Protozoa (protozoa)

PROTOZOA, protozoa (from Greek protos—first and zoon—animal), a phylum of the animal kingdom, the representatives of which consist of a single, to varying degrees differentiated, cell. Initially (17th–18th centuries) and for some time after the introduction of the term "Protozoa" into science (Goldfuss, 1817–20), due to the shortcomings of research techniques and the lack of definite and developed foundations of a system for the differential demarcation of this phylum from other groups of animals, diverse organisms were included in its composition, including those belonging to multicellular animals (e.g., rotifers, etc.). At the same time, typical representatives of Protozoa were described as highly organized creatures with the presence of Metazoa organs: an intestinal canal, reproductive organs, a vascular system, etc. The first undoubted Protozoa were seen by Leeuwenhoek in 1676. As a separate phylum, Protozoa were first established by Siebold in 1841. Phylogenetically, the representatives of various groups of this phylum, and especially parasitic forms with the presence of complex developmental cycles, have undoubtedly passed through a long process of evolution. However, despite the sometimes complex differentiation of the cellular body, Protozoa, compared with Metazoa, are primitive organisms and in this sense they are "protozoa" (simplest). The complexity of the structure of individual parts of the cellular body of Protozoa and the specialization of their functions served as the reason for the view of some authors (Dobell) that Protozoa represent a series of creatures parallel to Metazoa and are "acellular" organisms. The size of protozoa fluctuates within wide limits. Along with forms measured in a few microns, there are species visible to the naked eye. When determining the size of Protozoa, it is necessary to take into account the individual variability of vegetative individuals, which manifests itself primarily in size, and the existence of developmental forms that are diverse in function and size. The external form of Protozoa is extremely diverse. A number of Protozoa, such as heliozoans and radiolarians, due to the special arrangement of skeletal elements, have a radial plan of structure; others, such as trypanosomes, Lamblia, some sporozoans, etc., are bilaterally symmetrical. In the absence of skeletal formations, the body of a Protozoan can take the most diverse form (naked rhizopods). The protoplast—the cellular body of a Protozoan—consists of protoplasm and a nucleus (or nuclei in two- or multi-nucleated forms). In some Protozoa, the outer layer of protoplasm, which is in direct contact with the external environment—the ectoplasm—is more transparent, condensed, usually devoid of inclusions, and seems almost homogeneous compared to the endoplasm. However, the boundary between both layers is not sharp. Sections of endoplasm, upon entering the surface layer, take on the character of ectoplasm and vice versa. In heliozoans, the ectoplasm contains large vacuoles. The transformation of ectoplasm and endoplasm into one another indicates the existence of reversible changes in the protoplasm. Along with the latter, there exist in Protozoa changes in the protoplasm of an irreversible character. Such is the pellicle (or periplast), covering the protoplast in the form of a thin, elastic, strongly light-refracting layer, sufficiently strong to give the protozoan a certain form and at the same time sufficiently elastic not to hinder temporary changes in form, which are called metaboly. In some Protozoa, the pellicle (less differentiated forms of the pellicle, e.g., in amoebae, belong to the number of "reversible formations") is a smooth, structureless membrane; in others, it forms various changes in the form of elastic fibers, thickenings, ribbing, etc. (flagellates, ciliates), causing the striation of its outer layer and the diverse sculpture of the external parts of the Protozoa. Besides the pellicle, fibrils, fibers, rods, axial rods, etc., penetrating into the endoplasm play a skeletal role. In ciliates, fibrils in connection with the pellicle give rise to a whole system of supporting formations, causing the complex structure of these Protozoa and their individual parts, in particular the organelles of movement. Some authors attribute another significance to the complex system of fibrils in flagellates and ciliates; in their opinion, this system is part of the so-called neuromotor apparatus, regulated from a special neuromotor center. The latter, for example, in paramecia, is located in the endoplasm in front of the oral opening. The fibrils extending from the center are in connection with the basal granules of the cilia and the inner ends of the trichocysts. The neuromotor center is considered as the center of coordination of the organelles of movement. In some flagellates, the basal part of the flagellum, penetrating