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- Differentiate between groups of protists
In the span of several decades, the Kingdom Protista has been disassembled because sequence analyses have revealed new genetic (and therefore evolutionary) relationships among these eukaryotes. Moreover, protists that exhibit similar morphological features may have evolved analogous structures because of similar selective pressures—rather than because of recent common ancestry. This phenomenon, called convergent evolution, is one reason why protist classification is so challenging. The emerging classification scheme groups the entire domain Eukaryota into six “supergroups” that contain all of the protists as well as animals, plants, and fungi that evolved from a common ancestor (Figure 1). The supergroups are believed to be monophyletic, meaning that all organisms within each supergroup are believed to have evolved from a single common ancestor, and thus all members are most closely related to each other than to organisms outside that group. There is still evidence lacking for the monophyly of some groups.
The classification of eukaryotes is still in flux, and the six supergroups may be modified or replaced by a more appropriate hierarchy as genetic, morphological, and ecological data accumulate. Keep in mind that the classification scheme presented here is just one of several hypotheses, and the true evolutionary relationships are still to be determined. When learning about protists, it is helpful to focus less on the nomenclature and more on the commonalities and differences that define the groups themselves.
Many of the protist species classified into the supergroup Excavata are asymmetrical, single-celled organisms with a feeding groove “excavated” from one side. This supergroup includes heterotrophic predators, photosynthetic species, and parasites. Its subgroups are the diplomonads, parabasalids, and euglenozoans.
Among the Excavata are the diplomonads, which include the intestinal parasite, Giardia lamblia (Figure 2). Until recently, these protists were believed to lack mitochondria. Mitochondrial remnant organelles, called mitosomes, have since been identified in diplomonads, but these mitosomes are essentially nonfunctional. Diplomonads exist in anaerobic environments and use alternative pathways, such as glycolysis, to generate energy. Each diplomonad cell has two identical nuclei and uses several flagella for locomotion.
A second Excavata subgroup, the parabasalids, also exhibits semi-functional mitochondria. In parabasalids, these structures function anaerobically and are called hydrogenosomes because they produce hydrogen gas as a byproduct. Parabasalids move with flagella and membrane rippling. Trichomonas vaginalis, a parabasalid that causes a sexually transmitted disease in humans, employs these mechanisms to transit through the male and female urogenital tracts. T. vaginalis causes trichamoniasis, which appears in an estimated 180 million cases worldwide each year. Whereas men rarely exhibit symptoms during an infection with this protist, infected women may become more susceptible to secondary infection with human immunodeficiency virus (HIV) and may be more likely to develop cervical cancer. Pregnant women infected with T. vaginalis are at an increased risk of serious complications, such as pre-term delivery.
Euglenozoans includes parasites, heterotrophs, autotrophs, and mixotrophs, ranging in size from 10 to 500 µm. Euglenoids move through their aquatic habitats using two long flagella that guide them toward light sources sensed by a primitive ocular organ called an eyespot. The familiar genus, Euglena, encompasses some mixotrophic species that display a photosynthetic capability only when light is present. In the dark, the chloroplasts of Euglena shrink up and temporarily cease functioning, and the cells instead take up organic nutrients from their environment.
The human parasite, Trypanosoma brucei, belongs to a different subgroup of Euglenozoa, the kinetoplastids. The kinetoplastid subgroup is named after the kinetoplast, a DNA mass carried within the single, oversized mitochondrion possessed by each of these cells. This subgroup includes several parasites, collectively called trypanosomes, which cause devastating human diseases and infect an insect species during a portion of their life cycle. brucei develops in the gut of the tsetse fly after the fly bites an infected human or other mammalian host. The parasite then travels to the insect salivary glands to be transmitted to another human or other mammal when the infected tsetse fly consumes another blood meal. brucei is common in central Africa and is the causative agent of African sleeping sickness, a disease associated with severe chronic fatigue, coma, and can be fatal if left untreated.
Watch this video to see T. brucei swimming. Note that there is no audio in this video.
