Anaerobic Energy Metabolism in “Protozoa”: An Evolutionary Perspective

Energy metabolism plays a key role in all living organisms and is strictly related to their ecological niche and lifestyle. Anaerobic energy metabolism is broadly diffused among ‘Protozoa’: it is typical in commensal and parasite species, such as digestive tract symbionts and pathogens in general, but it is also present in free-living species adapted to anoxic or hypoxic habitats. Energy metabolism pathways in extant obligate or facultative anaerobes are varied and anaerobically functioning mitochondria or mitochondrion related organelles seem to be ubiquitous in such organisms. In fact, it has been recently suggested that the endosymbiosis which gave origin to mitochondria in the eukaryote ancestor, occurred as early as the nucleus acquisition and that organelles such as hydrogenosomes and mitosomes are degenerate forms of mitochondria which evolved independently in different eukaryotic lineages. While mitochondria and hydrogenosomes are directly involved in ATP generation, however, mitosomes are not, but arguably still retain such essential functions that no eukaryotic organism could do without them. This chapter critically reviews the present knowledge on the anaerobic energy metabolism in ’Protozoa’, in view of the current (although still debated) picture of the evolution of eukaryotes and of their mitochondrion-related organelles.