Biogenic Sulfur in the Environment - American Chemical Society

Chapter 10

Biogeochemical Cycling of Dimethyl Sulfide in Marine Environments Downloaded via TUFTS UNIV on July 24, 2018 at 11:35:41 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

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Stuart G. Wakeham and John W. H. Dacey 1

Skidaway Institute of Oceanography, P.O. Box 13687, Savannah, GA 31416 Department of Biology, Woods Hole Oceanographic Institution, Woods Hole, MA 02543 2

Dimethylsulfide (DMS) plays an important role in the global atmospheric sulfur cycle. This single compound contributes a major portion of the reduced biogenic sulfur transferred from the ocean to the atmosphere. For this reason, there is considerable interest in characterizing the biogeochemical processes by which DMS is produced and consumed. Numerous research projects are currently addressing aspects of DMS cycling in oceanic, coastal, and intertidal environments. This paper provides an overview of the biogeochemistry of DMS in marine systems and synthesizes the current state of knowledge in this area of research. The production of volatile reduced sulfur compounds in marine ecosystems and the subsequent efflux of these compounds to the marine atmospheric boundary layer is an important source of sulfur to the global atmosphere (1). Independent of its role in the atmospheric sulftir budget, Charlson et al. (2) have suggested that dimethylsulfide (DMS) also plays a major role in cloud formation over oceans. Oxidation products of DMS appear to serve as sites for cloud nucleation. In the ocean DMS is the predominant volatile sulfur compound (2:6). The range of DMS emission rates of 0.7-13 μπιοί S/mVyx from surface seawater (4.7-9) yields an estimated global flux of 1.1 ± 0.5 Tmol S/yr (Andreae, 1985; Tmol = 32xl0 g). This flux is a major component of the 1.1-1.6 Tmol S/yr of biogenic sulfur transferred to the atmosphere from the world ocean and of the 1.9-3.6 Tmol S/yr total global (marine + terrestrial) biogenic sulfur flux. In comparison, coastal wetlands and marshes emit sulfur at significantly higher rates on an aerial basis than does the ocean, e.g. for DMS, 1-246 umol/m /d (10-12). However wetlands occupy a relatively small area (4 χ 10 * km vs 3 χ 10 km of ocean) and their role in the global sulfur cycle is minor, with a contribution of some 2% of the total gaseousflux.Approximately half the sulfur flux is from marshes is DMS and half may be H S QQ). Interest in biogeochemical processes controlling the emission of DMS from the ocean has led to increased efforts to determine the sources and sinks of DMS. The precursor of DMS in the marine environment is dimethylsulfoniopropionate (DMSP), also known as dime thy lpropiothe tin (DMPT). DMSP occurs in many species of marine phytoplankton (13-16) and higher plants 12

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0097-6156/89/0393-0152$06.00/0 1989 American Chemical Society

Saltzman and Cooper; Biogenic Sulfur in the Environment ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

