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Chapter 28 Protists Lecture Outline Overview: A World in a Drop of Water In the past, taxonomists classified all protists in a single kingdom, Protista. However, it is now clear that Protista is in fact paraphyletic. Some protists are more closely related to plants, fungi, or animals than they are to other protists. As a result, the kingdom Protista has been abandoned. Various lineages are recognized as kingdoms in their own right. Scientists still use the convenient term protist informally to refer to eukaryotes that are not plants, animals, or fungi. Concept 28.1 Protists are an extremely diverse assortment of eukaryotes Protists exhibit more structural and functional diversity than any other group of organisms. Most protists are unicellular, although there are some colonial and multicellular ones. At the cellular level, many protists are very complex. This is to be expected of a single cell that must carry out the basic functions performed by all the specialized cells in a multicellular organism. Protists are the most nutritionally diverse of all eukaryotes. Some are photoautotrophs, containing chloroplasts. Some are heterotrophs, absorbing organic molecules or ingesting food particles. Some are mixotrophs, combining photosynthesis and heterotrophic nutrition. Protists can be divided into three groups, based on their roles in biological communities. These groups are not monophyletic. Protists include photosynthetic algal protists, ingestive protozoans, and absorptive protists. Protist habitats are also very diverse. The life cycles of protists vary greatly. Some are exclusively asexual, while most have life cycles including meiosis and syngamy. Endosymbiosis has a place in eukaryotic evolution. Much of protist diversity is the result of endosymbiosis, a process in which unicellular organisms engulfed other cells that evolved into organelles in the host cell. The earliest eukaryotes acquired mitochondria by engulfing alpha proteobacteria. The early origin of mitochondria is supported by the fact that all eukaryotes studied so far either have mitochondria or had them in the past. Later in eukaryotic history, one lineage of heterotrophic eukaryotes acquired an additional endosymbiont?a photosynthetic cyanobacterium?that evolved into plastids. This lineage gave rise to red and green algae. This hypothesis is supported by the observation that the DNA of plastids in red and green algae closely resembles the DNA of cyanobacteria. Plastids in these algae are surrounded by two membranes, presumably derived from the cell membranes of host and endosymbiont. On several occasions during eukaryotic evolution, red and green algae underwent secondary endosymbiosis. They were ingested in the food vacuole of a heterotrophic eukaryote and became endosymbionts themselves. For example, algae known as chlorarachniophytes evolved when a heterotrophic eukaryote engulfed a green alga. This process likely occurred comparatively early in evolutionary time, because the engulfed alga still carries out photosynthesis with its plastids and contains a tiny, vestigial nucleus called a nucleomorph. Concept 28.2 Diplomads and parabasalids have modified mitochondria Most diplomonads and parabasalids are found in anaerobic environments. These protists lack plastids, and their mitochondria lack DNA, an electron transport chain, and the enzymes needed for the citric acid cycle. In some species, the mitochondria are very small and produce cofactors for enzymes involved in ATP production in the cytosol. Diplomonads have two equal-sized nuclei and multiple flagella. Giardia intestinalis is an infamous diplomonad parasite that lives in the intestines of mammals. The most common method of acquiring Giardia is by drinking water contaminated with feces containing the parasite in a dormant cyst stage. The parabasalids include trichomonads. The best-known species, Trichomonas vaginalis, inhabits the vagina of human females. If the normal acidity of the vagina is disturbed, T. vaginalis can outcompete beneficial bacteria and infect the vaginal lining. The male urethra may also be infected but without symptoms. The infection is sexually transmitted. Genetic studies of T. vaginalis suggest that the species became pathogenic after some individuals acquired a particular gene through horizontal gene transfer from other vaginal bacteria. The gene allows T. vaginalis to feed on epithelial cells. Concept 28.3 Euglenozoans have flagella with a unique internal structure Euglenozoa is a diverse clade that includes predatory heterotrophs, photosynthetic autotrophs, and pathogenic parasites. Members of this group are distinguished by the presence of a spiral or crystalline rod inside their flagella. Most euglenozoans have disc-shaped mitochondrial cristae. The best-studied groups of euglenozoans are the kinetoplastids and euglenids. The kinetoplastids have a single large mitochondrion associated with a unique organelle, the kinetoplast. The kinetoplast