YANG Jie, and YANG Guipeng



Dissolved and Particulate Dimethylsulfoxide in the South China Sea During Winter

YANG Jie1), 2), and YANG Guipeng1), 3), *

1),,266100,2),266071,3),,266237,

The distributions of particulate and dissolved dimethylsulfoxide (DMSOp, DMSOd) were studied for the first time in the surface water of the South China Sea (SCS) in January 2010. The concentrations of DMSOp ranged from 2.6 to 56.8nmolL−1with an average of 11.1±2.2nmolL−1, and those of DMSOd ranged from 11.8 to 335.1nmolL−1with an average of 50.0±16.5nmolL−1. DMSOd dominated over both dimethylsulfide (DMS) and dissolved dimethylsulfoniopropionate (DMSPd) by 1–3 orders of magnitude and represented the major dissolved dimethyl sulfur pool. In addition, DMSOp/chlorophyll-ratios varied from 2.7 to 180.7 mmolg−1with an average of 30.5±9.6mmolg−1. DMSOd concentrations displayed a significant negative relationship with sea surface temperature (SST) and sea surfaces salinity (SSS) in the whole study area. The distribution of DMSOd in the coastal waters was obviously influenced by the Pearl River discharge, with high concentrations appearing around the river mouth. In the offshore waters, a significant correlation was observed between the DMSOp and DMSOd concentrations, suggesting that DMSOd was mainly from the diffusion of intracellular DMSO rather than from the photochemical and biological oxidation of DMS.

dissolved dimethylsulfoxide; particulate dimethylsulfoxide; phytoplankton; chlorophyll-; distribution; South China Sea

1 Introduction

Dimethylsulfoxide (DMSO) in marine environments has attracted increasing attention in the last two decades due to its important role in the biological cycle of dimethylsulfide (DMS), which is the most abundant reduced volatile sulfur compound in marine environments (Lovelock., 1972; Bates., 1987). The high DMS concentrations in the surface water, related to its atmospheric levels, drive a significant sea-to-air flux of DMS (Yang., 2000, 2005). Once in the atmosphere, DMS is oxidized to sulfur dioxide, methanesulfonate and sulfate (Berresheim., 1990; Yin., 1990; Putaud., 1993) which may influence the global climate and the acidity of rain (Charlson., 1987; Nguyen., 1992). Thus, DMS and its principle precursor dimethylsufoniopropionate (DMSP) have been widely investigated in marine environments (Scarratt., 2000).

In comparison to DMS and DMSP, DMSO receives less attention in the context of the biogeochemical cycle (Lee., 1999a). DMSO can be formed through biological oxidation (Zhang., 1991; Juliette., 1993)and photochemical oxidation of DMS (Brimblecombe and Shooter, 1986; Kieber., 1996; Hatton, 2002b; Toole., 2004). Recently, del Valle. (2007b) demonstrated that DMSO could be produced by the biological DMS consumption (BDMSC) in a natural aerobic system. DMSO was the major product of BDMSC (72%) in the surface mixed layer. On the other hand, DMSO can be reduced back to DMS by some bacteria (Zinder and Brock, 1978; Bilous and Weiner, 1985; McEwan., 1985) and eukaryotic algae (Spiese., 2009). As a result, DMSO has been considered as the potential source and sink for DMS. Because of the interaction between them, a significant relationship between DMSO and DMS is often observed (Hatton., 1996; Kiene., 2007; Hatton and Wilson, 2007).

Dissolved DMSO (DMSOd) is considered to be one of the most abundant methylated sulfur compounds in marine environments, and its concentration has been determined for a variety of oceanic water. DMSOd concentration often exceeds that of DMS and dissolved DMSP (DMSPd), sometimes even 1–2 orders of magnitude higherthan DMS concentration (Simó., 1995; de Mora., 1996; Lee and de Mora, 1996; Bouillon., 2002). Thus it has been hypothesized that direct biosynthetic pathway might exist to account for the high concentrations of DMSO (Simó., 1995; de Mora., 1996). Simó. (1998a) proved that DMSO could be produced by phytoplankton, and the subsequent determination of DMSO in the ice algae further supports this idea (Lee., 2001). The latest research in the Ross Sea has also demonstrated that 31.4% of DMSOd resulted from particle-associated processes (del Valle., 2007a).