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Current evidence suggests that species classified as chromalveolates are derived from a common ancestor that engulfed a photosynthetic red algal cell, which itself had already evolved chloroplasts from an endosymbiotic relationship with a photosynthetic prokaryote. Therefore, the ancestor of chromalveolates is believed to have resulted from a secondary endosymbiotic event. However, some chromalveolates appear to have lost red alga-derived plastid organelles or lack plastid genes altogether. Therefore, this supergroup should be considered a hypothesis-based working group that is subject to change. Chromalveolates include very important photosynthetic organisms, such as diatoms, brown algae, and significant disease agents in animals and plants. The chromalveolates can be subdivided into alveolates and stramenopiles.
Alveolates: Dinoflagellates, Apicomplexians, and Ciliates
A large body of data supports that the alveolates are derived from a shared common ancestor. The alveolates are named for the presence of an alveolus, or membrane-enclosed sac, beneath the cell membrane. The exact function of the alveolus is unknown, but it may be involved in osmoregulation. The alveolates are further categorized into some of the better-known protists: the dinoflagellates, the apicomplexans, and the ciliates.
Dinoflagellates exhibit extensive morphological diversity and can be photosynthetic, heterotrophic, or mixotrophic. Many dinoflagellates are encased in interlocking plates of cellulose. Two perpendicular flagella fit into the grooves between the cellulose plates, with one flagellum extending longitudinally and a second encircling the dinoflagellate (Figure 4). Together, the flagella contribute to the characteristic spinning motion of dinoflagellates. These protists exist in freshwater and marine habitats, and are a component of plankton, the typically microscopic organisms that drift through the water and serve as a crucial food source for larger aquatic organisms.
Some dinoflagellates generate light, called bioluminescence, when they are jarred or stressed. Large numbers of marine dinoflagellates (billions or trillions of cells per wave) can emit light and cause an entire breaking wave to twinkle or take on a brilliant blue color (Figure 5). For approximately 20 species of marine dinoflagellates, population explosions (also called blooms) during the summer months can tint the ocean with a muddy red color. This phenomenon is called a red tide, and it results from the abundant red pigments present in dinoflagellate plastids. In large quantities, these dinoflagellate species secrete an asphyxiating toxin that can kill fish, birds, and marine mammals. Red tides can be massively detrimental to commercial fisheries, and humans who consume these protists may become poisoned.
The apicomplexan protists are so named because their microtubules, fibrin, and vacuoles are asymmetrically distributed at one end of the cell in a structure called an apical complex (Figure 6). The apical complex is specialized for entry and infection of host cells. Indeed, all apicomplexans are parasitic. This group includes the genus Plasmodium, which causes malaria in humans. Apicomplexan life cycles are complex, involving multiple hosts and stages of sexual and asexual reproduction.
The ciliates, which include Paramecium and Tetrahymena, are a group of protists 10 to 3,000 micrometers in length that are covered in rows, tufts, or spirals of tiny cilia. By beating their cilia synchronously or in waves, ciliates can coordinate directed movements and ingest food particles. Certain ciliates have fused cilia-based structures that function like paddles, funnels, or fins. Ciliates also are surrounded by a pellicle, providing protection without compromising agility. The genus Paramecium includes protists that have organized their cilia into a plate-like primitive mouth, called an oral groove, which is used to capture and digest bacteria (Figure 7). Food captured in the oral groove enters a food vacuole, where it combines with digestive enzymes. Waste particles are expelled by an exocytic vesicle that fuses at a specific region on the cell membrane, called the anal pore. In addition to a vacuole-based digestive system, Paramecium also uses contractile vacuoles, which are osmoregulatory vesicles that fill with water as it enters the cell by osmosis and then contract to squeeze water from the cell.
Watch the video of the contractile vacuole of Paramecium expelling water to keep the cell osmotically balanced.
A link to an interactive elements can be found at the bottom of this page.
