Abstract
“Candidatus Megaira polyxenophila” is a recently described member of Rickettsiaceae which comprises exclusively obligate intracellular bacteria. Interestingly, these bacteria can be found in a huge diversity of eukaryotic hosts (protist, green algae, metazoa) living in marine, brackish or freshwater habitats. Screening of amplicon datasets revealed a high frequency of these bacteria especially in freshwater environments, most likely associated to eukaryotic hosts. The relationship of “Ca. Megaira polyxenophila” with their hosts and their impact on host fitness have not been studied so far. Even less is known regarding the responses of these intracellular bacteria to potential stressors. In this study, we used two phylogenetically close species of the freshwater ciliate Paramecium, Paramecium primaurelia and Paramecium pentaurelia (Ciliophora, Oligohymenophorea) naturally infected by “Ca. Megaira polyxenophila”. In order to analyze the effect of the symbiont on the fitness of these two species, we compared the growth performance of both infected and aposymbiotic paramecia at different salinity levels in the range of freshwater and oligohaline brackish water i.e., at 0, 2, and 4.5 ppt. For the elimination of “Ca. Megaira polyxenophila” we established an antibiotic treatment to obtain symbiont-free lines and confirmed its success by fluorescence in situ hybridization (FISH). The population and infection dynamics during the growth experiment were observed by cell density counts and FISH. Paramecia fitness was compared applying generalized additive mixed models. Surprisingly, both infected Paramecium species showed higher densities under all salinity concentrations. The tested salinity concentrations did not significantly affect the growth of any of the two species directly, but we observed the loss of the endosymbiont after prolonged exposure to higher salinity levels. This experimental data might explain the higher frequency of “Ca. M. polyxenophila” in freshwater habitats as observed from amplicon data.
Highlights
Paramecium (Ciliophora, Oligohymenophorea) is a unicellular protist with a broad, nearly global distribution in fresh and brackish water bodies
Some authors (Fokin and Sabaneyeva, 1990; Smurov and Fokin, 1998) reported a higher bacterial infection frequency in protists, i.e., Paramecium, living in brackish environments compared to those living in freshwater environments
All cultures were maintained at 19◦C in Cerophyll medium (CM) inoculated with Raoultella planticola DSM3069 (Enterobacteriales, Enterobacteriaceae) (Chiellini et al, 2019)
Summary
Paramecium (Ciliophora, Oligohymenophorea) is a unicellular protist with a broad, nearly global distribution in fresh and brackish water bodies This ciliate is studied, among other things (Karunanithi et al, 2019; Kelz and Mashour, 2019; Mayne et al, 2019; Soares et al, 2019; Arnaiz et al, 2020) for the abundance and diversity of its endosymbionts (Floriano et al, 2018; Garushyants et al, 2018; Grosser et al, 2018; Potekhin et al, 2018; Sabaneyeva et al, 2018; Schrallhammer et al, 2018; Castelli et al, 2019a,b; Fokin et al, 2019; Koehler et al, 2019; Lanzoni et al, 2019; Plotnikov et al, 2019). It could indicate that stressed protists are more susceptible to infection
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