The reasons why DMSO is biosynthesized by phytoplankton still remain unresolved. Similar to DMSP, DMSO could act as a cryoprotectant, a free-radical scavenger and an osmoregulator, and its potential biological functions have been discussed in detail by Lee and de Mora (1999). A significant correlation between DMSO and DMSP is frequently observed, and particulate DMSO (DMSOp), constituting only 8%–50% of the intracellular sulfur pool, is usually less abundant than particulate DMSP (DMSPp) (Simó and Vila-Costa, 2006; Hatton and Wilson, 2007). However, high DMSOp concentrations relative to DMSPp were also observed by Lee. (1999b) and Bouillon. (2002).

To date, very little information is available on the joint measurement of DMSOp and DMSOd in the surface seawater (Lee., 1999b; Bouillon., 2002; Hatton and Wilson, 2007). Although it is well accepted that DMSOp is an important source of DMSOd, evidence is scarce in marine environments especially where the DMS level is extremely low. More information is needed to confirm the biosynthesis source of DMSOd in seawater.

There have been some studies on DMS in the South China Sea (SCS) (Yang, 1999; Yang., 1999; Yang, 2000; Ma., 2005), yet no research on DMSO has been previously performed. Here we first report the distribution of DMSO in the SCS. The aim of this work is to study the spatial distributions of DMSOp and DMSOd simultaneously, to investigate the possible relationships between the dimethyl sulfur compounds, and to obtain a preliminary picture of DMSO biogeochemistry in the SCS.

2 Materials and Methods

2.1 Study Area

The SCS is the largest marginal sea in the world with an area of 3.5 million km2, located in the tropical-sub- tropical western North Pacific. The seasonally reversing winds play an important role in determining the upper ocean circulation (Chen., 2001). In winter the northeast monsoon prevails, driving a large-scale cyclonic gyre over the entire deep basin with a strong southward current along the coast of Vietnam (Shaw and Chao, 1994). Along the northern boundary of the basin, Kuroshio water (KW) with warm, saline and oligotrophic properties intrudes into the SCS through the Luzon Strait (Nitani, 1972). The surface KW from the Philippine Sea also intrudes into the SCS and flows westward along the continental margin of China (Shaw, 1991). Its central gyre is warm, permanently stratified and oligotrophic. The northern SCS receives an enormous amount of freshwater, nutrient and suspended material from the Pearl River which is the biggest river in southern China. The Pearl River Delta receives 64% of the industrial sewage and 74% of the municipal sewage of the entire Guangdong Province and the water in the Pearl River Estuary is severely polluted due to the rapid economic development and population growth (Ma., 2005).

2.2 Sampling and Methods

The surface (0–1m) seawater samples were collected during 6–30 January 2010 on board of the‘in the SCS. This cruise included 59 grid stations as shown in Fig.1. The environmental and hydrographic parameters including the site, sea surface temperature (SST) and sea surface salinity (SSS) are listed in Table 1. Water samples were collected using 8L Niskin bottles deployed on a standard conductivity-temperature- depth (CTD) rosette. An aliquot of 40mL of seawater sample was filtered under gravity through a Whatman GF/F filter. The filtrates were used to determine the DMSOd onboard. The filters for DMSOp measurement were cryostored at −80℃ before being analyzed in the laboratory.

Fig.1 Sampling stations in the SCS in January 2010.

DMSO was analyzed using NaBH4as the reducing agent following the procedures described by Simó. (1998b). Since the DMSO determination is based on the DMS produced by NaBH4reduction, the DMS existing in the sample before the reduction is an interference factor. Furthermore, DMSP in seawater also gives rise to DMS by NaBH4reduction, creating a second interference factor in the DMSO determination. Hence previous removal of DMS and DMSP is necessary. To remove DMS and DMSPd, the filtrate was transferred to a 42mL glass vial with 2.0 mL of 10molL−1KOH, and the vial was immediately sealed and kept in the dark at 4℃ for 24h to achieve the full transformation of DMSPd into DMS. After adjusting pH to 4 by addition of 36% HCl, the entire volume of solution was injected with a syringe into the stripping chamber. The newly generated DMS from the hydrolysis of DMSPd, as well as the intrinsic DMS in seawater, was removed by purging the sample with ultra-pure nitrogen.

For the removal of DMSPp, the GF/F filter was placed in a 42mL glass vial filled with 40mL Milli-Q water and 2.0mL of 10molL−1KOH and immediately sealed, similar to that of DMSPd.

Table 1 Detailed information at the sampling stations and their SST, SSS, Chl-a, DMSOd and DMSOp data

Note: na, not analyzed.

