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The effects of allochthonous dissolved organic matter(DOM)on the growth and photosynthesis of Phaeodactylum tricutum were investigated.P.tricutum incubated in f/2 medium was exposed to DOM additives,which were extracted from the plant and sediment samples of a salt marsh in North Branch of the Yangtze estuary,China.During 12 days incubation,the chlorophyll fluorescence parameters of P.tricutum were measured by a Phyto-PAM phytoplankton analyzer.Spectral properties of DOM in algae filtrates were also observed.The concentrations of chlorophyll a,active chlorophyll a,and the maximum quantum yield of photosystem Ⅱsignificantly decreased after four days of incubation,suggesting that the growth and photosynthetic efficiency of P.tricutum were inhibited.After adding sediment-DOM extract,both a250/a365(the ratio of the absorption coefficients at 250 and 365 nm)and S values(spectral slope coefficients)of algae filtrates declined in the first two days,which demonstrated a loss of low molecular weight DOM.Parallel factor analysis of fluorescence spectra of DOM in algae filtrates revealed that DOM could be classified into two humic-like and two protein-like components.The fluorescence intensity of tyrosine-like component originating from algae increased significantly during incubation.This study supports the hypothesis that allochthonous DOM derived from salt marsh plant and sediment have a strong influence on the adjacent aquatic ecosystems.