room lighting. After MeSH addition, incubation bottles were placed in the dark and sub-samples (1 or 4 ml) were periodically withdrawn with a glass syringe for determination of concentrations of sulfur gases over time. Sub-samples of whole water samples were syringe-filtered through Gelman AE filters while being injected into the sparger (Kiene and Service, 1991) whereas prefiltered samples were injected directly. The effects of Suwannee River humic acid (HA; International Humic Substances Society reference 1RlOlH; average MW N 1000 daltons) on the loss of MeSH from 0.2-pm-filtered seawater was tested by adding aqueous HA to a final concentration of 1.25 mg l- ’ added HA. An additional experiment evaluated the effects of a range of added HA concentrations (0.3-2.5 mg 1-l ) on the MeSH loss rate constants. The rate of MeSH loss in the HA treatments was compared with that in unamended samples. All samples used in these tests were incubated in the dark. 2.4. Calculation of apparent first-order rate constants Plots of the natural log of the MeSH concentration versus time were fit with a linear least-squares regression line. The slope of the line was taken as the apparent first-order rate constant (K,,,,). Correlation coefficients for such plots were > 0.90 in all cases. 2.5. Analytical methods DMS and MeSH were measured in 2-ml water samples with a purge-and-trap system identical to the one used previously for DMS (Kiene and Service, 1991). Separation of sulfur gases was achieved with a Teflon Chromosil 330 column (2 m X 3 mm i.d.; Supelco Inc.) which was maintained at either 35” or 55°C. The retention times were 0.9 or 0.6 min for MeSH and 1.9 or 1.2 min at the two temperatures, respectively. The lower temperature provided somewhat better resolution of MeSH from interfering peaks. The gases were detected with a flame photometric detector in which the flame was doped with CS, from a permeation source (VICI Metronics) held in the hydrogen line. For the New Hampshire samples, the experiments were carried out in the laboratory of Dr. Mark Hines using an analytical system (Kiene and Hines, 1995) which is very similar to the one used in Alabama. Standard curves for the sulfur gases were obtained using a permeation system as described in Kiene and Hines (1995). The slopes of the standard curves of MeSH and DMS agreed with each other to within +5%. In Alabama, DMS standards were prepared gravimetrically in ethylene glycol as described previously (Andreae, 1980) and MeSH concentrations were estimated using the standard curve for DMS. Detection limits for both MeSH and DMS were N 0.2 nM. The analytical precision near the detection limit was + 5% for DMS and + 20% for MeSH; the lower precision for MeSH was due to elution of small interfering peaks near the MeSH peak. DMSP(p) was measured by the alkaline hydrolysis method (White, 1982) which quantitatively cleaves

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73

DMSP into DMS and acrylic acid. The DMS so formed was quantified by gas chromatography as described previously (Kiene and Service, 1991). The procedure for DMSP(d) analysis was modified somewhat from that used previously. A 20-ml sub-sample was removed from the incubation bottle and allowed to drip through a 47-mm Gelman AE filter held in a glass filter tower (Gulf of Maine samples used GF/F filters). The filter was used for DMSP(p) determinations while the filtrate was used for DMSP(d) determinations. After all of the sample had passed the filter, _ 5 ml of the filtrate were placed in a small open sparge tube and bubbled with He (100 ml min- ’ for 2 min) to remove DMS. After the He flow was turned off, 1 ml of the sample was removed by pipette and placed in a 14-ml serum vial. One ml of 5 N NaOH was added to this vial and it was sealed quickly with a Teflon-faced butyl rubber septum. DMSP in the water sample was decomposed quantitatively to DMS (and acrylic acid) by the NaOH. After 30 min, the reaction was complete and the sample could be analyzed, although they were routinely analyzed the next day ( < 24 h). The DMS in the vials was measured by sweeping the headspaces of the serum vials into a cryotrap and subsequently into a gas chromatograph as described in Kiene and Gerard (1994). Standards were prepared using the same liquid volumes as the samples. This approach yielded excellent precision (typically better than &-5%) and low detection limits (OS-l.0 nM) for l-ml samples. 2.4. Terminology and calculation of yields The decrease of DMSP(d) concentrations was measured over time courses and this is referred to as net consumption or degradation. The added DMSP(d) was presumed to be degraded (as opposed to sequestered into particulate material) because particulate DMSP concentrations always held steady or declined slightly during the incubations which were carried out in the dark. The net yield (%) of MeSH or DMS was calculated as follows: (maximum net accumulation of sulfur compound) (net consumption of DMSP( d) during the same period)

