Lesson 8: Introduction to Protists PDF

LESSON 8 Introduction to Protista: General Characteristics and Classification of Protists. Beneficial and harmful Protozoa COURSE MATERIALS: German biologist Ernst Haeckel proposed in 1886 that a third kingdom of organisms be established to accommodate eukaryotic microorg...

LESSON 8 Introduction to Protista: General Characteristics and Classification of Protists. Beneficial and harmful Protozoa COURSE MATERIALS: German biologist Ernst Haeckel proposed in 1886 that a third kingdom of organisms be established to accommodate eukaryotic microorganisms that did not fit into the plant or animal kingdom, and he termed it Protista. The vast majority of protists are unicellular or form colonies consisting of one or a couple of distinct kinds of cells according to Alastair Simpson a professor in the Department of Biology at Dalhousie University. He further explained that there are examples of multicellular protists among brown algae and certain red algae. Cells Like all eukaryotic cells, protists have a central compartment called the nucleus, which houses the genetic material. They also have specialized cellular machinery called organelles. Photosynthetic protists such as the various types of algae contain plastids that differ in color pigments and even the number of membranes that enclose the organelle, as in the case of diatoms and dinoflagellates, which constitute the phytoplankton in the ocean. Nutrition Some protists are autotrophic others are heterotrophic. Heterotrophic protists are grouped as either: Phagotrophs or use their cell body to surround and swallow up food, often other cells; or Osmotrophs or absorb nutrients from the surrounding environment. A few photosynthetic forms are also phagotrophs like some members of the dinoflagellates, Euglena for example. Such organisms are called mixotrophs, reflecting the mixed nature of their nutritional habits. Reproduction Most protists produce asexually by budding and fission, either binary or multiple fission, where a mature cell splits into two (binary) or many (multiple) identical cells. Budding occurs when a mature cell produces a bud – a daughter nucleus – which then develops into a new separate cell. This is the basic premise of multiple fission; daughter nuclei dividing until they actually transform into a young version of the parent protists. However, binary fission is an unsustainable form of asexual reproduction that eventually necessitates a form of sexual reproduction called conjugation. Conjugation is the exchange of genetic materials between two protists in order to prevent death caused by performing binary fission more than a several times. Protists reproduce sexually through syngamy, which is a conjugation and an alternation of generations. In syngamy, two gametes -- reproductive cells each with half the required genetic material -- combine to form a zygote, a fertilized egg. Syngamy occurs in slime molds, green algae and similar organisms. The alternation of generations is essential to plants but used by protists as well for sexual reproduction. It requires two alternating generations, sporophytes and gametophytes, which work together to reproduce. Zoospores, created by the sporophytes, produce male and female gametophytes, which combine egg and sperm, producing a new sporophyte to re-initiate the cycle. Protists Diversity Protista is a grouping of convenience, containing organisms not easily accommodated elsewhere. It includes all unicellular and colonial eukaryotic organisms, but is often expanded to include multicellular algae. A. Algae (The Plant-like Protists) The algae is a collective name traditionally given to several phyla of primitive, and mostly aquatic plants, making up a highly diverse group of over 30,000 species. Most algae shared Most algae share a number of common features which caused them to be grouped together. Among these are: _ Possession of the pigment chlorophyll _ Deriving energy from the sun by means of oxygenic photosynthesis _ fixing carbon from CO2 or dissolved bicarbonate The characteristics used to place algal protists into different taxa include the type of chlorophyll present, the form in which carbohydrates is stored, and the structure of the cell wall. A group not considered here are the cyanophytes, known as the blue-green algae; although they carry out oxygenic photosynthesis, they are prokaryotes, and are more closely related to certain bacteria. Euglenophyta This is a group of unicellular flagellated organisms, which probably represent the most ancient group of algal protists. Individuals range in size from 