Protozoan Groups

Emergence of Eukaryotes
The first reasonable evidence for life on earth dates from approximately 3.5 billion years ago. These first cells were prokaryotic, bacteria-like organisms. After an enormous time span of evolutionary diversification at the prokaryotic level, unicellular eukaryotic organisms appeared. Although the origin of single-celled eukaryotes may never be known with certainty, it clearly involved a process of symbiosis. Certain aerobic bacteria may have been engulfed by other bacteria that were unable to cope with the increasing concentrations of oxygen in the atmosphere. The aerobic bacteria had the enzymes necessary for deriving energy in the presence of oxygen, and they would have become the ancestors of mitochondria. Most, but not all, genes of the mitochondria would come to reside in the host-cell nucleus. Almost all present-day eukaryotes have mitochondria and are aerobic.

Some ancestral unicellular eukaryotic cells engulfed photosynthetic bacteria, which evolved to become chloroplasts, and those eukaryotes thereby were able to manufacture their own food molecules using energy from sunlight. The descendants of one line, green algae, eventually gave rise to multicellular plants.

Some eukaryotes that did not become residences for chloroplasts, and even some that did, evolved animal-like characteristics and gave rise to a variety of phyla that are collectively call protozoa. Protozoa are a diverse assemblage of unicellular organisms with puzzling affinities. They are distinctly animal-like in several respects: they lack a cell wall, have at least one motile stage in the life cycle, and most ingest their food. Throughout their long history, protozoa have radiated to generate a bewildering array of morphological forms within the constraints of a single cell.

The organisms referred to as protozoa are united only on the basis of a single, negative characteristic: they are not multicellular. This concept was recognized, in a way, by the American zoologist Libbie Hyman (1940),* who preferred the term “acellular” rather than the traditional “unicellular” to describe protozoa. She distinguished them as “animals whose body substance is not partitioned into cells.” Although most zoologists have returned to describing protozoa as unicellular because of electron microscopic studies subsequent to Hyman’s book, the concept of acellularity is still valuable. It reminds us that the traditionally recognized phylum Protozoa was not a natural phylogenetic grouping. An enormous amount of information on protozoan structure, life histories, and physiology has accumulated in recent years, and the Society of Protozoologists published a new classification of protozoa in 1980, recognizing seven separate phyla. We adopt this classification because it comes closer to reflecting real evolutionary relationships than older, simpler systems, but we cannot give adequate treatment to all groups, even all phyla, of protozoa in a book of this scope. We will introduce the most important and largest phyla: Sarcomastigophora (containing the flagellates and amebas), Apicomplexa (important intracellular parasites, including the malarial organism), and Ciliophora (ciliates).

Protozoan phyla do demonstrate a basic body plan or grade—a single eukaryotic cell—and they amply demonstrate the enormous adaptive potential of that grade. Over 64,000 species have been named, and over half of these are fossils. Although they are unicellular, protozoa are functionally complete organisms with many complicated, microanatomical structures. Their various organelles tend to be more specialized than those of the average cell in a multicellular organism. Particular organelles may perform as skeletons, sensory structures, conducting mechanisms, and other functions.

Protozoa are found wherever life exists. They are highly adaptable and easily distributed from place to place. They require moisture, whether they live in marine or freshwater habitats, soil, decaying organic matter, or plants and animals. They may be sessile or free swimming, and they form a large part of the floating plankton. The same species are often found widely separated in time as well as in space. Some species may have spanned geological eras exceeding 100 million years.

Despite their wide distribution, many protozoa can live successfully only within narrow environmental ranges. Species adaptations vary greatly, and successions of species frequently occur as environmental conditions change. These changes may be caused by physical factors, such as drying of a pond or seasonal changes in temperature, or by biological changes, such as predator pressure.

Protozoa play an enormous role in the economy of nature. Their fantastic numbers are attested by the gigantic ocean soil deposits formed over millions of years by their skeletons. About 10,000 species of protozoa are symbiotic in or on animals or plants, sometimes even other protozoa. The relationship may be mutualistic (both partners benefit), commensalistic (one partner benefits, no effect on the other), or parasitic (one partner benefits at the expense of the other), depending on the species involved. Parasitic protozoa cause some of the most important diseases of humans and domestic animals.

A number of species are colonial and some have multicellular stages in their life cycles, which may lead one to wonder why such protozoa are not considered metazoans. The reasons are that they usually have clearly recognizable, noncolonial relatives and, more arbitrarily, that they do not have more than one kind of nonreproductive cell and they do not undergo embryonic development. By definition, metazoa have more than one kind of nonreproductive cell in their bodies and undergo embryogenesis.

Position Relative to the Animal Kingdom
A protozoan is a complete organism in which all life activities occur within the limits of a single plasma membrane. Because their protoplasmic mass is not subdivided into cells, protozoa sometimes have been termed “acellular,” but most people prefer “unicellular” to emphasize the many structural similarities to the cells of multicellular animals.

Protozoa was for many years the name of a phylum. Evidence from electron microscopy, life cycle studies, genetics, biochemistry, and molecular biology has shown that this group encompassed at least seven phyla (and according to some authors, up to 30). Combining all animal-like unicellular eukaryotes with the unicellular algae into a kingdom Protista simply created another, more massive, paraphyletic taxon. Thus we will use the terms protozoa and protozoan informally, covering these organisms in a single section as a convenience and not implying that they form a monophyletic group.

Biological Contributions

1. Intracellular specialization(division of labor within a cell) involves organization of functional organelles in the cell.
2. The simplest example of division of labor between cells is seen in certain colonial protozoa that have both somatic and reproductive zooids (individuals) in the colony.
3. Asexual reproduction by mitotic division appears in unicellular eukaryotes.
4. True sexual reproduction with zygote formation is found in some protozoa.
5. The responses (taxes) of protozoa to stimuli represent the simplest reflexes and instincts as we know them in metazoans.
6. The simplest animal-like organisms with exoskeletons are certain shelled protozoa.
7. All types of nutrition are developed in protozoa; autotrophic, saprozoic, and holozoic. Basic enzyme systemsto accomplish these types of nutrition are developed.
8. Means of locomotion in aqueous media are developed.