INVERTEBRATE ZOOLOGY - H. I. Shcherbak - 2008

PROTISTS - PROTISTA

PROTOZOA

General characteristics. Protozoa are eukaryotic organisms with a cellular level of Organization, complete and self-sufficient in both morphological and physiological terms. Unlike other protists, they are characterized by an animal-type Cell Structure and heterotrophic Nutrition (with the exception of euglenoids and dinoflagellates, which include autotrophic forms). They occur as unicellular and colonial forms, as well as multinucleated plasmodia. The microscopic cysts of protozoa are airborne, facilitating their dispersal and global distribution. Protozoa inhabit seas, freshwater bodies, and soils; many species are parasitic in nearly all eukaryotic groups.

The size of protozoa ranges from 1-2 µm to 27 cm. The smallest are intracellular parasites, while the largest are deep-sea marine xenophyophore rhizopods. Protozoan Cells exhibit A wide variety of shapes and Symmetry types, including spherical, radial, bilateral, and helical; asymmetrical forms also exist. Protozoa frequently form colonies that may be linear, lamellar, spherical, arborescent, free-swimming, or sessile.

The name "protozoa" does not reflect the true complexity of their organization. Protozoan cells are significantly more complex than the cells of Multicellular Organisms because, In addition to standard cellular Functions such as METABOLISM and Protein Synthesis, they also perform the functions of an entire Organism: locomotion, feeding, reproduction, dispersal, and defense against unfavorable environmental conditions. In multicellular organisms, specific life functions are carried out by Organs or Organ Systems, whereas in protozoa, these tasks are performed by specialized structural elements within the single cell, known as Organelles.

A protozoan cell, like any Introduction/5.html">Eukaryotic Cell, contains general cellular organelles such as The Cell membrane, nucleus, Ribosomes, Endoplasmic reticulum, Golgi apparatus, Lysosomes, and Mitochondria (the latter are absent in microsporidians and certain flagellates). However, they also possess specific organelles that ensure their functioning as complete, independent organisms.

Integumentary and skeletal structures. A protozoan cell is bounded by a Plasma Membrane, or Plasmalemma, which features a typical biomembrane structure. Externally, the plasmalemma is covered by a glycocalyx composed of mucopolysaccharides. In some groups, various extracellular surface structures are formed, such as organic scales, loricae, or shells (e.g., in testate amoebas and foraminifers). In most cells, a dense, elastic layer known as the pellicle lies beneath the membrane, maintaining the cell's constant shape. The structure of the pellicle varies among different protozoan taxa and will be discussed in the respective chapters. Some protozoa possess only a plasmalemma with no other integumentary or skeletal structures, resulting in a labile, constantly changing body shape (e.g., amoebas, plasmodia).

Flagellates, Ciliates, and active stages of Sporozoans possess a Cytoskeleton that gives their body a defined shape without sacrificing elasticity. The cytoskeleton is formed by a system of microtubules and microfilaments located in the peripheral layers of the Cytoplasm, with a structure specific to each major protozoan group. Microfilaments are strands containing Actin, Myosin, and other Proteins. They form contractile or non-contractile structures and, together with microtubules, enable the contraction of the cell or its specific parts. Microtubules are relatively rigid structures composed of the protein tubulin, performing both structural support and contractile functions within the cell. Located beneath the pellicle of many ciliates and flagellates, they determine cell shape and form an integral part of the flagellar and ciliary apparatuses, axopodia, and other structures. Certain protozoa, such as radiolarians, possess a mineral Skeleton consisting of spicules and capsules. This skeleton is formed through intracellular biocrystallization and comprises an organic matrix along with Mineral Substances, primarily silica and, more rarely, strontium sulfate.

Cytoplasm. The cytoplasm of protozoa is typically differentiated into two layers: a denser, more homogeneous outer layer known as ectoplasm, and a fluid, granular inner layer known as endoplasm. Ectoplasm can transform into endoplasm, and vice versa.

The nuclear apparatus consists of one or more nuclei, which may be morphologically and functionally identical or dissimilar. In some multinucleated protozoa, such as ciliates, two distinct types of nuclei are recognized: vegetative (macronuclei) and generative (micronuclei). This phenomenon is known as nuclear dualism. Vegetative nuclei regulate the metabolic and daily physiological activities of the cell, whereas generative nuclei store Genetic information and transmit it to daughter cells during reproduction. At certain stages of The life cycle, nuclei may contain haploid, diploid, or polyploid sets of Chromosomes.

Locomotory organelles. Protozoa move by means of temporary cytoplasmic protrusions—pseudopodia—or specialized structures such as flagella and cilia. These same organelles are also utilized for capturing food.

Nutrition types and digestive organelles. The majority of protozoa are heterotrophs, while only a few are autotrophic and capable of Photosynthesis. Autotrophic organisms possess chromatophores containing chlorophyll. Mixotrophic species (with mixed nutrition) are capable of photosynthesis in the presence of light, but switch to heterotrophic nutrition in the dark by absorbing preformed Organic compounds. Heterotrophic feeding occurs via phagocytosis or pinocytosis. Phagocytosis is an active mechanism for capturing microorganisms and solid food particles using pseudopodia or a cell Mouth (cytostome). A food vacuole forms around the engulfed food within the cytoplasm, into which lysosomes transport digestive Enzymes. Undigested food residues are expelled from the cell at any point on the membrane where the food vacuole makes contact, or through a specialized opening known as the cytoproct (cell anus). Pinocytosis involves the uptake of fluid through narrow channels formed by the invagination of the cell membrane into the cytoplasm.

