Following publication of our paper, we were made aware that we might not have sufficiently pointed out the potential role that microorganisms released with wastewater effluents could play in the increased tolerance of downstream biofilms toward micropollutants. Indeed, a large survey in 23 wastewater-impacted streams, including the three sampling sites investigated in our study, showed that downstream bacterial community profiles in the water column were a mixture between the upstream and the effluent. Whether these microorganisms go from a planktonic into a benthic biofilm state and play a role in the tolerance to micropollutants remains unclear. Nevertheless, it is plausible that such microorganisms might have developed a tolerance to micropollutants in the wastewater treatment plants (WWTP) before their release into the streams. Mussmann et al. have shown that, among all identified nitrifying bacteria in the WWTP effluent, only one taxa preferentially added to the downstream biofilm community whereas the others did not. Similarly, Chonova showed that less than 5% and not the most abundant bacterial taxa were found in the biofilms downstream of urban and hospital WWTP. Even if actively colonizing downstream biofilms, monitoring the diversity profile of these microorganisms alone does not answer the question whether they contribute to the community tolerance to micropollutants. Therefore, controlled experiments focusing on both benthic bacterial and algal communities are needed to address this issue.
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