“…We argue that this shaping force is largely caused by protistan predation. All of the most numerous protists in this study are known bacterivores (Böhme et al, 2009;Vaerewijck et al, 2011;Samba-Louaka et al, 2019), including sessile peritrich ciliates like Vorticella, which are considered important for treatment efficiency, effluent clarity, and nitrification, as well as Kinetoplastida and Cryomonadida, both of which are known to feed on wastewater bacteria (Pauli et al, 2001;Arregui et al, 2012;Foissner, 2016;Liu et al, 2016;Pohl et al, 2021;Freudenthal et al, 2022). The most abundant protistan taxon in this study, bacterivorous Vannelida, is often absent from wastewater surveys that rely on primer-based methods, but studies based on culturing or metatranscriptomic methods have repeatedly reported their presence in WWTP bioreactors (Ramirez et al, 2015;Freudenthal et al, 2022).…”