Various systematic groups of protozoa (photos of some organisms are presented in this article) unite unicellular animals belonging to the eukaryotic group. They differ from all other eukaryotes in that they usually consist of only one cell, although there may be several. Therefore, the cellular level of organization is their highest. Systematic groups of protozoa do not include organisms that do not satisfy this requirement. Almost all representatives are microscopic in size.
Morphophysiological features
In terms of their morphophysiological differentiation, they are different. For example, amoebas are relatively simply arranged (they do not have differentiated organoids for contraction, movement, food capture, etc.). And if we talk about ciliates, then their organization is quite complicated. These protozoa have superficial pellicular structures, contractile and supporting fibrils, as well as organoids for movement, defense, food capture, etc.). All systematic groups of protozoa are characterized by the presence of a peculiar cellular ultrastructure, as well as general purpose organoids: lysosomes, ribosomes, Golgi apparatus, endoplasmic reticulum, mitochondria. Their core is surrounded by a two-membrane shell, in which there are pores. It contains nucleoli, karyoplasm and chromosomes (usually they are despiralized in the interphase nucleus).
Number of species
To date, the main systematic groups of protozoa have been investigated. About 25-30 thousand of their species are known. And these are only those that have been discovered by scientists. The number of protozoa existing in nature should be several times larger, since they are not sufficiently studied due to technical difficulties and their microscopic dimensions. Hundreds of new species of protozoa are described annually. All of them are divided into five classes: knizosporidii, ciliates, sporozoans, flagellates and sarcodes. A comparative characteristic of the systematic groups of protozoa allows us to highlight their characteristic features. We will talk about many of them in this article.
Role in nature
Protozoa are very common in nature. They occupy a significant place in food chains, as well as in the biosphere as a whole. Many of them (ciliates, radiolarians, flagellates) are an integral part of marine plankton. Rapidly multiplying, often they reach a large number. These microorganisms are an important component of the diet of marine zooplankton, especially copepods. Often the simplest (ciliates, foraminifera) are included in the marine benthos, which occurs from the intertidal zone to very large depths. A number of microorganisms are part of freshwater benthos and plankton. The species composition of protozoa living in fresh waters is an indicator of saprobity, that is, pollution of water bodies with organic substances. The food of many fish fry, including commercial fish, is some of the simplest, most often ciliates.
Parasitic species
This systematic group of protozoa is rather numerous. Entirely of the parasites are two classes - knizosporidii and sporozoans. Of particular importance among these protozoa are parasites of humans, commercial and domestic mammals, fish and birds. The human diseases caused by them include giardiasis, leishmaniasis, amoebiasis, malaria. The most severe for cattle are the diseases that cause blood parasites - trypanosomiasis, theilerioses, pyroplasmidoses.
This systematic group of protozoa also causes great damage to poultry farming (coccidioses). Protozoal diseases in fish farming mainly affect juvenile commercial fish. In particular, ichthyophthirius, a parasitic ciliator (pictured above), can cause the death of fry. In a large part, the cnidosporidia class consists of fish parasites and beneficial insects - silkworm and bees. Today, ways are developed to use such protozoa, namely microsporidia, to combat insect pests. Promising results have already been obtained in this direction.
Marine protozoa
This systematic group of protozoa played an important role in the formation of sedimentary rocks. This applies in particular to radiolarians and foraminifers. Many Cretaceous deposits, limestones and other sedimentary rocks formed in different geological periods at the bottom of sea water bodies are partially or fully formed by skeletons (flint or calcareous) of protozoa. Micro-paleontological analysis in this regard is carried out during geological exploration, especially in oil exploration.
Laboratory use
In laboratory practice, various types of protozoa (ciliates, amoeba) are widely used in the study of biophysical, genetic and cytological problems. The technique of laboratory cultures is well developed. Protistology is a science that studies protozoa.
Microanatomy
The structure of the protozoan cell depends primarily on the lifestyle of a particular species, as well as on its physiological adaptations. Note that there is always one or more nuclei surrounded by a double membrane. This conclusion allowed scientists to make a comparative description of the systematic groups of protozoa. In the nuclei are chromosomes. Their obligatory presence in all species also made it possible to reveal a comparative characteristic of the systematic groups of protozoa. A photo of the ciliates is presented below.
