Abundance and distribution of complete ammonia oxidizers under a salinity gradient in the Yangtze Estuary and its adjacent sea

This study investigated the distribution, community composition, and environmental drivers of complete ammonia-oxidizing bacteria (comammox) in estuarine sediments, focusing on the Yangtze River Estuary. It examined how environmental gradients-particularly salinity, pH, and ammonium concentration-influence comammox abundance and diversity, thereby clarifying their ecological niche in transitional aquatic ecosystems. Seventeen sediment samples were collected across the estuary. Quantitative Polymerase Chain Reaction and high-throughput sequencing revealed that comammox clade A.2 dominated the community. Salinity showed a strong negative correlation with comammox abundance, while ammonia-oxidizing bacteria and ammonia-oxidizing archaea abundance increased with rising salinity. Comammox was the predominant ammonia oxidizer in low-salinity sediments, with its abundance also shaped by pH and ammonium levels. Comammox is widespread and important contributors to nitrification in Yangtze Estuary estuarine sediments, especially under low-salinity conditions. Their community structure and abundance are primarily shaped by salinity, indicating a distinct ecological niche compared to canonical ammonia oxidizers.