10−500μm. Euglenophytes are commonly found in fresh water, particularly that with a high organic content, and to a lesser extent, in soil, brackish water and salt water. Members of this group have a well-defined nucleus, and chloroplasts containing chlorophylls a and b. The storage product of photosynthesis is a β-1,3- linked glucan called paramylon, found almost exclusively in this group. Euglenophytes lack a cellulose cell wall but have instead, situated within the plasma membrane, a flexible pellicle made up of interlocking protein strips, a characteristic which links them to certain protozoan species. A further similarity is the way in which locomotion is achieved by the undulation of a structures situated near the base of the flagellum; these are the paraflagellar body and the stigma or eyespot. The latter is particularly conspicuous, as it is typically an orange-red color, and relatively large. Reproduction is asexual by binary fission. Division starts at the anterior end, and proceeds longitudinally down the length of the cell, giving the cell a characteristic ‘two-headed’ appearance. During mitosis, the chromosomes within the nucleus replicate, forming pairs that split longitudinally. Since the euglenophyte is usually haploid, it thus becomes diploid for a short period. As fission proceeds, one daughter cell retains the old flagellum, while the other one generates a new one later. https://euglenabiology.weebly.com/structure--function.html Dinoflagellates The dinoflagellates (also known variously as Pyrrophyta, or ‘fire algae’) are chiefly marine planktonic types, comprising some 2000 species. This is another unicellular group, but one whose cells are often covered with armored plates known as thecae (sing: theca). They are generally biflagellate, with the two dissimilar flagella lying in part within the longitudinal and lateral grooves that run around the cell. The beating of the flagella causes the cell to spin like a top as it moves through the water (the group takes its name from the Greek word ‘to whirl’). Although many non photosynthetic (chemoheterotrophic) types exist, most dinoflagellates are photosynthetic, containing chlorophyll a and c plus certain carotenoids and xanthophylls, which give them a red/golden appearance. As a group, they are second only to the diatoms as the primary photosynthetic producers in the marine environment. Some dinoflagellates form endosymbiotic relationships with marine animals such as corals and sea anemones; these are termed zooxanthellae. Reproduction by asexual means involves binary fission. In armored forms, the theca may be shed before cell division, or split along suture lines; in either case, daughter cells must regenerate the missing sections. Sexual reproduction is known to occur in some dinoflagellates, and is probably more widespread. Gametes produced by mitosis fuse to produce a diploid zygote; this undergoes meiosis to reinstate the haploid condition in the offspring. In some species we see isogamy, the fusion of identical, motile gametes, while in others, anisogamy occurs, in which gametes of dissimilar size fuse. Fusion may occur between genetically identical gametes, or only when the gametes come from genetically distinct populations. https://biology4isc.weebly.com/kingdom-protista.html Diatoms The diatoms, which belong to the division Chrysophyta (the golden-brown algae), make up the majority of phytoplankton in marine food chains, and as such are the most important group of algal protists in terms of photosynthetic production. Over 10 000 species of diatom are recognized, but some experts feel that the real number is many times greater than this. As with the dinoflagellates, chlorophylls a and c are present, but not chlorophyll b. Their color is due to carotenoids and xanthophylls (particularly fucoxanthin) masking the chlorophyll. Diatom classification is based almost entirely on the shape and pattern of these shells, which are uniform for a particular species, and often have a very striking appearance. When diatoms die, their shells fall to the bottom of the sea, and can accumulate in thick layers where they represent a valuable mineral resource. This fine, light material (diatomaceous earth) has a number of applications, or example in filtration systems, and also as a light abrasive in products such as silver polish or toothpaste. Reproduction is usually asexual by binary fission, but a sexual phase with the production groups of algae in that they are diploid. In diatoms, asexual reproduction involves mitotic cell division, with each daughter cell receiving one half of the parental frustule, and