Protozoa

Protozoa are a diverse group of single-celled organisms that include more than 50,000 species. The vast majority are microscopic, many measuring less than 1/200 mm, but some, such as the freshwater Spirostomun, may reach 0.17 in (3 mm) in length, large enough to be seen with the naked eye. Scientists have even discovered some fossil specimens that measured 0.78 in (20 mm) in diameter. Many protozoans form complex, exquisite shapes and their beauty is often greatly overlooked on account of their diminutive size.

All protozoans require water, but within this limitation they may live in oceans, rivers, lakes, stagnant ponds of water, moist soil and even decaying matter. Many are solitary but some are colonial; some are free-living others are sessile; and some are parasites of plants and animals—from other protozoans to humans.

The protozoan cell is often bounded by a thin pliable membrane, although some sessile forms may have a toughened outer layer formed of cellulose, or even distinct shells formed from a mixture of materials. The inside of the cell is filled with a fluidlike material called cytoplasm, in which a number of tiny organs are located. The most important of these is the nucleus, which is essential for growth and reproduction. Also present are one or more contractile vacuoles, whose job it is to maintain the correct water balance of the cytoplasm. If too much water enters the cell, these vacuoles swell up, move towards the edge of the cell wall, and release the water through a tiny pore in the membrane. Protozoans living in saltwater do not require contractile vacuoles as the concentration of salts in the cytoplasm is similar to that of seawater and osmosis is negligible. Food vacuoles develop whenever food is ingested and shrink as digestion progresses.

Some protozoans contain the green pigment chlorophyll more commonly associated with plants, and are able to manufacture their own carbohydrates in a similar manner to plants. Others feed by engulfing small particles of plant or animal matter. To assist with capturing prey, many protozoans have developed the ability for locomotion. Some, such as Euglena and Trypanosoma are equipped with a long whiplike flag-ella which, when quickly moved back and forth, pushes the body through the surrounding water body. Other protozoans such as paramecium have developed large numbers of tiny cilia around the membrane; the rhythmic beat of these hairlike structures propel the cell along and also carry food, such as bacteria, towards the gullet. Still others are capable of changing the shape of their cell wall. The amoeba, for example, is capable of detecting chemicals given off by potential food particles such as diatoms, algae, bacteria, or other protozoa. As the cell wall has no definite shape, the cytoplasm can extrude to form pseudopodia (Greek: pseudes, false; pous, foot) in various sizes and at any point of the cell surface. As the Amoeba approaches its prey, two pseudopodia extend out from the main cell and encircle and engulf the food, which is then slowly digested.

Various forms of reproduction have evolved among the protozoa. One of the simplest involves a splitting of the cell in a process known as binary fission or asexual reproduction. In species like amoeba, this process takes place over a period of about one hour: the nucleus divides and the two sections drift apart to opposite ends of the cell. The cytoplasm also begins to divide and the cell changes shape to a dumbbell appearance. Eventually the cell splits giving rise to two identical “daughter” cells, which then resume moving and feeding.

Some species may occasionally reproduce through sexual means, which involves the joining together, or fusion, of the nuclei from two different cells. In the case of paramecium, each individual has two nuclei: a larger macronucleus that is responsible for growth, and a much smaller micronucleus that controls reproduction. When paramecium reproduces sexually, two individuals join together in the region of the oral groove—a shallow groove in the cell membrane that opens to the outside. When this has taken place, the macronuclei of each begins to disintegrate, while the micronucleus divides in four. Three of these then degenerate and the remaining nucleus divides once again to produce two micronuclei that are genetically identical. The two cells then exchange one of these nuclei, which, upon reaching the other individual’s micronucleus, fuses to form what is known as a “zygote nucleus.” Shortly afterwards, the two cells separate but within each cell a number of other cellular and cytoplasmic divisions will continue to take place, eventually resulting in the production of four daughter cells from each individual.

Protozoans live in a great range of environmental conditions. When these conditions are unfavorable, such as when food is scarce, most species are able to enter an inactive phase, where cells become non-motile and secrete a surrounding cyst that prevents desiccation and protects the cell from extreme temperatures. The cysts may also serve as a useful means of dispersal, with cells being borne on the wind or on the feet of animals. Once the cyst reaches a more favorable situation, the outer wall breaks down and the cell resumes normal activity.

Many species are of considerable interest to scientists, not least because of the medical problems that many cause. The Plasmodium protozoan, the cause of malaria in humans, is responsible for hundreds of millions of cases of illness each year, with many deaths occurring in poor countries. This parasite is transferred from a malarial patient to a healthy person by the bite of female Anopheles mosquitoes. As the mosquito feeds on a victims’ blood the parasite passed from its salivary glands into the open wound. From there, they make their way to the liver where they multiply and later enter directly into red blood cells. Here they multiply even further, eventually causing the blood cell to burst and release from three to 36 infectious bodies into the blood plasma. A mosquito feeding on such a patient’s blood may absorb some of these organisms, allowing the parasite to complete its life cycle and begin the process all over again. The shock of the release of so many parasites into the human blood stream results in a series of chills and fevers—typical symptoms of malaria. Acute cases of malaria may continue for some days or even weeks, and may subside if the body is able to develop an immunity to the disease. Relapses, however, are common and malaria is still a major cause of death in the tropics. Although certain drugs have been developed to protect people from Plasmodium many forms of malaria have now developed, some of which are immune to the strongest medicines.

Amoebic dysentery is caused by Entamoeba histolytica; African sleeping sickness, which is spread by the bite of the tse-tse fly, is caused by the flagellate protozoan Trypanosoma; a related species T. cruzi causes Chagas’ disease in South and Central America; Eimeria causes coccidiosis in rabbits and poultry; and Babesia, spread by ticks, causes red water fever in cattle.

Some protozoan species form nondestructive relationships with other species such as the protozoans found in the intestine of wood-eating termites. Living in the termites’ intestines the protozoans are provided with a place to live as they ingest the wood fibers for their own nutrition. In the process of doing so, they also release proteins which can be absorbed by the termite’s digestive system, which is otherwise unable to break down the tough cellulose walls of the wood fibers. Through this mutualistic relationship, the termites benefit from a nutritional source that they could otherwise not digest, while the protozoans receive a place to live and a steady supply of food.

Protozoans are an important food source for a wide range of animals and are therefore essential for the ecological food webs. Many also play a role in medical research while others are used in industry, for example in the purification of filter and sewage beds.