[A-OS13] Marine ecosystems and biogeochemical cycles: theory, observation and modeling

[AOS13-06] Microbe-driven cycling of dissolved organic nitrogen in the Changjiang River estuary: the case study of polyamine and dissolved free amino acids

*Qian Liu^1,3、Yuhao Chen^2,1 (1.Second Institute of Oceanography, Ministry of Natural Resources, P. R. China、2.Shanghai Jiaotong University, P. R. China、3.Zhejiang University, P. R. China)

キーワード：Dissolved organic nitrogen cycling, Polyamines, Dissolved free amino acids, Metagenomic analysis, Microbe-driven, Changjiang River Estuary

The Changjiang River is the largest river in the East Asia. High water and sediment fluxes enrich the estuary with nutrients, organic matter, pollutions, etc. Dissolved organic nitrogen (DON) occupies a large fraction of total nitrogen pool, but their dynamics and contributions to nitrogen (N) and carbon (C) cycling are less investigated. Labile DON (LDON) is considered as the fraction of bioavailable DON, contributing significantly to N cycling in seawaters. In this study, we determined spatio-temporal distributions of turnover, uptake rates and oxidation rates of two representatives of LDON (polyamines and amino acids), and estimated their contributions to prokaryotic N and C demands in the continuum from the inner estuary to the continental shelf of the East China Sea in order to understand the dynamics and ecological role of DON in coastal waters. The relative abundances of functional genes in prokaryotic microorganisms and major microbes involved in LDON transport and transformation were estimated by metagenomic analysis. Results showed that both groups of LDON turned over rapidly within the estuary (<1 day) and decreased to the shelf waters along the salinity gradient. The mean turnover rates reached the peak in the summer (July). The large dissolved pools and rapid turnover of dissolved polyamines and amino acids resulted in fast microbial uptake, which contributed to prokaryotic N and C productions more significantly in March (N, 223%; C, 120%, median) and less in October (N, 22.6%; C, 9.76%). Polyamines were more significant in prokaryotic production within the estuary, while amino acids contributed more in shelf waters, indicating differences in LDON fate driven by microbes. The correlation between turnover or uptake rates of two LDON groups and physiochemical parameters suggested that polyamine cycling was more affected by salinity, while amino acids dynamics was partially associated with chlorophyll a, indicating the involvement of phytoplankton in amino acids uptake. Particle-associated and free-living microbial consortiums controlled LDON utilization in estuarine and shelf waters, respectively, according to both in-situ rate measurements and gene abundances. The coupling between rates and gene numbers of LDON suggests the active involvement of prokaryotic microorganisms, especially heterotrophic bacteria, in LDON transport and transformation. This study gives an insight into fates and contributions of DON to coastal ecosystems.