This study that investigates the impact of cadmium (Cd) stress on a crucial functional group in the activated sludge consortium, the ammonia oxidizing bacteria (AOB). The laboratory set-up consists of 3 identical continuous flow complete mixed activated sludge systems. Two systems were
shocked with Cd, while the third system served as an unstressed control. Nitrification was significantly affected by Cd during the stress events as exhibited by the increase in effluent ammonia levels. Cd stressed reactors showed a fast recovery followed by a significant increase in ammonia
oxidation activity as compared to the unstressed control. This pattern was reproducible for both shocked reactors during three independent shock events and could be attributed to an increased energy demand due to the expression of stress response functions or increased energy uptake efficiency
resulting from reduced AOB diversity. Molecular investigations to elucidate the mechanisms behind the observed trends are in progress.
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