Paramecium has two nuclei, a macronucleus and a micronucleus, in each cell. The micronucleus is essential for sexual reproduction, whereas the macronucleus directs asexual binary fission and all other biological functions. The process of sexual reproduction in Paramecium underscores the importance of the micronucleus to these protists. Paramecium and most other ciliates reproduce sexually by conjugation. This process begins when two different mating types of Paramecium make physical contact and join with a cytoplasmic bridge (Figure 8). The diploid micronucleus in each cell then undergoes meiosis to produce four haploid micronuclei. Three of these degenerate in each cell, leaving one micronucleus that then undergoes mitosis, generating two haploid micronuclei. The cells each exchange one of these haploid nuclei and move away from each other. A similar process occurs in bacteria that have plasmids. Fusion of the haploid micronuclei generates a completely novel diploid pre-micronucleus in each conjugative cell. This pre-micronucleus undergoes three rounds of mitosis to produce eight copies, and the original macronucleus disintegrates. Four of the eight pre-micronuclei become full-fledged micronuclei, whereas the other four perform multiple rounds of DNA replication and go on to become new macronuclei. Two cell divisions then yield four new Paramecia from each original conjugative cell.
Which of the following statements about Paramecium sexual reproduction is false?
- The macronuclei are derived from micronuclei.
- Both mitosis and meiosis occur during sexual reproduction.
- The conjugate pair swaps macronucleii.
- Each parent produces four daughter cells.
[reveal-answer q=”294017″]Show Answer[/reveal-answer]
[hidden-answer a=”294017″]Statement c is false.[/hidden-answer]
Stramenopiles: Diatoms, Brown Algae, Golden Algae and Oomycetes
The other subgroup of chromalveolates, the stramenopiles, includes photosynthetic marine algae and heterotrophic protists. The unifying feature of this group is the presence of a textured, or “hairy,” flagellum. Many stramenopiles also have an additional flagellum that lacks hair-like projections (Figure 9). Members of this subgroup range in size from single-celled diatoms to the massive and multicellular kelp.
The diatoms are unicellular photosynthetic protists that encase themselves in intricately patterned, glassy cell walls composed of silicon dioxide in a matrix of organic particles (Figure 10). These protists are a component of freshwater and marine plankton. Most species of diatoms reproduce asexually, although some instances of sexual reproduction and sporulation also exist. Some diatoms exhibit a slit in their silica shell, called a raphe. By expelling a stream of mucopolysaccharides from the raphe, the diatom can attach to surfaces or propel itself in one direction.
During periods of nutrient availability, diatom populations bloom to numbers greater than can be consumed by aquatic organisms. The excess diatoms die and sink to the sea floor where they are not easily reached by saprobes that feed on dead organisms. As a result, the carbon dioxide that the diatoms had consumed and incorporated into their cells during photosynthesis is not returned to the atmosphere. In general, this process by which carbon is transported deep into the ocean is described as the biological carbon pump, because carbon is “pumped” to the ocean depths where it is inaccessible to the atmosphere as carbon dioxide. The biological carbon pump is a crucial component of the carbon cycle that maintains lower atmospheric carbon dioxide levels.
Like diatoms, golden algae are largely unicellular, although some species can form large colonies. Their characteristic gold color results from their extensive use of carotenoids, a group of photosynthetic pigments that are generally yellow or orange in color. Golden algae are found in both freshwater and marine environments, where they form a major part of the plankton community.
The brown algae are primarily marine, multicellular organisms that are known colloquially as seaweeds. Giant kelps are a type of brown algae. Some brown algae have evolved specialized tissues that resemble terrestrial plants, with root-like holdfasts, stem-like stipes, and leaf-like blades that are capable of photosynthesis. The stipes of giant kelps are enormous, extending in some cases for 60 meters. A variety of algal life cycles exists, but the most complex is alternation of generations, in which both haploid and diploid stages involve multicellularity. Compare this life cycle to that of humans, for instance. Haploid gametes produced by meiosis (sperm and egg) combine in fertilization to generate a diploid zygote that undergoes many rounds of mitosis to produce a multicellular embryo and then a fetus. However, the individual sperm and egg themselves never become multicellular beings. Terrestrial plants also have evolved alternation of generations. In the brown algae genus Laminaria, haploid spores develop into multicellular gametophytes, which produce haploid gametes that combine to produce diploid organisms that then become multicellular organisms with a different structure from the haploid form (Figure 11). Certain other organisms perform alternation of generations in which both the haploid and diploid forms look the same.