deep into the body, forms its own system of fibrils. In heliozoans, radially arranged skeletal elements serve as the basis for ray-like pseudopodia. Other derivatives of the ectoplasm serve for the construction of external coverings of a cuticular character. In contrast to the pellicle, the cuticular covering does not participate in cell division; it either passes to one of the daughter cells or remains in the form of empty shells. Chemically, cuticular formations consist of chitin, pseudochitin, cellulose, etc. The organic framework of these formations is often impregnated with various salts (calcium, iron, strontium, salts of silicic acid, etc.), as a result of which the surface of the Protozoan body is covered with a strong protective shell. Among the protective formations are cysts, which are present in both free-living (ciliates) and parasitic forms (sporozoans) of Protozoa. Encystment in some Protozoa occurs from time to time as a reaction to unfavorable environmental conditions; in others, predominantly parasitic forms, it is one of the regular stages of their life cycle, arising at a certain stage of development (coccidia). In some cases, the transformation of vegetative individuals into cysts is preceded by a stage of so-called precystic forms, which are distinguished by their smaller size (dysentery amoeba). The role of a protective adaptation in ciliates can be played by a special mucous substance—tectin. The cellular body includes, as a necessary component, the nuclear apparatus, represented by one, two, or many nuclei. With the monomorphic type of nuclear apparatus, the nuclei have the same structure and the same function; all nuclei of this category are capable of forming chromosomes. With the dimorphic type, two or more nuclei differ from each other both in structure and in function. Nuclear dimorphism is especially sharply expressed in ciliates, where the formation of chromosomes takes place only in the smaller micronuclei. In the first type, the nuclear apparatus carries out all general physiological functions of the cell nucleus, including sexual functions; the latter, in the case of binuclearity, are performed exclusively by the small nuclei—micronuclei. Morphologically, two types of resting nuclei are distinguished in Protozoa—vesicular and massive. In the simplest case, in vesicular nuclei, the entire chromatin mass is concentrated in the center in the form of a rounded, homogeneous formation—an internal body, or karyosome (inner nucleus), surrounded by a zone of structureless nuclear sap (outer part of the nucleus). In other cases, linin threads lie between the karyosome and the nuclear membrane, due to which vacuolization of the outer nucleus arises. Chromatin, if it is located in the outer nucleus, lies either in the walls and nodes of the linin network or, in its main mass, adheres to the nuclear membrane. In some Protozoa, the inner nucleus has the same reticular structure as the outer one. The vesicular type of nucleus is observed in amoebae, coccidia, gregarines, and some ciliates (micronuclei). With a reduction of the inner body and a more significant development of the outer parts of the nucleus, a reticular nucleus is formed, in which chromatin granules are located under the nuclear membrane and in the crossbars of the linin network (e.g., in Entamoeba). Massive nuclei are characterized by a strong development of chromatin formations, as a result of which dense and opaque nuclei are obtained, in which chromatin is most often distributed evenly and in which there is one or several nucleoli. This category of nuclei is found in ciliates (macronuclei) and some flagellates. Between vesicular and massive nuclei, there are certain transitions, so that a sharp boundary between them cannot be drawn. Vesicular nuclei with a large number of chromatin inclusions, when they are densely arranged, take on the character of massive nuclei, as is observed in euglenids (Euglena). Division of the Protozoan nucleus occurs according to the same principles as in Metazoa, i.e., by mitosis or amitosis. The first method is characteristic of the majority of Protozoa, the second—for the macronuclei of ciliates. Besides that, in trypanosomes and some amoebae, nuclear division occurs according to the type of so-called promitosis or primitive mitosis, where the signs of typical mitosis are less sharply expressed. The movement of Protozoa, depending on the degree of differentiation of the cellular body, is carried out with the help of organelles having different structures. In the simplest case, in Protozoa devoid of hard form-determining elements and skeletal formations, movement occurs apolarly with the help of one or another form of protoplasmic protrusions—pseudopodia (naked rhizopods). In other Protozoa, flagella, cilia, and special contractile elements embedded in the protoplasm—myonemes—serve as organelles of movement.

