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. 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: the anal pore. In addition to a vacuole-based digestive system, Paramecium also uses contractile vacuoles: osmoregulatory vesicles that fill with water as it enters the cell by osmosis and then contract to squeeze water from the cell. Paramecium : Paramecium has a primitive mouth called an oral groove to ingest food and an anal pore to excrete it.
Contractile vacuoles allow the organism to excrete excess water. Cilia enable the organism to move. 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.
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, while 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 then become new macronuclei. Two cell divisions then yield four new paramecia from each original conjugative cell.
Paramecium : sexual reproduction : The complex process of sexual reproduction in Paramecium creates eight daughter cells from two original cells. Each cell has a macronucleus and a micronucleus. During sexual reproduction, the macronucleus dissolves and is replaced by a micronucleus.
Stramenophiles include photosynthetic marine algae and heterotrophic protists such as diatoms, brown and golden algae, and oomycetes. Describe characteristics of the following Stramenophiles: diatoms, brown algae, golden algae, and oomycetes. Current evidence suggests that chromalveolates have an ancestor which resulted from a secondary endosymbiotic event. The species which fall under the classification of chromalveolates have evolved from a common ancestor that engulfed a photosynthetic red algal cell.
This red algal cell had previously evolved chloroplasts from an endosymbiotic relationship with a photosynthetic prokaryote. 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.
A subgroup of chromalveolates, the stramenopiles, also referred to as heterokonts, includes photosynthetic marine algae and heterotrophic protists. Many stramenopiles also have an additional flagellum that lacks hair-like projections.
Members of this subgroup range in size from single-celled diatoms to the massive and multicellular kelp. Stramenophile structure : This stramenopile cell has a single hairy flagellum and a secondary smooth flagellum. 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. 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. Diatoms : Assorted diatoms, visualized here using light microscopy, live among annual sea ice in McMurdo Sound, Antarctica. 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.
The biological carbon pump is a crucial component of the carbon cycle that helps to maintain 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. For instance, compare this life cycle to that of humans.
In humans, 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. 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.
Terrestrial plants also have evolved alternation of generations. Brown algae life cycle : Several species of brown algae, such as the Laminaria shown here, have evolved life cycles in which both the haploid gametophyte and diploid sporophyte forms are multicellular. The gametophyte is different in structure from the sporophyte. 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 non-photosynthetic and include many saprobes and parasites. The saprobes appear as white fluffy growths on dead organisms. 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.
Rhizaria are a supergroup of protists, typically amoebas, that are characterized by the presence of needle-like pseudopodia. The Rhizaria supergroup includes many of the amoebas, most of which have threadlike or needle-like pseudopodia.
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. Ammonia tepida : Ammonia tepida, a Rhizaria species viewed here using phase contrast light microscopy, exhibits many threadlike pseudopodia. Foraminiferans, or forams, are unicellular heterotrophic protists, ranging from approximately 20 micrometers to several centimeters in length; they occasionally resemble tiny snails.
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. Foraminiferans are also useful as indicators of pollution and changes in global weather patterns. The life-cycle involves an alternation between haploid and diploid phases.
The haploid phase initially has a single nucleus, and divides to produce gametes with two flagella. The diploid phase is multinucleate, and after meiosis fragments to produce new organisms. The benthic forms has multiple rounds of asexual reproduction between sexual generations. Forams : These shells from foraminifera sank to the sea floor.
A second subtype of Rhizaria, the radiolarians, exhibit intricate exteriors of glassy silica with radial or bilateral symmetry. Radiolarians display needle-like pseudopods that are supported by microtubules which 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 meter-thick depths. Preserved, sedimented radiolarians are very common in the fossil record. Radiolarian shell : This fossilized radiolarian shell was imaged using a scanning electron microscope. Each diplomonad cell has two similar, but not identical haploid nuclei. Diplomonads have four pairs of locomotor flagella that are fairly deeply rooted in basal bodies that lie between the two nuclei.
A second Excavata subgroup, the parabasalids, are named for the parabasal apparatus, which consists of a Golgi complex associated with cytoskeletal fibers. Other cytoskeletal features include an axostyle, a bundle of fibers that runs the length of the cell and may even extend beyond it. Parabasalids move with flagella and membrane rippling, and these and other cytoskeletal modifications may assist locomotion. Like the diplomonads, the parabasalids exhibit modified mitochondria.
