The Protist Kingdom

During the last 20 years, an alternative basic classification to the pre-existing ones, which seeks to reflect more accurately the phylogeny without being ambiguous, has gradually acquired adherents and is now quite widespread. In this context, protists include ancient protozoa and certain algae, and modern classifications recognize between 27 and 45 phyla (RUPPERT & BARNES, 96). Next, we will briefly describe the main phyla encompassed by ancient protozoa, dealing with photosynthetic protists together with representatives of the Plant Kingdom.

Source: Planktonic protists: plants and grazers. The two yellowish protists are diatoms (plants) while the brown life forms are tintinnids (grazers). Examples of morphological diversity of protists. Sample from a plankton net tow in the Bay of Villefranche on the January 25 2011. The two diatoms (plants) and the three tintinnid ciliates, which feed on small plants, have very different shapes. John R. Dolan (NOAA)


Protozoa are eukaryotes that occur as single cells or in cell colonies. They have predominantly microscopic dimensions. Its name derives from the Greek protos and zoon, which mean, respectively, “first” and “animal”. This term, consecrated until today, was created to group together unicellular eukaryotic organisms with characteristics peculiar to animals, such as the ability to move and heterotrophy. However, some organisms with animal and plant characteristics such as Euglena, for example, were also considered protozoa. Currently, the term protozoan has been applied in a collective designation, with no taxonomic value in reference to heterotrophic unicellular eukaryotes (mostly), which obtain their food by ingestion or absorption. The autotrophic eukaryotic unicellular organisms, in turn, are collectively called algae, a term that also has no taxonomic value.

Source: Protozoan. Donald Hobern from Copenhagen, Denmark

In protozoa of colonial forms, individual cells can be linked by cytoplasmic filaments or embedded in a common matrix. Such cells are similar in structure and function, although in a few ways there is a differentiation of individuals into vegetative and reproductive cells. In other words, colonial protozoa are just independent cell aggregations.
The number of protozoan species was estimated at 45,000 in 1990, but protozoologists complain that the number often exists in a state of anonymity. Of the total cited, about 20,000 are fossil species, 18,000 are free-living and 7,000 parasites.

Protists generally play an important role in the food chains of natural communities where free water exists. Autotrophs (algae) are abundant in salt and fresh waters, as well as in symbiotic associations with animals, of various levels of organization, and protozoa. Some groups in particular form an important part of the diet of numerous animals. There are saprophytic protozoa and they also ingest bacteria, making use of substances and organisms involved in the final decomposition of food chains, and thus, causing the organic matter to recirculate. Of course, there are some protozoa that can cause disease in humans.

The main groups of protozoa are differentiated into groups based on the organism’s mode of locomotion. Amoebas move by expansions of their cytoplasm. Members of the Ciliophora group move through thin appendages, or cilia, which propel the cell through synchronous undulations, like a swimmer’s arms. Flagellates have threadlike appendages, or flagella, usually located at one end of the cell body, that move cells in a liquid medium. Sporozoa move by sliding, as they do not have external locomotion organelles.
Protozoa are found in the Protist kingdom (single-celled eukaryotic organisms, colonial or not).

The main phyla are:


Protozoa that move through cellular projections called pseudopods


Protozoa that move through flagella.


Protozoa that move around using cilia


Protozoa that do not have a locomotor structure.

Most protozoa are microscopic. Some, however, can be seen with the naked eye, as is the case of Spirostomum, a freshwater ciliate that reaches 3 mm in length, of some species of the genus Globigerina, a marine sarcodyne that lives in suspension in sea water. In addition to these, which are current organisms, there is a fossil record, from past geological eras, of a sarcodyne called Nummulites, 19 cm long.

Source: Spirostomum teres – 160x. Picturepest, 2021.

Protozoan Morphology

The size and shape of these organisms show considerable variation. In general, the individual protozoan cell represents a complete organism. Like all eukaryotic cells, the protozoan cell also consists of cytoplasm, separated from the environment by a special envelope, and a nucleus.