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(17-19). D M S P is involved i n the cycling of methionine (2Q). In saline environments, D M S P also appears to be important i n regulating cellular osmotic pressure (21), and it has been suggested that sulfur may be stored in that form (22). Yet another role for D M S P may be as a precursor of acrylic acid, which along with D M S is formed by the enzymatic cleavage of D M S P (22). Acrylic acid is a broad-spectrum bacteriocide (21) which, when excreted by marine phytoplankton, may inhibit colonization of healthy algal cells by bacteria Our investigations of the biogeochemistry of D M S have focussed on a variety of coastal marine environments. In this paper we present an overview of our research on the processes leading to D M S production and consumption. We discuss our results in the context of obtaining a broader understanding of D M S cycling in the marine environment. Distribution and Production of D M S in the Ocean Interest in evaluating the sea-air flux of D M S has resulted i n a very large data set (some 3000 measurements) on D M S concentrations i n surface seawater in various oceanic biogeographical zones (3.4.6.7.9). Concentrations i n surface waters range from about 1 n m o l / L in oligotrophic areas, 1-2 n m o l / L in coastal areas, to 2-5 n m o l / L in upwelling regions. Surface concentrations vary both seasonally and with latitude. The depth distribution of D M S i n the water column is characterized by a surface or near-surface concentration maximum of a few nmol/L, sharply decreasing at the base of the euphotic zone to low levels of less than 0.1 n m o l / L in the deep ocean. The depth profile in oligotrophic waters of the Cariaco Trench off Venezuela (Figure 1) is typical of the open ocean (see Wakeham et al. (S) for analytical details). We also determined the depth profile for D M S P , the algal precursor to D M S , i n the Cariaco Trench. D M S P associated with particulate matter, most likely algal cells, showed a subsurface concentration maximum, while free D M S P concentrations were highest at the sea surface. Total D M S P concentrations were about 2-4x greater than D M S . Several mechanisms may be involved i n the production of D M S i n seawater. Phytoplankton may produce D M S as a normal metabolic product (26.27). Neither the biochemical function of D M S production nor the rate at which D M S is released i n the ocean are understood. Furthermore there is extensive evidence that distributions of D M S in the ocean are poorly correlated with phytoplankton production or biomass (326). This has led to the view that D M S r synthesis and the subsequent release of D M S are highly species-specific and need not correlate with the abundance of phytoplankton. In fact, algal D M S P content and D M S production by various species of algae i n culture varies over several orders of magnitude (15.26-28). Early research into D M S production by marine algae led to the discovery that Polysiphonia fastigiata produces D M S by enzymatic cleavage of D M S P (22). Several coccolithophorids and dinoflagellates release D M S at particularily high rates. Thus, Barnard et al. (lé) suggested that high concentrations of D M S i n the Bering Sea result primarily from release ot D M S by Pheaocystis pouchetti. Similarity, Turner et al. (6) found a strong correlation between D M S i n the English Channel and abundances of the dinoflagellate Gwodinium aureolum. Actual release rates in the ocean are unknown, but could be expected to be of the order of 10* mol/cell/day based on laboratory data (27.28). For the two algal species for which there are published data (Hymenomonas carterae (22) and Gymnodinium nelsonii (28)), turnover of intercellular D M S P (releasing D M S into seawater) is 1.4% and 0.3% per day respectively. Subsequent work (Figure 2) suggests that 15


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Figure 1. Vertical concentration profiles for D M S , chlorophyll a and D M S P in the western basin of the Cariaco Trench off Venezuela (10° 39'N, 65°30'W). D M S was determined by sparging and gas chromatography with a flame photometric detector. Particulate D M S P was determined by base treatment of material collected on 0.22 τη filters and analysis of the D M S released; free D M S P was determined as D M S released upon base treatment of sparged water samples obtained after initial D M S analysis. Chlorophyll a data from W . Cooper and R . Z i k a (personal communication). μ

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Time (hr) Figure 2. Release of D M S by Prorocentrum micans i n culture. Experiments were conducted in 1-liter glass bottles with silicone rubber stoppers and with a phytoplankton cell density of 500 c e l l s / m L . The bottles were placed on a rotator (2 rev/min) in low light (2-30 /ieinstein/m sec). D M S increase in the headspace was measured by gas chromatqgraphy/flame photometric detector. A linear regression of the data yields a D M S production rate of 2.1 χ Ι Ο " jimol/cell day, corresponding to a D M S P turnover of 0.26%/day. 2

