The Bacterial Ectosymbionts of the Ciliate Kentrophoros

Kentrophoros is a single-celled eukaryote that has a symbiosis with sulfur-oxidizing bacteria. This thesis investigates the diversity and functional significance of this symbiosis through molecular ecology and genomics. The bacteria belong to a single lineage in the Gammaproteobacteria that is specifically associated with Kentrophoros. Kentrophoros also constitutes a monophyletic group, despite their morphological diversity, which includes new species discovered during this work. Mitochondrial genes of Kentrophoros show evidence of rapid diversification during their evolutionary history. Genomes of the symbionts encode pathways for sulfur oxidation, but none of the known pathways for autotrophic carbon fixation. Instead they have the genetic potential for uptake and usage of organic carbon substrates, suggesting that this symbiosis is based on chemolithoheterotrophy, instead of chemosynthesis, as previously thought. The ecophysiological consequences of such a metabolism are discussed.