Shifts in the relative abundance and potential rates of sediment ammonia-oxidizing archaea and bacteria along environmental gradients of an urban river-estuary-adjacent sea continuum.

Ammonia-oxidizing archaea (AOA) and bacteria (AOB) play important roles in N cycling in sediments globally. However, little is known about their ammonia oxidation rates along a river-estuary-sea continuum. In this study, we investigated how the potential ammonia oxidation rates (PARs) of AOA and AOB changed spatially along a continuum comprising three habitats: the Shanghai urban river network, the Yangtze Estuary, and the adjacent East China Sea, in summer and winter. The AOA and AOB PARs (0.53 ± 0.49 and 0.72 ± 0.69 μg N g d, mean ± SD, respectively) and their amoA gene abundance (0.47 ± 0.85 × 10 and 2.4 ± 3.54 × 10 copies g, respectively) decreased along the continuum, particularly from the urban river to the estuary, driven by decreasing sediment total organic C and N and other correlated inorganic nutrients (e.g., NH) along the gradient of anthropogenic influences. These spatial patterns were consistent between the seasons. The urban river network, where the anthropogenic influences were strongest, saw the largest seasonal differences, as both AOA and AOB had higher PARs and abundance in summer than in winter. The ratios between AOA and AOB PARs (0.87 ± 0.51) and gene abundances (0.25 ± 0.24), however, were predominantly <1, indicating an AOB-dominated community. Comparing the different NH consumption pathways, total aerobic oxidation accounted for 12-26% of the total consumption, with the largest proportion in the estuary, where the system was well oxygenated, and the lowest in the adjacent sea, where inorganic N was highly depleted. This study revealed the spatiotemporal patterns of AOA and AOB potential rates and gene abundance along gradients of human influences and identified organic matter and nutrients as key environmental factors that shaped the variation of AOA and AOB along the continuum.