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D M S production by another dinoflagellate, Prorocentrum micans, also occurs at a rate of 0.3% of D M S P per day. In recent surveys of D M S distributions in seawater, there has been concern artifacts might result from the disruption of algal cells during analysis of D M S (i.e. during filtration or sparging of unfiltered samples), presumably as a consequence of cell lysis and subsequent enzymatic decomposition of D M S P to D M S . These concerns led us to ask the question if natural processes i n the ocean lead to D M S release into seawater as a result of loss of integrity of algal cells? We investigated one such process, grazing of marine phytoplankton by zooplankton (28). In a series of experiments (Figure 3), there was a marked enhancement of D M S release when phytoplankton were ingested by zooplankton. In fact, D M S production rates i n treatments containing zooplankton grazing on phytoplankton were an order of magnitude higher than rates observed in treatments containing phytoplankton alone. W e have developed a simple model to evaluate the importance of zooplankton grazing i n the marine D M S cycle (Figure 4). In our grazing experiments, approximately one-third of the algal D M S P ingested by zooplankton was recovered as free D M S in seawater. Thus, D M S produced by zooplankton grazing on DMSP-containing phytoplankton exceeds the direct release of D M S by phytoplankton i f more than 3% of the DMSP-bearing phytoplankton in a parcel of water are ingested per day. In the ocean, however, zooplankton almost surely graze more than 3 % of the standing stock of phytoplankton per day. Assuming that phytoplankton biomass is approximately balanced (where zooplankton grazing is equivalent to phytoplankton growth), a phytoplankton production rate of 0.2 per day (e.g., worth Pacific Gyre, (22)) would require that about 20% of phytoplankton cells be ingested each day. Under such conditions, the rate of release of D M S during grazing could be six times greater than that released by the phytoplankton alone. Although our experiments demonstrated that grazing increases D M S production, the mechanism by which D M S is released is unclear. It may be that capture and handling of algal cells leads to cell disruption and to enzymatic decomposition of cellular D M S P . O r D M S P decomposition may occur during digestion in the intestinal tract of the zooplankton or by microbial activity in fecal material. As noted above, about one-third of the D M S P in ingested algal cells is converted to D M S . A t present, we are unable to account for the remaining two-thirds. Some may have been oxidixed to dimethylsulfoxide ( D M S O ) or degraded further. The disappearance of D M S during the zooplankton grazing experiment i n Figure 3 appears to have been due to oxidation of D M S by bacteria. O n the other hand, some cellular D M S P may have been released unaltered into seawater. Free (dissolved) D M S P has been measured i n several studies (5JL; Cariaco Trench reported here, Figure 1). Subsequent decomposition of free D M S P could lead to further production of D M S . The reaction of D M S P with O H - yields D M S and acrylic acid. Abiotic decomposition of D M S P with O H - at the p H of seawater is unlikely to be important; kinetic experiments by Dacey ana Blough (2Û) indicate that D M S P has a 8-year half-life when reacting with O H - i n seawater at 10°C. Biotic decomposition appears more likely. F o r example, W a k e h a m et a l . (£) supplemented unfiltered seawater from a coastal marine pond with D M S P and found complete conversion to D M S in several days. Turner et al. (6) reported similar biotic decomposition of D M S P to D M S i n 0.2um-filtered seawater. While both experiments indicate that a biological mechanism is required to decompose D M S P to D M S , with a biological half-life of several days for D M S P in seawater, it is unknown whether the reaction is catalysed by enzymes also released from disrupted algal cells or whether bacteria are responsible. Bacterial fermentation of D M S P has been reported by Wagner and Stadtman


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PRORO+CENTROPAGES

400-1

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Figure 3. D M S production during zooplankton grazing on phytoplankton:

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Centrophages hamatus grazing on Prorocentrum micans;

( m and A ) P. micans alone. Experimental conditions as for Figure 2 (phytoplankon 500 c e l l s / m L ) except zooplankton ( 4 0 / L ) added. D M S roduction is accelerated when the phytoplankton are ingested by zooplankton. lowever, when experiments are run for extended time intervals, D M S disappears from solution, presumably by microbial oxidation. A t the end of this experiment, zooplankton had grazed the phytoplankton to very low levels, and oxidation of D M S that had been createa in the process has resulted i n a net accumulation of less D M S than in the phytoplankton-alone treatments.