Osmoregulatory and excretory organelles. In marine and parasitic protozoa inhabiting nearly isotonic environments, Metabolic waste products are eliminated directly across the cell membrane. In freshwater protozoa, the intracellular salt concentration is significantly higher than in the surrounding medium, causing Water to continuously enter the cell. Excretion and osmoregulation in these organisms are carried out by contractile, or pulsating, vacuoles. By contracting, the vacuole expels excess water and dissolved metabolic wastes from the cell. The activity of the contractile vacuole also plays a vital role in Respiration by accelerating the inward Diffusion of Water and dissolved oxygen across the cell membrane.

Reproduction. Protozoa reproduce by both Selection/8.html">Asexual and sexual means. Asexual reproduction occurs via binary fission or multiple fission (schizogony). The simplest form of asexual reproduction is mitotic binary fission, resulting in two daughter individuals. In this process, The Nucleus divides first, followed by the cytoplasm. Certain organelles also divide in half (e.g., the basal body of the flagellum), while others are partitioned between the daughter cells, with any missing structures subsequently regenerated.

In multiple fission, or schizogony, the Cell Nucleus undergoes successive mitotic divisions, cytoplasm accumulates around each newly formed nucleus, and the parent cell breaks apart into multiple daughter cells. A variant of schizogony is endodyogeny, where mitotic nuclear division and organelle duplication occur beneath the membrane of the parent cell, and the newly formed daughter cells remain connected for some time. Multiple fission may also occur as palintotomy, where, unlike schizogony, nuclear and cytoplasmic divisions occur synchronously, producing small daughter cells that subsequently grow for a period before the next division cycle. Budding is observed in some protozoa, wherein a smaller daughter cell (the bud) detaches from a larger parent cell.

During the sexual process, parent cells typically produce A large number of Gametes. Gametes originating from different cells then fuse in pairs to form a zygote, a process known as copulation. The zygote either develops directly into an adult organism (e.g., in foraminifers) or divides to give rise to multiple offspring (e.g., in the malaria parasite). Copulation is termed isogamous if the gametes are identical, or anisogamous if a larger female gamete (macrogamete) fuses with a smaller male gamete (microgamete). When gametes are markedly dimorphic—with a large, immotile macrogamete and a small, motile microgamete—the process is called oogamy. In ciliates, the sexual process occurs via conjugation, which involves the temporary union of two individuals and the reciprocal exchange of nuclear material.

Unlike multicellular organisms, which exhibit individual development (ontogeny), ontogeny in protozoa refers solely to the regeneration of missing structures in daughter cells following asexual reproduction (such as the cytostome and cirri in ciliates, or skeletal elements in radiolarians). Instead, protozoa are characterized by the regular alternation of distinct unicellular generations—a life cycle.

A life cycle is the segment of a species' existence between two identical stages that repeats cyclically (e.g., from zygote to zygote). The simplest life cycle spans from one Cell Division to the next. More complex life cycles are found in unicellular organisms that exhibit a regular alternation of two distinct generations: one reproducing sexually, the other asexually (metagenesis).

Gametes are always haploid, whereas the zygote is diploid. Depending on the stage at which reductional division (Meiosis) occurs, life cycles are categorized into those with gametic chromosomal reduction (where meiosis occurs during gamete formation), zygotic reduction (where the zygote divides meiotically), and sporic or intermediate reduction (characterized by the alternation of Haploid and Diploid generations, Fig. 1).

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Fig. 1. MAIN TYPES OF nuclear cycles:

a - with zygotic reduction; b - gametic; c - intermediate (after K. Beklemishev):

1 - Stages of the haploid generation; 2 - stages of the diploid generation; 3 - zygote; 4 - agametes; 5 - gametes;

R! - site of reductional division

Protozoa are able to withstand adverse environmental conditions (such as the drying out of water bodies, or extreme temperatures) by forming cysts, a process known as encystment. During this stage, the cells shed or retract their locomotory organelles, round up, secrete a protective envelope, and enter a dormant state. Cysts not only protect protozoa from harsh conditions but also facilitate their dispersal.

Irritability. Protozoa are capable of perceiving environmental changes and responding to them in specific ways, a property known as irritability. These adaptive responses manifest as taxis—directional cell movements toward (positive taxis) or away from (negative taxis) the source of the stimulus.

Protozoa play a vital role in the production and breakdown of organic matter, participate in marine and freshwater food webs, and contribute to the self-purification of water bodies. They serve as a food source for numerous animals, particularly fish larvae. Soil-dwelling species, alongside other organisms, are involved in pedogenesis, enhance soil fertility, and release heteroauxins that stimulate Plant GROWTH AND DEVELOPMENT. Furthermore, certain groups of protozoa (such as foraminifers and radiolarians) are well-known for their contribution to sedimentary rock formation. Parasitic species cause severe diseases in animals, humans, and occasionally plants. In turn, protozoa themselves can host Viruses, Bacteria, and other protozoan species.

At the same time, humans employ certain species of parasitic protozoa in biological pest control Methods against crop pests and animal parasites. Protozoa are also utilized for the bioindication of organic pollution in aquatic environments, as many flagellate and Ciliate species are highly sensitive to organic load. The species composition of protozoan communities can be used to assess the eutrophication level of water bodies, reflecting organic contamination. An important application of protozoa is the biological Treatment of industrial and domestic wastewater using aeration tanks—artificial basins where activated sludge develops, consisting of a community of heterotrophic organisms. This sludge is primarily composed of bacteria and protozoa (such as stalked ciliates, free-swimming ciliates, and numerous species of sarcodines and flagestlates).

Based on their cellular architecture, protozoa are divided into five major groups: Flagellates, Sarcodines, Alveolates, Microsporidians, and Myxosporidians, most of which encompass several phyla. Let us examine those groups whose representatives are most widespread, hold practical significance, and are therefore thoroughly described in contemporary scientific literature.