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Abstract

Symbioses between eukaryotic hosts and chemosynthetic, sulfur-oxidizing bacteria are both phylogenetically and geographically widespread. Most of the known host species are animals (Metazoa), but ciliates (Ciliophora) are also known to be host organisms. The karyorelictean ciliate Kentrophoros has ectosymbionts that oxidize sulfide and fix carbon. They are found interstitially in shallow-water marine sediments, at the interface between sulfidic and oxic layers. Kentrophoros appears to phagocytose and digest its endosymbionts for nutrition. Although it was first described almost a century ago, the phylogenetic affiliation of the symbionts remains unknown. The aim of this study was to identify the ectosymbionts of Kentrophoros. Kentrophoros was collected from the islands of Elba (Mediterranean Sea) and Sylt (North Sea). 16S rRNA sequences show a diversity of bacterial types associated with Kentrophoros, including Proteobacteria from various classes, and Bacteroidetes. One cluster of sequences that falls within the basal Gammaproteobacteria was considered to be a likely candidate for the symbiont, and specific probes for fluorescence in-situ hybridization (FISH) were designed for it. FISH with the specific probes shows hybridization with the ectosymbionts of some, but not all, Kentrophoros individuals examined. The ectosymbionts are thus either belonging to more than one clade, or there is greater than expected sequence variation at the probe target site. We also reconstructed the phylogeny of Kentrophoros and related ciliates using 18S rRNA sequences. However, we could not recover a well-supported monophyletic clade containing all Kentrophoros sequences, suggesting that the genus as currently defined may be polyphyletic. The molecular and morphological diversity of Kentrophoros is consistent with the apparent diversity of its ectosymbionts. These unexpected results indicate that broader taxon and gene sampling are needed to understand the diversity of both host and symbiont in this system