Which of the following statements about the Laminaria life cycle is false?
- 1n zoospores form in the sporangia.
- The sporophyte is the 2n plant.
- The gametophyte is diploid.
- Both the gametophyte and sporophyte stages are multicellular.
[reveal-answer q=”65382″]Show Answer[/reveal-answer]
[hidden-answer a=”65382″]Statement c is false.[/hidden-answer]
The water molds, oomycetes (“egg fungus”), were so-named based on their fungus-like morphology, but molecular data have shown that the water molds are not closely related to fungi. The oomycetes are characterized by a cellulose-based cell wall and an extensive network of filaments that allow for nutrient uptake. As diploid spores, many oomycetes have two oppositely directed flagella (one hairy and one smooth) for locomotion. The oomycetes are nonphotosynthetic and include many saprobes and parasites. The saprobes appear as white fluffy growths on dead organisms (Figure 12).
Most oomycetes are aquatic, but some parasitize terrestrial plants. One plant pathogen is Phytophthora infestans, the causative agent of late blight of potatoes, such as occurred in the nineteenth century Irish potato famine.
The Rhizaria supergroup includes many of the amoebas, most of which have threadlike or needle-like pseudopodia (Ammoniatepida, a Rhizaria species, can be seen in Figure 13).
Pseudopodia function to trap and engulf food particles and to direct movement in rhizarian protists. These pseudopods project outward from anywhere on the cell surface and can anchor to a substrate. The protist then transports its cytoplasm into the pseudopod, thereby moving the entire cell. This type of motion, called cytoplasmic streaming, is used by several diverse groups of protists as a means of locomotion or as a method to distribute nutrients and oxygen.
Take a look at this video to see cytoplasmic streaming in a green alga. Note that there is no audio in this video.
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Foraminiferans, or forams, are unicellular heterotrophic protists, ranging from approximately 20 micrometers to several centimeters in length, and occasionally resembling tiny snails (Figure 14).
As a group, the forams exhibit porous shells, called tests that are built from various organic materials and typically hardened with calcium carbonate. The tests may house photosynthetic algae, which the forams can harvest for nutrition. Foram pseudopodia extend through the pores and allow the forams to move, feed, and gather additional building materials. Typically, forams are associated with sand or other particles in marine or freshwater habitats. Foraminiferans are also useful as indicators of pollution and changes in global weather patterns.
A second subtype of Rhizaria, the radiolarians, exhibit intricate exteriors of glassy silica with radial or bilateral symmetry (Figure 15). Needle-like pseudopods supported by microtubules radiate outward from the cell bodies of these protists and function to catch food particles. The shells of dead radiolarians sink to the ocean floor, where they may accumulate in 100 meter-thick depths. Preserved, sedimented radiolarians are very common in the fossil record.
Red algae and green algae are included in the supergroup Archaeplastida. It was from a common ancestor of these protists that the land plants evolved, since their closest relatives are found in this group. Molecular evidence supports that all Archaeplastida are descendents of an endosymbiotic relationship between a heterotrophic protist and a cyanobacterium. The red and green algae include unicellular, multicellular, and colonial forms.
Red algae, or rhodophytes, are primarily multicellular, lack flagella, and range in size from microscopic, unicellular protists to large, multicellular forms grouped into the informal seaweed category. The red algae life cycle is an alternation of generations. Some species of red algae contain phycoerythrins, photosynthetic accessory pigments that are red in color and outcompete the green tint of chlorophyll, making these species appear as varying shades of red. Other protists classified as red algae lack phycoerythrins and are parasites. Red algae are common in tropical waters where they have been detected at depths of 260 meters. Other red algae exist in terrestrial or freshwater environments.
