Abstract
Abstract. 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 β-proteobacteria (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 β-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.
Highlights
Nitrification, the biological oxidation of ammonia to nitrate, is a fundamental process in the nitrogen cycle and plays a key role in estuarine and marine ecosystems
The upper reaches receive a small amount of freshwater, sewage, and industrial effluent discharge; the middle reaches receive about half of the freshwater from the North and West rivers, tributaries of the Pearl River, with little salinity stratification; and the lower reaches are controlled mainly by estuarine mixing of freshwater and seawater (Wang et al, 2012)
The upper Pearl River estuary (PRE) was characterized by hypoxic waters containing sufficient nutrients; Dissolved oxygen (DO) concentrations increased seaward, while the nutrient and total suspended material (TSM) concentrations distinctly decreased seaward
Summary
Nitrification, the biological oxidation of ammonia to nitrate (the largest pool of fixed inorganic nitrogen in water bodies), is a fundamental process in the nitrogen cycle and plays a key role in estuarine and marine ecosystems. Nitrification includes both ammonia and nitrite oxidation, which are catalyzed by different microorganisms that may occupy broad niches in estuarine and marine environments. In sharp contrast to ammonia oxidation, nitrite oxidation, which is the second step in nitrification, has been investigated less in estuarine and marine ecosystems, despite bacterial nitrite oxidation being the only biochemical reaction known to form nitrate in aerobic conditions.
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