Once DMSP was eliminated via the DMS formation and sparging procedure, an amount of 0.12g sodium borohy- dride was added to the slightly acidic solution through a side port of the stripping chamber; after 15min reaction 1.0mL of 5% HCl was injected into the solution through the low bleed septum to react for 3min. The generated DMS was then purged and measured as described in Yang. (2008). In brief, the solution was stripped with ultra- pure nitrogen at a flow rate of 40mLmin−1for 30min and the volatiles were cryotrapped in a Teflon loop submerged in liquid nitrogen (−190℃). A glass tube filled with anhydrous calcium chloride was located between the purge chamber and the cryogenic trap to absorb the moisture. Once the sparging procedure was completed, the Teflon loop was quickly placed in hot water (90–100℃) for two minutes and then the desorbed gases were introduced to the GC system for the analysis. The produced DMS from the purge-and-trap system was analyzed using an Agilent 7890A GC with a flame photometric detector (FPD) and a HP-5 high resolution GC column (30m´0.32mm, 0.25mm). Detector gas flow rates were 50mLmin−1for synthetic hydrogen and 60mLmin−1for compressed air. The carrier gas was 11mLmin−1of 99.999% ultra-pure nitrogen with a split ratio of 10:1. The initial column temperature was held at 35℃ for 5min, and raised to 150℃ at 15℃min−1with the final temperature held for 5 min. The temperatures of injector port and detector were kept at 150℃ and 160℃, respectively. The precision of this method was within 5%. The concentration of DMSO was calculated from the DMS result based on the 100% reduction ratio of DMSO.

DMS was immediately analyzed on board after sampling using a purge and cryogenic trap technique and then quantified by a Shimadzu GC-14B gas chromatograph equipped with a FPD according to the procedures described in Yang. (2005). DMSPp and DMSPd were broken down to DMS using alkali hydrolysis and the produced DMS was then analyzed as described above.

Chlorophyll(Chl-) was determined fluorometrically with a Turner Designs Fluorometer after 300mL of seawater was filtered through Whatman GF/F glass fiber filter and then the filter was extracted in 90% acetone according to Parsons. (1984).

3 Results and Discussion

3.1 Distributions of DMSOp and DMSOd in the Surface Seawater

The DMSOp and DMSOd concentrations in the SCS surface water are listed in Table 1, together with the SST, SSS, depth and Chl-concentrations. The horizontal distributions of DMSOp and DMSOd in the investigated area are shown in Fig.2.

The DMSOp concentrations in the study area ranged from 2.6 to 56.8nmolL−1, with a mean value of 11.1±2.2 nmolL−1. The highest concentration of DMSOp (56.8 nmolL−1) was present at station E600 situated in the coastal waters with a moderate Chl-level (0.88mgL−1). The second highest DMSOp concentration (31.2nmolL−1) was observed at station QD02 located on the continental shelf. The phytoplankton identification and enumeration in the same cruise indicated that relatively high proportion of Dinophyta (11.5%) and Chrysophyta (coccolithophores) appeared at station QD02. Dinophyta and coccolithophorid species are known to be the prolific producers of DMSP. Furthermore, some phytoplankton species from three taxo- nomic groups (dinoflagellates, prymnesiophytes and diatoms) have been shown by laboratory experiments to have the ability to produce DMSO in common with DMSP (Hatton and Wilson, 2007). Thus we speculated that the two groups of algae might also be the main producers of DMSO and be responsible for the high DMSOp concentrations. DMSOp includes the DMSO in the phytoplankton cells as well as that bound to detrital particles. DMSOp in the sediment traps in the North Sea was determined by Hatton (2002a) who suggested that DMSO could be formed in sedimenting materialthe breakdown of DMSP to DMS, which would then be oxidized to DMSO by anaerobic or microaerophilic bacteria present in the material. Therefore, the highest DMSOp level at the coastal station E600 with a depth of only 29.4m could not preclude the source of sediments. As the water column was well mixed in winter, sediments might be resuspended and thus produce more DMSOp at the coastal stations. The low DMSOp concentrations (≤3.0nmolL−1) were found at stations E403 and E405 in the Luzon strait with water depths of 4163.0m and 3397.0m, respectively. Our results were in accordance with those previous measurements (Lee., 1999b; Bouillon., 2002; Simó and Vila-Costa, 2006; Yang and Yang, 2011).

