Protozoa

Protozoa are a group of single-celled organisms that live by taking in food. As a major group in the kingdom Protista, protozoa are described as having animal-like—rather than plantlike—qualities, since they move about to find and eat their food. Since they are protists, protozoa are eukaryotes (they contain a nucleus).

GROUPS OF PROTOZOA

The word protozoa literally means "first animal," indicating that they are considered to be the early ancestor cells from which more complex, multicelled animals evolved. As animal-like protists, protozoa are divided into four phyla (one of the seven major classification groups that biologists use to identify and categorize living things) based mainly on how they move about.

Flagellates. The first group is the flagellates called Mastigophora, which move about by the use of one or more flagella (a whiplike tail). This phylum, also known as Zoomastigina, lives in a watery environment. Flagellates are very diverse since some live as parasites (organisms that live in or on other organisms and benefit from the relationship) and others as free-living organisms.

CHARLES LOUIS ALPHONSE LAVERAN

French biologist Charles Laveran (1845–1922) discovered the parasite (an organism that lives in or on another organism and benefits from the relationship) that causes human malaria (a disease characterized by cycles of chills, fever, and sweating). This was the first time that a disease was shown to be caused by living animal cells called protozoa (a group of single-celled organisms that live by taking in food). His creative work not only led to understanding the disease and its transmission by certain species of mosquito but also directed many other researchers into this new field.

Charles Laveran was born in Paris, France, the son of a military doctor. When he was five years old, the family was transferred to Algeria. Returning to Paris at the age of eleven, Laveran eventually entered the same military medical school his father had attended and graduated in 1867. Continuing in his father's footsteps, he joined the military medical service and saw active duty during the Franco-Prussian War (1870–71). It was then that he saw how disease can ravage an army worse than any enemy. In 1878 he was sent to Algeria as his father had been, and it was there that he began a careful study of the disease malaria. Malaria was common in many parts of Algeria, and had affected humans for centuries, with no one able to do anything to prevent it. For a long time it was believed to be caused by mal aria, the Italian words for "bad air." During Laveran's time, it was thought that perhaps it had a bacterial cause since French chemist and microbiologist Louis Pasteur (1822–1895) was discovering more and more bacterial diseases.

Laveran went about his research in a careful, methodical way, and although he was limited by a primitive, low-powered microscope, he spent a great deal of time examining blood samples from malaria patients both living and dead. Finally, in November 1880, he first observed under the microscope tiny circular and cylindrical bodies that had moving flagella or hair-like filaments. This, he knew, was no bacteria. Instead, he knew it was a living animal cell, a minute, single-celled creature called a protozoon (plural, protozoa). Once inside the human body, protozoa act like parasites. A parasite is a species that lives in or on another species at the expense of that species. In other words, the parasite thrives and the host gets sick or even dies. The particular protozoan parasite that Laveran discovered was later named plasmodium.

Laveran had a great deal of trouble, however, convincing a skeptical medical community of his protozoan discovery, and eventually it took the great Pasteur to agree with him before everyone was convinced. Laveran was able to study the cycle of malaria in the red blood cell and discovered exactly what goes on there. He found that the protozoa increased in size inside the red blood cell until they almost filled it, at which time they then divided and formed spores. When these spores were freed from the destroyed blood cell, they invaded healthy blood cells and continued to do the same thing. Laveran had a strong feeling that the malaria protozoa were nurtured and transmitted to humans by certain types of mosquito, but it remained for the English physician, Ronald Ross (1857–1932), to finally prove this in 1897. Laveran went on to study other protozoan diseases and eventually joined the Pasteur Institute devoting the rest of his life to the study of tropical diseases. For his discovery of the protozoa plasmodium, Laveran was awarded the 1907 Nobel Prize in Physiology and Medicine.

Sarcodina. The second group of protozoa are the members of the phylum Sarcodina, commonly known as amoeba. An amoeba is recognized because it has no particular, fixed shape. It has been described as looking formless like a bag of jelly. The reason for this is that the amoeba's single cell is surrounded by a plasma membrane and is constantly changing shape. It is this constant shape-changing that allows the amoeba to move. Amoeba can be found in all types of water and even in the soil. >Some are parasites of humans and can cause fever, abdominal cramps, and diarrhea. There is also a species of shelled amoeba in which the organism's cytoplasm (jelly-like cell contents) is protected by a shell it has created out of its own mineral secretions. With names like foraminiferams, heliozoans, and radiolarians, the shells of these tiny water creatures represent some of the most intricate and beautiful designs of nature.

Sporozoa. The third group of protozoa belong to the phylum Sporozoa and are parasitic spore-formers. In their adult stage, they cannot move, but during another stage, they live in a host who transfers them to yet another organism who they infect. This is how the species Plasmodium vivax goes from a mosquito to a human and infects the latter with malaria. This particular protozoan kills between 2,000,000 and 4,000,000 people every year.

Ciliophora. Finally, the phylum Ciliophora makes up the fourth group of protozoa. These organisms are characterized by short, flexible, hair-like strands or filaments called cilia that cover their bodies. These move in a rhythmic, coordinated manner and are able to propel the protozoan through its liquid environment. Some species, like the Paramecium, have as many as 15,000 cilia per cell and can move very quickly. Ciliates are the most structurally complex of all the single-celled organisms on Earth, and although most ciliates reproduce asexually, some species conduct a complicated form of sexual reproduction called conjugation. Conjugation is similar to mating since one of the ciliates (the donor or "male") produces a small protein tube through which genes are transferred to the recipient or "female" when the cells are joined together. Although conjugation achieves gene mixing, it does not result in the production of an entirely new individual. Most ciliates are found in fresh and salt water and are fierce predators, eating bacteria and other small organisms.

Many protozoans serve as an essential food source for a wide range of animals. Therefore, they are very important to the ecological food web (the transfer of energy in an ecosystem) of higher organisms. Some are also used for medical purposes while others serve such practical uses as purification of sewage beds.

[See alsoProtists ]