Program     The geochemical and biological study of corals       Poster Nitrogen fixation and transfer of diazotroph derived nitrogen (DDN) in coral reef system P-G3-07 Hua-Xia Sheng* , State Key Laboratory of Marine Environmental Science, Xiamen University Xianhui Sean Wan, State Key Laboratory of Marine Environmental Science, Xiamen University Chichi Liu, State Key Laboratory of Marine Environmental Science, Xiamen University Bobo Zou, State Key Laboratory of Marine Environmental Science, Xiamen University Hui Huang, South China Sea Institute of Oceanology, Chinese Academy of Sciences Shuh-Ji Kao, State Key Laboratory of Marine Environmental Science, Xiamen University Presenter Email: hxsheng@xmu.edu.cn Coral reefs are essential ecosystems in tropical oceans, providing extensive ecosystem goods and services to around 500 million people. It is one of the most productive ecosystems but living in oligotrophic ocean. This has given rise to the ecosystem conundrum which is called as the "Darwin's paradox". A major hypothesis was that biological nitrogen fixation by cyanobacteria and heterotrophic bacteria provides the "new" N for the oligotrophic coral reef ecosystem and thus plays an important role in flourishing productivity. However, quantitative information on the contribution of N fixation to primary production remains sparse in field. Furthermore, transfer of the diazotrogh derived nitrogen (DDN) from diazotroph to non-diazotroph is poorly understood. Here we measured N fixation rate and its subsequent transfer in a typical tropical coral ecosystem by using 15N-N2 tracer methods. The result shows that, the symbiotic N fixation microbes in crustose coralline algae (CCA) and Pocillopora damicornis both contributed to the N fixation in coral system, and N fixation rate of CCA is significantly higher than coral halobiont. The DDN in coral holobiont can be transferred to symbiodiniums (zooxanthellae) and coral host cells, implying the redistribution and reuse of "new" N in coral holobiont.

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