This parasite lacks mitochondria but has hydrogen-producing organelles. Their discovery proved that mitochondria-like organelles evolved multiple times and that complex cells can survive without oxygen.
Marlene Benchimol, Ana Paula Gadelha, Wanderley de Souza, CC BY 4.0, via Wikimedia Commons
An engine without oxygen
Deep within the human urogenital tract lives a single-celled organism that challenges ideas about complex life. Trichomonas vaginalis is a flagellated protozoan, a parasite that causes the common sexually transmitted infection trichomoniasis. It measures between 7 and 30 micrometers long, with a pear-like or amoeboid shape, and propels itself with four anterior flagella. This organism is an anaerobe, meaning it thrives in environments with little to no oxygen. Because of this, it has no use for conventional mitochondria, the powerhouses of most eukaryotic cells.
Instead, T. vaginalis contains unique double-membraned organelles called hydrogenosomes. These structures, typically 0.5 to 1.0 micrometers in diameter, perform a specialized type of anaerobic metabolism. They take pyruvate—a product of glucose breakdown in the cytoplasm—and ferment it. The process generates adenosine triphosphate (ATP), the cell's energy currency, but also produces peculiar byproducts: acetate, carbon dioxide, and molecular hydrogen (H₂).
The discovery and characterization of the hydrogenosome occurred in the early 1970s at The Rockefeller University in New York City, led by researchers D. G. Lindmark and Miklós Müller. Their work identified hydrogenase, the hydrogen-producing enzyme, within these organelles for the first time in a eukaryotic cell. This finding changed the understanding cellular evolution and adaptation to anaerobic life.
A different kind of eukaryote
The existence of the hydrogenosome fundamentally altered the scientific view of eukaryotic evolution. Initially, it was thought that organisms like T. vaginalis represented a very early stage of life, from a time before the ancestral bacterium that became the mitochondrion was engulfed. Later research showed that hydrogenosomes and mitochondria share a common evolutionary origin. Hydrogenosomes are now understood to be highly modified mitochondria, adapted for a life without oxygen. They lack a genome, the Krebs cycle, and the respiratory chain components found in their oxygen-using counterparts.
All proteins required for the hydrogenosome's function are encoded by the parasite's nuclear DNA. A 2011 proteomic analysis identified 569 distinct proteins associated with the hydrogenosome, a number far lower than the 1,000-1,500 found in typical mitochondria. Despite its specialized nature, the parasite is remarkably successful. Trichomonas vaginalis is the most prevalent non-viral sexually transmitted pathogen in the world. In 2020, the World Health Organization estimated 156 million new cases among people aged 15-49.
The parasite's success is partly due to its large and complex genome, which contains approximately 60,000 genes. This genetic toolkit allows it to adapt to the changing environment of its human host. While the hydrogen it produces is simply a metabolic waste product, the organelle that makes it shows how life can evolve to operate in extreme, oxygen-deprived niches.
💡Fun Facts
*T. vaginalis* has five flagella in total; four at the front for movement and a fifth integrated into an undulating membrane along its side.
The parasite does not have a dormant cyst stage and can only survive for a few hours outside a human host in moist environments.
The name of its unique organelle, "hydrogenosome," literally means "hydrogen-body."
Despite its anaerobic nature, *T. vaginalis* has enzymes and proteins to respond to and manage oxygen-related stress.
The research is ongoing; the parasite itself is not on public display. The coordinates point to The Rockefeller University, where hydrogenosomes were discovered.
Admission
Free to learn about this organism's biology.
Accessibility
The coordinates lead to an urban university campus with paved walkways.