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intact.undisturbed phytoplankton: Prob.Survival χ DMSP Turnover* 0.8 χ 0.01s 0.008

ingested phytoplankton: ProbJngestton χ OMSP Conversions 02 χ 0.3 s 0.06

Figure 4. Conceptual model illustrating the importance of zooplankton grazing on phytoplankton to D M S production i n the ocean. F o r undisturbed phytoplankton, the probability of releasing D M S from intercellular D M S P is estimated as the product of survival against herbivory (0.8 assuming a algal turnover of 0.2/day) times the turnover of intercellular D M S P ( < 1%/day). For zooplankton grazing, the probability of D M S production is g a z i n g rate (0.2 per day) times the DMSP-to-DMS conversion efficiency (0.3). Thus the production of D M S v i a herbivory and ingestion o f D M S r containing algal cells is approximately 6x the release rate by undisturbed algal cells.


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(21). Dacey and Blough (unpub. results) have isolated a bacterium which grows on D M S P as its sole source of carbon, growing on acrylate while quantitatively liberating D M S . If D M S concentrations at the surface of the ocean are presumed to be at steady state, production must balance loss. The fate of D M S is thought to be evasion across the sea surface into the marine atmospheric boundary layer. However, since rates of D M S production are unknown, it is impossible to compare production with flux to the atmosphere, which is relatively well constrained. A n alternative sink for D M S in seawater is microbial consumption. The ability of bacteria to metabolize D M S in anaerobic environments is well documented (32-341 Data for aerobic metabolism of D M S are fewer (there are at present none for marine bacteria), but Sivela and Sundman (25) and de Bont et al. (26) have described non-marine aerobic bacteria which utilize D M S as their sole source of carbon. It is likely that bacterial turnover of D M S plays a major role in the D M S cycle in seawater. D M S Cycling in a Stratified Coastal Salt Pond To better characterize processes involved in the production and consumption of D M S i n m a r i n e environments, we are c o n d u c t i n g a study of D M S biogeochemistry in a seasonally stratified coastal marine basin, Salt Pond, Cape Coa, M A (Wakeham et al., 5.371 The rationale for studying a coastal pond is that as it becomes thermally stratified i n summer, the water column is effectively partitioned into three biogeochemical zones (Figure 5): an oxygenated epilimnion (0-2 m water depth), an oxygen-deficient metalimnion (2-3.5 m) ana a strongly anoxic hypolimnion (3.5-5 m). Thus a variety of both aerobic and anaerobic processes occur simultaneously, but at different depths in the same water column. The epilimnion of Salt Pond is probably typical of moderately productive and oxic coastal seawater i n general. Summer D M S concentrations in the epilimnion are in the range of 5-10 n m o l / L D M S P concentrations were higher than D M S concentrations, and free D M S P was more abundant than particulate D M S P . The mechanisms of D M S production i n the epilimnion are also probably the same as those occurring in most oceanic regimes, that is some combination of direct release by phytoplankton and formation of D M S as algal cells are ingested by herbivores. It is in the oxygen-deficient metalimnion, however, that the highest D M S concentrations were found (Figure 5). Particulate D M S P also showed a strong concentration peak i n the metalimnion, but i n contrast to the overlying epilimnion, free D M S P was significantly less abundant. It is possible that the particulate D M S P peak in the metalimnion arises from algal cells which settle out of the epilimnion and are concentrated near the metafimnion-hypolimnion boundary, as indicated by the strong peak in chlorophyll a at this depth (Figure 5). Production of D M S in this zone could then result from physiological stress on the algal cells under conditions of reduced oxygen tensions or to bacterial decomposition of algal material. In either case the peak in D M S concentration tracks the seasonal vertical migrations of the oxic-anoxic boundary as the pond mixes down in winter and then restratifies in summer (22). A marked decrease in the concentration of D M S was always observed at the top of the hypolimnion (Figure 5) where H 2 S concentrations begin to increase as a result of sulfate reduction in sediments and bottom waters of the pond. A concentration profile of this type strongly suggests a significant removal process at the top of the hypolimnion. G i v e n the H^S and D M S profiles, it was suspected that the anaerobic phototrophic bactena, which are abundant i n the hypolimnion (e.g. bacteriocnlorophyll d i n Figure 5), might


