5.1. Phylum Protozoa

The Protozoa – “first animals” – are one-celled heterotrophs possessing typical cellular structures. Classically, the Protozoans have been treated as a single phylum within the Animal Kingdom. But the unicellular level of organization is the only characteristic by which the phylum can be described; in all other respects the phylum displays extreme diversity. Protozoans exhibit all types of symmetry, a great range of structural complexity and adaptations for all types of environmental conditions. As organisms, the protozoans have remained at the unicellular level, but have evolved along numerous lines through the specialization of parts of the protoplasm. That is, specialization has occurred through the evolution of organelles. Protozoan evolution parallels that of multicellular animals in which specialization has occurred through the differentiation of cells within a multicellular body.

Protozoans occur wherever moisture is present—in the sea, in all types of fresh water and in soil. There are commensal, symbiotic and many parasitic species. In fact, the sporozoans are entirely parasitic. Although most protozoans occur as solitary individuals, there are numerous colonial forms. Both solitary and colonial species may be either free-moving or sessile.

The great majority of protozoans are microscopic. Plasmodium, the malarial parasite, is so small that it occupies only 1/4 or 1/5 of a human red-blood cell. At the other extreme, the freshwater ciliate, Spiroslomum, may reach a length of 3 mm. and be seen with the naked eye. The protozoan body is usually bounded only by the cell membrane. The rigidity or flexibility of the protozoan body is largely dependent upon the nature of the underlying cytoplasm. This cortical cytoplasm is usually gelatinous and called ectoplasm, in contrast to the more fluid, internal cytoplasm called endoplasm. Nonliving external coverings or shells occur in many different groups. Such coverings may be simple gelatinous or cellulose envelopes; or they may be distinct shells, composed of various inorganic materials or sometimes foreign particles cemented together.The nucleus is most commonly vesicular containing considerable nucleoplasm and one or more nucleoli.

Characteristic of many protozoans is an organelle called the contractile vacuole. Contractile vacuoles are water-balancing structures acting as pumps to remove excess water from the cytoplasm. These usually spherical vacuoles periodically collapse, releasing their fluid contents to the outside. Contractile vacuoles are most commonly encountered in freshwater protozoans with cytoplasm hypertonic to the aqueous environment. However, contractile vacuoles are also present in some marine groups.

All types of nutrition occur in protozoans. Some are autotrophic, and others are saprozoic; many are holozoic, and digestion occurs intracellularly within food vacuoles. Intracellular digestion has been most studied in amoebas and ciliates.

Gas exchange occurs by the diffusion of oxygen across the cell membrane. Protozoans that live in water where there is active decomposition of organic matter, or live in the digestive tract of other animals, can exist with little or no oxygen present. Some protozoans are facultative anaerobes, utilizing oxygen when present but also capable of anaerobic respiration. Metabolic wastes are diffused to the outside of the organism. Ammonia is the principal nitrogenous waste, and the amount of protein consumed.

The protozoan reproductive processes and life cycles are varied.

Asexual reproduction occurs in all protozoans and is the only known mode of reproduction in some species. Division of the animal into two or more daughter cells is called fission. When this process results in two similar daughter cells, it is termed binary fission; when one daughter cell is much smaller than the other, the process is called budding.

Sexual reproduction may involve fusion (syngamy) of identical gametes (called isogametes), or gametes that differ in size and structure. In ciliate protozoans there is no formation of distinct gametes; instead two animals adhere together in a process called conjugation, and they exchange nuclei.

Encystment is characteristic of many protozoans, including the majority of freshwater species. In forming a cyst, the protozoan secrets a thickened envelope about itself and becomes inactive. Depending on the species, the protective cyst is resistant to desiccation or low temperatures, and encystment enables the animal to pass through unfavorable environmental conditions. The simplest life cycle includes only two phases: an active phase and a protective, encysted phase.

Classification of the Protozoa into three of their major groups is based upon their characteristically different methods of locomotion:

1) by flagellar movement (the zooflagellates — “animal flagellates” — or mastigophores),

2) by pseudopodia (the sarcodines),

3) by ciliary movements (the ciliates).

The fourth major group, the sporozoans, are nonmotile during the major phases of their lives and all are parasites.

