A hydrogenosome is an organelle of ciliates, trichomonads and fungi. It produces hydrogen and ATP. This organelle may have evolved from mitochondria.

Certain anaerobic protozoa and fungi possess membrane-bound organelles known as "hydrogenosomes". Hydrogenosomes were discovered in the early 1970s by Lindmark and Müller in the US and by Kulda and others in Prague.Lindmark D G & Müller M, "Hydrogenosome, a cytoplasmic organelle of the anaerobic flagellate Tritrichomonas foetus, and its role in pyruvate metabolism" J Biol Chem 248:7724−7728. These organelles are approximately 1 micrometre in diameter and are so called because they produce molecular hydrogen. The anaerobic ciliated protozoan Nyctotherus ovalis, found in the hindgut of several species of cockroach, has numerous hydrogenosomes that are intimately associated with endosymbiotic methane-producing Archaea, the latter using the hydrogen produced by the hydrogenosomes.

The best studied hydrogenosomes are those of the sexually transmitted parasite Trichomonas vaginalis and those from rumen chytrids such as Neocallimastix.

Like mitochondria, the hydrogenosomes are bound by distinct double membranes and have an inner membrane with some cristae-like projections.

It had been postulated that hydrogenosomes evolved from mitochondria by the concomitant loss of their respiration and organellar genomes, and indeed so far no hydrogenosome has been found that has a genome.

The matrix of N. ovalis hydrogenosomes contains ribosome-like particles of the same size as a numerous type of ribosome (70s) of the endosymbiotic methanogenic Archaea. This suggested the presence of an organellar genome which was discovered indeed by Akhmanova and later partly sequenced by Boxma.Akhmanova et al, Nature 1998 396:527.Boxma et al, Nature 2005 434:74-79.

This finding was the final confirmation that hydrogenosomes are indeed derived from mitochondria. van der Giezen et al, Int Rev Cytol 244:175-225.

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