Green Algae: Chlorophytes and Charophytes
The most abundant group of algae is the green algae. The green algae exhibit similar features to the land plants, particularly in terms of chloroplast structure. That this group of protists shared a relatively recent common ancestor with land plants is well supported. The green algae are subdivided into the chlorophytes and the charophytes. The charophytes are the closest living relatives to land plants and resemble them in morphology and reproductive strategies. Charophytes are common in wet habitats, and their presence often signals a healthy ecosystem.
The chlorophytes exhibit great diversity of form and function. Chlorophytes primarily inhabit freshwater and damp soil, and are a common component of plankton. Chlamydomonas is a simple, unicellular chlorophyte with a pear-shaped morphology and two opposing, anterior flagella that guide this protist toward light sensed by its eyespot. More complex chlorophyte species exhibit haploid gametes and spores that resemble Chlamydomonas.
The chlorophyte Volvox is one of only a few examples of a colonial organism, which behaves in some ways like a collection of individual cells, but in other ways like the specialized cells of a multicellular organism (Figure 16). Volvox colonies contain 500 to 60,000 cells, each with two flagella, contained within a hollow, spherical matrix composed of a gelatinous glycoprotein secretion. Individual Volvox cells move in a coordinated fashion and are interconnected by cytoplasmic bridges. Only a few of the cells reproduce to create daughter colonies, an example of basic cell specialization in this organism.
True multicellular organisms, such as the sea lettuce, Ulva, are represented among the chlorophytes. In addition, some chlorophytes exist as large, multinucleate, single cells. Species in the genus Caulerpa exhibit flattened fern-like foliage and can reach lengths of 3 meters (Figure 17). Caulerpa species undergo nuclear division, but their cells do not complete cytokinesis, remaining instead as massive and elaborate single cells.
The amoebozoans characteristically exhibit pseudopodia that extend like tubes or flat lobes, rather than the hair-like pseudopodia of rhizarian amoeba (Figure 18). The Amoebozoa include several groups of unicellular amoeba-like organisms that are free-living or parasites.
A subset of the amoebozoans, the slime molds, has several morphological similarities to fungi that are thought to be the result of convergent evolution. For instance, during times of stress, some slime molds develop into spore-generating fruiting bodies, much like fungi.
The slime molds are categorized on the basis of their life cycles into plasmodial or cellular types. Plasmodial slime molds are composed of large, multinucleate cells and move along surfaces like an amorphous blob of slime during their feeding stage (Figure 19). Food particles are lifted and engulfed into the slime mold as it glides along. Upon maturation, the plasmodium takes on a net-like appearance with the ability to form fruiting bodies, or sporangia, during times of stress. Haploid spores are produced by meiosis within the sporangia, and spores can be disseminated through the air or water to potentially land in more favorable environments. If this occurs, the spores germinate to form ameboid or flagellate haploid cells that can combine with each other and produce a diploid zygotic slime mold to complete the life cycle.
The cellular slime molds function as independent amoeboid cells when nutrients are abundant (Figure 20). When food is depleted, cellular slime molds pile onto each other into a mass of cells that behaves as a single unit, called a slug. Some cells in the slug contribute to a 2–3-millimeter stalk, drying up and dying in the process. Cells atop the stalk form an asexual fruiting body that contains haploid spores. As with plasmodial slime molds, the spores are disseminated and can germinate if they land in a moist environment. One representative genus of the cellular slime molds is Dictyostelium, which commonly exists in the damp soil of forests.
Watch this video to see the formation of a fruiting body by a cellular slime mold. Note that there isn’t any narration in the video.
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The opisthokonts include the animal-like choanoflagellates, which are believed to resemble the common ancestor of sponges and, in fact, all animals.