In parabasalids these structures function anaerobically and are called hydrogenosomes because they produce hydrogen gas as a byproduct. The parabasalid Trichomonas vaginalis causes trichomoniasis, a sexually transmitted disease in humans, which appears in an estimated 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. Some of the most complex of the parabasalids are those that colonize the rumen of ruminant animals and the guts of termites. These organisms can digest cellulose, a metabolic talent that is unusual among eukaryotic cells. They have multiple flagella arranged in complex patterns and some additionally recruit spirochetes that attach to their surface to act as accessory locomotor structures. 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. The chloroplast of Euglena descends from a green alga by secondary endosymbiosis.
In the dark, the chloroplasts of Euglena shrink up and temporarily cease functioning, and the cells instead take up organic nutrients from their environment. Euglena has a tough pellicle composed of bands of protein attached to the cytoskeleton.
The bands spiral around the cell and give Euglena its exceptional flexibility. The human parasite, Trypanosoma brucei , belongs to a different subgroup of Euglenozoa, the kinetoplastids. The kinetoplastid subgroup is named after the kinetoplast, a large modified mitochondrion carrying multiple circular DNAs. 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.
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. The process of classifying protists into meaningful groups is ongoing, but genetic data in the past 20 years have clarified many relationships that were previously unclear or mistaken. The majority view at present is to order all eukaryotes into six supergroups: Archaeplastida, Amoebozoa, Opisthokonta, Rhizaria, Chromalveolata, and Excavata.
The goal of this classification scheme is to create clusters of species that all are derived from a common ancestor. At present, the monophyly of some of the supergroups are better supported by genetic data than others. Although tremendous variation exists within the supergroups, commonalities at the morphological, physiological, and ecological levels can be identified.
Figure Which of the following statements about Paramecium sexual reproduction is false? Figure Which of the following statements about the Laminaria life cycle is false? What genus of protists appears to contradict the statement that unicellularity restricts cell size? A marine biologist analyzing water samples notices a protist with a calcium carbonate shell that moves by pseudopodia extension.
The protist is likely to be closely related to which species? The chlorophyte green algae genera Ulva and Caulerpa both have macroscopic leaf-like and stem-like structures, but only Ulva species are considered truly multicellular.
Explain why. Unlike Ulva , protists in the genus Caulerpa actually are large, multinucleate, single cells. Because these organisms undergo mitosis without cytokinesis and lack cytoplasmic divisions, they cannot be considered truly multicellular. Why might a light-sensing eyespot be ineffective for an obligate saprobe?
Suggest an alternative organ for a saprobic protist. By definition, an obligate saprobe lacks the ability to perform photosynthesis, so it cannot directly obtain nutrition by searching for light. Instead, a chemotactic mechanism that senses the odors released during decay might be a more effective sensing organ for a saprobe. Opisthokonta includes animals and fungi, as well as protists. Describe the key feature of this phylum, and an example of how an organism in each kingdom uses this feature.
Describe two ways in which paramecium differs from the projected traits of the last eukaryotic common ancestor. Student View. Preview Copy. Save Please log in to save materials. Show More Show Less. Course Alignments. Biology 2e Biological Diversity Preface The Chemistry of Life The Cell Genetics Evolutionary Processes Biological Diversity Plant Structure and Function Animal Structure and Function Ecology The Periodic Table of Elements Geological Time Measurements and the Metric System Protists Groups of Protists Introduction Eukaryotic Origins Characteristics of Protists Groups of Protists Ecology of Protists Groups of Protists Overview By the end of this section, you will be able to do the following: Describe representative protist organisms from each of the six presently recognized supergroups of eukaryotes Identify the evolutionary relationships of plants, animals, and fungi within the six presently recognized supergroups of eukaryotes Identify defining features of protists in each of the six supergroups of eukaryotes.
Eukaryotic supergroups. This diagram shows a proposed classification of the domain Eukarya. Currently, the domain Eukarya is divided into six supergroups. Within each supergroup are multiple kingdoms.
Although each supergroup is believed to be monophyletic, the dotted lines suggest evolutionary relationships among the supergroups that continue to be debated. Glaucophytes Glaucophytes are a small group of Archaeplastida interesting because their chloroplasts retain remnants of the peptidoglycan cell wall of the ancestral cyanobacterial endosymbiont Figure. Green Algae: Chlorophytes and Charophytes The most abundant group of algae is the green algae.
Volvox aureus is a green alga in the supergroup Archaeplastida. This species exists as a colony, consisting of cells immersed in a gel-like matrix and intertwined with each other via hair-like cytoplasmic extensions. Ralf Wagner True multicellular organisms, such as the sea lettuce, Ulva , are also represented among the chlorophytes.
