Spatial variability pattern of the anaerobic ammonia-oxidizing bacterial community across a salinity gradient from river to ocean

In natural habitats, the diversity of anaerobic ammonia-oxidizing (anammox) bacteria could be affected by multiple environmental variables. In this study, we investigated the distribution of the anammox bacterial community in surface sediment from the Dongjiang River (riverine sediment, DJ) to the Pearl River Estuary (estuarine sediment, PRE) and then to the South China Sea (coastal sediment, SCS). The results revealed evident differences in the structural diversity of anammox bacteria in three different habitats. Candidatus Brocadia accounted for approximately 90% of the total anammox bacteria in DJ, conversely, Ca . Scalindua dominated in the SCS. Nevertheless, Ca . Scalindua, Ca . Brocadia and Ca . Kuenenia coexisted in the PRE. The qPCR results indicated that anammox bacterial 16S rRNA gene abundance ranged from 2.23 × 10 5 to 1.19 × 10 7 copies g −1 of wet weight, but no significant correlation was found between the abundances and environmental variables ( p > 0.05). The relative abundances of Ca . Brocadia gradually decreased with increasing salinity, and Ca . Scalindua showed the opposite trend, suggesting that salinity was a crucial factor in sculpturing the community composition of anammox bacteria in natural environments. Ca . Brocadia should be able to live in freshwater ecosystems, but it can also tolerate a certain level of salinity. Ca . Scalindua was halophilic anammox bacterium and exists only in saline environments. Ca . Kuenenia could adapt to a wide range of salinity and preferred to live in high DIN level conditions according to our search. The distribution pattern of anammox bacteria may be the result of microbial migration and long-term adaptation to salinity.