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

## TL;DR

Peritrich ciliates in the Danube River act as ecosystem engineers, creating complex biofilms that support diverse microorganisms and enhance community resilience.

## Contribution

This study reveals the autogenic ecosystem-engineering role of peritrichs in forming biofilms and promoting functional redundancy in lotic ecosystems.

## Key findings

- Peritrichs create three-dimensional structures that host filter feeders and predators, enhancing biofilm function.
- Warmer months see increased epibiont abundance, particularly choanoflagellates, on peritrich surfaces.
- Predatory ciliates like Trachelius ovum influence biofilm structure by consuming colonial peritrichs.

## Abstract

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.

Colonial peritrichs act as physical ecosystem engineers in middle Danube biofilms, generating three‐dimensional structures during 1–28 days of colonisation. Their surfaces host filter feeder epibionts (choanoflagellates, other peritrichs, rotifers), expanding filtration capacity and promoting functional group establishment including swimming and attached predators, especially in warmer environments. By Days 11–28, functional redundancy emerges, enhancing community resilience.