Nuclei in ciliates are usually of two types: one or more small micronuclei and one large macronucleus. Cells in some protozoa are multi-core. The main systematic groups of protozoa are characterized by the presence of vacuoles. These are spaces separated by a membrane from the rest of the cytoplasm. They differ in function and size. Vacuoles serve to digest incoming food, as well as to accumulate and remove solid and liquid metabolic products. In addition, they can be used to settle algae, which provide the hosts, that is, the simplest organisms, with photosynthesized organics. In vacuoles or cytoplasm of some species, pigment granules are contained. In addition, plastids - intracellular specialized structures that contain pigments associated with the implementation of photosynthesis - are distinguished in their composition. Their number and shape depend on the species, that is, they relate to classification features. Under the outer membrane of some protozoa, there are extrusions that "shoot" organelles. These are, for example, mucocysts that secrete mucus, as well as trichocysts that eject thin threads. Perhaps they serve as protection. The protozoa also have flagella. Depending on their type, there can be from one to many. These are locomotor filiform organelles having a complex system of longitudinal internal microtubules. According to the principle of operation and their structure, they significantly differ from flagella of bacteria. In particular, they are characteristic of flagella. Let's say a few words about them separately.
Flagella use flagella to create water currents and / or locomotion, due to which they receive food. This systematic group of protozoa includes many parasites, as well as free-living forms and symbionts of animals. There are single-celled polyenergy and mono-energy forms, as well as multicellular and colonial. Flagella generally have a tendency toward osmotrophic nutrition and small cell sizes. Although phagotrophic, very large forms are found among them.
Cilia resemble flagella, however, they are shorter and always form entire complexes on the cell, coordinated work. The nature of the distribution of cilia on the surface is an important classification feature applicable to systematic groups of protozoa.
Crustaceans do not belong to them, however, some representatives of protozoa are covered with mineralized or organic scales, shells or shells, different in composition and structure. In addition, they may have a rigid intracellular skeleton, which is usually a complex lattice structure.
Food
As for physiology, representatives of this systematic group are either autotrophs (that is, they photosynthesize organic matter), or heterotrophs (absorb it from the environment), or mixotrophs (use the two above-mentioned nutritional methods). Heterotrophs can absorb dissolved substances on the cell surface. In this case, they are called osmotrophs. In addition, they can swallow solid food through various mechanisms, and then they are called phagotrophs. Food phagotrophs may include other organisms. In particular, they can eat other protozoa. Sometimes, in order to capture prey, they pursue it.
Selection
Solid metabolic products are removed from the cell into the external medium by the fusion of vacuoles containing them with the outer membrane. This sometimes happens in its specialized area (cytoproct, cell powder). The contractile vacuoles actively pump out the excess water with metabolic waste dissolved in it.
Breath
The characterization of the systematic groups of protozoa can be supplemented by the characteristics of respiration. It can be both oxygen-free (anaerobic) and requiring an oxygen medium, that is, aerobic. Oxygen for obligate anaerobes is harmful, and they die in an environment rich in them. Some species in aquatic habitats poor for them breathe thanks to symbiotic bacteria. In this case, the intensity with which this process occurs is directly proportional to the temperature and depends on the type of substrate being metabolized, i.e., split molecules, as well as the taxonomic group.
Breeding
The description of the systematic groups of protozoa is completed by the features of reproduction. They can reproduce asexually. In this case, the cell is divided into several daughter cells. However, they can carry out this process sexually, with the participation of two cells. These methods sometimes alternate depending on what stage the life cycle is at. Sex cells (they are called gametes) merge during sexual reproduction or there is a temporary connection (conjugation) of two ordinary cells, which leads to the exchange of hereditary material between them. If the externally merging gametes are almost the same, we are talking about isogamy. If one of them is noticeably larger in size, they talk about anisogamy.
Of course, we only briefly described the systematic groups of protozoa. Biology is a science, having studied which, you can discover a whole new world. Our eyes are not able to distinguish between microorganisms, but science allows us to see them. We see whole worlds. Examples of these are various systematic groups of protozoa. Roots, sporozoans, ciliates, flagellates, and other organisms are quite interesting in many ways.