houses extranuclear DNA. Kinetoplastids are symbiotic and include pathogenic parasites. For example, Trypanosoma causes African sleeping sickness, a disease spread by the African tsetse fly, and Chagas? disease, which is transmitted by bloodsucking bugs. Trypanosomes evade immune detection by switching surface proteins from generation to generation, preventing the host from developing immunity. One-third of Trypanosoma?s genome codes for these surface proteins. Euglenids are characterized by an anterior pocket from which one or two flagella emerge. They also have a unique glucose polymer, paramylon, as a storage molecule. Many species of the euglenid Euglena are autotrophic but can become heterotrophic in the dark. Other euglenids can phagocytose prey. Concept 28.4 Alveolates have sacs beneath the plasma membrane Members of the clade Alveolata have alveoli, small membrane-bound cavities, under the plasma membrane. Their function is not known, but they may help stabilize the cell surface or regulate water and ion content. Alveolata includes flagellated protists (dinoflagellates), parasites (apicomplexans), and ciliates. Dinoflagellates are abundant components of marine and freshwater phytoplankton. Dinoflagellates and other phytoplankton form the foundation of most marine and many freshwater food chains. Other species of dinoflagellates are heterotrophic. Most dinoflagellates are unicellular, but some are colonial. Each dinoflagellate species has a characteristic shape, often reinforced by internal plates of cellulose. Two flagella sit in perpendicular grooves in the ?armor? and produce a spinning movement. Dinoflagellate blooms, characterized by explosive population growth, can cause ?red tides? in coastal waters. The blooms are brownish red or pinkish orange because of the presence of carotenoids in dinoflagellate plasmids. Toxins produced by some red-tide organisms have produced massive invertebrate and fish kills. These toxins can be deadly to humans as well. Some dinoflagellates form mutualistic symbioses with coral polyps, the animals that build coral reefs. Photosynthetic products from the dinoflagellates provide the main food resource for reef communities. All apicomplexans are parasites of animals, and some cause serious human diseases. The parasites disseminate as tiny infectious cells (sporozoites) with a complex of organelles specialized for penetrating host cells and tissues at the apex of the sporozoite cell. Apicomplexans have a nonphotosynthetic plasmid called the apicoplast, which carries out vital functions including the synthesis of fatty acids. Most apicomplexans have intricate life cycles with both sexual and asexual stages and often require two or more different host species for completion. Plasmodium, the parasite that causes malaria, spends part of its life in mosquitoes and part in humans. The incidence of malaria was greatly diminished in the 1960s by the use of insecticides against the Anopheles mosquitoes, which spread the disease, and by drugs that killed the parasites in humans. However, resistant varieties of Anopheles and Plasmodium have caused a malarial resurgence. About 300 million people are infected with malaria in the tropics, and up to 2 million die each year. The search for malarial vaccines has had little success because Plasmodium is evasive. It spends most of its time inside human liver and blood cells, and continually changes its surface proteins, thereby changing its ?face? to the human immune system. The need for new treatments for malaria led to a major effort to sequence Plasmodium?s genome. By 2003, researchers had identified the expression of most of the parasite?s genes at specific points in its life cycle. This research could help scientists identify potential new targets for vaccines. Identification of a gene that may confer resistance to chloroquine, an antimalarial drug, may lead to ways to block drug resistance in Plasmodium. Ciliates are a diverse group of protists, named for their use of cilia to move and feed. The cilia may cover the cell surface or be clustered into rows or tufts. Some ciliates scurry about on leglike structures constructed from many cilia. A submembrane system of microtubules coordinates ciliary movements. The cilia are associated with a submembrane system of microtubules that may coordinate movement. Ciliates have two types of nuclei, one or more large macronuclei and tiny micronuclei. Each macronucleus has dozens of copies of the ciliate?s genome. The genes are not organized into chromosomes but are packaged into small units with duplicates of a few genes. Macronuclear genes control the everyday functions of the cell such as feeding, waste removal, and water balance. Ciliates generally reproduce asexually by binary fission of the macronucleus, rather than mitotic division. The sexual shuffling of genes occurs during conjugation, during which two individuals exchange haploid micronuclei. In ciliates, reproduction and conjugation are separate processes. In a