synthesizing a new one to complement it. The newly formed half, however, always acts as the hypotheca (lower half) of the new cell; consequently, one in two daughter cells will be slightly smaller than the parent, an effect which is heightened over a number of generations. This process continues until a critical size is reached, and the diatoms undergo a phase of sexual reproduction, which reestablishes the normal frustule size. In species whose frustules have a degree of elasticity, the daughter cells are able to expand, and the problem of cell diminution does not arise. In bilaterally symmetrical (long, thin) diatoms, meiosis in parental cells produces identical, non-motile gametes, which fuse to form a zygote. https://www.northcountrypublicradio.org/news/story/6522/20180712/life-within-the-quot-glass-houses-quot-of-diatoms Chlorophyta The green algae share something in common with the higher plants of ultrastructure, metabolism and photosynthetic pigments, pointing to the likelihood of a common ancestor. They possess both chlorophyll a and b and certain carotenoids, store carbohydrate in the form of starch, and generally have a rigid cell wall containing cellulose. The starch is stored in structures called pyrenoids, which are found within chloroplasts. There are two phylogenetically distinct lines of green algae, the Charophyta and the Chlorophyta; the latter are much the bigger group, but the charophytes seem to be more closely related to green plants. Chlorophytes demonstrate a wide variety of body forms, ranging from unicellular types to colonial, filamentous, membranous and tubular forms. The vast majority of species are freshwater aquatic, but a few marine and pseudoterrestrial representatives exist. Desmid or (Order Desmidiales), order of single-celled (sometimes filamentous or colonial) microscopic green algae, comprising some 5,000 species in about 40 genera. The genus usually chosen to illustrate the unicellular condition in chlorophytes is Chlamydomonas. This has a single chloroplast, similar in structure and shape to that of a higher plant and contains a pyrenoid. Situated together at the anterior end is a pair of smooth or whiplash flagella, who’s regular, ordered contractions propel it through the water. A further structural feature found in Chlamydomonas and other motile forms of green algae is the stigma or eye-spot; this is made up of granules of a carotenoid pigment and is at least partially responsible for orienting the cell with respect to light. Reproduction in Chlamydomonas and other unicellular types under favorable conditions of light, temperature and nutrients, occurs asexually by the production of zoospores. A single haploid adult loses its flagella and undergoes mitosis to produce several daughter cells, which then secrete cell walls and flagella and take up an independent existence of their own. This can result in a tremendous increase in numbers; a single cell can divide as many as eight times in one day. Sexual reproduction in Chlamydomonas, which occurs when conditions are less favorable, differs in details according to the species.  http://phytoguide.blogspot.com/p/chl.html Phaeophyta The brown algae are multicellular, large and complex seaweeds, which dominate rocky shores in temperate and polar regions. Apart from one or two freshwater types, they are all marine. The presence of fucoxanthin masks the presence of chlorophylls a and c. (In this context it must be stated here that not all ‘brown’ seaweeds look brown, nor indeed do all the ‘red’ ones look red). Unlike the higher plants and green algae, which use starch as a food reserve, the phaeophytes use an unusual polysaccharide called laminarin (β-1,3-glucan). The level of tissue organization in brown algae is greatly advance of any type of algae. The simplest thallus of a brown alga resembles the most complex structure found in green algae. Laminaria is one of the kelps, the largest group of brown algae. It grows attached to underwater rocks or other objects by means of holdfasts, root-like structures which anchor the plant. The thallus is further subdivided into a stalk- like stipe and a broader, blade-like lamina. The phaeophytes also represent an advance in terms of sexual reproduction; here oogamy is the usual state of affairs and alternation of generations has developed to such an extent that diploid and haploid stages frequently assume separate morphological forms. Reproduction in Laminaria involves sporophyte and gametophyte plants that are morphologically quite distinct; (heteromorphic alternation of generations). Reproductive areas called sori develop on the blade