A multinucleate alga. Caulerpa taxifolia is a chlorophyte consisting of a single cell containing potentially thousands of nuclei. An interesting question is how a single cell can produce such complex shapes. Link to Learning Take a look at this video to see cytoplasmic streaming in a green alga. Gymnomoebae The Gymnamoeba or lobose amoebae include both naked amoebae like the familiar Amoeba proteus and shelled amoebae, whose bodies protrude like snails from their protective tests.
Amoebae with tubular and lobe-shaped pseudopodia are seen under a microscope. These isolates would be morphologically classified as amoebozoans. Slime Molds A subset of the amoebozoans, the slime molds, has several morphological similarities to fungi that are thought to be the result of convergent evolution. Plasmodial slime molds. The life cycle of the plasmodial slime mold is shown. The brightly colored plasmodium in the inset photo is a single-celled, multinucleate mass.
Cellular Slime Mold. The image shows several stages in the life cycle of Dictyostelium discoideum , including aggregated cells, mobile slugs and their transformation into fruiting bodies with a cluster of spores supported by a stalk. A Colonial Choanoflagellate. Ammonia tepida , a Rhizaria species viewed here using phase contrast light microscopy, exhibits many threadlike pseudopodia. It also has a chambered calcium carbonate shell or test.
Foraminiferan Tests. These shells from foraminifera sank to the sea floor. Radiolarian shell. This fossilized radiolarian shell was imaged using a scanning electron microscope. A Chlorarachniophyte. This rhizarian is mixotrophic, and can obtain nutrients both by photosynthesis and by trapping various microorganisms with its network of pseudopodia. Alveolates: Dinoflagellates, Apicomplexians, and Ciliates A large body of data supports that the alveolates are derived from a shared common ancestor.
The dinoflagellates exhibit great diversity in shape. Many are encased in cellulose armor and have two flagella that fit in grooves between the plates. Movement of these two perpendicular flagella causes a spinning motion. Dinoflagellate bioluminescence. Bioluminescence is emitted from dinoflagellates in a breaking wave, as seen from the New Jersey coast. They have a characteristic apical complex that enables them to infect host cells.
Paramecium has a primitive mouth called an oral groove to ingest food, and an anal pore to eliminate waste. Contractile vacuoles allow the organism to excrete excess water. Cilia enable the organism to move. Art Connection Conjugation in Paramecium. The complex process of sexual reproduction in Paramecium creates eight daughter cells from two original cells.
Each cell has a macronucleus and a micronucleus. During sexual reproduction, the macronucleus dissolves and is replaced by a micronucleus. 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. Stramenopiles: Diatoms, Brown Algae, Golden Algae and Oomycetes The other subgroup of chromalveolates, the stramenopiles, includes photosynthetic marine algae and heterotrophic protists.
Stramenopile flagella. This stramenopile cell has a single hairy flagellum and a secondary smooth flagellum. Assorted diatoms, visualized here using light microscopy, live among annual sea ice in McMurdo Sound, Antarctica. Gordon T.
Art Connection Alternation of generations in a brown alga. Several species of brown algae, such as the Laminaria shown here, have evolved life cycles in which both the haploid gametophyte and diploid sporophyte forms are multicellular.
The gametophyte is different in structure than the sporophyte. The sporophyte is the 2 n plant. The gametophyte is diploid. Both the gametophyte and sporophyte stages are multicellular. A saprobic oomycete engulfs a dead insect. Diplomonads Among the Excavata are the diplomonads, which include the intestinal parasite, Giardia lamblia Figure.
The spores germinate and grow into a haploid gametophyte, which then makes gametes by mitosis. The gametes fuse to form a zygote that grows into a diploid sporophyte. Alternation of generations is seen in some species of Archaeplastid algae, as well as some species of Stramenopiles Figure. In some species, the gametophyte and sporophyte look quite different, while in others they are nearly indistinguishable.
Glaucophytes are a small group of Archaeplastida interesting because their chloroplasts retain remnants of the peptidoglycan cell wall of the ancestral cyanobacterial endosymbiont Figure. Red Algae Red algae, or rhodophytes lack flagella, and are primarily multicellular, although they range in size from microscopic, unicellular protists to large, multicellular forms grouped into the informal seaweed category. Red algae have a second cell wall outside an inner cellulose cell wall.
Carbohydrates in this wall are the source of agarose used for electrophoresis gels and agar for solidifying bacterial media. Other protists classified as red algae lack phycoerythrins and are parasites. Both the red algae and the glaucophytes store carbohydrates in the cytoplasm rather than in the plastid. Red algae are common in tropical waters where they have been detected at depths of meters. Other red algae exist in terrestrial or freshwater environments. The red algae life cycle is an unusual alternation of generations that includes two sporophyte phases, with meiosis occurring only in the second sporophyte.