On the whole, the distribution feature of DMSOp in the SCS was that the DMSOp concentrations decreased from inshore to offshore sites similar to the distribution pattern in the southwest East China Sea (ECS), but in contrast to the distribution pattern in the northeast ECS (Yang and Yang, 2011). This might be caused by different Chl-distribution patterns and different phytoplankton species compositions. In the northeast ECS, the proportion of Dinophyta increased in the offshore direction (Guo., 2012) and the Chl-concentrations were also very high in the open sea. However, in the SCS, the Chl-concentrations decreased sharply in the offshore direction. Therefore, the DMSOp distribution pattern was controlled mainly by the Chl-concentration distribution.

The DMSOd concentrations varied widely in the whole study area, ranging from 11.8 to 335.1nmolL−1with a mean of 50.0±16.5nmolL−1(Table 1). In the coastal water the concentrations were remarkably higher than those in the open sea. The three highest DMSOd concentration values higher than 200.0nmolL−1were all observed at the coastal shelf stations. The highest value (335.1nmolL−1) was found at station A9 which was at the mouth of Lingdingyang Bay, the biggest estuary of the Pearl River. The second one (229.5nmolL−1) was determined at station C7a near the mouth of Yamen, one of the outlets of the Pearl River. The third highest DMSOd concentration (212.3nmolL−1) was recorded at station S601 which was also located in the coastal waters. The DMSOd concentrations in this study were comparable with those in the ECS (3.2–357nmolL−1with a mean of 61.9nmolL−1) and the average value compared well with those previous results (Simó., 1995; Lee and de Mora, 1996; Simó., 1997; Hatton., 1998; Bouillon., 2002; Kiene., 2007; del Valle., 2009).

Fig.2 Distributions of Chl-a, SSS, SST, DMSOp and DMSOd in the sea surface water of the SCS.

The dominance of DMSO was evident as its concentration was typically 2–3 orders of magnitude higher than that of DMS, and 1–2 orders of magnitude higher than that of DMSPd. The DMSOd distribution pattern in the SCS is similar to that in the ECS. The highest DMSOd concentrations were recorded in the estuaries where the DMS concentrations were very low (0.12nmolL−1) and the environmental parameters were very similar. Recent study demonstrated that DMSO was the main product (72%) of the BDMSC in the surface mixed layer (del Valle., 2007b). In fact, the biological DMS consumption rate at station C7a was only 1.72nMd−1. Assuming that there was no DMSO loss and the BDMSC was the only source of DMSO, it would take about 185 days to build up the measured DMSO pool. This meant that DMSO produced from the BDMSC was not the main source of DMSO in the Pearl River Estuary. DMSO in the estuary was mainly from the release of cellular DMSO and the product of the photochemical oxidation of DMS. This might be ascribed to the fact that, in the Pearl River Estuary, the DOC concentrations which influenced the DMS photolysis rate were also very high (>240mmolL−1) (Lin, 2007). Hatton (2002b) demonstrated that under visible wavelengths of light most of the photo-chemically removed DMS would be photo-oxidised to DMSO, and the photolysis rate constants were affected by a combination of light intensity, DOC concentration and the initial DMS concentrations. Among these factors, high DOC concentration would increase the initial photolysis rates in surface waters (Brugger, 1998). So in coastal marine areas with high DOC concentrations the photolysis of DMS might become the dominant removal mechanism and therefore this may be a reason for the high DMSO concentrations. DMSO can also be produced directly within cells of marine phytoplankton, and is released into seawater after algal death. Considering that the Chl-concentrations were relatively high (0.54–3.57mgL−1), this pathway may be another important source for the high dissolved DMSO concentrations.Finally, the high DMSOd concentrations might be due to the sampling artifacts, as described in Yang and Yang (2011).

3.2 Ratios of DMSOp/Chl-a and DMSPp/DMSOp

The ratio of DMSOp/Chl-can be used to compare the distribution of DMSOp in different trophic regimes (Iverson., 1989; Simó., 1997). In this study DMSOp/ Chl-ratios varied from 2.7 to 180.7mmolg−1with an average of 30.5±9.6mmolg−1(Table 1; Fig.3a). This average value was comparable with those reported in the literature (Simó., 1998a; Simó., 2000; Bouillon., 2002; Besiktepe., 2004; Riseman and DiTullio, 2004; Yang and Yang, 2011).

Fig.3 Diagram of DMSOp/Chl-a (a) and DMSPp/DMSOp (b) ratios at different transects in the SCS. Details about transects E5, E6, C, A, S1, S4, S6 and E4 were listed in Table 1. Station A–I corresponded to each station from top to bottom at each transect in Table 1.