of the diploid sporophyte at certain times of year. These consist of many sporangia, interspersed with thick protective hairs called paraphyses. As the sori develop, meiosis occurs, leading to the production of haploid zoospores. These in turn develop into haploid filamentous gametophyte plants, much smaller and quite different in morphology from the more highly organized sporophyte. Indeed, in contrast to the large sporophyte, the gametophyte is a microscopic structure. The gametophytes are dioecious, that is the male and female reproductive structures are borne on separate individuals. The female plant bears a number of oogonia, each of which produces a single egg, which escapes through a pore at the apex of the oogonium, but remains attached in a sort of cup, formed by the surrounds of the pore. In similar fashion the male plant bears several antheridia, each liberating a single antherozoid; this however is motile by means of flagella and fertilizes the egg. The diploid zygote so produced grows immediately into a new sporophyte. http://botanystudies.com/characteristics-of-phaeophyta/brown-alage-members/ Rhodophyta The red coloration of the Rhodophytes or red algae, is due to the pigments phycoerythrin and phycocyanin, which mask the chlorophylls present, in this case chlorophyll a and d. The biggest single difference between the red algae and the other groups is that they lack flagella at any stage of their life cycles. Thus, they are completely lacking in any motile forms, even in the reproductive stages; the gametes rely on being passively dispersed. Almost all the red algae are multicellular marine species, inhabiting habitats ranging from shallow rock pools to the ocean’s deeps. Life cycles vary considerably, and may be quite complex, with variations on the alternation of generations theme. Several species of the more primitive red algae reproduce asexually by releasing spores into the water. These attach to an appropriate substrate and mature into an adult. Red algae are the source of several complex polysaccharides of commercial value. Agar and agarose are used in the laboratory in microbial growth media and electrophoresis gels respectively, while carrageenan is an important thickening agent in the food industry. In addition, Porphyra species are cultivated in Japan for use in sushi dishes. The fronds of five species of Rhodophyta collected from Abu Qir Bay. a) Corallina elongata, b) Corallina officinalis, c) Jania rubens, d) Polysiphonia elongata, e) Pterocladia capillacea / Frondas de las cinco especies de Rhodophyta colectadas en la bahía de Abu Qir. a) Corallina elongata, b) Corallina officinalis, c) Jania rubens, d) Polysiphonia elongata, e) Pterocladia capillacea https://www.researchgate.net/figure/The-fronds-of-five-species-of-Rhodophyta-collected-from-Abu-Qir-Bay-a Corallina_fig2_311677701 B. The Protozoa (Animal-like Protists) The name Protozoa comes from the Greek, meaning ‘first animal’, and was originally applied to single-celled organisms regarded as having animal-like characteristics other party. (Multicellular animals were termed Metazoa). Protozoans as a group have evolved an amazing range of variations on the single-celled form, particularly with respect to the different means of achieving movement. They are a morphologically diverse group of well over 50 000 species; although the majority are free-living, the group also includes commensal forms and some extremely important parasites of animals and humans. Most protozoans are found in freshwater or marine habitats, where they form a significant component of plankton, and represent an important link in the food chain. Although water is essential for the survival of protozoans, many are terrestrial, living saprobically in moist soil. One of the important structural features of protozoans is the contractile vacuole, whose role is to pump out excess amounts of water that enter the cell by osmosis. The activity of the contractile vacuole is directly related to the osmotic potential differential between the cell and its surroundings. This is vitally important for freshwater protozoans, since the hypotonic nature of their environment means that water is continually entering the cell. The contractile vacuole often has a star-shaped appearance, the radiating arms being canals that drain water from the cytoplasm into the vacuole. Most protozoans have a heterotrophic mode of nutrition, typically ingesting particulate food such as bacteria, and digesting them in phagocytic vacuoles. Since they actively ‘hunt’ their food rather than simply absorbing it across the cell surface, it is not