real sense, ciliates have ?sex without reproduction.? Conjugation provides an opportunity for ciliates to eliminate transposons and other types of ?selfish? DNA that can replicate within a genome. During conjugation, foreign genetic elements are excised when micronuclei develop from macronuclei. Up to 15% of a ciliate?s genome may be removed every time it undergoes conjugation. Concept 28.5 Stramenopiles have ?hairy? and smooth flagella The clade Stramenopila includes both heterotrophic and photosynthetic protists. The name of this group is derived from the presence of numerous fine, hairlike projections on the flagella. In most cases, a ?hairy? flagellum is paired with a smooth flagellum. In most stramenopile groups, the only flagellated stages are motile reproductive cells. The heterotrophic stramenopiles, the oomycetes, include water molds, white rusts, and downy mildews. Many oomycetes have multinucleate filaments that resemble fungal hyphae. However, there are many differences between oomycetes and fungi. Oomycetes have cell walls made of cellulose, while fungal walls are made of chitin. The diploid condition, reduced in fungi, is dominant in oomycete life cycles. Oomycetes have flagellated cells, while almost all fungi lack flagella. Molecular systematics has confirmed that oomycetes are not closely related to fungi. Their superficial similarity is a case of convergent evolution. In both groups, the high surface-to-volume ratio of filamentous hyphae enhances nutrient uptake. Although oomycetes descended from photosynthetic ancestors, they no longer have plastids. Instead, they acquire nutrients as decomposers or parasites. Water molds are important decomposers, mainly in fresh water. They form cottony masses on dead algae and animals. White rusts and downy mildews are parasites of terrestrial plants. They are dispersed by windblown spores, and form flagellated zoospores at another point in their life cycles. One species of downy mildew threatened French vineyards in the 1870s. Another species causes late potato blight, which contributed to the Irish famine in the 19th century. Late blight continues to cause crop losses today. Researchers are working to develop resistant potatoes by transferring genes from wild potatoes that confer resistance to blight. Diatoms are unicellular algae with unique glasslike walls composed of hydrated silica embedded in an organic matrix. The wall is divided into two parts that overlap like a shoebox and lid. These walls allow live diatoms to withstand immense pressure, providing a defense for them from the crushing jaws of predators. Most of the year, diatoms reproduce asexually by mitosis with each daughter cell receiving half of the cell wall and regenerating a new second half. Some species form cysts as resistant stages. Sexual stages are not common. When it occurs, it involves the formation of eggs and amoeboid or flagellated sperm. Diatoms are a highly diverse group of protists, with an estimated 100,000 species. They are abundant members of both freshwater and marine plankton. Diatoms store food reserves as the glucose polymer laminarin or, in a few diatoms, as oil. Massive accumulations of fossilized diatoms are major constituents of diatomaceous earth. Golden algae, or chrysophytes, are named for their yellow and brown carotenoids. Their cells are biflagellated, with both flagella attached near one end of the cell. Some species are mixotrophic, absorbing organic molecules or ingesting bacteria by phagocytosis. Many chrysophytes live among freshwater and marine plankton. While most are unicellular, some are colonial. At high densities, they can form resistant cysts that remain viable for decades. Brown algae, or phaeophytes, are the largest and most complex protists known. All brown algae are multicellular, and most species are marine. Brown algae are especially common along temperate coasts in areas of cool water and adequate nutrients. They owe their characteristic brown or olive color to carotenoids in their plastids, which are homologous to the plastids of golden algae and diatoms. The largest marine algae, including brown, red, and green algae, are known collectively as seaweeds. Seaweeds inhabit the intertidal and subtidal zones of coastal waters. This environment is characterized by extreme physical conditions, including wave forces and exposure to sun and drying conditions at low tide. Seaweeds have a complex multicellular anatomy, with some differentiated tissues and organs that resemble those in plants. These analogous features include the thallus, or body, of the seaweed. The thallus typically consists of a rootlike holdfast and a stemlike stipe, which supports leaflike photosynthetic blades. The term ?seaweed? refers to brown algae as well as some species of green and red algae. The giant seaweeds known as kelps live in deep water beyond the intertidal zone. The stipes of these algae may be as long as 60 m. Seaweeds living in the intertidal zone must cope with rough water as well as