x 100

The net changes in the sulfur compound pools were calculated from the change in concentrations during the incubations minus any changes which occurred in non-DMSP treated controls. In most cases, < 100% of the DMSP-S was recovered in the maximum net accumulations of DMS plus MeSH during the incubation. This could have been due to degradation to non-volatile compounds or to simultaneous removal of DMS and MeSH. As shown below, removal of MeSH during the incubations may be significant. For DMS on the other hand, losses were relatively slow in Gulf of Mexico waters, and < 25% was lost on the time scale of most incubations (5-S h). Estimates of DMS losses in Gulf of Maine waters were not determined. The DMSP(d) added to water samples is referred to as exogenous DMSP(d), while that which is naturally present in the water samples is referred to as endogenous DMSP(d).

3. Results 3.1. Degradation of DMSP and production of sulfur gases After amendment of water samples with DMSP(d), the concentration of DMSP(d) declined rapidly and within 2-6 h approached those observed in parallel, unamended samples (Fig. I>. The apparent first-order rate constants for DMSP(d) losses in 15 similar experiments during the course of this study ranged from 0.1 to 1 h-‘. The turnover rates of DMSP(d) will be discussed elsewhere (Kiene, 1996). The loss of DMSP(d) was accompanied by increases in the concentrations of both DMS and MeSH (Fig. 1). Control. samples without DMSP additions did not show appreciable changes in DMS, MeSH or DMSP(d) during the incubation, compared to amended samples (data not shown). The general patterns in the data in Fig. 1 are representative of

R.P. Kiene / Marine Chemistry 54 (1996) 69-83

0

2

4

6

a

HOURS Fig. 1. Time courses of DMSP(d), DMS and MeSH in a water sample amended with - 40 n M DMSP(d). The sample was collected on July 6, 1994 near the mouth of Mobile Bay, located on the Gulf of Mexico and had a salinity of 16. Incubation was in the dark at the in situ temperature of 28°C. DMSP(d) was added at time zero. The endogenous pools of DMS and DMSP(d) in parallel control samples (no DMSP added) remained below 4 n M. MeSH was < 0.5 n M and DMS remained near 3.5 nM in non-DMSP-treated controls. Results are from a single bottle but are representative of many similar incubations (see text).

a large number of experiments conducted with coastal and shelf waters from sub-tropical and temperate regions; however, the relative magnitude of the DMS and MeSH accumulations varied in different water samples (Table 1). The net yields, in terms of sulfur from DMSP, ranged from 12% to 66% for DMS and from 3% to 64% for MeSH in different experiments (Table 1). The mean yields were 32% and 22% for DMS and MeSH, respectively. There was no apparent relationship between the maximum net accumulations of MeSH and DMS, and both compounds taken together could account for 15-109% (mean 54%) of the sulfur in the DMSP(d) consumed (Table 1). In four out of seventeen experiments, MeSH was the major sulfur gas which accumulated from DMSP (Fig. 2; Table 11. When DMSP(d) degradation was inhibited by 50 p_M glycine betaine (Kiene and Service, 1993; Kiene and Gerard, 19951, both MeSH and DMS production were substantially lower (Fig. 21, indicating that DMSP degradation was required to produce these sulfur gases. MeSH did not arise from the degradation of DMS produced from DMSP since direct additions of DMS from lo-50 nM to Gulf of Mexico waters showed relatively slow turnover of this compound (loss rate constants of 0.027-0.083 h- ’ >, and did not cause appreciable increases in MeSH concentrations (data not shown). MeSH production was stimulated by additions of 50 nM MMPA and L-methionine (Fig. 3), but the rate of production caused by these compounds was slower than that observed with a 50 nM addition of DMSP. In contrast, the methyl ester of MMPA caused only a small accumulation of MeSH which was detectable only after 5 h of incubation.