5.1.1. Class Mastigophora

The superclass Mastigophora (or Flagellata) includes those protozoans that possess flagella as adult locomotor organelles; it is generally considered to be the most primitive of the major protozoan groups. Since flagellates cannot make their own food, most of them take in tiny pieces of food from the water where they live. But some are parasites. These flagellates live on or inside other living things and feed on them. Many cause disease. Mastigophorans are conveniently divided into the phytoflagellates and the zooflagellates.

The zooflagellates (class Zoomastigophorea) possess one to many flagella, lack chromoplasts and are either holozoic or saprozoic. The zooflagelates are regarded as the most primitive of the Protozoa and also as a link between the algae and the protozoans. They are thought to have been derived from photosynthetic forms. Some are free-living, but the majority of species are commensal, symbiotic, or parasitic in other animals, particularly arthropods and vertebrates. Many groups have become highly specialized. It is generally agreed that this division does not represent a closely related phylogenetic unit. The zooflagellates have probably evolved from a number of different holophytic groups through the loss of pigments. The zooflagelates multiply asexually by mitosis and cell division. The parasitic forms include Trypanosoma gambiense, a flagellate that causes African sleeping sickness, and members of the genus Trichonympha, complex and beautiful flagellates that live as symbionts in the digestive tracts of termites where they digest the wood ingested by the termite.

5.1.2. Class Sarcodina

The sarcodines are amoeba-like organisms which have no coat or wall outside their cell membranes and which generally move and feed by the formation of pseudopodia. They move by changing shape. The shape changes to form fingerlike parts known as pseudopodia, or “false feet”. For the amoeba to move in another direction, it must form new pseudopodia on that side of the body. Despite their uncomplicated appearance, they are complex cells and are even capable of some complex behavior patterns, as when sensing and pursuing prey organisms. Amoebas take in food by forming pseudopodia around each peace of food. When the food is surrounded, a food vacuole is formed around it and it is moved inside the cell. Reproduction may be sexual or asexual. Many of the sarcodines cover their amoeba-like characteristics with firm bright shells. Their shells are made of calcium carbonate, extracted from water. It is possible to date a particular stratum by the type of Foraminifera that it contains, a fact that has proved of immense practical value in locating oil-bearing strata in Texas, Oklahoma and many other oil-rich areas.

Some cause amoebic dysentery.

5.1.3. Class Ciliatea

The subphylum Ciliophora has only one class, the Ciliatea. This is the largest and the most homogeneous of the protozoan classes.

The ciliophores, or ciliates, are the most highly specialized and complicated of the Protozoa, and indeed probably represent the most complex of all living cells. They are characterized by cilia. The rows of cilia are called cirri, which can be used for walking or jumping. Cilia, membranelles, and cirri move in a coordinated fashion. Most ciliates possess a cell mouth or cystosome.

Most ciliates live in freshwater or seawater. They do not have any parts to help them move. Instead, they move by changing shape. They move by means of many short hair-like parts called cilia.

Perhaps the best known ciliate is the Paramecium. It is shaped like the sole of a slipper. It has a long groove on one side. The groove is lined with cilia that help pull food along the groove and into the mouth-like gullet. At the end of the gullet, a bubble-like food vacuole forms around the food. When the vacuole is full, it carries the food into the cell where the food is broken down

Cilia have the same structure as do flagella; they differ from flagella chiefly in that they are generally more numerous and are considerably shorter. Compound ciliary organelles, evolved from the adhesion of varying numbers of individual cilia, are of common occurrence.

Some ciliates also have myonems, contractile threads. All have a complex skin, the cortex, which includes the cell membrane. In some groups, the cortex contains small barbs known as trichocysts, which are discharged when the cell is stimulated in certain ways. The ciliates have another unusual feature: they have two kinds of nuclei, macronuclei and micronuclei. One or more of each kind is present in all cells. They also have a complex system for exchange of genetic information, in which cells conjugate and the macronuclei undergo meiosis. The macronucleus in certain ciliates contains 50 to 100 times as much DNA as the micronucleus and so is believed to represent multiple copies of it.

The body shape is usually constant and in general is asymmetrical. The ciliate body is typically covered by a complex, living pellicle, usually containing a number of different organelles.

Mucigenic bodies are the group of pellicular organelles found in many ciliates. They are arranged in rows like trichocysts and discharge a mucoid material that may function in the formation of cysts or protective coverings.

About 6,000 species of ciliates are known, both freshwater and saltwater forms. Almost all are free-living (non-parasitic).