Protozoa

The protozoa are a vast assemblage of extremely diverse organisms whose only common characteristic is that they are all unicellular. They were previously classified as members of the animal kingdom but are now recognized as a distinct group and occupy a kingdom of their own, Protista.

Protozoa have been known only since the seventeenth century, when Dutch naturalist Antoni van Leeuwenhoek observed them with simple microscopes of his own making. Leeuwenhoek’s discovery was accidental; he was simply curious about the microscopic nature of such common substances as rainwater and scrapings from his mouth and teeth. His observations of small, active creatures associated with these substances were the first recorded descriptions of microscopic life. Although Leeuwenhoek made no effort to classify protozoa (which he called “little beasties”), his observations were later confirmed by scientists who recognized the importance of his discoveries.

Protozoan Diversity

The diversity of protozoan types cannot be overemphasized. Though some have a close evolutionary relationship, others have followed very different paths. Some protozoa are animal-like, obtaining energy by consuming food from external sources, and others resemble plants in that they contain chlorophyll and use the energy of the sun to manufacture food.

Protozoa occur in saltwater and freshwater, in the soil, and inside larger organisms. In short, they are wherever moisture is present. Most are motile, although some are sessile (attached), and others, like those in the genus Volvox, form colonies. The great majority of protozoa are microscopic, although the very largest are just visible to the naked eye. The blood parasite Anaplasma is only one-sixth to one-tenth the size of a human red blood cell, while the ciliate in the genus Spirostomum (freshwater protozoan that uses tiny hairlike cilia for locomotion) is up to three millimeters long and can be seen without a microscope.

An individual protozoan, because it consists of only one cell, does not contain any organs. Instead, it harbors analogous structures called organelles, which enable it to carry out its life functions. Central among these organelles is the nucleus, which contains the genes and serves as a sort of control center for cell activities. The cells of protozoa contain from one to many nuclei, depending on the species. Other organelles that distinguish the protozoa are those used for locomotion. These include whiplike flagella, hairlike cilia, and flowing extensions of the cell called pseudopodia, or false feet, which are generated in the direction of movement. Locomotory organelles are important in the classification of the protozoa. Many protozoa, especially freshwater species, possess an organelle called the contractile vacuole, which serves to pump excess water out of the cell. This organelle is especially important in freshwater protozoa, which tend to accumulate water by osmosis because their bodies contain more dissolved substances than the water in which they live. As water flows into these protozoa, the contractile vacuole absorbs it, expands, and finally contracts to flush the water through the cell membrane and into the surrounding environment.

Protozoa can be divided into two groups based on how they get their food: autotrophic and heterotrophic. Autotrophic protozoa contain chlorophyll and can convert solar energy into usable energy. The freshwater Euglena are autotrophs. Heterotrophic protozoa must capture their food, either funneling it through mouthlike openings or, like the amoeba, engulfing food particles with their pseudopods. This latter method of obtaining food is called phagocytosis.

Gas exchange in protozoa occurs through simple diffusion of gases across the cell membrane. Some protozoa live in environments such as stagnant water, or the guts of animals where little oxygen is present. These protozoa are called facultative anaerobes, which means they can live with or without oxygen.

Protozoan Reproduction

Most protozoa reproduce asexually: a single parent produces offspring that are identical genetic copies of itself. Asexual reproduction in protozoa is accomplished in two ways—fission and budding. In fission, the parent cell simply divides into two offspring cells of equal size. In budding, the offspring is a smaller cell that grows off the larger parent cell. Some protozoa carry out sexual reproduction, in which two parent cells release smaller sex cells called gametes. Two gametes fuse and grow into a new protozoan that is genetically different from either parent.

Conjugation is an interesting process that occurs in ciliate protozoa and may be linked to the reproductive process. It involves the temporary linking of two cells, followed by an exchange of nuclei, after which the cells separate. However, no offspring are formed. It is thought that conjugation is a means of renewing the nuclei of certain protozoa so that they may continue to reproduce asexually. Another process that many protozoa undergo is called encystment, in which the protozoa secrete a thickened sheath around themselves and enter a period of dormancy. Encystment enables protozoa to survive adverse environmental conditions such as drought or cold. When conditions become favorable again, the protozoa emerge from their cysts and resume their normal activities.

Protozoan Phyla

Though the classification of these organisms is debated, the kingdom Protista is generally divided into five groups or phyla: Euglenophycota, Sarcomastigophora, Apicomplexa, Craspedophyta, and Ciliophora. The subphylum Mastigophora contains the flagellates: protozoa that move using one or more whiplike flagella. The subphylum mastigophorans are of two types: phytoflagellates, which contain chlorophyll and are primarily autotrophic; and zooflagellates, which are primarily heterotrophic. Many zooflagellates are parasites of insects and vertebrates. The flagellates of the subphylum Mastigophora show such a high degree of structural diversity that it is impossible to describe a typical representative. However, among the more interesting phylum members are the dinoflagellates, which occur in marine and freshwater environments. They have two flagella—one points away from the cell, and the other lies transversely around it. Many dinoflagellates contain a golden-brown photosynthetic pigment called xanthophyll and are autotrophic, but others are unpigmented and heterotrophic. These protozoa are encased in a cellulose sheath, or pellicle, which is often highly ornamented with spikes, hooks, or other prominences. Certain marine species are responsible for red tides, a population explosion that is responsible for massive fish kills and causes paralytic poisoning in humans who consume shellfish taken from red tide waters.