3.2. Rates of MeSH loss and some controlling factors MeSH produced from DMSP(d) typically declined rapidly after an initial period of accumulation (Figs. 1 and 2). The loss of MeSH was investigated in a series of experiments in which MeSH was directly added to water samples. Concentrations of exogenous MeSH declined rapidly in whole, unfiltered water samples from the Gulf of Mexico and Gulf of Maine, and these loss curves could be modeled fairly well with a first-order decay law (Fig. 4). The loss of MeSH was slowed, but not stopped, by filtration of the water through 0.2~p,m membrane filters prior to the addition of MeSH (Fig. 4). Apparent first-order rate constants for MeSH losses, obtained for different water samples during this study, are given in Table 2. The rate constants ranged over a factor of 10 for both filtered and unfiltered samples, with mean values of 0.43 and 0.21 h-’ for unfiltered and filtered water samples, respectively. Filtration decreased the rate constant by a factor of 1.3-4.5 depending on the water sample. There was no significant correlation between rate constants and either salinity or in situ temperature. The loss of MeSH from 0.2~pm-filtered seawater was not accompanied by accumulation of either DMS or DMDS, both of which remained at nearly constant levels during incubations (Fig. 5). Removal of oxygen from

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Chemistry 54 (1996) 69-83

75

Table 1 The net yields of DMS and MeSH obtained during degradation of exogenous dissolved DMSP in water samples from the Gulf of Mexico and the Gulf of Maine Sample location

Date

Salinity

Temp.

Net yield of DMS from DMSP

(“0

% of DMSP-S accounted for

(%)

Net yield of MeSH from DMSP (%)

Gulf of Mexico sites: Santa Rosa Sound, FL Dauphin Island, AL Dauphin Island, AL Dauphin Island, AL Fort Morgan Beach, AL Dauphin Island, AL Dauphin Island, AL Dauphin Island, AL Dauphin Island, AL Sea Buoy, 10 km south of Dauphin Is. Dauphin Island, AL Dauphin Island, AL Dauphin Island. AL

5/11/94 5/23/94 6/28/94 7/6/94 7/19/94 S/8/94 9/5/94 9/8/94 10/26/94 11/2/94 11/28/94 12/l/94 12/7/94

33 14 21 16 28 12 22 19 17 32 22 18 11

26 26 27 28 28 27 26 26 21 22 15 14 17.5

66 36 28 41 30 56 22 29 41 28 42 34 35

43 10 64 25 29 24 36 25 11 15 7 10 8

109 46 92 66 59 80 58 54 52 43 49 44 43

7/4/94 7/5/94 6/ 17/95 6/21/95

28 28 31 32

17 17 12.5 15

15 12 12 13

29 23 3 9

44

Gulfof Maine sites:

Great Bay, NH Piscataqua River, NH Stelwagon Basin, Gulf of Maine Wilkinsons Basin, Gulf of Maine

35 15 22

5i

Mean yield

Dissolved DMSP was added at concentrations of 30-50 n M, depending on the experiment. The yield is given as a percentage and is defmed as: (maximum net accumulation of sulfur gas) Xl00 (net consumption of DMSP(d) during the same period) Also shown is the % of DMSP-sulfur accounted for by the sum of DMS and MeSH at their respective maxima during the incubation. The sum was frequently < 100% because some of the DMS and MeSH may have been consumed during the incubation.

