Peritrichs (Ciliophora, Peritrichia) in the Danube: Keystone Organisms in the Formation of Diverse Protist Biofilms.

Peritrichs are widely studied ciliates; however, their eukaryotic epibionts have not yet been examined in detail. Our study investigates the colonisation patterns and seasonal dynamics of peritrich ciliates and their epibionts in lotic environments over 14 sampling periods. In the early stages of colonisation, settlement was likely characterised by random establishment, consistent with the lottery model of Chesson and Warner. In later stages, the autogenic, ecosystem-engineering role of peritrich species facilitated the settlement of additional organisms on the substrate. During warmer months, the structurally complex surface of peritrichs hosted a greater abundance of epibionts-particularly choanoflagellates. Colonisation of the peritrich stalks by attached filter feeders and other functional groups increased the filtration-capable surface area, thereby enhancing biofilm function. Additionally, the presence of predatory ciliates such as Trachelius ovum can alter biofilm structure by consuming colonial peritrichs. These findings highlight the crucial role of peritrichs in biofilm dynamics and their contribution to community complexity in lotic ecosystems.