Different Responses of Ammonia-Oxidizing Archaea and Bacteria to Oxygen Depletion During Niche Separation in Water Column of Bohai Sea

Summer hypoxia in the bottom water of the Bohai Sea is accompanied by marked changes in water-column structure and nutrient conditions, which may affect ammonia-oxidizing microorganisms and thus alter nitrification-related nitrogen cycling. To examine these responses, seawater samples were collected along an inshore–offshore transect from the coastal waters off Qinhuangdao City to the central Bohai Sea in June, July, and August 2018. Surface, middle, and bottom waters were sampled, and physicochemical variables, including temperature, salinity, dissolved oxygen (DO), and inorganic nitrogen concentrations, were measured. Ammonia-oxidizing archaea (AOA) and ammonia-oxidizing bacteria (AOB) were then analyzed using amoA amplicon sequencing to compare their diversity, community structure, genus-level composition, and dominant OTU patterns. From June to August, along with temperature increase, the water column showed stronger stratification and oxygen depletion in the bottom water, together with shifts in salinity and inorganic nitrogen distributions. AOA and AOB showed different temporal responses. AOA diversity surpassed that of AOB in July, but AOB became more diverse in August. Principal coordinates analysis showed clear separation of AOA communities between July and August, and sampling month had a significant effect on AOA community structure (PERMANOVA, p = 0.009). In contrast, AOB communities did not show significant month-related separation (p = 0.157). According to environmental fitting analysis, AOA community structure was predominantly associated with temperature and salinity, with only a marginal link to NH4+, while pH and salinity were the key factors shaping AOB communities. At the genus level, AOA was dominated by Nitrosopumilus and AOB was dominated by Nitrosospira. Overall, the results indicate that AOA showed stronger temporal variation than AOB during the development of summer hypoxia in seawater of Bohai Sea.