The DMSOp/Chl-ratios usually depend on the dominant algal species in seawater, since intracellular DMSO contents in algae vary largely with species (Hatton and Wilson, 2007). The higher DMSOp/Chl-values appearedat the offshore stations (E604, E605, KK1, SEATS), where- as the lower values occurred at the inshore stations (A9, S102). According to the phytoplankton data collected in the same cruise, coccolithophores exhibited the highest cell abundance at the offshore station SEATS and decreased from this station to the inshore stations, and the proportion of Dinophyta was relatively high (10.8%) at station SEATS. The low abundance of coccolithophores appeared at transect E4 where the proportion of Dinophyta (5.6%) was also relatively low here. As a consequence, the low DMSOp/Chl-values were observed at transect E4. Moreover, the lowest abundance of coccolithophores was found at station E403, and thus the lowest DMSOp/Chl-value was found here.

Under many circumstances, DMSPp dominated over DMSOp in the surface water. The DMSPp/DMSOp ratios of 1 (coastal NW Mediterranean and Equatorial Pacific) to 12–13 (Arctic and Gulf of Maine) were reported by Simó and Vila-Costa (2006). Lee. (1999b) found that the average DMSPp/DMSOp ratios were from 0.009 to 0.038 in the Saguenay Fjord, Quebec. Bouillon. (2002) also obtained similar ratios from 0.033 to 3.03 in May and from 0.024 to 2.56 in June in the North Water. Yang and Yang (2011) determined the ratios from 0.008 to 2.220 in the ECS in winter. Our DMSPp/DMSOp ratios varied from 0.07 to 8.40 with an average of 1.02±0.35 (Fig.3b), which was comparable with those previous results. The highest ratio was obtained at Station E403 where the minimum DMSOp/Chl-ratio (2.7mmolg−1) appeared. The lowest DMSPp/DMSOp ratio (0.07) occurred at station S405 where the relatively high DMSOp/ Chl-ratio (47.1mmolg−1) was observed. Hatton and Wilson (2007) demonstrated that the DMSPp/DMSOp ratio was greatly affected by phytoplankton taxonomy. The DMSPp/ DMSOp ratio would be a rather good indicator for oxidative stress; for instance, the ratio increased as temperature decreased with latitude (Simó and Vila- Costa, 2006). However, the inverse relationship between the ratio and temperature was not found in our study. Simó and Vila-Costa (2006) also speculated that the inverse relationship might result from compositional patterns of the phytoplankton assemblage. Variation in the DMSPp/DMSOp ratios with different phytoplankton species and under different environmental conditions deserves further study.

3.3 Ecological and Environmental Factors Influencing DMSO Distributions

Ecological factor such as Chl-and environmental factors including temperature and salinity which may influence the distribution of DMSO in the surface seawater were examined in the present study. The relationships of DMSOp and DMSOd with other dimethylated-sulphur compounds such as DMS and DMSP were also investigated (Table 2).

Table 2 Correlations of DMSOp and DMSOd with other dimethylated-sulphur compounds and environmental factors

Note:†, DMSOd and DMSOp concentrations at the offshore stations (depth>50m).

As is well known, DMSOp results mostly from phytoplankton, hence a relationship between DMSOp and Chl-would be expected. However, in the present study no significant relationship was found between DMSOp and Chl-(Table 2). According to the phytoplankton data obtained in the same cruise, the phytoplankton community was mainly composed of Bacillariophyta, Dinophyta, Chrysophyta and Diazotrophs, and the phytoplankton species composition varied significantly among stations. Therefore, taxon-dependent production of DMSOp was further proved by our results.

Some researchers have found significant relationships between DMSOp and DMSPp (Riseman and DiTullio, 2004; Simó and Vila-Costa, 2006; Hatton and Wilson, 2007). They speculated that these two compounds might have a common origin in phytoplankton. However, in our study no relationship was found between the two compounds (Table 2). A similar result was also obtained in the Black Sea (Besiktepe., 2004), in the North Water (Lee., 1999b; Bouillon., 2002) and in the ECS (Yang and Yang, 2011). This may be interpreted by the variation of DMSPp/DMSOp ratio. The ratios (0.07–8.40) in our study varied by a factor of 120. Similarly, Lee. (1999b) and Bouillon. (2002) found that the ratios varied by a factor of 21 to 107. However, other studies showed that this ratio varied by a factor of less than 5 (Riseman and DiTullio, 2004; Hatton and Wilson, 2007).