Figure 5. Water column profiles in coastal Salt Pond on August 27,1985. Bact. chlis primarily bacteriochlorophyll d. Saltzman and Cooper; Biogenic Sulfur in the Environment ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

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consume D M S as well as H?S. Initial experiments confirmed that D M S is degraded by inocula from the hypolimnion of the pond (5). In a subsequent study (21), enrichment cultures of phototrophic purple bacteria from Salt Pond oxidized D M S to D M S O , with D M S serving as an electron acceptor for photosynthesis and the oxidation of D M S to D M S O being used to reduce COo to biomass. Furthermore, a pure strain of bacteria has been isolated which is capable of both phototrophic oxidation of D M S to D M S O and chemotrophic reduction of D M S O back to D M S i n the dark (J. Zeyer, pers. commun.). A comparison of estimated rates of D M S production and consumption i n Salt Pond allows us to calculate a first-order budget for D M S (Figure 6). We assume that p r o d u c t i o n of D M S is restricted to the e p i l i m n i o n and metalimnion, and that the major removal processes are tidal exchange with adjacent Vineyard Sound, gas exchange to the atmosphere, and microbial consumption i n the hypolimnion. In the case of D M S production, it is impossible to derive realistic estimates of release by phytoplankton because the species composition of the pond is virtually uncharacterized and there are too few data on D M S release rates by algae, certainly none in nature. O n the other hand, we do know the inventory of D M S P and the turnover rate of particulate organic carbon (POC) in the pond. If particulate D M S P in the epilimnion and metalimnion (100 μ π ι ο ΐ / m in August, 1985) decomposes to produce D M S at a rate similar to that of P O C turnover (0.2/day; 28), then a DMUS production of 20 /*mol/m /day would result. This production rate includes contributions from both microbial decomposition of algal cells and grazing; direct exudation by phytoplankton would produce some unknown amount of additional D M S . It the D M S inventory i n Salt Pond is at steady state i n summer (£), production should approximately balance removal. Tidal removal of D M S to Vineyard Sound is minimal. Outflow from Salt Pond is thought to be primarily surface water, and using a maximum tidal range of 0-0.2 m / d ana a mean surface water concentration of 10 nmol/L, we calculate an export rate of less than 2 μΐηο1/πι /& The water-air flux ot D M S may be calculated using the two film model of Liss and Slater (22; flux = -ki C^). With the same surface water D M S concentration (C^) and an estimated mass transfer coefficient (k{) for D M S of 1.5 c m / h , the projected flux of D M S from the pond into the atmosphere would be 4 Mmol/m /d. This compares with the range of estimated emissions from the ocean of 5-12 Mmol/m /d (1). Microbial consumption of D M S in the hypolimnion is more difficult to estimate since our laboratory experiments showing degradation have not been rigorous enough to yield degradation rates applicable to natural conditions. However, if we assume that D M S is transported into the hypolimnion by eddy diffusion, with an arbitrary eddy diffusion coefficient of 1 m / d , the observed concentration gradient across the oxic/anoxic boundary would support a sink for 30 M m o l / m / d D M S produced i n the h y p o l i m n i o n . W e therefore hypothesize that anaerobic microbial consumption in the hypolimnion of Salt Pond may be the major sink for D M S produced in the metalimnion. A t this point we cannot estimate the potential for D M S oxidation in the epilimnion. 2

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D M S Emission from Coastal Salt Marshes Coastal wetlands have long been noted for their relatively high emission of volatile sulfur gases to the atmosphere; indeed the typical odor of marshes often is due largely to D M S . Several studies have reported emissions of D M S , H S , and other sulfur compounds, dimethyldisulfide, carbonyl sulfide, and carbon disulfide (10-12.40-421 D M S and H S constitute the bulk of the flux, with D M S predominating i n vegetated areas and H S in mud flats. Fluxes of D M S 2

