# Identification and Feeding Characterization of Sterkiella histriomuscorum (Protozoa, Ciliophora, Hypotrichia) Isolated from Outdoor Mass Culture of Scenedesmus dimorphus

## TL;DR

This study identifies and characterizes the ciliate Sterkiella histriomuscorum, which grazes on algae in outdoor cultures and may impact algal growth.

## Contribution

The study provides new insights into the morphology, excystment, and feeding preferences of Sterkiella histriomuscorum in algal cultures.

## Key findings

- Sterkiella histriomuscorum occurs frequently in Scenedesmus cultures during spring and summer.
- The ciliate prefers to graze on chlorophytes and the diatom Phaeodactylum tricornutum.
- Molecular analysis shows that Sterkiella and S. histriomuscorum are non-monophyletic.

## Abstract

Herbivorous protistan grazers are ubiquitous and abundant in marine and temperate freshwater environments. However, little is known about the algivorous ciliates and their feeding habits in outdoor mass algal cultures. In this study, we report on one hypotrich ciliate, identified as Sterkiella histriomuscorum, from the outdoor mass culture of Scenedesmus in Arizona, USA. A long-term field survey revealed that this species often occurs in Scenedesmus culture in spring and summer, and can graze very heavily on Scenedesmus cells. By isolating Sterkiella cells and then observing them via light microscopy and electron microscopy, detailed information about the morphology, ultrastructure, excystment process, and feeding characteristics of the ciliate was obtained. Specifically, it seems that S. histriomuscorum has a range of different strategies for excystment, and the sharp change in the ion concentration in the environment around the cyst results in osmotic shock, which likely facilitates the excystment. Feeding experiments revealed that S. histriomuscorum preferred to graze on chlorophytes as well as the diatom Phaeodactylum tricornutum and had no interaction with chrysophytes or cyanobacteria. Molecular phylogenetic analysis based on the SSU rRNA gene sequence indicated that both the genus Sterkiella and the species S. histriomuscorum are non-monophyletic. The information obtained from this study will help advance our understanding of the biodiversity and ecological function of S. histriomuscorum, and will also be very useful in the development of early warning systems and control measures for preventing or treating this contaminant in microalgal mass cultures.