10

C

za ‘56 5 %4

HOURS

Fig. 2. Effects of 50 u M glycine betaine, a structural analog of DMSP, on degradation of added DMSP(d) (A) and the production of DMS (B) and MeSH (C) in water samples collected from the Piscataqua River located in New Hampshire, U.S.A. The symbols are defmed in (A) and are the same for all three panels. Water samples were incubated in the dark at the in situ temperature.of 17°C. The salinity was 28.

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Chemistry 54 (1996) 69-83

Fig. 3. Time course of MeSH concentrations in water samples from the Dauphin Island Pier treated with: 0 = no addition; 0 = 50 nM DMSP; A = 50 nM L-methionine; A = 50 n M 3-methiolpropionate; and 0 = 50 nM of methyl-3-methiolpropionate. MeSH concentrations were at or below the detection

limit (0.3 n M) in the control (no addition)

samples.

the water by sparging with helium had no effect on the rate of MeSH loss in filtered water (data not shown). The addition of Suwannee River humic acid (1.25 mg l- ’ final concentration) greatly accelerated the loss of MeSH in dark-incubated, 0.2~km-filtered water (Fig. 6). The apparent first-order loss rate constant increased from 0.1 to 0.43 hh ’ with increasing concentrations of added Suwannee River humic acid over the range O-2.7

A -0

Y

0.0

U V

0.6

5

0.4

=

0.2

=%-

0

6

HOURS

0

2

4

6

10

12

HOURS Fig. 4. Kinetics of MeSH loss in filtered and unfiltered water samples collected from the Dauphin Island Pier (A) and the Gulf of Maine (B). MeSH was added to initial concentrations of 13- 15 n M in the Dauphin Island samples and 6-9 n M in the Gulf of Maine samples. Results are expressed as the concentration (C) measured at each time divided by the initial concentration C,. The curves are exponential tits. The Dauphin Island samples had a salinity of 20 and a temperature of 25°C. The Gulf of Maine samples had a salinity of 32 and a temperature of 10°C. The Gulf of Maine data represent two separate bottles for each treatment.

R.P. K&e/Marine Table 2 MeSH loss rate constants

in unfiltered

Sample location

and 0.2~pm-filtered Date

Chemistry 54 (1996) 69-83

water samples from different Salinity

1-I

locations

Incubation temp.

Apparent first-order constant, K (h- ’ )

(“a

unfiltered

0.58 _

IO 18 24 24

25 24 24 14 18 10 9 13.5 14.5 24 24

32 32

10 13

0.38 0.14

water

loss rate

0.2-pm-filtered

water

Gulf of Mexico sires: Dauphin Island, AL Dauphin Island, AL Sea Buoy, IO km south Dauphin Island, AL 50 km south of Dauphin Dauphin Island, AL Dauphin Island, AL Dauphin Island, AL Dauphin Island, AL Sea Buoy, 10 km south Sea Buoy, 10 km south

of Dauphin Is. Island

of Dauphin Is. of Dauphin Is.

g/29/94 10/4/94 10/6/94 12/l/94 12/8/94 l/4/95 l/5/95 l/17/95 2/2/95 3/27/95 3/29/95

20 14 20 18 37 17 19

a

0.22 0.11 0.29 0.32 0.43 0.42 1.4 0.47

0.15 0.21 0.24

_ 0.13 0.14 0.23 0.54 0.36

Gulfof Maine sires: Georges Basin Wilkinsons Basin

6/18/95 6/20/95

a Water used on this date was the same as that collected on 3/27/95 but was incubated to revoval of sub-samples on 3/29/95 for the MeSH loss determinations.

0.08 0.04

in a 20-I carboy in natural sunlight for 2 days prior

mg 1-l (Fig. 7). Similar increases in rate constants for MeSH loss were obtained when different amounts of filtered Suwannee River water (20 mg 1-l DOC> were added to Gulf of Mexico water to give final added DOC concentrations in the range of those used in Fig. 7 (data not shown).