Choanoflagellates include unicellular and colonial forms, and number about 244 described species. These organisms exhibit a single, apical flagellum that is surrounded by a contractile collar composed of microvilli. The collar uses a similar mechanism to sponges to filter out bacteria for ingestion by the protist. The morphology of choanoflagellates was recognized early on as resembling the collar cells of sponges, and suggesting a possible relationship to animals. The Mesomycetozoa form a small group of parasites, primarily of fish, and at least one form that can parasitize humans. Their life cycles are poorly understood.
These organisms are of special interest, because they appear to be so closely related to animals. In the past, they were grouped with fungi and other protists based on their morphology. Some phylogenetic trees still group animals and fungi into the Opisthokonta supergroup though this is also considered a protist specific group in other phylogenies.
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The descriptions of protists are presented in the following paragraphs. Important examples of such organisms include the amoeba, diatoms, euglena, and paramecium.
Amoeba: Discovered by August Johann Rösel von Rosenhof in the year 1757, amoeba was referred to as Proteus animalcule by the naturalists of earlier times. The Amoeba proteus is a commonly found species of this microbe. Its size ranges from 220 – 740 micrometers. Their body structure is characterized by the presence of a single or more than one nuclei. Reproduction takes place asexually, in the form of cytokinesis.
Euglena: It is a unicellular microbe, which has more than 1000 species. These organisms exhibit both autotrophy and heterotrophy. The former ones produce sugars by the means of photosynthesis. Raw materials used in this process include the carotenoid pigments, chlorophyll ‘a’ and chlorphll ‘b’. Owing to the dual characteristics of plants and animals possessed by the euglena, there is confusion over how to classify them. Reproduction takes place asexually in the form of binary fission. Flagella are the organs used for locomotion. Eyespot is the part of euglena’s body that is photo-sensitive. Light is detected with the help of this part, and necessary adjustments for photosynthesis are made.
Diatom: It is a phytoplankton that forms one of the important groups of algae. Most of the diatoms are unicellular in nature. Their cell wall is known as frustule, which is made up of hydrated silicon dioxide. There is a great variety in the forms of these frustules. Diatoms are found in freshwater bodies like rivers and lakes, and also in oceans. The 100,000 species of diatoms are grouped under 200 genera. They prove to be useful from the point of studying water quality of a particular area. Most number their species are found in the tropical regions. Binary fission is the mode of reproduction used by diatoms.
Paramecium: These are unicellular microorganisms, which possess the locomotory organ called cilia. Their body length ranges from 50 – 350 micrometers. Contractile vacuoles are used by the paramecium for the purpose of osmoregulation. The oral groove is a part of this organism present on the side of its body. Intake of food (with a sweeping motion) is the function of the oral groove. Yeasts, algae, and bacteria form the diet of this organism. These microbes are commonly found in freshwater regions. Few of the paramecium species can also be found in oceans. Bacterial endosymbionts and Paramecium aurelia share symbiotic relationship with each other.
Microbes are amongst important living beings found on earth. The examples of protists and their characteristics presented in the above paragraphs should help you to understand more about these organisms.
Types and examples of Protists
Biologists consider protists as a polyphyletic group, which means they probably do not share a common ancestor. The word protists comes from the Greek word for first, indicating that researchers believe protists may have been the first eukaryotes to evolve on Earth. Now, the Protists are classified in to three main types or subdivisions on the basis of their similarity with other kingdoms. These are
- Protozoa (animal like protists)
- Molds (Fungus Like Protists)
- Algae ( Plants like Protists)
A) Protozoa (animal like protists)
Protozoa are single-celled organisms. These are also called animal like protists. All protozoa are heterotrophic, that is, they feed on other organisms to obtain nutrition. There are also parasitic protozoa that live in the cells of larger organisms.
Protozoa can be divided into four main groups:
- Phylum Sporozoa (Parasitic Protozoans): e.g. malaria
- Phylum Ciliophora (Ciliated Protozoans): e.g. paramecia
- Phylum Rhizopoda (Amoeboid Protozoans): e.g. amoeba
- Phylum Zoomastigophora (Flagellate Protozoans): e.g. Trypanosoma
1- Phylum Rhizopoda (Amoeboid Protozoans): e.g. amoeba
- These are a group of protozoa characterized by their amoeboid movement through temporal projections called pseudopodia.
