Comparative analysis of niche adaptation strategies of AOA, AOB, and comammox along a gate-controlled river-estuary continuum.

Ammonia oxidizers are key players in the biogeochemical nitrogen cycle. However, in critical ecological zones such as estuaries, especially those affected by widespread anthropogenic dam control, our understanding of their occurrence, ecological performance, and survival strategies remains elusive. Here, we sampled sediments along the Haihe River-Estuary continuum in China, controlled by the Haihe Tidal Gate, and employed a combination of biochemical and metagenomic approaches to investigate the abundance, activity, and composition of ammonia-oxidizing archaea (AOA), ammonia-oxidizing bacteria (AOB), and complete ammonia oxidizers (comammox). We also conducted an extensive comparison of the salinity adaptation mechanisms of different ammonia oxidizers. We found that AOB (57.55 ± 11.46 %) dominated the nitrification process upstream of the tidal gate, while comammox (68.22 ± 14.42 %) played the major role downstream. Redundancy analysis results showed that total nitrogen, ammonium, and salinity were the primary factors influencing the abundance, activity, and contribution of ammonia oxidizers. The abundance and activity of AOB were significantly positively correlated with ammonium. KEGG annotation results showed that AOA Nitrososphaera, AOB Nitrosomonas, and comammox Nitrospira had 7, 31, and 22 genes associated to salinity adaptation, respectively, and were capable of employing both the "salt-in" and "salt-out" strategies. Metagenome assembly results indicated that comammox outperformed AOA primarily in compatible solute accumulation; AOA can synthesize glutamate, whereas comammox Nitrospira can additionally synthesize glycine betaine, choline, and trehalose. The tidal gate caused sharp shifts in ammonium (from 4.10 ± 3.28 mg·kg to 0.45 ± 0.10 mg·kg) and salinity (from 1.64 ± 0.48 ppt to 3.26 ± 0.89 ppt), playing a dominant role in driving niche differentiation of ammonia oxidizers along the Haihe River-Estuary continuum. These findings provide profound insights into the nitrogen cycle in freshwater-saltwater transition zones, especially in today's world where estuaries are widely controlled by tidal gates.