4. Discussion 4.1. MeSH production

from DMSP and its implications for the DMS cycle

This is the first study to present data on the production of MeSH in aerobic seawater. The results suggest that a significant proportion of the DMSP(d) undergoing degradation is converted to MeSH, and that MeSH may, at

Fig. 5. Loss of MeSH and lack of accumulation of either DMS or DMDS in 0.2 km-filtered seawater (salinity of 32) obtained from the Sea Buoy site, 10 km south of Dauphin Island in the Gulf of Mexico. Symbols: 0 = MeSH; A = DMS; * = DMDS.

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Marine Chemisrry 54 (1996) 69-83

0.2 pm filtered water + MeSH

0.2 JJ~ filtered water + M&H + 1.25 mg~1 humic ac‘kl

OT

0.5

0

1.5

2

H&S

Fig. 6. The effect of Suwannee River humic acid (1.25 mg I- ’ added concentration) on MeSH loss kinetics in 0.2~km-filteredshelf water from the Gulf of Mexico. The incubation

temperature

was 25°C.

times, be the major sulfur gas produced from DMSP (Fig. 1; Table 1). This finding is important because MeSH production represents a diversion of DMSP-sulfur away from climatically active DMS, and represents a source of a reactive thiol species in surface waters. The degradation of DMSP(d) and the associated production of sulfur gases has been shown to be a biological process (Kiene, 1990; Kiene and Service, 1991). This was further substantiated in this study by the use of 50 FM GBT which strongly inhibited loss of DMSP(d) and the accumulation of DMS and MeSH (Fig. 2). GBT probably acts as a competitive inhibitor of DMSP transport into bacterial cells (Kiene and Gerard, 1995). The pathway responsible for MeSH production most likely involves initial demethylation of DMSP to yield 3-methiolpropionate (Taylor, 1993). Some fraction of this compound is then demethiolated, yielding MeSH as the sulfur product:

(CH&S+CH2CH,COO-

demeFCH$CH,CH,COO-

demet~latlo”CHjSH + ?

Support for the operation of this pathway comes from the fact that additions of 50 n M MMPA significantly stimulated MeSH production, albeit at a slower rate than from DMSP (Fig. 3). Methionine, another compound which contains a methiol group, also caused MeSH to accumulate, suggesting that demethiolation of these natural sulfur compounds may readily occur in seawater. In contrast, the methyl ester of MMPA caused much less MeSH to accumulate, suggesting it is not as readily metabolized. In theory, MMPA (and methionine) could yield more MeSH than DMSP because some fraction of DMSP is degraded by lyases to DMS (Fig. 1; Table 1).

n

0

0.5

1

1.5

2

Humic Acid Added (mgtl

2.5 )

Fig. 7. The effects of different concentrations of added Suwannee River humic acid on the loss rate constant for MeSH in O.Z~m-filtered water. Water was collected at the Sea Buoy site. The salinity was 20 and the samples were incubated at 24°C.