The protozoa in the phylum Sarcomastigophora move about and engulf their food by means of pseudopodia. Amoebas belong to this phylum, as well as many marine, freshwater, and terrestrial species. The members of this phylum are the simplest protozoa and have few organelles. Some have exquisitely crafted external skeletons. The radiolarians, which are restricted to the marine environment, have beautifully sculptured exoskeletons made of silicon dioxide or strontium sulfate. The skeleton is riddled with tiny pores through which the living organism within is able to extrude thin filaments of cytoplasm for purposes of feeding, locomotion, and anchorage. Radiolarians exist in great numbers in the surface waters of the ocean and, upon dying, sink to the bottom where, in some areas, their skeletons form a large proportion of ocean bottom sediments.

The phylum Apicomplexa (formerly phyla Sporozoa and Cnidospora) are parasitic protozoa formerly referred to as sporozoans because some members of both groups go through sporelike infective stages. This phylum contains parasites that infect cells of the intestine and blood and are responsible for causing malaria in humans. These protozoa belong to the genus Plasmodium, a group containing more than fifty species, four of which infect humans. All require a mosquito for transmission of the disease-causing protozoan. When the mosquito bites a person, the parasite is introduced into the blood with the saliva of the insect. From the blood, the protozoan travels to the individual’s liver, where it reproduces and then escapes to invade red blood cells. Once in the red blood cells, it consumes hemoglobin, reproduces, and escapes to invade additional red blood cells. This cycle of invasion and escape is responsible for the debilitating chills and fever characteristic of malaria. The phylum also contains amoeboid parasites of fishes and insects.

The largest phylum is Ciliophora, which has more than eight thousand species. Of all the phyla, this one has the most homogeneous members as far as their evolutionary relationships are concerned. They all appear to have arisen from a common ancestor. Members of this phylum are commonly referred to as ciliates because they use cilia for locomotion and for sweeping food particles into the cytostome (cell mouth). They are also characterized by the presence of two nuclei—a small micronucleus and a larger macronucleus whose sole purpose is the synthesis of deoxyribonucleic acid (DNA, the genetic material). Although some ciliophorans are sessile, or attached, many are nimble, quick, and carnivorous and are, therefore, perhaps the most animal-like of all the protozoa. They reproduce either asexually by fission or through sexual conjugation, but they never release free gametes into the surrounding environment. Ciliophorans are common in fresh and saltwater. Some are parasitic.

Protozoology

Although Antoni van Leeuwenhoek had little sense of what he was looking at when he first observed protozoa in the seventeenth century, he used the same tool that later scientists did, the microscope. Protozoology—the study of organisms too small to be observed with the naked eye—has become an area of specialization within microbiology.

Protozoa are best examined while alive, as specimens that have been killed, fixed, and stained often become distorted. Some of the smaller species can be viewed under the cover slip of a microscope slide; others, especially the larger specimens, must be viewed under less restrictive conditions: in a petri dish with a dissecting microscope or under a raised cover slip with corners that have been supported with paraffin or some other material.

One of the challenges of studying protozoa with the microscope is the speed with which they move and the tedium of keeping track of them by constantly adjusting the position of the slide. For this reason, a solution of methyl cellulose is often added to the slide preparation to increase the viscosity of the water and slow down the organisms.

As many protozoa are almost transparent, it is necessary to highlight their features by using dyes or stains that will not kill them. Such substances, called vital stains, render various structures in the protozoa visible under the microscope. For example, the vital stain Janus Green B selectively stains mitochondria, the energy-producing organelles, while neutral red has an affinity for vacuoles, the organelles that contain engulfed food particles. In studying protozoa structure, scientists use an assortment of vital stains and regular stains, which are used when protozoa are killed and preserved on a slide. Electron microscopy, used to examine the extremely fine structure of protozoa, has special stains and fixatives of its own for maintaining the structure of these organisms under the sustained electron bombardment required to illuminate the specimen.

Protozoa are ideal subjects of study for cytologists, geneticists, and developmental biologists because they reproduce asexually and, therefore, provide the researcher with an endless succession of clones, or genetically identical individuals. Scientists can subject these clones to various environmental conditions and observe the effects in their offspring.

The primary method of maintaining protozoa in a laboratory is a culture, which usually consists of one species of protozoa and the bacterium on which it feeds. The object is to exclude other organisms that might harm the protozoan. As toxic waste products build up in the culture dish, a small number of protozoa are transferred to a clean culture medium. This procedure is called subculturing and continues if the culture is maintained. There are two types of cultures: monoxenic and axenic. In monoxenic cultures, a single species of protozoa is maintained with a single species of bacteria. In axenic cultures, a single species of protozoa lives in isolation from any other organisms. The axenic culture is of value in certain types of nutritional studies.

