Diversity of Endosymbiotic Bacteria inParamecium

Abstract

For about one and a half centuries now microbial symbioses in Paramecium have been observed and investigated. Meanwhile a great diversity of endosymbiotic bacteria is known in many of the different species of Paramecium. Paramecium is a unicellular but complete eukaryotic organism. For microbial symbionts, the large cells offer plenty of space and a variety of niches of different metabolic conditions. Bacteria living in the cytoplasm, in micronuclei or macronuclei, or even in the perinuclear space show quite different life strategies. Peculiar adaptations to the symbiotic mode of life have stimulated research in different fields, such as microbiology, cell biology, physiology, ecology, and phylogenetics. The symbionts of Paramecium not only turn out to belong to different taxa of Eubacteria, but some appear to be related to human pathogens. As a special highlight, an endonuclear symbiont of Paramecium, Holospora obtusa, may be the closest relative of mitochondria known to date.