The flagellum is a thread-like formation, usually exceeding the length of the cell body. In cross-section, the flagellum has a round or oval shape. In peridineans, it is ribbon-like. The flagellum is most often pointed towards the free end. Like cilia, it consists of a strong axial filament surrounded by a layer of liquid protoplasm. At the base of the flagella, as well as the cilia, lies a thickening of the axial filament—the basal body, located either directly under the pellicle or in deeper parts of the protoplast; in the latter case, the part of the flagellum enclosed in the protoplasm is called the rhizoplast. In many flagellates, at the point of origin of the flagellum, there lies a parabasal body of various shapes, which, together with the basal body, forms the parabasal apparatus. The function of the latter has not yet been sufficiently clarified (see Kinetoplast). In trypanosomes, the flagellum is located along the edge of a special fold of the pellicle—the undulating membrane, and in some species, it protrudes from the front part of the body in the form of a free lash. The part of it that is connected to the membrane and runs along its edge is called the marginal filament. Flagella can be present in the number of one (Oicomonas), two (Trypanoplasma), three (Tricercomonas), four (Tetramitos), and only in rare cases more. Cilia differ from flagella by their shorter length. Their size is significantly smaller than the cell body. Their distribution is either uniform, or they are localized only in certain places, sometimes forming special organelles (membranelles) by their fusion. Myonemes differ from flagella in their structure in that their contractile elements do not occupy a central position but are located at the periphery. The process of nutrition, depending on the composition of nutrients, occurs in Protozoa in two different ways. The holotrophic (holophytic) type of nutrition is common for Protozoa possessing chlorophyll. The process in this case proceeds, as in green plants, by means of photosynthesis. In the heterotrophic (holozoic) type, organic substances in the form of living organisms or products of their decay are used to build the cell body. Chlorophyll, found in holophytic Protozoa, is enclosed in chromatophores. Besides the green pigment, pigments of red, yellow, and brown color are also found in the protoplasm of colored Protozoa. Colored Protozoa, multiplying in large numbers, cause the phenomenon of "blooming" of the water bodies they inhabit (Euglena viridis, Dunaliella salina), as well as the red coloration of snow (Chlamydomonas nivalis). In the chromatophores of Protozoa, there are often embedded sharply defined, rounded inclusions—pyrenoids, which contain starch. In some cases, they appear as solid, homogeneous, "naked" formations or ones enclosed in a membrane; in others, they consist of separate small elements. They show an affinity for nuclear stains. They reproduce by division. Chromatophores are present almost exclusively in flagellates. For some forms of flagellates, it has been established that the amount and color of the pigment depend on the reaction of the environment. Euglena sanguinea loses its red pigment in an environment poor in phosphorus and nitrogen and acquires a green color. Among chlorophyll-bearing flagellates, besides forms that develop normally in purely mineral environments, there are organisms that require mixed nutrition, i.e., the temporary (Euglena gracilis) or permanent (Chlamydomonas) presence of organic compounds in the environment. An abundance of the latter in some forms entails the loss of green pigment and a transition to saprozoic nutrition. These facts indicate the absence of a sharp boundary between the two main types of nutrition in Protozoa.

In the protoplasm of certain groups of Protozoa, algae with green chromatophores—zoochlorellae—are encountered as intracellular symbionts. They belong to the protococcous algae and are observed in amoebae, heliozoans, and ciliates; moreover, some species are encountered both with and without zoochlorellae, while for others the presence of green symbionts is of a more permanent nature (Paramaecium bursaria, Stentor polymorphus). Another group of symbionts with yellow or brown pigment (zooxanthellae) has a wide distribution among Radiolaria, is common in Foraminifera, and in certain species of Ciliata. In a systematic respect, zooxanthellae do not represent a single group; insofar as this question has been clarified, some of them belong to Cryptomonadina, others to Dinoflagellata. The food of holozoic Protozoa consists of bacteria, algae, protozoa, individual cells of higher animals, small Metazoa, and formed particles resulting from the primary decay of organisms (detritus). Saprozoic forms also feed at the expense of decaying organic substances of the organism, but those that have reached deeper stages of decomposition with a transition into a dissolved state. Parasitic forms combine both purely holozoic and saprozoic types of nutrition. For example, trypanosomes feed on organic substances dissolved in the blood, the dysentery amoeba on blood cells (and partially on dissolved organic compounds), coccidia and endoglobular parasites on products of cellular decay, etc. The process of food intake in Protozoa lacking special adaptations occurs by osmosis (sporozoans, part of the flagellates, Astomata); in others, which feed predominantly on formed elements, either any part of