It is a more or less homogeneous material, composed of globular protein molecules, loosely assembled to form a three-dimensional molecular frame. Embedded in its interior, there are several structures that give the protozoa its characteristic appearance. Submicroscopic protein fibrils (fibrillary bundles, myonemas, microtubules) are groups of parallel fibrils existing in the cytoplasm. The contractibility of protozoa is due to the presence of these fibrils.
In various forms of protozoa, pigments diffuse into the cytoplasm. Its hues are numerous: green, brown, blue, purple or pink.

In most of these organisms, cytoplasm differentiates into ectoplasm and endoplasm. Ectoplasm is more like a gel, while endoplasm is more bulky and fluid. Changing from one layer to another is gradual. Cell structures are found in the endoplasm.

Like other eukaryotic cells, protozoa have membranous systems in the cytoplasm. These form a more or less continuous network of channels and lacunae, giving rise to the cell’s endoplasmic reticulum. Other cytoplasmic structures include ribosomes, Golgi complexes, mitochondria, kinetosomes or blepharoplasts (intracytoplasmic basal bodies of the cilia or flagella), nutritive vacuoles, contractile vacuoles, and nucleus.


Every protozoan has at least one eukaryotic nucleus. Many protozoa, however, have multiple nuclei (almost all ciliates) during most of their life cycles. Cores come in many shapes, sizes and structures. In several species each individual has 2 similar nuclei. In the ciliates, two different nuclei: a large one (macronucleus) and a small one (micronucleus). Macronuclei control metabolic activities and regeneration processes; micronuclei are related to the processes of reproduction.

Cell envelope

The cytoplasm, with its various structures, is separated from the external environment by the cell envelope. This works not only in protection, but also in controlling the exchange of substances and is the site of perception of chemical and mechanical stimuli, serving equally to establish contacts with other cells. This envelope sometimes contains a film, intimately affixed to the cell membrane. Its thickness, flexibility and number of layers are variable.

In its simplest form, the film is the cell membrane or plasmalemma. The ciliate skin is thick and often stiff and structured in different ways. There may even be the presence of elevated rows of platelets and modular thickenings.
There are other types of protective membranes, produced by protozoa, which are outer coverings to the film. They are carapaces, foreheads, loric and cysts. Such envelopes consist of different materials and, in general, have an organic matrix, reinforced by the incrustation of substances such as calcium carbonate or silica.

It must be remembered that cysts are only temporary envelopes. Many protozoa can encyst and, in this way, increase their protection against external aggressions. In some cases, food shortages and desiccation favor encystment. In others, reproduction is regularly linked with the formation of cysts. The developed stages of parasitic species are often transmitted to another host enveloped by a resistant cyst.


Protozoa can move through three structures: pseudopods, flagella and cilia. In addition, some species can develop a sliding movement by flexing the body, without the use of specialized structures.

Pseudopods: A pseudopod is a temporary projection of part of the cytoplasm of those protozoa that do not have a rigid film. It is used by amoebas (sarcodine) for locomotion and food capture. Amoeboid movement can also be seen in cells of other organisms, such as human white blood cells.

Flagella and Cilia: The flagellum is an extremely thin, filamentous extension of the cytoplasm. As a general rule, the number of flagella present in an individual varies from one to eight (1 or 2 are the most frequent numbers). The flagellum is composed of two parts: an elastic filament, called the axoneme, and the contractile cytoplasmic sheath that surrounds the axoneme.
In certain parasitic mastigophores, such as the Trypanosoma genus, there is a delicate membrane that extends along the side of the body, with a flagellum on its outer margin. When this membrane vibrates, it shows a typical undulating movement and is therefore called the undulating membrane.

The cilia, in addition to their locomotor function, also aid in food intake and often serve as tactile organelles. They are thin, short and filiform cell extensions, and the length may be uniform or not. In general, the cilia are arranged in longitudinal rows, oblique or spiral, inserted in edges or in furrows.
Electron microscopy showed the existence of the same basic designs of the fine flagellar and ciliary structure present in all eukaryotic cells. The sections show two central fibers and nine peripheral double fibers (“9+2” structure), surrounded by a continuous membrane like film.


Whittaker, Robert H., and Lynn Margulis. “Protist classification and the kingdoms of organisms.” Biosystems 10, no. 1-2 (1978): 3-18.

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