Studies On The Distribution And Influencing Factors Of Dimethylsulfoxide In The China Shelf Seas

Dimethylsulfoxide (DMSO) is thought to be an environmentally significantcompound due to the potential role it plays in the biogeochemical cycle of theclimatically active trace gas, dimethylsulphide (DMS). It has been shown that DMSOin seawater is not only formed through photochemical oxidation and biologicaloxidation of DMS, but also produced directly in the cells of marine phytoplankton. Inaddition, DMSO can be reduced to DMS under both aerobic and anaerobic conditions.The interactions between DMSO and DMS are likely to be important in controllingsea surface concentrations of DMS, and thus DMSO could influence the role playedby DMS in global climate regulation.In the present dissertation, we choose the East China Sea (ECS), the Yellow Sea(YS) and the Bohai Sea (BS) as the study areas that are affected seriously by humanactivities. First, the spatial and temporal variations of distributions of dissolved andparticulate dimethylsulfoxide (DMSOd and DMSOp) and factors influencing them aresystematically studied. Secondly, we focus on the production of DMS,dimethylsulfoniopropionate (DMSP) and DMSO during the growth stages of threetypical offshore algae by laboratory culture experiment. The main conclusions aredrawn as follows:1. Temporal and spatial variations of DMSO in the ECS and the South YS (SYS)(1) The distributions of DMSOd and DMSOp and the factors influencing themwere determined in the ECS and the SYS during July,2011and December,2011-January,2012. The results showed that the concentrations of DMSOd and DMSOp inthe surface water in summer were13.8(3.72-46.7) and10.9(2.53-56.2) nmol L-1,respectively. Winter concentrations were10.1(2.31-26.2) and8.72(2.42-30.8) nmolL-1, respectively. DMSOd and DMSOp concentrations showed a notable seasonalvariation with higher values in summer than in winter, the spatial distributions ofDMSOd and DMSOp in the ECS and the SYS were obvious influenced by theYangtze River runoff and the variation of phytoplankton species. The horizontal distributions of DMSOd displayed a general decrease in concentration from inshore tooffshore sites, and high concentrations of DMSOd appeared close to the mouth ofYangtze River in summer and winter. The distribution patterns of DMSOp werecompared in different seasons. In summer, it showed a gradually increasing trendfrom southeast to northwest due to the temporal variation of phytoplankton speciesand biomass. In winter it was obviously influenced by anthropogenic inputs and theoligotrophic Kuroshio waters, decreasing from inshore to offshore stations.(2) The vertical distributions of DMSOd and DMSOp in the stations of Yangtzeestuary transect P were observed. The results showed that DMSOd concentrationswere substantially higher within the surface mixed layer than below it. Highconcentrations of DMSOp in the water column corresponded well with the high levelsof chlorophyll a (Chl-a), which might be attributed to the influences of phytoplanktonbiomass. Additionally, DMSOd and DMSOp exhibit a consistent diurnal variation indifferent seasons, with higher levels in day and lower ones at night, indicating that theproduction processes of DMSO might be related to the photochemical oxidation ofDMS.(3) DMSOp/Chl-a ratios showed seasonal difference between the summer andwinter, with winter average higher than summer one, which could be attributed to thevariation in the phytoplankton species composition and abundance. Data observed inthe same cruises show that the species and biomass of dinoflagellates(high-DMSO-producers) decrease sharply from winter to summer, which consolidatethe dominant status of diatoms in the phytoplankton community. Therefore, thesemight explain why DMSOp/Chl-a ratios were higher in winter than in summer. Apositive correlation was found between DMSOp/Chl-a and salinity in these twocruises, suggesting that salinity changes influenced phytoplankton taxonomiccomposition and hence DMSOp concentrations. No correlation was observed betweenthe DMSOp and Chl-a concentrations in the two studied seasons, which might beattributed to variations in the taxonomic composition of phytoplankton. In fact, theconcentrations of DMSOp in seawater were largely influenced by the distributions ofcertain species of phytoplankton. In summer, a positive correlation was foundbetween DMS and DMSOd concentrations, however, no correlation was observedbetween them in winter, which might be attributed to complicated process ofphotochemical oxidation of DMS, such as sunshine radiation, concentrations of dissolved organic carbon (DOC) and NO3-.2. Temporal and spatial variations of DMSO in the BS and the YS(1) The distributions of DMSOd and DMSOp and the factors influencing themwere determined in the BS and the YS during May,2012and November,2012. Theresults showed that the concentrations of DMSOd and DMSOp in the surface water inspring were19.7(4.84-144) and25.1(5.09-108) nmol L-1, respectively. Autumnconcentrations were6.95(2.07-30.58) and4.01(0.73-13.89) nmol L-1, respectively. Ingeneral, average concentrations of DMSOd and DMSOp in the BS and YS showedobvious seasonal variations, with spring average values being2.8and6.3times higherthan autumn values. In the two