Deciphering the Greek roots results in defining protozoa as 'first' (proto)
'animals' (zoa). Although molecular phylogenetic studies indicate that protozoa
are among the earliest branching eukaryotes (see phylogenetic
tree), such a definition does not provide much descriptive
information. Generally protozoa are unicellular (ie, single-cell) organisms
with a few species exhibiting colonial forms. For the most part the protozoa
can be distinguished from other unicellular eukaryotes. However, there is sometimes
confusion and controversy. The protozoa are a very diverse group and it is difficult
to formulate definitions and descriptions which will include the majority of
this group. Questions one might ask in regards to defining protozoa include:
Protozoa exhibit a wide variety of morphologies (click figure for larger image and description). There is no one shape or morphology which would include a majority of the protozoa. Shapes range from the amorphous and ever-changing forms of ameba to relatively rigid forms dictated in part by highly ordered cytoskeletons or secreted walls or shells. Several protozoan species express photosynthetic or other pigments and thus are colored. Many protozoan species exhibit complex life cycles with multiple stages. Sometimes the different life cycle stages are so dissimilar that they have been mistaken for completely different species.
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Protozoa--except for a few colonial forms--are unicellular, or single-celled, organisms; although, some argue that they are actually 'acellular'. Thus, the vast majority of protozoa are microscopic. However, they do exhibit an incredibly large range of sizes. Extant species range in size from <1 µm (10-6 meter) to several mm. Fossilized Forminiferida of several cm have been identified. (Extinct protozoa can be detected because of a secreted calcium carbonate shell.) Most of the organisms discussed in this course will be 3-50 µm. This small size necessitates the use of a microscope to detect protozoa. An electron microscope is needed for detailed morphological studies.
Protozoa are found virtually everywhere. As a group, the protozoa are extremely adaptable. Individual species, though, generally have very specific niches. Like all other organisms, protozoa must be able to acquire and metabolize nutrients from their environment. Many protozoa simply absorb solutes (i.e., osmotrophy) from their media, while some are scavengers that ingest solid material (i.e., phagotrophy). Predatory protozoa either actively hunt down or passively ambush other organisms (typically bacteria or other protozoa). Some protozoa are photosynthetic and can capture the energy of the sun and convert it to usable chemical energy (i.e., autotrophic or phototrophic). Many protozoa are not restricted to a single feeding mechanism and can utilize combinations of the above (i.e., heterotrophic, mixotrophic).
Protozoa can also be viewed as free-living or symbiotic. Generally free-living organisms are found in the soil or aqueous environments, whereas symbionts live in close association with another organism. Symbiosis implies a physiological dependency of one organism on another organism and not just a close physical association between two organisms. Generally this dependency is in the form of nutrition. Different forms of symbiosis can be distinguished which reflect the nature of the association between the two organisms (Box).
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The earliest observations of protozoa noted their motility. This motility resulted in their classification as 'animals', which were distinguished from the non-motile 'plants'. However, motility is not a universal feature of protozoa and different protozoa utilize different mechanisms for their movement (Table). In fact, protozoa were initially classified based in part on their mechanism of motility (see Taxonomy).
Cilia and flagella are subcellular structures which propel protozoa through a fluid medium. Flagella are long whip-like structures which propel the organism as a result of wave-like beat which is propagated through their length. Flagellated protozoa typically have one or a few flagella per organism. In contrast, ciliated protozoa are usually covered with rows of numerous cilia. The beats of these cilia are coordinated and function like oars to propel the organism. Cilia and flagella can also assist in the procurement of food, reproduction and other functions. Cilia and flagella are made up of the same protein components and are actually equivalent structures. Both are membrane bound filamentous projections from the cell. The filament, known as an axoneme, is composed of a series of parallel microtubules, typically exhibiting a '9 + 2' arrangement. Movement is produced when the microtubules slide past each other. The force which mediates this sliding motion is generated by a protein called dynein. Dyneins are 'motor proteins' which convert the chemical energy released by ATP hydrolysis into a mechanical energy. Microtubules are cytoskeletal elements which also play important roles in cell shape and are a major component of the mitotic spindle.
In contrast to the swimming exhibited by flagellates and ciliates, ameba are protozoa that crawl along a solid substratum in a fashion known as 'ameboid movement'. The ameba projects out a pseudopodium, or false foot, from the cell body. The pseudopodium then attaches to the substratum and then pulls the rest of the cell body forward. The force involved in this movement is generated by another cytoskeletal system, which is comprised of actin and myosin. Actin forms long filaments, also known as microfilaments, and myosin is a motor protein which moves along the microfilaments in an ATP dependent manner. Muscle contraction is another example of the force generation via actin-myosin cytoskeletal elements. In a mechanistic sense, phagocytosis is a form of ameboid movement also involving microfilaments. In this case the pseudopodia are extended to surround the particle being ingested. Fusion of the pseudopodia with the cell body results in the internalization of the particle within a vacuole.
