Study On Biogeochemistry Of Dimethylsulfoxide In China Seas

The spatial and temporal variations of distributions of DMSO were studied in the East China Sea (ECS), the Yellow Sea (YS), the Bohai Sea and the South China Sea (SCS). In order to obtain a preliminary feature of DMSO biogeochemistry, the possible relationships between the dimethyl sulfur compounds and the environmental factors such as temperature and salinity were also investigated. The main research results were summarized as follows:(1) A method of chemoreduction-purge-and-trap followed by gas chromatographic analysis for the determination of trace dimethylsulfoxide (DMSO) in seawater has been developed. It has a DMSO detection limit of 2.7 pmol of sulfur, corresponding to a concentration of 0.75 nM for a 40 mL sample, and has a precision of 4%~5%.(2) Distributions of DMSO were determined in the ECS from December 2009 to January 2010 and in the waters adjacent to the Yangtze Estuary in June and November 2010. The results showed that the average concentrations of DMSOd and DMSOp in the surface waters of the ECS wer 61.90±26.37 and 21.31±4.51 nmol L-1,respectively. In the waters off the Yangtze River Estuary, those were 30.18±9.08 and 16.61±6.24 nmol L-1 in summer, 21.39±11.58 and 5.18±1.36 nmol L-1 in autumn, respectively. In general, average concentrations of DMSOd and DMSOp in the ECS showed obvious seasonal variations, with higher values in summer and lower values in autumn. In the three studied seasons, the horizontal distributions of DMSOd in the ECS were obviously influenced by the Yangtze River, where high concentrations of DMSOd appeared around the mouth of it, and it decreased from inshore to offshore stations. The distribution patterns of DMSOp were compared in different seasons. In winter, it increased offshore in the northeast ECS due to the increasing temperature coupled to the change of phytoplankton species, but decreased offshore in the southwest ECS due to the poor DMSO-producer phytoplankton species in the open sea. In summer, DMSOp distributions did not show the same pattern as Chl-a which could be attributed to the seasonal change in phytoplankton community structure coupled with salinity. In autumn, DMSOp and Chl-a displayed a similar distribution pattern ascribed to the dominance of diatoms. No correlation between DMSOp and Chl-a was observed in winter and summer, while significant relationship between them was found in autumn.The ratios of DMSOp to Chl-a exhibited obvious seasonal variations, with an average value 3.41 times higher in summer than in autumn. DMSOp/Chl-a ratio correlated significantly with temperature in the northeast ECS in winter, while no relationship between them was found in summer and autumn. In summer, DMSOp/Chl-a was correlated with dissolved inorganic nitrogen and salinity, indicating that DMSO might act as an osmolyte in algal cells, while no relationship was found in autumn. DMSOd was correlated significantly with NO3-, dissolved organic carbon (DOC) and bacterial abundance in summer, but no correlation with DMSOp, indicating that DMSOd was mainly from photo-oxidation and bacterial oxidation of DMS rather than from DMSOp. DMSOd may have the same source as those materials such as the inputs of continent-derived organic matter via Yangtze River. In autumn, no relationship between DMSOd and those parameters was found, suggesting that the source of DMSOd was complicated and none of those parameters was a major source for DMSOd. Both in summer and autumn DMSOd and salinity had a significant negative relationship.The vertical profiles of DMSOd and DMSOp did not exhibit the same pattern. The maximum DMSOd was present at the depth of 2~25 m. Photochemical and biological oxidation of DMS together with the exudation of DMSO by phytoplankton might contribute to the high levels of DMSO observed near the surface. In contrast, the maximum DMSOp appeared at the bottom, probably due to the release of resuspending sediments. DMSOp and DMSOd exhibited a strong diurnal variation in the surface seawater. The highest DMSOp concentration appeared around noontime and then decreased sharply, presumably due to its antioxidant function. The DMSOd concentrations were much lower in the daytime than in the night, suggesting that there were significant loss processes of DMSOd such as microbial consumption and photo-oxidation during the daytime.(3) Distributions of DMSOd and DMSOp were determined during two cruises in the YS and the Bohai Sea in April and September 2010. The average concentrations of DMSOd in the surface waters in the two cruises were 16.99±1.96 and 10.45±1.77 nmol L-1; those of DMSOp were 17.67±2.24 and 17.20±3.54 nmol L-1, respectively. The average concentrations of DMSOd showed obvious seasonal variations, with the higher value in spring than in autumn, while seasonal variations of DMSOp were not distinct. The distribution patterns of DMSOp were controlled mainly by the phytoplankton biomass, with high DMSOp concentrations appearing in the waters containing high Chl-a levels.DMSOp and Chl-a had significant relationships in these two seasons, and even had significant relationships at all stations in two seasons, indicating that phytoplankton biomass might play a major role in controlling the distributions of DMSOp in the study area. In spring DMSOd was correlated with Chl-a, and had a significant relationship with DMSOp, suggesting that DMSOp might be the major source for the DMSOd in the surface water. In autumn, DMSOd was not correlated with Chl-a and DMSOp, but correlated with bacterial abundance, indicating that the bacterial oxidation of DMS might be responsible for the source of DMSOd. The relationship between DMSOd and other environmental factors such as temperature, NO3- and DOC concentrations was not found, which might be due to the complex production and consumption of DMSO.The seasonal variation of DMSOp/Chl-a was not distinct, because the phytoplankton were dominated by diatoms in these two seasons. In autumn, DMSOp/Chl-a and salinity had a significant relationship, similar to the results in the ECS in summer. No relationship between DMSOp/Chl-a and other environmental factors such as temperature, salinity and nutrients was found, suggesting that the biosynthesis of DMSO was influenced by many complex factors.(4) Distributions of DMSOd and DMSOp were investigated in the surface water of the SCS in January 2010. The average concentrations of DMSOd and DMSOp were 49.97±16.47 and 11.08±2.20 nmol L-1, respectively. The distribution of DMSOd was obviously influenced by Pearl River effluent, with high concentrations around the mouth of the river. The concentrations of DMSOp also decreased in the offshore direction due to the variation of phytoplankton biomass and the phytoplankton species composition. At the inshore stations the resuspending sediments might also account for the high DMSOp concentrations.The DMSOp/Chl-a ratios varied in a large range due to the different phytoplankton species. The three highest DMSOp/Chl-a values appeared at the offshore stations where Dinophyta and Chrysophyta (coccolithophores) were the dominant algal species indicating that these algal may the main DMSO producers. In the surface water DMSOd and DMS did not correlate significantly, indicating that in winter DMS was not the major source for DMSOd. At the offshore stations (depth> 50 m), DMSOd correlated significantly with DMSOp, suggesting that DMSOp might be the major source of DMSOd. No relationship between DMSOp and Chl-a, DMSOp and DMSPp was found in the SCS.