Effects of dam building on the occurrence and activity of comammox bacteria in river sediments and their contribution to nitrification

The recent discovery of complete ammonia oxidizers (comammox) has fundamentally changed our understanding of nitrification. However, studies on the occurrence and activity of comammox bacteria and their contribution to nitrification remain unclear. Here, we investigated the abundance, activity, and diversity of comammox bacteria and their contribution to nitrification in sediments from dammed rivers in winter and summer. Our results indicated that comammox clade A was ubiquitous in all sediment samples and the community structure in comammox varied between the upper and lower reaches, but not on the time scale (winter and summer). Comammox activity in the dammed river sediments in summer was prominently higher than in winter (summer: 1.08 +/- 0.52; winter: 0.197 +/- 0.148 mg N kg-1 day-1). Furthermore, the activity of comammox bacteria in summer appeared higher in the vicinity of the dammed river and in the Sanjiang estuary, which is located downstream of the dammed river. The activity of ammonia-oxidizing bacteria (AOB) (0.77 +/- 0.478 mg N kg-1 day-1) was higher compared to comammox (0.639 +/- 0.588 mg N kg-1 day-1) and ammonia-oxidizing archaea (AOA) (0.026 +/- 0.022 mg N kg-1 day-1) in both winter and summer. In terms of contribution to the nitrification process, AOB (winter: 67.13 +/- 12.21 %; summer: 50.57 +/- 16.14 %) outperformed comammox (winter: 28.59 +/- 12.51 %; summer: 48.38 +/- 16.62 %) and AOA (winter: <7.39 %; summer: <2.09 %). These findings indicated that the nitrification process in dammed river sediments was mainly dominated by AOB. Additionally, comammox activity was significantly affected by temperature and NH4+, suggesting that these variables were key determinants of the niche partitioning of comammox. Collectively, our findings provide novel perspectives into the widespread distribution and contribution of comammox to nitrification in dammed river ecosystems, thus broadening our understanding of the nitrification processes.