Bioreactor function under perturbation scenarios is affected by interactions between bacteria and protozoa.

This study investigated the impact of transient cadmium perturbations on the structure and function of the microbial community in an activated sludge system. The impact of cadmium perturbation on the bioreactor performance, bacterial activity, bacterial community structure, and bacteria-protozoa interactions was examined. The bacterial community exhibited a short-term inhibition following a pulse perturbation of cadmium. Process recovery was associated with an increase in bacterial abundance above the unperturbed control reactor, followed by high biomass activity after the washout of cadmium. This trend was seen for multiple experiments at both laboratory- and pilot-scale. The increase in biomass activity could not be explained by changes in bacterial community structure. Independent experiments showed that the increase in bacterial abundance, and by association biomass activity, was caused by the decrease in the protozoal grazing due to the higher inhibition of ciliated protozoa as compared to bacteria when exposed to cadmium. This paper highlights the importance of expanding the investigative boundaries of the microbial ecology of bioengineered systems to include protozoal grazing, especially under perturbation scenarios.