Testing alternative benefits in Chlorella-ciliate symbiosis

Ciliates in symbiosis with algae (the so-called `zoochlorellae` and `zooxanthellae`) or with sequestered functional chloroplasts have been found to be a common component of pelagic food webs in oceanic and freshwater systems. These organisms play a dual role in aquatic ecosystems, i.e., as primary and secondary producers. However, little is known about the factors regulating their dynamics and distribution in the water column of different aquatic ecosystems. Symbiosis between ciliates and algae is often considered as an adaptation allowing exploitation of oligotrophic environments. Yet, the occurrence of these organisms also in sunlit waters of eutrophic environments, suggests the existence of other benefits beyond the advantage to struggle starvation when food is scarce. Based on the analysis of the existing scientific literature and our preliminary results on the presence of UV sunscreen agents in symbiotic ciliates, we hypothesize that symbiosis with algae of the genus Chlorella in this group of organisms may have evolved to obtain other benefits, such as a mutual protection against the damaging effects of solar ultraviolet radiation (UVR). During this 3 years project, we will combine novel approaches including field and laboratory experiments to test seven hypotheses that will elucidate several aspects of the ecology and adaptations of Chlorella-bearing ciliates to live under high UV exposure conditions such as those found in many mountain lakes. In particular, our research activities will be characterized by a combination of photobiological, ecological, and molecular techniques including direct estimations of DNA damage and phylogenetic characterization of algal endosymbionts. Thus, we propose a more integrated approach to advance understanding of the link between environmental biology of UVR and this group of organisms. The relatively rapid generation time of ciliates and the possibility to obtain symbiont-containing and symbiont-free (aposymbiotic) clones provide an excellent framework to experimentally test our hypotheses. Our results will provide relevant information not only to the ecology and photobiology of symbiotic ciliates but also to other scientific areas such as global change (i.e., sensitivity of species and populations to UV-B radiation, adaptation strategies) and physiology and evolution-phylogenetics related aspects of phototrophic symbioses.

Ciliates living in symbiosis with algae are common members of planktonic food webs in aquatic ecosystems. These organisms are important in the flux of energy and matter because they play a dual role in the aquatic ecosystem as primary producers and as consumers. Current understanding of the main benefit in such mutualistic symbioses between ciliates and algae is a nutritional advantage to exploit nutrient-poor environments. By their very nature, however, mutualistic organisms with photosynthetic symbionts are exposed to the damaging effects of solar UV radiation (UVR). In our project, we showed that other benefits exist beyond the advantage of struggling starvation when food is scarce. Among them, the symbiosis of freshwater ciliates with green algae of the genus Chlorella results in a mutual protection against the damaging effects of solar UVR. Combining different methodologies, we first provided breakthrough evidence for the direct (e.g. by self-shading) and indirect (reduction of photo-oxidative stress) photoprotective role of symbiotic Chlorella, and second, for the symbiotic origin of specific UV photoprotective substances in several ciliate symbioses. In addition, we showed through phylogenetic analyses, the existence of a homogenous Chlorella group among different ciliate species from one lake, but clearly different Chlorella in the same ciliate species from two lakes differing in their underwater UV transparency. This genetic difference was coupled to a distinct physiological trait such as the ability to synthesize UV photoprotective substances. Further analyses in the most studied ciliate-algal-symbiosis, Paramecium bursaria, showed the existence of a biogeographic pattern of the symbionts. These results showed a high degree of species-specificity in ciliate symbioses, but also highlighted the importance of physiological adaptation in symbiotic Chlorella. The high degree of species-specificity in the P. bursaria symbiosis was also demonstrated in laboratory experiments. Generally, our results provided relevant information not only on the ecology and photobiology of symbiotic ciliates but also to other scientific areas such as global change (i.e., sensitivity and adaptation strategies of species and populations to UVR) and physiology and phylogenetic - related aspects of such phototrophic symbioses.