A Primary Study On The Fatty Acid As Biomarker And Its Application In The Study Of Bacteria's Role In The Nutrients Interface Exchange Process At The Area Of HAB High-frequency-occurrence In East China Sea (ECS)

With the use of sophisticated apparatus like GC —MS and GC —IsoMS, inter-seasonal (Aug. Nov. 2002; Febr. 2003) surface and core sediment samples were analyzed, in order to give a preliminary measurement of the content, composition, horizontal, vertical variation and seasonal fluctuation pattern of the fatty acid at the Yangtze River Estuary and its adjacent area, which is well known for its high-frequency occurrence of harmful algae bloom (HAB). Faced with huge quantities of parametric data, appropriate statistical method was successfully employed here, to elucidate the complex relationship between the fatty acid compositions, content and relative environmental parameters. Assisted with the advanced carbon stable isotope techniques, the source and sink of these fatty acids were discriminated clearly, and more over, the microbial-origin acids were specified and taken into account in the nutrients exchange process across the sediment-seawater interface so that the role of microbe was evaluated indirectly concerning the nutrients exchange rate and species transformation. The results are listed as follows:1 Rich fatty acids are detected in the surface and core sediment samples in EastChina Sea, ranging from 0.1~100ug/g(per dried sediment).As for the fatty acid from the surface sediment, its total content and composition undergo a strong seasonal variation pattern. The principle component analysis (PCA) of the bulk of fatty acids data through summer, autumn and winter shows that the largest contributor of fatty acids in summer are the phytoplankton and zooplankton in the marine ecosystem while the smallest is the terrigenous inputs. However, the terrigenous contribution increased from summer to autumn to winter while the fresh fatty acids originated in summer begin to undergo sedimentation anddegradation at the sediment, approved by the factor analysis of those influential factors on the total variance of ￡ FA content.2 Cluster analysis was applied to the data of 23 kinds of fatty acids due to their relative abundance in sediment. The results demonstrated that three typical clusters are formed, which are considered to represent three sources of fatty acids. The first cluster is composed of long-chain fatty acids, the carbon skeleton of which ranges from 20 to 26. The second cluster comprises branched fatty acids and odd-numbered carboxyl acid, dominated by pentadecnoic acid, heptadecnoic acid and their isomers and anteisomers. The third cluster mainly consists of fatty acid of even-carbon-number, monounsaturated and polyunsaturated fatty acids. Hereby, a rough conclusion would be like that fatty acid with carbon number over 20 is regarded as terrigenous origin, and cluster the second is of microbial nature, and the last cluster is characterized of marine algae and zooplankton. This is definitely approved by the compound-specific stable carbon isotope ratio analysis-81 C%o. The cluster one was detected with an average of — 31.7%o of 13C relative abundance, while the second cluster is —25.3%o and the third is —22.3%o. This apparent isotope discrepancy was the direct results of isotope bio-fractionation, which is totally different with respect to specific sources.3 The ratio between SFA and USFA varies at depth, coupled with the vertical 813C fluctuation. This is going to cast light on the diagenesis process of fatty acids in the sediment. Since the research area is characterized by an average of shallow water, high productivity and sedimentation rate, the fatty acids undergo only a few diagenesis processes while falling through the water column, before their final arrival at the seabed. The insufficient diagenesis would definitely lead to a comparatively reduction condition. The isotope data had re-approved the relative strong organic matter diagenesis activities at the depth of 2-3cm, since the isotope ratio or in another word, the isotopic fractionation was radically changed at this depth.The microbial-origin FA was used here to give an indirect assessment on the bacterial abundance at the sediment-seawater interface. In this way and combined with hydrological, geological, chemical and biological parametric data, PCA and multivariate linear regression model are employed, to give a quasi-quantification of the role of bacteria on the nutrients exchange process across the sediment-seawater interface.> A feint influence was observed on the silicate exchange process and there is no significant variance before and after the biological factor is considered in the regression model. The controlling factors of the exchange are categorized into two groups: the first stands for the dissolution process and is composed of temperature and clay content-another important silicate source. The other is the diffusion process, composed of interstitial water content and silicate concentration gap between the interstitial water and the overlying water.> Bacterial played a significant role of limitation on the phosphor exchange rate and flux, together with grain size and clay content of sediment. All these factors suppress the release of PO4-P from the sediment by adsorption or absorption. This mechanism seems reasonable since it would like to accounted for the most negative PO4-P fluxes in summer, when a high productivity occurred usually. Temperature is the only factor that promotes the release of PO4-P while the other possible positive contribution from the other factorAGAP-nutrients is greatly suppressed and is small enough to be neglected. This result is consist with the PCA results. Unusually, DO is not considered here to be the essential factor that controls the redox potential while the organic content is the right one.> Bacteria play such a critical role on the species transformation of nitrogen at the seawater-sediment interface. Ammonia is promoted below the interface under a reduction state, while nitrate is favored above the interface under oxidation condition. The mean grain size and clay content ofsediment limit the exchange process by physical adsorption, and the interstitial water serves as a pool to accommodate all species of nitrogen. Temperature speeds up the diffusion rate and to some extent influence the nitrification and denitrification process indirectly.To summarize, fatty acid is a kind of very informative biomarker to discriminate the source and sink of organic matter. The application of stable carbon isotope would assist the interpretation of fatty acid data. Bacterial-origin fatty acid could be used to give a satisfactory assessment on the species and abundance of bacterial at the sediment, thus it could be used to evaluate the role of bacteria in the nutrients exchange process at quasi-quantification level.