Program     PACECS: Processes and Approaches of Coastal Ecosystem Carbon Sequestration/ MEMCS: Mechanisms and Environmental Effects of Microbes on Carbon Sequestration       Poster Niche differentiation of ammonia and nitrite oxidizers along a salinity gradient from the Pearl River estuary to the South China Sea P-SPS2-04-S Lei Hou* , State Key Laboratory of Marine Environmental Science, Xiamen University, Xiamen 361101, China; College of Ocean and Earth Sciences, Xiamen University, Xiamen 361101, China Xiabing Xie, State Key Laboratory of Marine Environmental Science, Xiamen University, Xiamen 361101, China Xianhui Wan, State Key Laboratory of Marine Environmental Science, Xiamen University, Xiamen 361101, China Shuh-Ji Kao, State Key Laboratory of Marine Environmental Science, Xiamen University, Xiamen 361101, China; College of Ocean and Earth Sciences, Xiamen University, Xiamen 361101, China Nianzhi Jiao, Yao Zhang, State Key Laboratory of Marine Environmental Science, Xiamen University, Xiamen 361101, China; College of Ocean and Earth Sciences, Xiamen University, Xiamen 361101, China Presenter Email: houleiwz@139.com The niche differentiation of ammonia and nitrite oxidizers is controversial because they display disparate patterns in estuarine, coastal, and oceanic regimes. We analyzed diversity and abundance of ammonia-oxidizing archaea (AOA) and betaproteobacteria (AOB), nitrite-oxidizing bacteria (NOB), and nitrification rates to identify their niche differentiation along a salinity gradient from the Pearl River estuary to the South China Sea. AOA were generally more abundant than beta-AOB; however, AOB more clearly attached to particles compared with AOA in the upper reaches of the Pearl River estuary. The NOB Nitrospira had higher abundances in the upper and middle reaches of the Pearl River estuary, while Nitrospina was dominant in the lower estuary. In addition, AOB and Nitrospira could be more active than AOA and Nitrospina since significantly positive correlations were observed between their gene abundance and the nitrification rate in the Pearl River estuary. There is a significant positive correlation between ammonia and nitrite oxidizer abundances in the hypoxic waters of the estuary, suggesting a possible coupling through metabolic interactions between them. Phylogenetic analysis further revealed that the AOA and NOB Nitrospina subgroups can be separated into different niches based on their adaptations to substrate levels. Water mass mixing is apparently crucial in regulating the distribution of nitrifiers from the estuary to open ocean. However, when eliminating water mass effect, the substrate availability and the nitrifiers' adaptations to substrate availability via their ecological strategies essentially determine their niche differentiation.