**E** ff**ectiveness of Photoprotective Strategies in Three Mixotrophic Planktonic Ciliate Species**

**Bettina Sonntag 1,2,\* and Ruben Sommaruga 2**


Received: 15 May 2020; Accepted: 17 June 2020; Published: 20 June 2020

**Abstract:** Mixotrophic ciliate assemblages often prevail in summer in the surface layers of lakes. During this time, they are potentially exposed to damaging levels of incident solar ultraviolet radiation (UVR) and need e fficient photoprotective mechanisms to minimize the damage. Herein, we tested the algal-bearing species of *Pelagodileptus trachelioides*, *Stokesia vernalis,* and *Vorticella chlorellata* for how they handled stress under exposure to the artificial sunlight spectrum (i.e., UV treatment), just photosynthetically active radiation (PAR), or in the dark (i.e., control). In addition to measurements of their survival, changes in behavior, shape, and whether dark or photoenzymatic repair (PER) mechanisms are present, we measured the concentration of UV-absorbing compounds (i.e., mycosporine-like amino acids). In contrast to the response in the PAR and dark treatments, sublethal e ffects were observed in all species when exposed to UVR. A wavelength-specific test for *P. trachelioides* revealed that UV-B was especially lethal. These results sugges<sup>t</sup> that the photoprotective mechanisms found in these ciliates are not su fficient to allow for their survival directly at the surface and that, accordingly, they need to shift their position further down in the water column.

**Keywords:** algal-ciliate symbiosis; mycosporine-like amino acids; *Pelagodileptus trachelioides*; planktonic freshwater ciliates; *Stokesia vernalis*; *Vorticella chlorellata*
