Effectiveness of Photoprotective Strategies in Three Mixotrophic Planktonic Ciliate Species

Bettina Sonntag,Ruben Sommaruga

doi:10.3390/d12060252

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 efficient 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 effects 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 suggest that the photoprotective mechanisms found in these ciliates are not sufficient to allow for their survival directly at the surface and that, accordingly, they need to shift their position further down in the water column.

Highlights

It is well documented for many groups of organisms that exposure to solar radiation might cause severe direct and indirect negative effects [1,2]
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 mycosporine-like amino acid (MAA) and numerous algal symbionts that suggest efficient protection from ultraviolet radiation (UVR)

Summary

Introduction

It is well documented for many groups of organisms that exposure to solar radiation might cause severe direct and indirect negative effects [1,2]. To escape high UVR levels at the lake surface, some organisms such as zooplankton perform diel vertical migrations [5] This adaptation is unknown for ciliates, and the species assemblage is believed to move along the water column and over the season according to food availability or water temperature [6,7,8]. During summer/autumn, mixotrophic ciliates that live in symbiosis with algal endosymbionts can be detected in the epilimnion of temperate lakes [6,9,10] Such a mutualistic relationship between green algae and a ciliate host has different advantages for both partners, namely, the ciliate receives nutrients from its partners and the algae are transported into sunlit areas, ensuring a positive photosynthetic balance. Another putative advantage of the algal–ciliate relationship, but less known, is photoprotection against UVR [14,15,16]

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