The domain Eukarya is closely associated with the kingdoms Plantae, Animalia, and Fungi, but under the kingdom Protista there are some diverse organisms that do not follow all of the characteristics of the typical eukaryote. The domain Eukarya is known for its cells with a membrane-enclosed nucleus and membrane-enclosed organelles. One of these many organelles is the mitochondrion. It is usually assumed that all eukaryotes have some form of mitochondria to produce the needed ATP to perform normal cellular functions and work. Although there are some eukaryotes that live in low-oxygen environments and have less active mitochondria, they still have the ‘essential’ organelle. However, scientists have discovered that not all eukaryotic organisms function with mitochondria.
For over eighty years, scientists have been trying to find a eukaryotic organism that does not have mitochondria like many other eukaryotes. It seemed like an impossible task for scientists and researchers to find such an unusual organism, but they eventually found the microbes from the genus Monocercomonoides. These microorganisms are typically found in the digestive systems of animals such as insects, snakes, and small mammals, like chinchillas. They are also closely related to human pathogens such as Giardia and Trichomonas. Giardia is a protozoan parasite that has small remnants of what could have been mitochondria. Instead of helping produce ATP, adenosine triphosphate, these microbial remnants help in the maturation of proteins. Similar to Giardia, Trichomonas belongs to the protist kingdom and lack mitochondria. However, scientists are still unsure whether Trichomonas shares a common ancestor with an organism that has mitochondria or if they descended from some sort of endosymbiotic relationship.
Image from flickr
In the image from the light microscope of the Monocercomonoides, you can see the flagella that helps the microbe maneuver through its surroundings by moving the liquid around it. Another feature to the Monocercomonoides is its membrane-enclosed nucleus like almost all other eukaryotes have. But, the Monocercomonoides are mostly known for their unusual lack of mitochondria. The mitochondrion is the organelle site where cellular respiration and the production of ATP takes place, so some people might think that since these microbes do not have the ability to produce ATP. However, some scientists believe, that like the Giardia protists, that Monocercomonoides also have the microbial remnants from an original mitochondrion. Since the environment Monocercomonoides are usually found in have plenty of nutrients, it is also believed that enzymes in the cytoplasm break down the abundance of nutrients to produce the needed energy for the organism.
Monoceromonoides have a cytosolic sulfur mobilization system also called SUF that is used as a substitution to mitochondria by performing similar functions to it. This system is highly complex and scientists are still studying how the whole process works to provide the microbes with the necessary energy for cellular work. Another important function of the mitochondria is the ability to synthesize clusters of sulfur and iron, but the Monocercomonoides survive without it by borrowing bacterial genes that perform the same exact function. Scientists are continuing the research more eukaryotes that lack mitochondria, and are delving deeper into the study of microbial protists.