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The reasons for the slower rate of MeSH production observed with MMPA and methionine, as compared to DMSP (Fig. 3), are presently unknown. Possible explanations might include slower uptake and metabolism of MMPA and methionine by microorganisms as compared to DMSP. Additionally, some fraction of the MMPA could be further demethylated to yield the hydrophilic thiol 3-mercaptopropionate, which was not measured during this study but which has been observed as a product of DMSP and MMPA degradation in anoxic sediments and cultures of aerobic methylotrophs (Kiene and Taylor, 1988b; Taylor and Gilchrist, 1991; Visscher and Taylor, 1994). Marine bacteria capable of demethylating DMSP have recently been isolated (Taylor and Gilchrist, 1991; Diaz et al., 1992) and a large proportion (5-66%) of DMSP-degrading bacteria in surface waters of the Caribbean Sea have been found to be demethylators (Visscher et al., 1992). DMSP has been estimated to comprise l-5% of the carbon biomass in living phytoplankton in surface ocean waters (Kiene, 1993; Bates et al., 1994) and an even larger percentage in some DMSP-producing phytoplankton (Matrai and Keller, 1994). The role of DMSP as a labile carbon substrate, particularly one which sustains C, metabolism in the ocean should not be overlooked (Kiene, 1993; Visscher and Taylor, 1994). The results presented here are consistent with previous studies with coastal and oceanic waters, which showed that DMS was often a minor product of DMSP degradation (Kiene and Service, 1991; Kiene, 1992). In the present study, DMSP(d) degradation was found to be primarily associated with the < I.O+m-size fraction of Gulf of Mexico seawater (Kiene, 1996), suggesting that bacteria may be mainly responsible for cleavage and demethylation of DMSP(d). However, low yields of DMS have also been observed when microzooplankton consume and degrade DMSP-producing algae (Belviso et al., 1990; Wolfe et al., 1994). The microbial food web appears to consume DMSP without a large production of DMS (Kiene, 1993; Wolfe et al., 1994). Oceanic DMS production in temperate and sub-tropical waters may therefore be less dependent on DMSP dynamics than previously believed. The net yields of DMS and MeSH from DMSP degradation varied considerably in different water samples (Table l), and there was no obvious relationship between these variables. The net yield of DMS or MeSH depends on the relative fraction of DMSP converted to that gas, the rate of sulfur gas production and the rate of its loss. There are insufficient data to generalize about relative yields of sulfur gases in different waters, particularly because the factors which affect the net accumulation of each gas are poorly understood. The mean net yields of 32% for DMS and 22% for MeSH, given in Table 1, illustrate that both compounds are produced in comparable amounts, but these percentages may not be representative of all water types. In the present study, the rate of DMS consumption in Gulf of Mexico waters was relatively slow (R. Kiene, unpublished data), such that during typical 5-8-h incubations, absolute losses were small ( < 25%). Thus, the net yields for DMS presented in Table 1 represent a close approximation of the actual yield (though they are still underestimates because some loss of DMS occurs). In contrast, MeSH losses appeared to be rapid (see Figs. 1 and 2), and the net yields given in Table 1 should be considered significant underestimates of the actual fraction of DMSP converted to MeSH. With this interpretation in mind, it may be concluded that MeSH production may rival or even exceed DMS production from DMSP(d) in some cases. Whether MeSH accumulates significantly will depend on whether production rates exceed loss rates. In the present study, increases in MeSH were easily observable because DMSP concentrations were increased 5-lo-fold above ambient levels and the DMSP(d) degradation kinetics were relatively fast. At low DMSP(d) concentrations and or with slow DMSP degradation kinetics, MeSH loss processes might dominate, and MeSH concentrations would remain low. 4.2. Turnover of MeSH Very little is known about the biogeochemistry of methanethiol (MeSH) in oxygenated marine waters, where its concentration is generally < 0.5 nM (L.eck et al., 1990). The results presented here suggest that, despite low steady-state concentrations, MeSH may turn over rapidly. Using the mean MeSH yield observed here (22%),