the cell body (naked rhizopods) or variously differentiated adaptations (temporary or permanent) serve for their intake, such as: a mouth opening of varying complexity, or cytostome, a pharynx in ciliates, or the sucking tubes of Suctoria. Digestion occurs in vacuoles, which arise in the endoplasm. The excretory organelles of Protozoa are usually contractile vacuoles having a definite localization. They empty through a rupture of the pellicle. The need for oxygen in Protozoa fluctuates within wide limits. Along with forms inhabiting an environment rich in oxygen, there are Protozoa that develop normally in an environment almost devoid of it, for example, commensals of the intestine and gallbladder, as well as so-called sapropelic forms living in an environment rich in hydrogen sulfide. Reproduction of Protozoa occurs by asexual and sexual methods. In Protozoa, the following types of asexual reproduction take place: 1) simple division of the vegetative individual, or trophozoite, into two equal daughter cells; 2) multiple division, or schizogony, in which the nucleus of the maternal individual gives rise to several daughter nuclei, with a corresponding portion of protoplasm going to each of them; at this stage, the protozoan is called a schizont or agamont. As a result of schizogony, several small, identical daughter individuals—merozoites—are produced; 3) simple budding, occurring by the separation of a smaller daughter form from the maternal individual; 4) multiple budding, in which several smaller daughter nuclei separate from the nucleus, followed by the separation of several buds from the maternal individual; in all the indicated methods of reproduction, the division of the maternal individual is preceded by the division of the nucleus; 5) plasmotomy, observed in some multinucleate ciliates and cnidosporidians, can proceed in the form of simple division or multiple budding; the main difference from the previous methods of division consists in the fact that in plasmotomy, the maternal individual divides independently of the division of the nuclei. Sexual reproduction, associated with the presence of a sexual process, arises in Protozoa from time to time after a more or less significant series of ordinary divisions of vegetative individuals. In essence, the sexual process in Protozoa reduces to the fusion of two generative nuclei, in the majority having passed through stages of reduction division and containing half the number of chromosomes compared to the trophozoite. The process of reduction division characterizes the stage of maturation of the nucleus. An individual with a nucleus having a reduced number of chromosomes is called a gamete, or sexual individual; the stage preceding it is called a gametocyte. In the presence of sexual differentiation of gametocytes, the male individual is designated as a microgametocyte and the female as a macrogametocyte. In Protozoa, the following forms of the sexual process are distinguished: 1) Copulation (see), consisting of the fusion of two individuals (gametes). The degree of differentiation of gametes in a sexual respect is extremely diverse. In some cases, the gametes do not differ from each other morphologically and are externally identical to vegetative individuals (hologamy); in others, the copulating individuals reveal a significant degree of sexual differentiation into male and female gametes (anisogametes). The process itself in the latter case is called anisogamy. A special type of sexual process is merogamy, in which gametes originate by schizogony from vegetative individuals. 2) Paedogamy, observed in some heliozoans and cnidosporidians—gametes arise from one maternal individual. 3) Autogamy, or self-fertilization, where the gametocyte contains two nuclei that arose from the nucleus of the vegetative individual in the absence of division of the cell body; the fusion of the daughter nuclei occurs after the reduction of chromosomes (cnidosporidians). 4) Parthenogenesis (virgin reproduction)—maturation without fertilization of a macrogametocyte after the reduction of chromatin; daughter cells arise by schizogony (Plasmodium). 5) Conjugation (see)—temporary connection of two vegetative individuals, during which an exchange of migratory micronuclei occurs between them, followed by the fusion of the latter with stationary micronuclei and the separation of the conjugants. After fertilization, a zygote is formed, which in sporozoans, by way of sporogony, breaks down into sporozoites, which subsequently transform into vegetative forms. Sometimes, sporoblasts are initially formed in the zygote, giving rise to spores. In the life cycle of Protozoa possessing sexual processes, the asexual generation, usually represented by vegetative individuals, is replaced by the sexual generation. The alternation of generations, characterized by the diversity of the forms included in them, proceeds particularly complexly in parasitic Protozoa, for example, in the causative agents of malaria (see); the same occurs in the causative agent of avian malaria (Proteosoma), with the difference that the sexual generation develops in the organism of a mosquito of the genus Culex, or in the blood parasite of pigeons, Haemoproteus columbae, the sexual generation of which develops in a fly of the genus Lynchia. We have a certain analogy with these processes in piroplasms, the sexual cycle of development of which occurs in the organism of ticks of the family Ixodidae.

