Hydrogenosome

A hydrogenosome is a double membrane-enclosed organelle found in some anaerobic eukaryotes such as ciliates, flagellates and fungi. Species of one of the latest discovered animal phylum named Loricifera also contain hydrogenosome-like structures. Hydrogenosome is a type of mitochondrion that produces energy (ATP molecules) but in the absence of oxygen and also lacks membrane foldings (cristae) unlike typical mitochondria. Thus, hydrogenosomes are considered to have evolved from mitochondria to produce molecular hydrogen and ATP in anaerobic conditions.^[1]

Hydrogenosomes were discovered in 1973 by Donald Gustav Lindmark and Miklós Müller. Because hydrogenosomes hold evolutionary lineage significance for organisms living in anaerobic or oxygen-stressed environments, many research institutions have since documented their findings on how the organelle differs in various sources.^[2]

Hydrogenosomes were isolated, purified, biochemically characterized and named in 1973 by Donald Gustav Lindmark and Miklós Müller at the Rockefeller University.^[3] In addition to this discovery of novel organelle, they also demonstrated for the first time the presence of pyruvate:ferredoxin oxido-reductase and hydrogenase in eukaryotes.^[2] Further studies were subsequently conducted on the biochemical cytology and subcellular organization of several anaerobic protozoan parasites (ex:Trichomonas vaginalis, Tritrichomonas foetus, Giardia lamblia, and Entamoeba sp.).^[1]

Using information obtained from hydrogenosomal and biochemical cytology studies these researchers determined the mode of action of metronidazole (Flagyl). Today, metronidazole is recognized as a standard chemotherapeutic agent for the treatment of anaerobic infections.^[4][5]

Since their discovery, hydrogenosomes have been found in a variety of anaerobic unicellular ciliates, flagellates, and fungi. The most notable amongst these is the parasitic Trichomonas vaginalis.^[6]

In 2010, new species of microscopic animals in the phylum Loricifera, such as species of Rugiloricus species and Pliciloricus which known for the first time as animals (metazoans) capable of living exclusively in oxygenless conditions.^[7] They contain hydrogenosome-like organelles by which they produce energy (ATP) by metabolizing hydrogen deep in the Mediterranean sediments.^[8] Other new species subsequently discovered such as Spinoloricus cinziae also contained the same organelle.^[8]

Hydrogenosomes are organelles that are speculated to have evolved from mitochondria to provide a different mechanism for anaerobic ATP synthesis utilizing pyruvate. The reaction results in the production of molecular hydrogen, from which the organelle receives its name.^[2]

Hydrogenosomes range from 0.5-2 micrometers and are bound by a double membrane. They are most often dumbell-shaped and found in large complexes of stacked hydrogenosomes. These stacks range from 4 or 5 (called juvenile complexes) to 20 or more hydrogenosomes.^[1]

In most cases, hydrogenosomes are genomeless, as a majority of the mitochondrial genome was transferred to the nucleus; because of this, all hydrogenosomal proteins are imported to the organelle.^[9][10] However, a hydrogenosomal genome has been detected in the cockroach ciliate Nyctotherus ovalis, and the stramenopile Blastocystis.^[11]

Due to the fact that many organisms have evolved to fit their anaerobic environments, a multitude of organisms have independently evolved hydrogenosomes or structures with similar functions. The similarity between Nyctotherus and Blastocystis, which are only distantly related, is believed to be the result of convergent evolution, and calls into question whether there is a clear-cut distinction between mitochondria, hydrogenosomes, and mitosomes (another kind of degenerate mitochondria).^[1][11]

Hydrogenosomes are present in different organisms across three kingdoms such as Protista, Fungi and Animalia. The vast variety of source organisms can be accredited to the theorized convergent evolution of hydrogenosomes from mitochondria to fit an anaerobic environment.^[1][9][11] Some of the best understood organisms are:

• Parabasalid flagellates like Trichomonas vaginalis, Tritrichomonas foetus, Histomonas meleagridis.
• Preaxostylid flagellates like Trimastix pyriformis.
• Heterolobosean amoeboflagellates likePsalteriomonas lanterna.
• Anaerobic ciliates like Nyctotherus ovalis, Metopus palaeformis, Trimyema compressum, Caenomorpha uniserialis, Dasytricha ruminantium.

• Anaerobic chytridiomycetes including species of Neocallimastix and Piromyces.

• Species of the phylum Loricifera, like Spinoloricus cinziae, Rugiloricus species and Pliciloricus species.^[8]

The hydrogenosomes of trichomonads (the most studied of the hydrogenosome-containing microorganisms) produce molecular hydrogen, acetate, carbon dioxide and ATP by the combined actions of pyruvate:ferredoxin oxido-reductase, hydrogenase, acetate:succinate CoA transferase and succinate thiokinase. Superoxide dismutase, malate dehydrogenase (decarboxylating), ferredoxin, adenylate kinase and NADH:ferredoxin oxido-reductase are also localized in the hydrogenosome.^[12]

• Plastid