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Biogeochemical Cycling oj Dimethyl Sulfide


and H S from coastal wetlands are typically 1-2 orders of magnitude higher than from oceanic regions (e.g. 2-10 μπιο1/πι /α for the ocean vs 100-200 /*mol/m /d for marshes), but on a global scale wetlands contribute only a small fraction (2%) of the total biogenic sulfur emission. D i u r n a l , tidal, and seasonal variations in D M S flux have been reported (12.40-42). Since some D M S is produced during the decomposition of sulfur-containing organic matter, primarily amino acids, in waterlogged soils (42), it had been generally assumed that, like H S , D M S in marshes was derived from anaerobic decomposition. However, whereas a turnover of approximately 0.1% per day of the H S typically present in marsh pore waters (500 μΐηοΙ/L at 0-2 cm) can support the observed flux (150 Mmol/m /d), a turnover of 100-30,000% per day of pore water D M S (50 n m o l / L ) would be needed to give the observed D M S flux (1-246 μ m o l / m / d (12)). This discrepency suggests that the factors regulating emission of H S and D M S from saltmarshes must be very different. This observation and results from other investigators (42) suggest that emission of D M S may be controlled by the physiology of marsh grasses. D M S P is present i n a number of marsh plants, but only i n Spartina alterniflora (12) and S. anglica (22) is it particularly abundant. Spartina alterniflora is the dominant species in temperate marshes of North America but not i n marshes or wetlands at lower latitudes. T o date, most D M S emission measurements have been made i n marshes dominated by S. alterniflora. Emission rates in areas having other marsh grasses, with lower D M S P content, are likely to be considerably lower. Estimates of D M S emission from saltmarshes in general which are based on fluxes from S. alterniflora without considering the species of grass are likely to be considerably overestimated. 2

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Figure 6. Schematic of the cycle of D M S in Salt Pond. See text for discussion.


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Concentrations of D M S P in S. alterniflora are highest in leaves and lowest in roots and rhizomes (12, Figure 7). D M S P content of leaves also generally increases with increasing salinity (19.44) and also appears to depend on the nutritional status of the grass (12). These several factors are consistent with a resumed osmoregulatory function for D M S P . A n osmoregulatory role for >MSP has been postulated by analogy with the quaternary ammonium analogs of D M S P , most notably glycine betame, a well-documented osmoticum (21). D i c k s o n et al. (45.) reported shifts in the D M S P content of the marine macroalga Ulva lactuca i n response to changes i n salinity. Certainly the concentrations of several 10's of m m o l / L in water contained in plant tissue mean that D M S P contributes to the osmotic pressure of the plant. It is yet unknown how dynamic this solute pool is, and recent evidence with a green macroalga Enteromorpha intestinalis (4£) and the European marsh grass Spartina anglica (22) have cast doubt on the capability of plants to adjust their D M S P content in response to shifts in salinity. Regardless of the physiological role of D M S P in these plants, virtually nothing is known about the factors controlling the in vivo decomposition of D M S P which results i n the liberation of D M S . Presumably enzymatic decomposition in leaves is the major mechanism resulting in D M S emission to the atmosphere from areas of S. alterniflora (Figure 8). If D M S P decomposition and subsequent release of D M S by leaves of marsh grasses are influenced by plant physiology, then understanding the mechanism of this conversion has important implications with respect to design of experiments for measuring D M S emission in the field. It is likely that the production and release of leaf D M S P are influenced by short-term changes in heat, incident solar radiation, and water balance. Clearly, chamber experiments must be designed to minimize unnatural perturbations of the environment around the plant. There is also evidence that soils are a sink for D M S (12, Figure 8). Kiene and Visscher (42) found consumption of D M S in anaerobic saltmarsh sediments. Uptake and consumption of D M S by soil microorganisms will affect the net emission rates and must be considered when extrapolating D M S flux rates to different geographical regions.