twice-daily low tides that expose the algae to hot sun and risk of desiccation. Seaweeds are important sources of food and commodities. Many seaweeds are eaten by coastal people, including Laminaria (?kombu? in Japan) in soup and Porphyra (Japanese ?nori?) for sushi wraps. A variety of gel-forming substances are extracted in commercial operations. Algin from brown algae and agar and carrageen from red algae are used as thickeners in food, lubricants in oil drilling, or culture media in microbiology. Some algae have life cycles with alternating multicellular haploid and diploid generations. The multicellular brown, red, and green algae show complex life cycles with alternation of multicellular haploid and multicellular diploid forms. A similar alternation of generations had a convergent evolution in the life cycle of plants. The complex life cycle of the kelp Laminaria provides an example of alternation of generations. The diploid individual, the sporophyte, produces haploid spores (zoospores) by meiosis. The haploid individual, the gametophyte, produces gametes by mitosis that fuse to form a diploid zygote. In Laminaria, the sporophyte and gametophyte are structurally different, or heteromorphic. In other algae, the alternating generations look alike (isomorphic) but differ in the chromosome number. Concept 28.6 Cercozoans and radiolarians have threadlike pseudopodia A newly recognized clade, Cercozoa, contains the amoebas. The term ?amoeba? used to refer to protists that move and feed by means of pseudopodia, cellular extensions that bulge from the cell surface. When an amoeba moves, it extends a pseudopodium and anchors the tip. Cytoplasm then streams into the pseudopodium. It is now clear that amoebas are not a monophyletic group. Those that belong to the clade Cercozoa are distinguished by their threadlike pseudopodia. Cercozoans include chlorarachniophytes and foraminiferans and are closely related to radiolarians, which also have threadlike pseudopodia. Foraminiferans, or forams, are named for their porous shells, or tests. Forams have multichambered, porous shells, consisting of organic materials hardened with calcium carbonate. Pseudopodia extend through the pores for swimming, shell formation, and feeding. Many forams form symbioses with algae. Forams live in marine and fresh water. Most live in sand or attach to rocks or algae. Some are abundant in the plankton. More than 90% of the described forams are fossils. The calcareous skeletons of forams are important components of marine sediments. Fossil forams are often used as chronological markers to correlate the ages of sedimentary rocks from different parts of the world. Radiolarians are mostly marine protists whose siliceous skeletons are fused into one delicate piece. Pseudopodia known as axopodia radiate from the central body and are reinforced by microtubules. The microtubules are covered by a thin layer of cytoplasm, which phagocytoses organisms that become attached to the axopodia. After death, radiolarian tests accumulate as an ooze that may be hundreds of meters thick in some seafloor locations. Concept 28.7 Amoebozoans have lobe-shaped pseudopodia Many species of amoebas that have lobe-shaped pseudopodia belong to the clade Amoebozoans, which includes gymnamoebas, entamoebas, and slime molds. Gymnamoebas are a large and varied group of Amoebozoans. They are common in soil as well as freshwater and marine environments. Most are heterotrophs that actively seek and consume bacteria and protists, while some feed on detritus. Entamoebas include free-living and parasitic species. Humans host at least six species of Entamoeba. One, E. histolytica, causes amebic dysentery, spread through contaminated drinking water and food. This disease kills 100,000 people each year. Slime molds were once thought to be fungi because they produce fruiting bodies that disperse their spores. However, this resemblance is due to evolutionary convergence. Molecular systematics places slime molds in the clade Amoebozoa and suggests that they descended from unicellular, gymnamoeba-like ancestors. Slime molds have diverged into two lineages with distinctive life cycles: plasmodial slime molds and cellular slime molds. The plasmodial slime molds are brightly pigmented, heterotrophic organisms. The feeding stage is an amoeboid mass, the plasmodium, which may be several centimeters in diameter. The plasmodium is not multicellular, but rather a single mass of cytoplasm with multiple diploid nuclei. The diploid nuclei undergo synchronous mitotic divisions, thousands at a time. Because of this characteristic, plasmodial slime molds have been used for studies of the molecular details of the cell cycle. Within the cytoplasm, cytoplasmic streaming distributes nutrients and oxygen throughout the plasmodium. The plasmodium phagocytoses food particles from moist soil, leaf mulch, or rotting logs. If the habitat begins to dry or if food levels drop, the plasmodium stops growing and differentiates into a stage of the life