Studies On Effects Of Zooplankton And Bacteria On DMSP Transformation In Seawater

Dimethylsulfoniopropionate (DMSP), an important sulfur compound in marinewaters, was considered to play significant roles in microbial food chain and served asprimary carbon/sulfur source for phytoplankton. It was generally known that DMSPcould mainly product dimethylsulfide (DMS) through enzymatic cleavage pathway.DMS has key effects on global climate regulation and the formation of acid rain. Theoccurrence of DMSP and its further turnover to DMS are complex web in whichbiological processes were thought to be the principal factor involved. Biologicalprocesses control the concentration of DMS/DMSP in seawaters and flux of DMSrelease to the atmosphere. Studies on how biological processes were act on the DMSand DMSP contribute to understanding of the biogeochemical cycles of sulfur in theocean.In the present dissertation, we focused on zooplankton grazing and bacteriaconsumption which were the two main factors influence the distribution of DMSP,aimed at exploring the mechanism of how zooplankton grazing and bacteriaconsumption affected the release of DMSP into the water column. At first, we chosethe Qingdao coastal water-Jiaozhou Bay as the study area. Zooplankton compositionand abundance were studied based on the zooplankton in situ samples collected inJiaozhou Bay during the time period from July to November,2010. The distributionand seasonal varied of zooplankton, and their relationships with the environmentalfactors of the seawater (water depth, temperature, salinity and Chl-a concentrations),as well as concentrations of dimethysulfide (DMS), dissolveddimethylsulfoniopropionate (DMSPd), particulate dimethylsulfoniopropionate(DMSPp) were analyzed. Secondly, we used the axenic phytoplankton culture ofdifferent DMSP contents fed copepods. By using MPN (Most Probable Numbers)method, we studied whether the concentration of DMSP-consuming bacteria (DCB)associated with copepod was related to DMSP content in the food under laboratorial experiments. DCB isolated from copepod body were incubated in different carbonsources afterwards. By observing their growth situation, we could get to know thepossible pathway of DCB utilize DMSP. Another objective of this dissertation was toquantitatively study the free-living DCB and Acartia tonsa-associated DCB in YorkRiver, Chesapeake Bay, USA. A. tonsa was the dominant species in this area,furthermore, we monitored the distribution and monthly variation of DCB. The mainconclusions were drawn as follows:1. Study on relationship between zooplankton abundance and DMSdistribution in Jiaozhou Bay in summer and autumn of2010Zooplankton composition and abundance were studied in Jiaozhou Bay during thetime period from July to November,2010. The dominant species in this ara wereBrachyura zoea larva、Penilia avirostris Dana、Macrura larva、 Sagitta crassaTokioka、Labidocera bipinnata Tanaka、Centropages dorsispinatus Thompson etScott、Acartia pacifica Steuer、Muggiaea atlantica Cunningham、Pleurobrachiaglobosa Moser、Noctiluca scientillans Kofoid et Swezy、Calanus sinicus Brodsky, etal.(1) Zooplankton distribution in Jiaozhou Bay were impacted by environmentalfactors of the water column (water depth, temperature, salinity and Chl-aconcentrations, et al) by varying degrees. As it was shown by the results, the averagezooplankton abundance in Jiaozhou Bay was unevenly distributed, with highestzooplankton abundance observed at Bay month (E3station), and the lowest at Easterncoast (B5station). The average zooplankton abundance of eastern stations (B5, C5,D4) were generally low which all below100ind/m~3. For the monthly variation ofzooplankton abundance, on the one hand, because the dominant species of water bodycontroled the total abundance of zooplankton, on the other hand, periodicityzooplankton larvae and adult emergence greatly impacted zooplankton abundance, thezooplankton abundance of Jiaozhou Bay showed significant seasonal variation. Theaverage zooplankton abundance from July to November ranged from87.2~246.2ind/m~3, with highest value found in summer at August (246.2ind/m~3), and lowest one in autumn at October (87.2ind/m~3). Zooplankton abundance in summer was higher thanthat in autumn.(2) Zooplankton abundance didn’t show strong relationships with water depth,temperature, salinity, Chl-a concentration, bacteria number. There was significantlypositive correlation between zooplankton abundance and DMS in October (p<0.05),and zooplankton abundance and DMSPp in Novemeber (p<0.05). Zooplanktonbiomass could accelerate DMSP release. But in other months (July, August,September), the correlations were not significant (p>0.05). Our results showed thatthe influence of zooplankton grazing on the release of DMS from water column wasaffected by complex environmental factors and further research was needed betweenzooplankton and DMS.