studied seasons, the horizontal distributions ofDMSOd in the BS were obviously influenced by anthropogenic activities, highconcentrations of DMSOd appeared around the Yellow River estuary. When thedistribution patterns were compared with each other in different seasons, they werenearly synoptic and decreased from inshore to offshore sites, without being stronglybiased by temporal change. Overall, in spring, elevated concentrations of DMSOdappeared on the northwest of the Cheju Island, where high levels of dissolvedinorganic nitrogen (DIN) and phosphate occurred, high DMSOp concentrationappeared in Jiangsu coastal water, under the combined actions of the high runoff andthe prevailing southerly winds, part of Yangtze River diluted water flowed toward thenortheast into the Jiangsu coastal area, the Yangtze River diluted water and SubeiCoastal Current brought abundant nutrients, promoted phytoplankton growth andaltered the composition of the phytoplankton community, thus inducing higherDMSOp concentrations in the coastal seawater. In autumn, the relatively highDMSOd concentrations were observed near the Lianyungang, corresponding torelatively high DOC concentrations, which might be related to the photochemicaloxidation of DMS. High DMSOp concentrations appeared in the middle of the YSwith relatively high concentrations of Chl-a, suggesting that the biosynthesis ofDMSO by phytoplankton appeared to be an important source of DMSOp.(2) DMSOp/Chl-a ratios showed clear seasonal patterns with an average value7times higher in spring than in autumn, which might be due to the different algalspecies in the BS and the YS. The species and biomass of dinoflagellates decreasesharply from spring to autumn, the lost proportion of dinoflagellates might be responsible for the lower ratios of DMSOp/Chl-a in autumn.(3) Linear regression analyses for DMSOd, DMS, DMSOp and bacterialabundance data showed that DMSOd did not correlated with DMS and DMSOp, butcorrelated well with bacterial abundance, indicating that the bacterial oxidation ofDMS might be responsible for the source of DMSOd during these two cruises. Nosignificant correlation was found between DMSOp and Chl-a, which might be due tothe algae bloom in the YS, the concentrations of DMSOp varied widely, thus nocorrelation between DMSOp and Chl-a was observed in these two cruises.3. Studies on DMS, DMSP and DMSO produced by three species of marine algaeThe production of DMS, DMSP and DMSO were studied in different growthstages of Amphidinium carterae, Chaeteceros curvisetus and Prymnesium saltans.Furthermore, we have studied the basic physiology of DMS, DMSP and DMSO inaxenic cultures. Concomitant analysis of dimethyl sulfur compounds provides furtherinsight into the role and fate of DMSP and DMSO in the marine sulphur cycle. Theresults are drawn as follows:(1) The growth cycles of the Amphidinium carterae, Chaetoceros curvisetus andPrymnesium saltans in the f/2media were20days. They reached the largestabundance on Day11,9and11, respectively, after having been inoculated, theaverage growth rates were0.218,0.196,0.219d-1, respectively. The concentrations ofDMSPp and DMSPd in the Amphidinium carterae were3.32(0.57-9.68) and6.41(0.96-18.32) μmol L-1, respectively. The concentrations of DMSPp and DMSPd in theChaetoceros curvisetus were0.47(0.17-1.25) and1.13(0.49-3.23) μmol L-1, thehighest concentration appeared on Day9and9. The concentrations of DMSPp andDMSPd in the Prymnesium saltans were2.72(0.56-7.62) and5.27(1.02-14.25) μmolL-1. The average concentrations of DMSPp per cell in Amphidinium carterae,Chaetoceros curvisetus and Prymnesium saltans were46.6,4.85and65.2nmol,respectively, suggesting the production capacity of DMSP in Amphidinium carteraeand Prymnesium saltans was higher than Chaeteceros curvisetus.(2) It is obvious that the DMS concentrations showed marked differences amongthe three microalgae as well as among the different phases of growth. The averageconcentrations of DMS in Amphidinium carterae, Chaetoceros curvisetus and Prymnesium saltans were0.80(0.01-2.89),0.61(0.005-2.03),0.74(0.008-1.76) μmolL-1. The DMS values increased in paralled with cell numbers and DMSPdconcentrations during the logarithmic phase, but there were some days time lagbetween the peaks of DMS and DMSPd concentrations, the highest concentration ofDMS appeared in the senescent period for all the cultures, natural senescence mightstimulate the liberation of DMSP from cells into the water where it is partly brokendown to DMS. DMS concentrations correlated well with cell numbers in the culturesof Amphidinium carterae, but not for Chaetoceros curvisetus and Prymnesium saltans,indicating that the conversion might be affected by many factors.(3) The average concentrations of DMSOd (DMSOp) in Amphidinium carterae,Chaetoceros curvisetus and Prymnesium saltans were7.28(7.89),4.89(2.93) and13.80(12.78) μmol L-1. The average concentrations of DMSOp per cell in the culturesof Amphidinium carterae, Chaetoceros curvisetus and Prymnesium saltans were99.91,30.28and275.45nmol, respectively, suggesting the production capacity ofDMSOp in Amphidinium carterae and Prymnesium saltans was higher thanChaeteceros curvisetus. Significant correlation was found between DMSOp andDMSOd concentrations in the three cultures.