cycle that produces fruiting bodies, which function in sexual reproduction. Plasmodial slime molds are primarily diploid. Cellular slime molds straddle the line between individuality and multicellularity. The feeding stage consists of solitary cells that feed and divide mitotically as individuals. When food is scarce, the cells form an aggregate (?slug?) that functions as a unit. Each cell retains its identity in the aggregate. The dominant stage in a cellular slime mold is the haploid stage. Most cellular slime molds lack flagellated stages. Dictyostelium discoideum is a common cellular slime mold that has become a model organism for addressing the evolution of multicellularity. As the fruiting body forms, cells that form the stalk dry out and die, cells at the top survive, form spores, and have the potential for future reproduction. Scientists have found that mutations to a single gene can turn individual Dictyostelium cells into ?cheaters? that never become part of the stalk. Since these mutants gain a strong reproductive advantage over noncheaters, why do all Dictyostelium cells not cheat? A group of scientists recently found a possible answer to this puzzle. Cheating mutants lack a protein on their cell surface, and noncheating cells can recognize this difference. Noncheaters preferentially aggregate with other noncheaters, depriving cheaters of the opportunity to exploit them. Such recognition systems may have been important in the evolution of multicellular animals and plants. Concept 28.8 Red algae and green algae are the closest relatives of land plants More than a billion years ago, a heterotrophic protist acquired a cyanobacterial endosymbiont. The photosynthetic descendents of this ancient protist evolved into the red and green algae. At least 475 million years ago, the lineage that produced green algae gave rise to the land plants. Unlike other eukaryotic algae, red algae have no flagellated stages in their life cycle. There are more than 6,000 known species of red algae, which are reddish due to the accessory pigment phycoerythrin. Coloration varies among species and depends on the depth that they inhabit. Some species lack pigmentation and are parasites on other red algae. Red algae are the most common seaweeds in the warm coastal waters of tropical oceans. Red algae inhabit deeper waters than other photosynthetic eukaryotes. Their photosynthetic pigments, especially phycobilins, allow them to absorb blue and green wavelengths that penetrate down to deep water. One red algal species has been discovered off the Bahamas at a depth of more than 260 m. Some red algae live in fresh water or on land. Most red algae are multicellular, with some reaching a size large enough to be called ?seaweeds.? The thalli of many red algal species are filamentous. The base of the thallus is usually differentiated into a simple holdfast. The life cycles of red algae are especially diverse. In the absence of flagella, fertilization depends entirely on water currents to bring gametes together. Alternation of generations is common in red algae. Green algae are named for their grass-green chloroplasts. These are similar in ultrastructure and pigment composition to those of plants. Molecular systematics and cellular morphology provide considerable evidence that green algae and land plants are closely related. In fact, some systematists advocate the inclusion of green algae into an expanded ?plant? kingdom, Viridiplantae. Green algae are divided into two main groups, chlorophytes and charophyceans. Most of the 7,000 species of chlorophytes have been identified. Most live in fresh water, but many are marine inhabitants. Some chlorophytes inhabit damp soil, while others are specialized to live on glaciers and snowfields. These snow-dwelling chlorophytes carry out photosynthesis despite subfreezing temperatures and intense visible and ultraviolet radiation. They are protected by radiation-blocking compounds in their cytoplasm and by the snow itself, which acts as a shield. Some chlorophytes live symbiotically with fungi to form lichens, a mutualistic collective. Large size and complexity in chlorophytes has evolved by three different mechanisms: Formation of colonies of individual cells (e.g., Volvox). The repeated division of nuclei without cytoplasmic division to form multinucleate filaments (e.g., Caulerpa). The formation of true multicellular forms by cell division and cell differentiation (e.g., Ulva). Some multicellular marine chlorophytes are large and complex enough to qualify as seaweeds. Most green algae have complex life cycles, with both sexual and asexual reproductive stages. Most sexual species have biflagellated gametes with cup-shaped chloroplasts. Alternation of generations evolved in the life cycles of some green algae. The other main group of green algae is most closely related to land plants. Lecture Outline for Campbell/Reece Biology, 7th Edition, © Pearson Education, Inc. 28- PAGE 13
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