Hydrogenosomes of Anaerobic Ciliates

Ciliates are highly complex unicellular eukaryotes. Most of them live in aerobic environments and possess mitochondria. However, in several orders of ciliates, anaerobic species evolved that contain “hydrogenosomes”, organelles that produce hydrogen and ATP. These hydrogenosomes of ciliates have not been studied in the same detail as those of trichomonads and chytrid fungi. Therefore, generalizations on the characteristics of hydrogenosomes of ciliates are somewhat premature, especially since phylogenetic studies suggest that hydrogenosomes have arisen independently several times in ciliates. In this chapter, the hydrogenosomes of the anaerobic, heterotrichous ciliate Nyctotherus ovalis from the hindgut of cockroaches will mainly be described as these are the ones that are, at the moment, the most thoroughly studied. Thus far, this is the only hydrogenosome known to possess a genome and this genome is clearly of mitochondrial origin. In fact, the hydrogenosome of N. ovalis unites typical mitochondrial features such as a genome and an electron-transport chain with the most characteristic hydrogenosomal property, the production of hydrogen. The hydrogenosomal metabolism of N. ovalis will be compared with that of two other ciliates that have been studied in less detail, i.e. the holotrichous rumen ciliate Dasytricha, and the free-living plagiopylid ciliate Trimyema. All studies combined indicate that it is likely that the various types of hydrogenosomes in ciliates evolved by modifications of aerobic mitochondria when the different ciliates adapted to anaerobic or micro aerobic environments. Furthermore, it is clear that the hydrogenosomes of anaerobic ciliates are different from those of chytrid fungi and from the well-studied ones in trichomonads.