Protozoa are widely distributed throughout the globe. A necessary condition for the existence of vegetative forms is the presence of moisture. In the encysted state, Protozoa are capable of enduring desiccation for a long time. Depending on their localization, Protozoa form more or less definite ecological groups that have adapted to known environmental conditions. They inhabit soil, freshwater bodies, and seas. A special biological group is constituted by parasitic Protozoa. The relationships between Protozoa and their hosts are defined as symbiosis, commensalism, and parasitism. In terms of external relationships, one usually distinguishes external and internal symbionts and commensals, as well as ecto- and endoparasites. Numerous Protozoa are localized on the surface of many animals and plants (Rhizopoda), but typical ectoparasitism among Protozoa is comparatively rare (Costia). Typical endoparasites belong to the inhabitants of the intestine and the circulatory system. According to their way of life among parasites, one distinguishes cavity-dwelling and tissue-dwelling; the latter can be between cells or inside cells. However, a sharp boundary between these groups cannot be drawn, since one and the same parasite at different moments of its development can parasitize in different parts of the host's body; thus, Trypanosoma cruzi spends part of its development in the blood plasma and part inside cells. Intestinal Protozoa belong to the cavity-dwelling, sarcosporidia, myxo- and microsporidia to the tissue-dwelling, and plasmodia and coccidia to the intracellular. Intracellular parasites usually lie in the protoplasm, but some species also penetrate into the nucleus (Cyclospora caryolytica). Some parasitic Protozoa can live in diverse organs and tissues of their host, for example, Nosema bombycis, which causes nosematosis in silkworms, while others affect only specific tissues, organs, and cells; for example, Lamblia of humans and animals is localized in the small intestine, other flagellates and amoebae in the oral cavity, for example, Trichomonas buccalis and Amoeba gingivalis.

The pathogenic effect of parasitic Protozoa on their hosts is extremely complex and basically boils down to mechanical action and intoxication. With mechanical action, direct destruction of infected cells occurs; thus, Ichthyophthirius, parasitizing on the skin of fish, destroys the epithelium; in intracellular parasitism, cells are often subjected to destruction (coccidia); intoxication arises on the basis of the excretion by parasites of the products of their vital activity. An example of a protozoan toxin is sarcocystin from sarcosporidia. As a result of parasitic invasion, in particular under the influence of the toxic action of Protozoa, a number of clinical phenomena arise that characterize this or that protozoan disease (fever, bloody urine, jaundice, etc.). Among blood diseases, human malaria, trypanosomiases, and various piroplasmoses are the most common; further protozoan diseases include coccidioses, amoebic dysentery, balantidiasis, leishmaniases, skin diseases of fish, sarcosporidiosis, etc. The routes of infection in protozoan diseases are: infection through food and drink (coccidioses), through mucous membranes (dourine of horses), through vectors that belong to insects, arachnids, and leeches; most parasitic blood diseases are spread by vectors. Infection through contact in protozoan diseases is of subordinate importance. The systematics of Protozoa has undergone various modifications. Regardless of the mode of nutrition (plant or animal), protozoa were united into the kingdom of Protista with two groups: Protophyta with holophytic and Protozoa with holozoic types of nutrition. The desire to recreate the continuity of the development of organisms served as the reason for including colonial forms between the protists, placed at the foundation of the evolutionary development of organisms, and the Metazoa, also as 'transitional' organisms. The establishment of a natural system of Protozoa, which would be based both on the ontogeny of currently existing forms and on paleontological facts that make it possible to trace the phylogeny of this or that group of organisms, encounters difficulties due to the comparative scarcity of paleontological data. Fossil remains of Protozoa are known only for a limited circle of groups having a hard skeleton, such as Foraminifera and Radiolaria. Therefore, one or another of the proposed systems of Protozoa will inevitably suffer from a certain artificiality and not fully reflect the natural kinship of individual systematic units. Doflein proposed the following systematics of Protozoa: Phylum Protozoa, I subphylum Plasmodroma Doflein: 1. Class Mastigophora Diesing; 2. Class Rhizopoda v. Siebold; 3. Class Sporozoa Leuckart. II subphylum Ciliophora Doflein: 1. Class Ciliata Bütschli; 2. Class Suctoria Bütschli. To the subphylum Plasmodroma belong Protozoa with pseudopodia and flagella, and one or several, for the most part, vesicle-like nuclei. Fertilization is of the iso- or anisogamy type. In many, there is an alternation of sexual and asexual generations. Class Mastigophora—movement by means of flagella. Class Rhizopoda—movement by means of pseudopodia. Class Sporozoa—'movement by various means, often reduced under the influence of a parasitic mode of life. Reproduction by spores arising after a sexual process.' To the subphylum Ciliophora belong Protozoa 'with cilia. The nuclear apparatus is represented by one or several macronuclei and one or several micronuclei. Fertilization in the form of anisogamous fusion of two individuals or by conjugation with mutual exchange of parts of the nucleus. Reproduction by simple division or budding. Fertilization is not accompanied by the formation of special forms of reproduction.' Class Ciliata—'cilia exist throughout the entire life. Nutrition is osmotic or through a cytostome.' Class Suctoria—'cilia are present only in young forms. Nutrition by means of suctorial tubes.' Lit.—see lit. to the art. Protozoology.