(3) Copepod was the dominant group of zooplankton in Jiaozhou Bay, it wasobserved in each month during our observation period and could compose about3~60.4%of total zooplankton abundance, with the highest percentage (60%) inSeptember. The copepods were inhomogeneous in their horizon distribution, it couldvaried up to an order of magnitude. The highest copepod abundance, which reach to80.21ind/m~3was found in northeast (A2station), followed by Bay mouth. Generally,copepod abundance of coastal water in Jiaozhou Bay was low, all below20ind/m~3,with the lowest value (8.79ind/m~3) state on the west (C1station).(4) There was no strong correlation between copepod abundance and water depth,temperature, surface salinity or Chl-a concentration (p>0.05). Copepod abundancewas significantly positive related to water depth in October and November (p<0.05),while negatively related to water temperature in November (p<0.05). In other months(July, August, September), the correlations between copepod abundance andenvironemtnal factors were not significant (p>0.05). It only showed significantpositive correlation between copepod abundance and DMS in July, DMSP inNovember (p<0.05), in other months (August, September, October), the correlationsbetween copepod abundance and DMS/DMSPd/DMSPp were not significant(p>0.05).2. Dietary effects on abundance and carbon utilization ability of DMSP-consuming bacteria associated with the copepod Acartia tonsa DanaIn this part, we chose five axenic phytoplankton culture: Skeletonema marinoi、Tetraselmis sp.、Alexandrium tamarense、Rhodomonas salina、Dunaliella tertiolectaas food. Under the laboratorial experiments, we measured DMSP-consuming bacteria（DCB） number associated with A. tonsa by using MPN method. Then studied howDCB growth on different carbon sources. The results showed that:(1) DCB were recovered from all treatments, among which the S. marinoitreatment yielded the highest abundance (3.64×10~6DCB copepod-1) and the R. salinatreatment the lowest (2.40×10~4DCB copepod-1). We found comparable number ofDCB in D. tertiolecta treatment and A. tamarense, while D. tertiolecta had nodetectable amount of DMSP and A. tamarense was a DMSP high producer, DCBabundance associated with A. tonsa was independent from dietary DMSP content.(2) The DCB were able to grow on carbon sources other than DMSP, differentgrowth rates indicated different substrates utilize abilities. Among the five substrates,Glycine betaine and DMSP had similar molecular structure in which all containedcarbon in the carbon chain form, and MMA, DMA, DMSO all contained carbon inmethyl form. The ability of utilize all the substrates indicating their ability toassimilate both carboxyl chain carbon and methyl carbon.(3) The grazing experiment results showed that DCB abundance associated withzooplankton was independence from food DMSP, this would allow DCB to maintaintheir populations among zooplankton even under DMSP-poor conditions, and theywould readily consume DMSP when it became available. This coupling between DCBand zooplankton represented a persistent and potentially important sink of DMSP inthe marine system.3. The diel migration and seasonal distribution of DMSP-consuming bacteriaassociated with copepod in York RiverDCB were ubiquitous in ocean environment and play important roles on DMSPtransportation. In this section, we studied free-living DCB and zooplankton-associatedDCB in York River, Virginia, USA, during May to October,2012. As it was shown inresults, DCB were present in all the samples indicating their universal existence in the seawater. For DCB dial migration, we found it was interestingly that free-living DCBwas lower in the daytime when sample at the beginning of the month, on the contrary,it would be lower in the nighttime when sample at the end of the month.The specificmechanism which controled this process needed to be further studied. Free-livingDCB showed positive correlations with temperature and salinity (p<0.05), whilezooplankton-associated DCB only showed positive correlation with salinity, indicatedthat substrates availability could be as important as or more important thantemperature in regulating heterotrophic microbial process. Association withzooplankton might give attached bacteria access to resources, thereby moderatingtheir responses to environmental temperature.