1 Niche differentiation of ammonia and nitrite oxidizers along a salinity gradient 2 from the Pearl River estuary to the South China Sea 3 4 Lei Hou1,2,†, Xiabing Xie1,†, Xianhui Wan1, Shuh-Ji Kao1,2, Nianzhi Jiao1,2, Yao Zhang1,2 5 1State Key Laboratory of Marine Environmental Science, Xiamen University, Xiamen 361101, China 6 2College of Ocean and Earth Sciences, Xiamen University, Xiamen 361101, China 7 8 Correspondence to: Yao Zhang (
[email protected]) 9 †Contributed equally 10 1 1 Abstract 2 The niche differentiation of ammonia and nitrite oxidizers are controversial because they display 3 disparate patterns in estuarine, coastal, and oceanic regimes. We analyzed diversity and abundance of 4 ammonia-oxidizing archaea (AOA) and β-proteobacteria (AOB), nitrite-oxidizing bacteria (NOB), and 5 nitrification rates to identify their niche differentiation along a salinity gradient from the Pearl River 6 estuary to the South China Sea. AOA were generally more abundant than β-AOB; however, AOB more 7 clearly attached to particles compared with AOA in the upper reaches of the Pearl River estuary. The 8 NOB Nitrospira had higher abundances in the upper and middle reaches of the Pearl River estuary, 9 while Nitrospina was dominant in the lower estuary. In addition, AOB and Nitrospira could be more 10 active than AOA and Nitrospina since significantly positive correlations were observed between their 11 gene abundance and the nitrification rate in the Pearl River estuary. There is a significant positive 12 correlation between ammonia and nitrite oxidizer abundances in the hypoxic waters of the estuary, 13 suggesting a possible coupling through metabolic interactions between them.