Apicomplexa also crawl along a substratum, but by a different mechanism than the ameba. The mechanism of this so-called 'gliding motility' is just beginning to be understood and probably involves both microfilament and microtubule based cytoskeletal systems. Apicomplexa also exhibit intracellular forms and invasion of the host cell also involves this gliding motility. (See also discussion of host cell invasion by the malaria parasite.)
Cellular motility involves force generation through either the microtubule-based cytoskeletal elements or the microfilament-based cytoskeletal elements. This is true for protozoa and other eukaryotes. The involvement of microtubules and microfilaments in both cell shape and cell movement make these subcellular structures more analogous to the musculoskeletal system.
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Protozoa, like all other organisms, reproduce. The most common form of reproduction in protozoa is asexual binary fission. In other words, a single organism will divide into two equal organisms. A slight modification of this binary fission, called budding, is when one of the newly formed cells is smaller than the other. Typically the larger cell is called the mother and the smaller is the daughter. Some protozoa will form an intracellular bud and essentially give birth. Another variation of binary fission is a multiple fission or segmentation. In this situation, several rounds of nuclear replication occur without cytokinesis. This multinucleated cell will then form multiple progeny simultaneously.
Many protozoa exhibit sexual reproduction in addition to the asexual forms of reproduction. This sexual reproduction can involve the production and fusion of gametes in processes similar to higher organisms. The Ciliophora undergo a conjugation in which opposite mating types will pair and directly exchange genetic material (i.e., DNA). Sometimes sexual reproduction is an obligatory step in the life cycle, whereas in other cases the organism can reproduce asexually with an occasional round of sexual reproduction.
In summary, protozoa are unicellular eukaryotic microorganisms. However, the amount of diversity in terms of morphology, size and life styles exhibit by protozoa makes it difficult to develop a more precise definition. Their long evolutionary history (see phylogenetic tree) accounts for much of this diversity. However, protozoa do exhibit features common to all eukaryotes. (Link to a series of powerpoint presentations on eukaryotic cell biology.)
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Taxonomy, or systematics, is the science of naming and classifying organisms. In addition to assigning taxonomic classifications, systematics also attempts to place organisms into groups reflect evolutionary relationships or phylogenies. However, taxonomic criteria are often arbitrary and taxonomy is always changing to reflect new discoveries and interpretations. Furthermore, utilitarian features, such as type of disease, host range and geographical distribution, are frequently used in the systematics of pathogenic micro-organisms. This is especially true for protozoan taxonomy. In addition, there is some debate on the overall philosophy in the classification of protozoa and the relationships between many protozoan groups are not known (see D.J. Patterson in Am. Nat. 154, S96-124).
Historically protozoa were divided into four major groups: the ameba, the flagellates, the ciliates, and the sporozoa. The distinguishing features between the groups was based on motility (i.e., ameboid, flagella, cilia). The sporozoa were a heterogeneous group that produced spores during one stage of their life cycles and exhibited a 'gliding' motility. However, such a classification scheme is quite arbitrary and does not necessarily reflect true evolutionary relationships between organisms. One problem with using motility as a taxonomic criteria is that many protozoa utilize different types of motility during different stages of their life cycles. For example, Naegleria exists in an ameba form when food is plentiful and transforms into a flagellate when food is absent. In general, the ameba are a heterogeneous group and are all probably dervived from flagellates. Among these four original protozoan groups only the ciliates are still considered a valid taxon.
Beginning the 1960's the electron microscope was used to identify ultrastructural features which could serve as criteria for grouping protozoa. In many cases morphology leads to a classification which places organisims into monophyletic groups. Monophyletic means that all of the organisms in that group are probably derived from a common ancestor. For example, many of the protozoa formerly called sporozoa possess subcellular structures, collectively known as the apical organelles, and now form a monophyletic group called apicomplexa.
More recently molecular techniques are being applied to taxonomy. Possible evolutionary histories and relationships can be derived by comparing DNA or protein sequences. Molecular sequence data has confirmed phylogenies based on other criteria, settled some debates, and led to a few surprises. For example, molecular data confirms that the apicomplexa are monophyletic, and furthermore, indicates that they are related to the ciliates and dinoflagellates. These three groups are now combined into a larger monophyletic group called alveolata. This relationship had been previously suspected and the name is in reference to morphological structures known as alveolar sacs.
Protozoa are often grouped within the protista, which includes other unicellular eukaryotes, such as algae and fungi. Many eukaryotic microbes do not clearly fit into one of these major groups. For example, dinoflagellates and euglenoids are claim by both protozoologists and phycologists as protozoa and algae, respectively. Another example is Pneumocystis carinii, which exhibits features common to both protozoa and fungi. DNA sequencing of several Pneumocystis genes clearly indicates that it is more closely related to the fungi than the protozoa and has resolved this long debate. DNA sequence data also indicates that microsporidia--which have always been considered protozoa and not the subject of debate--are a highly derived fungus. In many cases, though, molecular data does not resolve evolutionary relationships between the various protozoa/protists (see molecular phylogeny). Protists are monophyletic in the sense that they are all eukaryotes and therefore probably evolved from a common descendent, but they do exhibit very long evolutionary histories and many of the relationships still need to be resolved.