surprising that the majority of protozoans are capable of movement. The structural features used to achieve locomotion (e.g. cilia, flagella) are among the characteristics used to classify the protozoans. The Zooflagellates (Mastigophora) Members of this, the biggest and most primitive group of protozoans, are characterized by the long flagellum (mastigos = ‘a whip’), by which they propel themselves around. Although typical zooflagellates have a single flagellum, some types possess several. The prefix ‘zoo-’ distinguishes them from plant-like flagellates such as Euglena, but as we have already mentioned, such a distinction is not necessarily warranted on molecular and structural grounds. Zooflagellates may be free-living, symbiotic or parasitic. An example of the latter is the causative agent of African sleeping sickness in humans, Trypanosoma brucei. This belongs to the kinetoplastids, a group characterized by the possession of a unique organelle called the kinetoplast, found within the cell’s single, large, tubular mitochondrion, and containing its own DNA. The flagellum extends back to form the edge of a long, undulating membrane that gives Trypanosoma its characteristic locomotion. The infectious form of T. brucei develops in the salivary glands of the intermediate host, the tsetse fly, and is passed to the human host when a bite punctures the skin. Here, it eventually reaches the central nervous system by way of the blood or lymphatic systems. Inflammation of the brain and spinal cord results in the characteristic lethargy, coma and eventual death of the patient. Reproduction in the zooflagellates is generally by binary fission. In the case of T. brucei, this occurs both in the human host and in the gut of the tsetse fly, from which it migrates to the salivary glands to complete its life-cycle. The Cilliates (Cilliophora) The largest group of protzoans, the ciliates, are also the most complex , showing the highest level of internal organization in any single-celled organism. Most are free-living types such as Paramecium, and as the name suggests, they are characterized by the possession of cilia, which may be present all over the cell surface or arranged in rows or bands. They beat in a coordinated fashion to propel the organism, or assist in the ingestion of food particles. A unique feature of the ciliates is that they possess two distinct types of nuclei: *Macronuclei are concerned with encoding the enzymes and other proteins required for the cell’s essential metabolic processes. They are polyploid, containing many copies of the genome. * Micronuclei, of which there may be as many as 80 per cell, are involved solely in sexual reproduction by conjugation. As might be expected, removal of the macronucleus leads quickly to the death of the cell; however, cells lacking micronuclei can continue to live, and reproduce asexually by binary fission. Most ciliates possess a specialized ‘mouth’ structure, the cytostome, through which food particles are ingested. The beating of cilia directs them to a cytopharynx, a membrane-covered passage or tube, which enlarges and detaches to form a food vacuole. Fusion with lysosomes and digestion by enzymes occurs as described earlier. Undigested particles are ejected from a region on the surface (the anal pore or cytoproct). As well as cilia, some members of the group have trichocysts projecting from the cell surface, harpoon-like structures that can be used for attachment or defense. Some ciliated protozoans are anaerobic, such as those found in the rumen of cattle. The only ciliate known to cause disease in humans is Balantidium coli, which causes a form of dysentery. The amoebas (Sarcodina) The amoebas are characterized by the possession of pseudopodia (= ‘false feet’), temporary projections from the cell into which cytoplasm flows until the organism has moved forward. This means that amoebas are continually changing their body shape and the position of their internal organelles. Pseudopodia are also used to capture and engulf food, forming a vacuole around it. Once again, digestive enzymes are released from lysosomes and the food particle dissolved. Once absorption of soluble nutrients has taken place, undigested waste is ejected by the vacuole moving back to the cell surface. Reproduction in the amoeba is by simple binary fission. Most amoebas are free living, in aquatic environments; their mode of movement and feeding makes them well adapted to life on the bottom of ponds and lakes, where there is a good supply of prey organisms and suspended organic matter. Also included in the group are some important parasites, including