The most abundant group of algae is the green algae. The green algae exhibit features similar to those of the land plants, particularly in terms of chloroplast structure.
In both green algae and plants, carbohydrates are stored in the plastid. 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.
The familiar Spirogyra is a charophyte. 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. Volvox colonies contain to 60, cells, each with two flagella, contained within a hollow, spherical matrix composed of a gelatinous glycoprotein secretion.
Individual cells in a Volvox colony 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.
Daughter colonies are produced with their flagella on the inside and have to evert as they are released. True multicellular organisms, such as the sea lettuce, Ulva , are also 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. Caulerpa species undergo nuclear division, but their cells do not complete cytokinesis, remaining instead as massive and elaborate single cells. Link to Learning Take a look at this video to see cytoplasmic streaming in a green alga.
Like the Archaeplastida, the Amoebozoa include species with single cells, species with large multinucleated cells, and species that have multicellular phases. Amoebozoan cells characteristically exhibit pseudopodia that extend like tubes or flat lobes. 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 is similar to the cytoplasmic streaming used to move organelles in the Archaeplastida, and is also used by other protists as a means of locomotion or as a method to distribute nutrients and oxygen.
The Amoebozoa include both free-living and parasitic species. The Gymnamoeba or lobose amoebae include both naked amoebae like the familiar Amoeba proteus and shelled amoebae, whose bodies protrude like snails from their protective tests.
Although Pelomyxa may have hundreds of nuclei, it has lost its mitochondria, but replaced them with bacterial endosymbionts. The secondary loss or modification of mitochondria is a feature also seen in other protist groups. Slime Molds 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. 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. When food is depleted, cellular slime molds aggregate 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 Figure. 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. View this video to see the formation of a fruiting body by a cellular slime mold. The Opisthokonts are named for the single posterior flagellum seen in flagellated cells of the group.
The flagella of other protists are anterior and their movement pulls the cells along, while the opisthokonts are pushed. Protist members of the opisthokonts include the animal-like choanoflagellates, which are believed to resemble the common ancestor of sponges and perhaps, all animals.
Choanoflagellates include unicellular and colonial forms Figure , and number about described species. In these organisms, the single, apical flagellum is surrounded by a contractile collar composed of microvilli.
The collar is used to filter and collect bacteria for ingestion by the protist. A similar feeding mechanism is seen in the collar cells of sponges, which suggests a possible connection between choanoflagellates and 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. The next three supergroups all contain at least some photosynthetic members whose chloroplasts were derived by secondary endosymbiosis. They also show some interesting variations in nuclear structure, and modification of mitochondria or chloroplasts. The Rhizaria supergroup includes many of the amoebas with thin threadlike, needle-like or root-like pseudopodia Figure , rather than the broader lobed pseudopodia of the Amoebozoa.
Many rhizarians make elaborate and beautiful tests—armor-like coverings for the body of the cell—composed of calcium carbonate, silicon, or strontium salts. Rhizarians have important roles in both carbon and nitrogen cycles.
When rhizarians die, and their tests sink into deep water, the carbonates are out of reach of most decomposers, locking carbon dioxide away from the atmosphere. The biological carbon pump is a crucial component of the carbon cycle that maintains lower atmospheric carbon dioxide levels. Foraminiferans are unusual in that they are the only eukaryotes known to participate in the nitrogen cycle by denitrification, an activity usually served only by prokaryotes.
Foraminiferans Foraminiferans, or forams, are unicellular heterotrophic protists, ranging from approximately 20 micrometers to several centimeters in length, and occasionally resembling tiny snails Figure. 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. Radiolarians A second subtype of Rhizaria, the radiolarians, exhibit intricate exteriors of glassy silica with radial or bilateral symmetry Figure.
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 meter-thick depths. Preserved, sedimented radiolarians are very common in the fossil record. Cercozoa The Cercozoa are both morphologically and metabolically diverse, and include both naked and shelled forms.
The Chlorarachniophytes Figure are photosynthetic, having acquired chloroplasts by secondary endosymbiosis. The chloroplast contains a remnant of the chlorophyte endosymbiont nucleus, sandwiched between the two sets of chloroplast membranes.
Chromalveolata 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. 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. The chloroplast of photosynthetic dinoflagellates was derived by secondary endosymbiosis of a red alga.
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.
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. Dinoflagellates have a nuclear variant called a dinokaryon. The chromosomes in the dinokaryon are highly condensed throughout the cell cycle and do not have typical histones.
Mitosis in dinoflagellates is closed, that is, the spindle separates the chromosomes from outside of the nucleus without breakdown of the nuclear envelope.
Some dinoflagellates generate light, called bioluminescence , when they are jarred or stressed.
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