Regression analyses revealed that there were no statistically significant relationships between the DMSOp con- centrations and the environmental parameters including SST and SSS. Our results suggested that the DMSOp concentrations were not mainly controlled by these parameters.

No significant correlation was found between DMSOd and DMS as well (Table 2). Similar results were also reported in the literature (de Mora., 1996; Simó., 1997; Kiene., 2007). Thus we speculated that DMS- linked production was not the only source of DMSOd in the SCS, whereas other important sources for the high DMSOd concentrations such as cellular exudation may exist (de Mora., 1996).

DMSO is a dipolar aprotic hydroscopic substance and can permeate biological membranes of healthy cells. Previous study has showed that the particulate associated DMSOd production is an important source of DMSOd in the surface water of the Ross Sea (del Valle., 2007a). Joint measurement of DMSOp and DMSOd in surface water has rarely been reported up to now. In a recent study on the west coast of Scotland, Hatton and Wilson (2007) found that DMSOp concentrations were not correlated with DMSOd in surface waters, whereas in culture experiments a significant relationship was observed. In the present study a significant correlation (=0.544,=37,<0.001) was found between DMSOp and DMSOd concentrations in the surface water (Table 2), after excluding some coastal stations (depth<50m). Our result suggested that DMSOd at the offshore stations in the SCS might result mainly from production by phytoplankton.

The influences of SST and SSS on the distributions of DMSOd and DMSOp were also examined in this study. DMSOd exhibited a significant negative relationship with temperature (=−0.576,=53,<0.001) and salinity (=−0.507,=53,<0.001), respectively. DMSOp exhibited a weaker significant negative relationship with temperature (=−0.286,=56,=0.033) and salinity (=−0.327,=56,=0.014) (Table 2). Simó and Vila-Costa (2006) hypothesized that the SST could reflect the degree of exposure to solar radiation. It has been suggested that DMSO together with DMSP, DMS and methanesulfinic acid may constitute a cascade reaction system against oxidative stress in the algal cell (Sunda., 2002). Harmful reactive oxygen species (ROS) are produced as by-products of photosynthesis in the chloroplast, particularly so if there is an excess of UV radiation. Conversion of DMSP into DMS would supply in situ ROS scavengers by oxidation of DMS to DMSO and oxidation of the latter to methanesulfinic acid (Simó and Vila-Costa, 2006). However, whether this would result in a net increase or decrease of the DMSO pool in the cell is unclear. The function of DMSO in the phytoplankton cells remains to be solved.

So far, very little is known about the relationship between the DMSOd concentration and the in situ seawater salinity. DMSO in the cell has a potential role as an osmoregulator (Lee and de Mora, 1999), thus in the low salinity seawater the excess of intracellular DMSO may be released from cells by passive diffusion. In addition, as mentioned above, the photolysis of DMS might be another source of DMSOd at the inshore stations near the mouth of the Pearl River, and hence higher concentrations of DMSO were reasonably expected in the low salinity water.

4 Conclusions

The present study provides important information on the distributions of DMSOp and DMSOd in the SCS. The concentrations of DMSOp and DMSOd in the surface water of the SCS varied greatly. No significant relationship was found between the DMSOp and Chl-concentrations, suggesting that the production of DMSO might be dependent upon different phytoplankton species. Moreover, the DMSOp/Chl-ratio varied greatly in the study area, providing further evidence suggesting that the DMSO was produced preferentially by some algal groups such as Dinophyta and Chrysophyta (coccolithophores). The DMSOd concentrations dominated over the DMS and DMSPd concentrations by 1–3 orders of magnitude, indicating that DMSOd acted as the major nonvolatile dimethyl sulfur pool in the study area. The highest DMSOd concentration appeared in the coastal water, presumably due to the influence of terrestrial input. In addition, DMSOd was significantly correlated with DMSOp at the offshore stations, implying that phytoplankton biosynthesis was the major source of DMSOd in the open sea.

Acknowledgements

We thank the captain and crew of the‘for their help and cooperation during theinvestigation. We are grateful to Dr. Jun Sun for providing the phytoplankton data in the cruise.This work was financially supported by the National Natural Science Foundation of China (No. 41576073), China-ASEAN ma- ritime cooperation fund: Comparative study of Holocene Sedimentary Evolution of the Yangtze River Delta and the Red River Delta, the National Key Research and Development Program of China (No. 2016YFA0601301), and AoShan Talents Program of Qingdao National Laboratory for Marine Science and Technology (No. 2015ASTP).

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(Edited by Chen Wenwen)