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and reasonable DMSP(d) turnover rates of 2-50 nM day- ’ for temperate and sub-tropical waters (Kiene, 1996), a production of 0.4-l 1 n M MeSH per day could be predicted. MeSH concentrations were recently found to be 0.3-0.7 n M in near-surface waters from Georges Basin (42”23’N, 67”27’W) in the Gulf of Maine (R. Kiene, unpublished data). In order to sustain these concentrations a production rate of between 2.3 and 6.4 nM day-’ would be required, because a whole water loss rate constant of 0.38 hh’ was measured in this area (Table 2). Both these calculations suggest that MeSH may be produced at substantial rates in surface waters. Further studies will be needed to determine actual MeSH production rates in the ocean. MeSH does not substantially contribute to sulfur fluxes from the ocean (Andreae, 1990) because most of the MeSH is consumed on short time scales and steady-state concentrations are very low (Leek et al., 1990). Several loss mechanisms contributed to MeSH consumption in dark-incubated seawater since 0.2~km filtration slowed, but did not stop, the loss (Fig. 4). At this time it is unclear whether filtration removed biological consumption processes or physicochemical consumption processes, such as adsorption or reaction with particle surfaces. MeSH and other thiols have been found to partition extensively on to sediment particles (Mopper and Taylor, 1986; Kiene, 1991), but MeSH is also a biologically labile compound (Suylen et al., 1987). Autooxidation of MeSH in filtered water did not appear to explain the losses of MeSH that were observed since deoxygenation of the water had no effect on the rate of loss (data not shown), and DMDS did not accumulate in oxygenated water (Fig. 5). 4.3. Fate of MeSH and its potential ramifications The fate of MeSH in seawater deserves close scrutiny because thiols in general are very reactive. One of the fates of MeSH may include abiological binding or adsorption to seawater DOM since experimental additions of Suwannee River humic acid greatly increased the loss rate of MeSH in O.Zl.Lm-filtered seawater (Figs. 6 and 7). At this time, very little is known about the interaction of MeSH with humic materials, but it is likely that this will be influenced by physical conditions such as pH, temperature, salinity and concentrations of other dissolved organic and inorganic components in seawater. It is also unclear whether MeSH reacts with humic acids in a reversible manner or whether it is bound firmly or transformed in some way. If the association between MeSH and humic matter is irreversible, then this type of reaction might be responsible for enrichment of sulfur in complex organic materials (Francois, 1987). DMSP may be considered an abundant source of MeSH for this reaction in marine systems. The apparent reaction of MeSH with humic acid raises questions as to whether bound MeSH might be photolabile, since humic materials are known to be some of the principal photoreactive components of DOM (Zika, 1981). It is possible, that MeSH could be converted to carbonyl sulfide (COS) photochemically; recent studies showed that cysteine (a thiol) and methionine were among the best precursors of COS in laboratory irradiations of seawater (Zepp and Andreae, 1994). COS is the most abundant sulfur gas in the atmosphere where it contributes to the climatically-important stratospheric sulfate layer. In seawater COS hydrolyzes fairly rapidly to CO, and H,S (Radford-Knoery and Cutter, 1994), with rate constants similar to, but slightly smaller than, those observed for MeSH loss. Both COS and H,S have recently been measured in surface seawater and concentrations range from pM to low nM levels (Luther and Tsamakis, 1989; Cutter and Radford-Knoery, 1993; Radford-Knoery and Cutter, 1994). Walsh et al. (1994) indicated that phytoplankton might be the main source of H,S in seawater, but the mechanisms of H,S formation were not identified. Because conversion of MeSH to H,S could also be carried out by bacteria (Suylen et al., 19871, it remains a possibility that MeSH, derived from DMSP or methionine, could be a significant precursor of H,S in seawater. H,S may exchange with the atmosphere, but the majority of it is strongly complexed in seawater, probably with trace metals (Dyrssen and Wedborg, 1989; Luther and Tsamakis, 1989). Thiols in general have large stability constants with a number of biogeochemically-important trace metals including Fe, Cu, Hg, Zn, and Cd (Martell and Smith, 1974; Shea and MacCrehan, 19881, with Cu having the largest. Unfortunately, stability

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constants for MeSH and these metals are not, to my knowledge, available. Concentrations of individual trace metals are typically in the low nM range in seawater and their speciation is dominated by organic complexes (Bruland, 1991). Because MeSH production rates in surface waters may fall in the range of I-10 nM day-‘, the interaction of this thiol with trace metals should be evaluated.

Acknowledgements I thank Ghislain Gerard for excellent technical assistance during this study. Mark Hines provided the analytical system and strong inspiration during the work in New Hampshire. I also thank Emile “Skeet” Lores for providing the initial evidence of reaction of MeSH with humic acid and for the gift of the Suwamree River humic acid. Financial support for this research was provided by grant OCE 92-03728 from the National Science Foundation. This is contribution No. 279 of the Dauphin Island Sea Lab.

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