Entamoeba histolytica, which causes amoebic dysentery in humans. Ingested in fecal contaminated water is responsible for some 50,000 – 10,000 deaths world-wide every year. Unlike its free-living relatives, Entamoeba is unable to reproduce outside of its host. The sporozoans (Apicomplexa) Members of this group are all parasitic, infecting a range of vertebrates and invertebrates. They have complex life cycles involving both haploid and diploid phases and infecting more than one host. Probably the best known is Plasmodium, the causative agent of malaria, which spends part of its life in a species of mosquito. Sporozoans are characterized by a spore-like stage called a sporozoite, which is involved in the transmission of the parasite to a new host. The tip of the sporozoite contains a complex of structures that assist in the penetration of the host’s tissues. Unlike the protozoans discussed above, sporozoans are generally non-motile, and absorb soluble nutrients across the cell surface rather than ingesting particulate matter. Plasmodium, the causative agent of malaria, has two hosts. Four different species of Plasmodium cause malaria, the most widespread human infectious disease. Each is transmitted by the Anopheles mosquito. Asexual reproduction (schizogony) takes place in the human host; the sexual cycle occurs in the mosquito, following ingestion of blood containing gametocytes. C. Slime mold and the Water mold (Fungus-like Protists) The last group are the fungus-like protists. There are two principal groupings, the slime molds and the water molds. Oomycota (Water molds) Water molds resemble true fungi in their gross structure, comprising a mass of branched hyphae. At the cellular and molecular level however, they bear very little resemblance, and are not at all closely related. The Oomycota derive their name from the single large egg cell that is fertilized to produce a diploid zygote as part of the sexual reproduction cycle. Many water molds play an important role in the decomposition of dead plants and animals in freshwater ecosystems, while others are parasitic on the gills of fish. Terrestrial members of the Oomycota include a number of important plant pathogens, such as rusts and mildews, which can have a devastating effect on crops such as tobacco and potatoes. Myxomycota (Plasmodial slime molds) At one stage in their life cycle, the plasmodial or acellular slime molds exist as a single celled amoeboid form. Two of these haploid amoebas fuse to give a diploid cell, which then undergoes repeated divisions of the nucleus, without any accompanying cell division; the result is a plasmodium, a mass of cytoplasm that contains numerous nuclei surrounded by a single membrane. This retains the amoeboid property of cytoplasmic streaming, so the whole multinucleated structure is able to move in a creeping fashion. The ‘feeding plasmodium’ which may be several centimeters in length, and often brightly colored, feeds phagocytically on rotting vegetation. Fruiting bodies develop from the plasmodium when it is mature or when conditions are favorable, and a cycle of sexual reproduction is entered. When favorable conditions return, meiosis gives rise to haploid spores, which germinate to produce the amoeboid form. Dictyostelida (Cellular slime molds) A unicellular amoeboid form also figures in the life cycle of the other group of slime molds, the Dictyostelida. This haploid amoeba is the main vegetative form, but when food supplies are scarce, large numbers aggregate to form a slug-like blob, superficially not unlike the plasmodium described above. Unlike the plasmodium, however, this aggregate is fully cellular, so each component cell retains its plasma membrane. Compare the life cycle of cellular slime molds with that of the plasmodial kind, fruiting bodies again develop, giving rise to spores that germinate into new amoebas. No meiosis step is required, however, because the whole cycle comprises haploid forms, and this is therefore a form of asexual reproduction. A simple sexual cycle may also occur, when haploid amoebas fuse to give a diploid zygote. READINGS: What are Protists? Retrieved from: https://www.livescience.com/54242protists.html#:~:text=Because%20it%20has%20characteristi cs%20of,the%20plant%20or%20animal%20kingdom.&text=Protists%20are%20a%20diverse% 2 0collection%20of%20organisms. How do organisms reproduce in the Kingdom Protista? https://sciencing.com/do-organisms-reproduce-kingdom-protista-8788583.html Watch: How do Protozoa Get Around? https://www.youtube.com/watch?v=bPwVOggUp4M Protists Biology https://www.youtube.com/watch?v=-zsdYOgTbOk