**5. Conclusions**

The three ciliate species tested here for their photoprotective strategies are common members of a mixotrophic assemblage that frequently occur in temperate lakes in summer/autumn. These planktonic ciliates were equipped with MAAs and numerous algal symbionts that sugges<sup>t</sup> e fficient protection from UVR. However, none of the ciliates survived under simulated UVR conditions similar to natural doses found at the lake surface. Considering that UVR is rapidly attenuated in the water column, the photoprotection level that these species have is probably enough for survival. However, an acclimation to higher doses does not seem feasible and is energetically too costly in the short term. Based on our results, we argue that the ciliates tested here need to actively shift their position along the depth (UV) gradient and to rely on vertical migration in the water column to escape the highest incident solar radiation levels around solar noon. Future studies on the photoprotective strategies of other mixotrophic ciliate species will shed more light on the role of MAAs and shading by algal symbionts under in situ conditions.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/1424-2818/12/6/252/s1, Table S1: UV-A and UV-B doses (kJ m<sup>−</sup>2) over the experimental period, Figure S1: Scheme of the morphological changes of *Pelagodileptus trachelioides* observed under irradiation conditions in the photoenzymatic repair experiment, Figure S2: Seasonal and spatial occurrence of *Pelagodileptus trachelioides*, *Vorticella chlorellata*, and *Stokesia vernalis* in Piburgersee in 2004–2005 [54].

**Author Contributions:** Conceptualization, B.S.; methodology, B.S. and R.S.; investigation, B.S.; data curation, B.S. and R.S.; writing—original draft preparation, B.S.; writing—review and editing, R.S.; project administration, B.S. and R.S; funding acquisition, B.S. and R.S. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the Austrian Science Fund FWF, gran<sup>t</sup> numbers P21013-B03, P28333-B25, and P16559-B06. Open Access Funding by the Austrian Science Fund (FWF).

**Acknowledgments:** The authors thank Josef Franzoi, Barbara Kammerlander, Gry Larsen, Salvador Morales-Gomez, and Monika Summerer for help with sampling and in the laboratory. The authors also thank Michael Butkay for providing unpublished material and helpful comments on *Pelagodileptus trachelioides*. The authors express their gratitude to the three anonymous reviewers who suggested valuable and critical suggestions and comments on an earlier version of the manuscript. Open Access Funding by the Austrian Science Fund (FWF).

**Conflicts of Interest:** The authors declare no conflict of interest.
