Amoeba

An amoeba is a single-celled organism known for its ability to change shape.

These organisms are often called amoeboid because they move by extending and retracting parts of their body. While people often use the terms "amoeba" and "amoeboid" interchangeably, they are not all part of one single group. Instead, amoeboid cells appear across many different lineages of eukaryotic life. You can find them in fungi, algae, animals, and protozoa. This ability to shift shape makes them highly adaptable in many environments.

The primary way an amoeba moves and eats is through pseudopods. These are temporary bulges of cytoplasm, the jelly-like substance inside the cell. Pseudopods form when actin microfilaments push against the plasma membrane, which is the thin outer layer of the cell. Because amoebae lack a rigid cell wall, they can stretch these parts freely. The specific shape of these pseudopods helps scientists tell different groups apart. For example, Amoebozoans have lobose pseudopods, which are rounded and tube-like. In contrast, Cercozoans have filose pseudopods, which are thin and thread-like. Some groups, like Foraminifera, use reticulose pseudopods that form a branching, net-like structure. Others, such as Radiolaria, have stiff axopodia supported by internal bundles of microtubules.

Amoebae use these same pseudopods to acquire nutrition through a process called phagocytosis. Since they do not have a mouth or a fixed feeding spot, they must surround their food. They extend their pseudopods to encircle prey, such as bacteria or other protists, and then engulf it. Some species are detritivores, meaning they eat dead organic material. Other amoebae use pinocytosis, which is a way of absorbing dissolved nutrients through small vesicles. To survive in freshwater, many amoebae must manage water levels using a contractile vacuole. Because the surrounding water has fewer solutes than the cell's cytosol, water enters the cell through osmosis. The contractile vacuole acts as a pump to expel this excess water so the cell does not burst.

The history of studying these organisms is quite long. In 1755, August Johann Rösel von Rosenhof made the first recorded observation of an amoeboid. He named his discovery "Der Kleine Proteus," or "the Little Proteus." In 1822, the naturalist Bory de Saint-Vincent created the genus name Amiba, derived from a Greek word meaning "change." Later, in 1841, Félix Dujardin described the thick substance inside these cells as "sarcode." For much of the 20th century, scientists used the class Sarcodina to group amoebae together based on their flexible bodies. However, modern molecular phylogenetic studies have shown that Sarcodina is not a single group sharing a common descent. Today, amoeboid organisms are classified into several different distinct groups.

Amoebae vary significantly in size. The marine species Massisteria voersi is extremely small, measuring only 2.3 to 3 micrometers in diameter. This puts it in the same size range as many bacteria. On the other end of the scale, some deep-sea xenophyophores can reach 20 centimeters in diameter. Most common freshwater amoebae are microscopic, but some "giant" species are visible to the naked eye. For instance, Chaos carolinense can reach 2000 micrometers, and Pelomyxa palustris can grow up to 5000 micrometers. This massive range shows how diverse single-celled life can be.

Some amoebae can also impact the health of other living things. Certain species are pathogenic, meaning they can cause disease. Entamoeba histolytica causes amoebic dysentery, while Naegleria fowleri can be fatal to humans if it enters the nose. Acanthamoeba can cause infections in the eyes or the brain. Interestingly, some amoebae can act as hosts for other microbes. Microorganisms can live and multiply inside an amoeba, using it as a shield from harsh outside conditions. This makes amoebae important players in the movement of bacteria and other pathogens.

Recent research into the genetics of amoebae has also revealed secrets about their reproduction. Scientists have found genes in Acanthamoeba that suggest it can undergo meiosis, a type of cell division used in sexual reproduction. In the species Dictyostelium discoideum, mating and meiosis occur when food becomes scarce. Because amoeboid lineages diverged so early in the history of life, these findings suggest that sexual reproduction may have been present very early in eukaryotic evolution. This connects the study of tiny single cells to the very beginnings of complex life on Earth.