Oxygen depletion shifts niche partitioning of AOA and AOB in Bohai Sea sediments

Nitrification is a keystone pathway that oxidizes reduced nitrogen to nitrate in the ocean. Under oxygen depletion, the balance between ammonia-oxidizing archaea (AOA) and bacteria (AOB) likely determines where and how strongly nitrification persists. Here, we surveyed surface sediments along a five-site inshore-offshore transect across the Bohai Sea hypoxic zone from early to late summer to resolve AOA/AOB niche separation during oxygen depletion, combining hydrography with amoA amplicon profiling, phylogenetic reconstruction of the top 25 OTUs, and monthly co-occurrence networks. As bottom-water oxygen declined, AOA showed higher α-diversity than AOB. When pooled across months, the ratio AOA/(AOA + AOB) showed no monotonic relationship with DO. In contrast, within-month regressions diverged markedly, with a negative trend in August, when DO was lowest. Community profiles showed that AOA remained dominated by Nitrosopumilus but varied in composition among months, whereas AOB was consistently dominated by Nitrosospira across sites and months. Phylogenetic reconstruction showed that Bohai AOA OTUs clustered mainly with OMZ- or hypoxia-associated reference clades, suggesting a tighter phylogenetic linkage to oxygen-depleted habitats. In contrast, AOB OTUs grouped with both typical coastal sediment and terrestrial/soil references, indicating broader marine-terrestrial affinities. August networks showed negative correlations between AOA and AOB, consistent with stronger niche segregation under oxygen depletion. Together, these results support active selection of AOA during oxygen depletion, whereas AOB appeared to respond more passively under resource competition.