- They are found mainly in bodies of water, either fresh or saline.
- They have pseudopodia (false feet) that help change their shape and capture and wrap food. e.g. Ameba “Amoeboid cells may also produce in fungi, algae, and animals”
2- Phylum Zoomastigophora (Flagellate Protozoans): e.g. Trypanosoma
- As the name suggests, These protozoans have one or more flagella for locomotion and sensation. A flagellum is a structure resembling hair capable of lashing movements similar to lashes that provide locomotion.
- They can be free-living (Euglena) as well as parasites (Trypanosoma).
- Parasitic forms live in the intestine or bloodstream of the host.
- They may also be colonial (volvox), Solitary (Phaeocystis)
3- Phylum Ciliophora (Ciliated Protozoans): e.g. paramecia
- The ciliates are a group of protozoa characterized by the presence of hair-like organelles called cilia, whose structure is identical to that of eukaryotic flagella, but which are generally shorter and present in much greater numbers, with a wavy pattern.
- The cilia help in locomotion and obtaining nutrition.
- These are single-celled organisms and are always aquatic.
- Paramecium is a model ciliate living in freely in freshwater. The most widely distributed species are Paramecium caudatum and Paramecium aurelia.
4- Phylum Sporozoa (Parasitic Protozoans) e.g. the malaria parasite, Plasmodium
- These organisms are named so because of the presence of spores in their life cycle.
- Sporozoa have neither flagella, eyelashes, nor pseudopodia. They are able to slip movements.
- All Sporozoa are parasites of animals and cause disease.
B) Molds (Fungus Like Protists)
Molds are saprophytic organisms (they feed on the dead and decomposing matter). These are small organisms that have many nuclei. Molds are usually characterized by the presence of spores and are even visible to the naked eye. Basically they are divided into two types, viz. Water molds and Slime molds.
Oomycota or oomycetes (generally called water molds)
- These are a group of filamentous protists that physically resemble fungi and are heterotrophic.
- They are microscopic, absorptive organisms that reproduce both sexually and asexually and are made of a tube-like vegetative body called mycelia.
- These may be free-living or parasitic. The parasitic form may grow on the scales or eggs of fish, or on amphibians or plant bodies.
- A notorious example of water molds is Phytophthora infestans, a microorganism that causes the serious potato and tomato disease known as late blight or potato blight.
Myxomycota or myxomycetes ( generally called as Slime mold)
- Slime molds are several kinds of unrelated eukaryotic organisms that can live freely as single cells but can aggregate together to form multicellular reproductive structures.
- These grow as a naked network of protoplasm that engulf bacteria and other small food particles by phagocytosis.
- Slime molds are common in moist, organic-rich environments such as damp, rotten wood, where there is an abundance of bacteria as a food source. They are mostly seen as they begin to sporulate because of their conspicuous and brightly colored fruiting bodies.
- They may be
- Plasmodial slime molds such as Physarum species
- Cellular slime molds which are unicellular amoeboid organisms such as Dictyostelium
- Endoparasitic slime molds such as the Plasmodiophora brassicae that causes clubroot disease of cruciferous crops.
C) Algae ( Plants like Protists)
These form another category under the Protista kingdom. These are generally unicellular or multicellular organisms. These are photosynthetic, they are found mainly in freshwater sources or marine lakes. They are characterized by a rigid cell wall.
Types of Algae
There are seven main types of algae that are following.
- Green algae (Chlorophyta)
- Euglenophyta (Euglenoids)
- Golden-brown algae and Diatoms (Chrysophyta)
- Fire algae (Pyrrophyta)
- Red algae (Rhodophyta)
- Yellow-green algae (Xanthophyta)
- Brown algae (Phaeophyta)
Green algae (Chlorophyta)
Examples: Chlorella, Chlamydomonas, Spirogyra, Ulva. Green algae.
