Algal Diversity inParamecium bursaria: Species Identification, Detection ofChoricystis parasitica, and Assessment of the Interaction Specificity

The 'green' ciliateParamecium bursarialives in mutualistic symbiosis with green algae belonging to the speciesChlorella variabilisorMicractinium conductrix. We analysed the diversity of algal endosymbionts and theirP. bursariahosts in nine strains from geographically diverse origins. Therefore, their phylogenies using different molecular markers were inferred. The green paramecia belong to different syngens ofP. bursaria. The intracellular algae were assigned toChl. variabilis,M. conductrixor, surprisingly,Choricystis parasitica. This usually free-living alga co-occurs withM. conductrixin the host's cytoplasm. Addressing the potential status ofChor. parasiticaas second additional endosymbiont, we determined if it is capable of symbiosis establishment and replication within a host cell. Symbiont-freeP. bursariawere generated by cycloheximid treatment. Those aposymbioticP. bursariawere used for experimental infections to investigate the symbiosis specificity not only betweenP. bursariaandChor. parasiticabut including alsoChl. variabilisandM. conductrix. For each algae we observed the uptake and incorporation in individual perialgal vacuoles. These host-symbiont associations are stable since more than five months. Thus,Chor. parasiticaandP. bursariacan form an intimate and long-term interaction. This study provides new insights into the diversity ofP. bursariaalgal symbionts.