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Figure 7. D M S P inventory in S. alterniflora and above-ground biomass at four 5. alterniflora sites in Great Marsh, Lewes, Delaware, i n June, 1986. Most above ground D M S P is concentrated in live leaves although stems and dead leaves represent the majority of above-ground biomass at this time. Roots have less D M S P per unit biomass but may represent a larger pool on an aerial basis. Below-ground D M S P is less likely, however, to play a role in D M S emission as discussed by Dacey et al. (12).




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Time (hrs) Figure 8. D M S accumulation in headspace above enclosed S. alterniflora cores. The straight line (φ) is a linear fit of data from three runs with sediment flooded; the curved line (Q) is a hyperbolic fît with sediment exposed. These data suggest that once ambient concentrations of D M S in air reach about 4 mol/mL, the sediment surface has the capacity to consume 1.7 μπιοί >MS/m2/h.

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By analogy to zooplankton grazing in the ocean, herbivory may also be involved in D M S release in salt marshes. Freshly-deposited Canada goose feces yielded D M S within 48 hours i n amounts equivalent to 80-190 μταοΐ/β dry weight. Leaves of S. alterniflora were a principal component of material in the feces, and the quantity of D M S released from feces suggests that a major portion of the D M S P contained in the leaves (80-300 Aimol/g dry weight; 12) is decomposed to D M S . However, since the role of herbivory in the turnover of organic matter is less in marshes than in the ocean, it is unlikely that herbivory plays a significant role in D M S flux in these sites. In salt marsh grass, the liberation of D M S is probably controlled primarily by physiological and decompositional processes.

Summary W e propose a model (Figure 9) to illustrate key factors controlling the formation and removal of D M S i n marine systems. D M S i n the marine environment appears to arise primarily from D M S P , although other sulfonium compounds have been identified. T h e d i s t r i b u t i o n of D M S P among phytoplankton species and its physiological role need further investigation. We propose that under most circumstances D M S released by phytoplankton represents a small turnover of intercellular D M S P . In situations where cells are stressed, as in the hypoxic zone of Salt Pond, D M S release may be accelerated. Our work with zooplankton grazing and the recent observations of significant soluble D M S P pools suggests that factors other than algal physiology are important in the formation of D M S . The high concentrations of free D M S P are almost certainly the consequence of the physical disruption of cells, most likely during grazing. The apparent chemical stability of D M S P in seawater suggests that the removal of D M S P is controlled by bacteria.


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metabolism decomposition, and ' herbivory

Phytoplankton DMSP

Figure 9. Schematic of biogeochemical processes producing and consuming D M S i n marine environments. D M S P i n certain phytoplankton and marsh grasses may be slowly metabolized and released directly. Disruption of D M S P containing cells, either by herbivory or by microbial decomposition, results in D M S P release into solution and enhanced production of D M S . D M S may be oxidized m i c r o b i a l l y to D M S O , and perhaps D M S O o ; photochemical decomposition of D M S i n surface waters may also occur. Residual D M S may escape into the atmosphere, where it undergoes further photochemical degradation. Very little is known about the removal of D M S from seawater. Bacterial and photochemical oxidation undoubtedly result in D M S O and further-oxidized products. It is not clear what percentage of D M S formed in seawater actually reaches the atmosphere. Its flux to the atmosphere certainly plays a major role in the chemistry of the atmosphere. A thorough understanding of the global distribution and dynamics of that flux will depend on obtaining an increased understanding of the processes that control the dynamics of D M S in marine ecosystems. Acknowledgments We thank W . Cooper and R . Z i k a for chlorophyll a data from the Cariaco Trench, and M . Hardisky for biomass data from Great Marsh, Lewes, D E . Ε . C a n u e l and L . H a r e provided laboratory assistance. This research was


10.

WAKEHAMANDDACEY

Biogeochemical Cycling of Dimethyl Sulfide

supported by National Science Foundation grants OCE-84-16203, OCÈ-87-14170 and OCE-87-19000, and National Aeronautics and Space Administration grant NAGW-606.

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RECEIVED September 15, 1988


Biogenic Sulfur in the Environment - American Chemical Society

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