Diplomonads lack functional
Diplomonads represent anaerobic flagellated protists within Excavata lacking classical aerobic mitochondria. Evolutionary adaptation to low-oxygen intestinal habitats led to loss of oxidative phosphorylation, electron transport chain and tricarboxylic acid cycle enzymes. Instead retains double membrane-bound mitosomes devoid of genome, involved solely in iron-sulfur cluster assembly. They possess nuclei, flagella, ribosomes but not functional ATP-generating mitochondria. Giardia lamblia exemplifies this reductive evolution, explaining absence of respiration-competent organelle and reliance on substrate-level phosphorylation in cytosol for energy. This genomic reduction reflects long-term adaptation to parasitic lifestyle within host small intestine requiring efficient iron sulfur metabolism.
Ref: NCBI Bookshelf, Microbiology: Diplomonads and Retortamonads Mitosomes; Campbell Biology 12th ed., Chapter 28 Excavata