We tested whether mixotrophic ciliates are more resistant to solar ultraviolet radiation (UVR) than heterotrophic ones because symbiotic algae can provide self-shading by cellular matter absorption and finally by direct UV screening from mycosporine-like proteins (MAAs). and so are then accompanied by a highly different community of several co-existing species (Mller et al. 1991; Sonntag et al. 2006). Besides net heterotrophic species, many mixotrophic ciliates that either bear algal symbionts or sequester algal plastids (kleptoplasts) prevail through the summertime in surface area waters where they are able to account for a lot more than 50% of the full total ciliate abundance (electronic.g. Carrias, Amblard, and Bourdier 1998; Dolan 1992; Mller et al. 1991; Speed 1982; Sonntag et al. 2006). Surviving in sunlit waters enables mixotrophic ciliates and their symbionts to optimally make use of nutrition and light (Modenutti et al. 2005; W?lfl and Geller 2002). This mutualistic romantic relationship is certainly primarily regarded as a dietary benefit and adaptation to reside in oligotrophic conditions (Dolan and Prez 2000). Nevertheless, exploiting the higher layers of the drinking water column also means that the ciliates face damaging degrees of incident ultraviolet MK-0822 pontent inhibitor radiation (UVR, 280C400 nm). Ultraviolet radiation may harm DNA and cell compartments, but organisms have developed manifold strategies of avoidance, protection, or repair. Planktonic organisms, for example, carry out vertical downward migrations in the water column to escape high irradiation intensities (e.g. Leech and Williamson 2001) or they may accumulate photoprotective compounds. The importance of UVR as a natural stress factor for mixotrophic ciliates is usually reflected in the recent finding that can reduce damage of sensitive cell compartments by self-shading (Sommaruga and Sonntag 2009; Summerer et al. 2009). Consequently, the mutualistic relationship between ciliates and their symbiotic algae should be considered not only from a nutritional, but MK-0822 pontent inhibitor also from a photobiological perspective (Sommaruga and Sonntag 2009). Yet, these recent findings were obtained in experiments with and there is usually little information on how freshwater euplanktonic species respond to UVR. In particular, the total number of symbiotic differs among ciliates, and also their arrangement and MK-0822 pontent inhibitor volume occupied in the host. Therefore, we may expect to find different tolerance among mixotrophic species. Only a few studies have addressed how different ciliate species respond to UVR, and those results available indicate that sensitivity is usually species-specific (Giese et al. 1965; Mostajir et al. 1999; Sanders et al. 2005; Sommaruga et al. 1999; Sonntag, Summerer, and Sommaruga 2011; Wickham and Carstens 1998). For example, Sanders et al. (2005) tested the UV sensitivity of two ciliate species from an oligotrophic lake. After exposure to UV-B (280C315 nm) and photoreactivating radiation (UV-A, 315C400 nm and visible light, 400C700 nm), sp. was able to photorepair, whereas sp. was not and died. Indirect effects of UVR on the structure FJH1 and the dynamics of planktonic food webs have also been observed (Mostajir et al. 1999; Wickham and Carstens 1998). In a mesocosm experiment, Mostajir et al. (1999) found a strong decline of predatory ciliates under UV-B stress leading to a positive feedback on their prey. In terms of UV transparency, there are large differences between subalpine and alpine lakes, i.e. located above the treeline (Rose et al. 2009). In subalpine lakes, UVR is usually strongly attenuated within the uppermost meters of the mixed layer and therefore, UV sensitive species may remain below the highest UVR levels close to the surface (Rose et al. 2009). By contrast, organisms in alpine lakes are exposed to elevated UVR levels throughout the water column, mainly because of the low concentration in chromophoric dissolved organic matter that acts as an external filter and of the higher incident UVR, particularly in the UV-B range, at higher elevations (Sommaruga and Psenner 1997). In the subalpine lake Piburgersee (PIB), the ciliate assemblage found in July is usually characteristically dominated by mixotrophic, mainly sp. (yet undescribed species), and (not separable during routine counting), and spp. Most individuals of the latter were ((Fig. 2 and 3). After exposure to UVR at the surface in PIB, cell numbers significantly decreased in ((((spp. (spp. MK-0822 pontent inhibitor (B), (C), and (D) at T0 and after 5 h of exposure at the surface and at the depth corresponding to 10% of 320 nm in GKS and PIB. Exposure conditions were: full solar.