Parasitic protozoa can be examined separately from their hosts in a laboratory but only when kept under conditions that provide them with a chemical and physical environment that will keep them alive for as long as necessary. For example, some parasitic protozoa of warm-blooded animals must be kept in a warm culture medium to prevent their being distorted by cooling.

The Base of the Food Chain

Protozoa are of first and foremost importance to ecology. These organisms make up the lowest links of the food chain that eventually leads to humans. Autotrophic protozoa are called producers because, like plants, they can use the energy of sunlight to manufacture food substances. Other protozoa are heterotrophs, or consumers, and must take food from their environments. Dinoflagellates are a group of autotrophs with a notorious role in the marine environment. These organisms, which are responsible for red tides, can pass their potent toxins up the food chain to cause paralytic shellfish poisoning in humans. The toxic materials they release into the ocean during periods of intense reproductive activity also lead to massive fish kills, which mean losses for the fishing industry.

Foraminifers are a type of shelled amoeba in the superclass (in some classifications, class or subphylum) Sarcodina. Some foraminifers existed millions of years ago when the earth’s fossil fuel deposits were being generated. Therefore, the presence of their fossil remains in drill cores helps indicate where oil is likely to be found.

A symbiotic relationship exists between protozoa and termites. Cellulolytic (cellulose-breaking) protozoa inhabit the guts of termites and are the organisms actually responsible for digestion of the wood that termites eat. Without these protozoa, the termites would not be able to feed on wood and would no longer be able to damage homes and other structures.

Protozoan Infections

In people, protozoa cause numerous infections, generally limited to four sites in the body: the intestines, the genital tract, the bloodstream and tissues, and the central nervous system (brain and spinal cord). Not all parasitic protozoa cause death in humans, but they are responsible for untold misery in wide areas of the world, especially underdeveloped countries where preventive medicine is unknown and too expensive for these societies to employ.

The two most common protozoan infections are malaria and amoebic dysentery. The five malarial parasites belong to the genus Plasmodium, though Plasmodium falciparum and Plasmodium vivax are the most common. These parasites are transmitted to humans through the bite of mosquitos in the genus Anopheles, sometimes called marsh mosquitos. They eventually destroy the body’s red blood cells, causing anemia and recurrent debilitating chills and fever. Malaria is a major health problem in parts of Africa, Asia, and Central and South America. More than 240,000 people worldwide are infected with the illness every year. Amoebic dysentery, also called amebiasis, flourishes in overcrowded, unsanitary conditions. The disease is caused by the Entamoeba histolytica parasite, and gradually erodes the inner lining of the intestines, causing ulcers. Amoebic dysentery is entirely avoidable when food and water are handled carefully to prevent contamination or are purified if contamination is suspected.

Other protozoa of medical importance are Trichomonas vaginalis, which are parasites that infect the genital tract, causing a common sexually transmitted infection called trichomoniasis. Giardia lamblia, is a parasite of the small intestine which causes the infection known as giardia. The single-celled Toxoplasma gondii causes toxoplasmosis, which infects the fetus and can cause congenital abnormalities. This disease is known to be contracted through cat feces. Cryptosporidiosis is an intestinal infection caused by Cryptosporidium parvum.

Further research involving protozoa in medicinal settings includes using Neospora caninum therapy as a cancer treatment, the successful use of medicines like disulfiram in treating Trypanosoma, Leishmania, and Giardia, and the use of the herbicide called oxadiazon in the treatment of toxoplasmosis.

Protozoa play an important role in the degradation of both human and industrial wastes in the environment. In sewage treatment, for example, anaerobic protozoa (those that flourish in the absence of oxygen) and bacteria play a role in the degradation of raw sewage. After they have done their work, the effluent is passed to a set of tanks with aerobic (oxygen-requiring) protozoa for further processing. Autotrophic protozoa have been used to deal with industrial wastes containing high levels of nitrates and phosphates. The settling tanks are illuminated to promote the growth of the autotrophs, which absorb and metabolize the industrial chemicals as part of their normal life processes.

Principal Terms

Autotroph: Any organism capable of synthesizing its own food by using either solar or chemical energy

Ciliate: A protozoan that uses short, hairlike structures called cilia for locomotion

Flagellate: A protozoan that uses long, whiplike structures called flagella for locomotion

Heterotroph: Any organism that must consume other organisms or organic substances to obtain nutrition

Nucleus: A cellular organelle that coordinates the cell’s activities and contains the genetic information

Organelle: Any of several structures found inside an individual cell, analogous to the organs of multicellular organisms

Phagocytosis: Obtaining food by engulfing it

Pseudopods: Cytoplasmic extensions of a protozoan’s body used for locomotion and the engulfing of food

Unicellular: Consisting of only one cell

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