Paramecium

A paramecium (plural, paramecia), is a microscopic single-cell organism found in aquatic environments that is commonly used in scientific studies. Paramecia are oval-shaped protists, a category of unicellular life that lacks tissues. They have short hairy structures covering their outer cell called cilia that enable them to move freely in their aquatic environs at speeds of up to two millimeters per second. Cilia also help in feeding. Paramecia may be found in freshwater, marine, and saltwater environments around the world, although they are capable of living in animals or moist soil. They are most typically found in warm, stagnant water.

Despite their small size, paramecia are relatively complex life-forms. They are classified as eukaryotes, which are organisms in which the cell or cells contain both a nucleus and chromosomes composed of DNA tied together by proteins. Additionally, the cells of eukaryotes contain organelles such as Golgi bodies, lysosomes, and mitochondria. Organelles are structures within individual cells that simulate the functions of true organs like those found in multicellular animals. Most forms of life, including plants, animals, and fungi, are categorized as eukaryotes. This is in contrast to prokaryotes such as bacteria and single-cell microorganisms called archaea that lack these organelles.

Background

According to taxonomy, which is the system of categorizing all life based upon shared characteristics, paramecia collectively form a single genus of protozoans also called Paramecium. Protozoans are unicellular organisms that have animal-like behaviors, including mobility, heterotrophy (meaning that they ingest organic compounds for nutrition), and complex internal structures.

Dutch biologist Antonie van Leeuwenhoek is credited as the first to identify protozoans, in the seventeenth century. Van Leeuwenhoek was one of the founders of the science of microbiology. It is thought that he may have discovered the paramecium possibly as early as 1674, although he never published a paper on his discoveries. Many scientists explored this new science of microbiology in the eighteenth century. During this era, the paramecium was called the "slipper animalcule" after its oblong shape and the word animalcule (meaning little animal) that van Leeuwenhoek had coined for all single-cell organisms. In 1773, O. F. Müller officially provided these organisms with their Linnaean name of paramecium, which was drawn from the Greek word for oblong.

Paramecia are characterized by their elongated shapes. The ends of paramecia may be either rounded or pointed. Based upon these morphological differences that were made through visual observation, paramecia were formerly classified as either belonging to the P. aurelia or P. bursaria species. However, more recently, scientists have combined these observations with genetic and molecular analyses to redefine the classification of the various types of paramecia. Based on the results of these studies, paramecia were divided into the aurelia and bursaria complexes of species. Since 2012, biologists have further separated the species of paramecium into five subgenera—Chloroparamecium, Cypriostomum, Helianter, Paramecium, and Viridoparamecium—based upon studies of their DNA.

As eukaryotes, paramecia contain many of the same organelles (such as mitochondria) as multicellular life-forms. The basic structures of the paramecium are contained within its cell by a stiff elastic membrane called a pellicle. The paramecium's pellicle is completely encircled by cilia that emerge from small sacs called alveoli. Cilia are hairlike organelles. In addition to providing them with independent movement, these cilia help feed paramecia by pushing food such as bacteria and other microscopic material toward an opening called the oral groove. At the back of the oral groove is the paramecium's mouth, which connects to the gullet. Within the gullet, food particles are broken down in organelles called food vacuoles. Another form of vacuole known as the contractile vacuole is responsible for releasing excess water from the cell in a process called osmoregulation.

Paramecia contain two nuclei: the macronucleus and micronucleus. The micronucleus contains two sets of the paramecium's genetic DNA, one of which is passed along to its descendants during sexual reproduction. The macronucleus is responsible for the control of all biological functions of the cell.

Overview

Paramecia are prized as model organisms. Model organisms are species of living organisms that are easily bred in laboratories that may be helpful in experiments used to make biological discoveries. The paramecium is considered valuable for its comparatively large size for a ciliate and complex cellular organization. Paramecia are used to study such cellular structures as cilia and basal bodies, while allowing scientists to examine the basic molecular processes that drive all forms of eukaryotic life.

Paramecium tetraurelia, for instance, is an example of a common laboratory subject. Its sexual cycle is easily manipulated through the management of its nutritional intake. Paramecia like P. tetraurelia have two different forms of sexual reproduction: conjugation, which is a form of cross-fertilization that occurs through the cell-to-cell contact of two paramecia; or autogamy, a form of asexual reproduction in which self-fertilization occurs through the division of the nucleus into two parts that reunite to create a new zygote. As a result, scientists have found this species useful in both genetic analyses and sequencing.

Several species of paramecia are known to be the hosts for a variety of bacteria called endosymbionts. Endosymbionts are tiny organisms that live with the cellular structures of larger organisms with which they often form a beneficial relationship. These life-forms may assist their host organisms by facilitating important vital biological processes. In the case of the paramecium, endosymbiont bacteria within their macronucleus and other cell structures help them avoid potentially dangerous lytic enzymes and carnivorous ciliates like those from the genus Didinium. They simultaneously provide their hosts with the ability to process the nutrients they need for survival. During conjugation, these bacteria may also enable their hosts to kill clones that lack endosymbiont bacteria, a process that helps to ensure the continued survival of the species. As these ciliates are comparatively large in comparison to other protozoans, they allow scientists to observe these endosymbiont processes more easily.

The biological mechanisms, number of species, and taxonomic categorization of paramecia are still active areas of research. For instance, paramecia contain a set of organelles called trichocysts, located in the ectoplasm underneath the pellicle. Paramecia will occasionally dispel the contents of these trichocysts, which take the form of thin, spiky threads. The function of these trichocysts is not fully understood, although studies have linked them to a form of protection against other microscopic predators. Similarly, the number of species of paramecia is not certain; a new species, Paramecium buetschlii, was identified in 2015. Such discoveries have served to help further delineate the taxonomic tree of the Paramecium genus as a whole.

Bibliography

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