- The green color pigments i.e. chlorophyll a and b are present in the Chlorophyta.
- Food reserves of Chlorophyta are starch, some fats or oils like higher plants.
- Green algae are believed to have the parents of higher green plants.
- Green algae can be unicellular (having one cell), multicellular (having many cells), colonial (many single cells living as an aggregation), or coenocytic (composed of a large cell with no crossed walls the cell can be uninucleated or multinucleated).
Examples: Euglena mutabilis or Colacium Sp.
- Euglenoids are single-celled protists that occur in freshwater habitats and wet soils.
- These actively swim in an aquatic environment with the help of their long flagellum. They can also perform creeping movements by expanding and contracting their body. This phenomenon is called the euglenoid movement.
- They have two flagella at the anterior end of the body.
- There is a small light-sensitive eyespot in their cell.
- They contain photosynthetic pigments like chlorophyll and therefore can prepare their own food. However, in the absence of light, they behave similarly to heterotrophs when capturing other small aquatic organisms.
- They have characteristics similar to those of plants and animals, which makes them difficult to classify and, therefore, are called connecting links between plants and animals.
Golden-brown algae and Diatoms (Chrysophyta)
Examples: Ochromonas sp., Chrysosaccus sp.
- Chrysophyta includes single-celled algae in which chloroplasts contain large amounts of fucoxanthin pigment, giving the algae their brown color.
- These are flagellated, with one tinsel-like flagellum and a second whiplash-like flagellum, which can be reduced to a short stub.
- Resting cysts or spores with ornamented spines are formed in Chrysophyta. The cyst walls contain silica.
- Chrysophytes are found mainly in low-calcium freshwater habitats.
Fire algae (Pyrrophyta)
Examples: Pfiesteria piscicida, Gonyaulax catenella, Noctiluca scintillans, Chilomonas sp., Goniomonas sp
- Fire algae are single-celled algae commonly found in the oceans and some freshwater sources that use flagella to move.
- They are divided into two classes: dinoflagellates and cryptomonads.
- Dinoflagellates can cause a phenomenon known as red tide, in which the ocean appears red due to its high abundance. Like some fungi, some Pyrrophyta species are bioluminescent. At night, they make the ocean seem a flame. Dinoflagellates are also toxic because they produce a neurotoxin that can alter the proper functioning of muscles in humans and other organisms.
- Cryptomonads are similar to dinoflagellates and can also produce harmful algal blooms, giving the water a red or dark brown appearance.
Red algae (Rhodoph yta)
Example Gelidium, Gracilaria, Porphyra, Palmaria, Euchema
- Red algae are commonly found in tropical marine areas.
- Unlike other algae, these eukaryotic cells lack flagella and centrioles.
- It grows on a solid surface, including a tropical reef or attached to other algae.
- The cell wall of Red algae is made up of cellulose and many different types of carbohydrates.
- These algae reproduce asexually by monospores (walled spherical cells without flagella) that are carried by streams until germination.
- Red algae also reproduce sexually and undergo alternation of generations.
Yellow-green algae (Xanthophyta)
Examples: Vaucheria, Botrydium, Heterococcus,
- They are single-celled organisms with cellulose and silica cell walls and contain one or two flagella for movement.
- Its chloroplasts do not have a certain pigment, which gives them a lighter color.
- Yellow-green algae generally live in freshwater but can be found in saltwater and wet soils.
Brown algae (Phaeophyta)
Examples: Kelp (Laminariales), Bladderwrack (Fucus vesiculosus), Sargassum vulgare
Agents of Decomposition
The fungus-like protist saprobes are specialized to absorb nutrients from nonliving organic matter, such as dead organisms or their wastes. For instance, many types of oomycetes grow on dead animals or algae. Saprobic protists have the essential function of returning inorganic nutrients to the soil and water. This process allows for new plant growth, which in turn generates sustenance for other organisms along the food chain. Indeed, without saprobe species, such as protists, fungi, and bacteria, life would cease to exist as all organic carbon became “tied up” in dead organisms.