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Chapter 28 Protists
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Overview: Living Small
Even a low-power microscope can reveal a great variety of organisms in a drop of pond water. Protist is the informal name of the kingdom of mostly unicellular eukaryotes, but there are some colonial and multicellular species. Protists constitute a paraphyletic group, and Protista is no longer valid as a kingdom. Protists exhibit more structural and functional diversity than any other group of eukaryotes.
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Protists, the most nutritionally diverse of all eukaryotes, include:
Protists can reproduce asexually or sexually, or by the sexual processes of meiosis and syngamy. Protists, the most nutritionally diverse of all eukaryotes, include: Photoautotrophs - contain chloroplasts. Heterotrophs - absorb organic molecules or ingest larger food particles. Mixotrophs - combine photosynthesis and heterotrophic nutrition.
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Endosymbiosis in Eukaryotic Evolution
There is considerable evidence that much protist diversity has its origins in endosymbiosis. Mitochondria evolved by endosymbiosis of an aerobic prokaryote. Plastids evolved by endosymbiosis of a photosynthetic cyanobacterium.
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Endosymbiosis --> Eukaryotic Evolution
Plastid Dinoflagellates Secondary endosymbiosis Apicomplexans Cyanobacterium Red alga Primary endosymbiosis Stramenopiles Heterotrophic eukaryote Plastid Secondary endosymbiosis Figure 28.2 Diversity of plastids produced by secondary endosymbiosis Over the course of evolution, this membrane was lost. Euglenids Secondary endosymbiosis Green alga Chlorarachniophytes
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Protist Diversity Figure 28.3 Protist diversity
Diplomonads Parabasalids Excavata Euglenozoans Dinoflagellates Alveolates Apicomplexans Ciliates Diatoms Chromalveolata Golden algae Stramenopiles Brown algae Oomycetes Chlorarachniophytes Forams Rhizaria Radiolarians Red algae Chlorophytes Green algae Archaeplastida Figure 28.3 Protist diversity For the Cell Biology Video Demonstration of Chemotaxis, go to Animation and Video Files. Charophyceans Land plants Slime molds Amoebozoans Gymnamoebas Entamoebas Nucleariids Unikonta Fungi Opisthokonts Choanoflagellates Animals
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Protist Diversity - Diplomonads intestinal parasites
Figure 28.3 Protist diversity 5 µm
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Diatoms - unicellular algae / cell walls are glass-like and made of silica
Figure 28.3 Protist diversity 50 µm
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Forams - have porous shells called tests
Figure 28.3 Protist diversity
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Volvox - colonial freshwater chlorophytes
20 µm 50 µm Figure 28.3 Protist diversity
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Amoeba - have pseudopods for ingestion and motility
Figure 28.3 Protist diversity 100 µm
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Flagella Undulating membrane 5 µm
Trichomonas vaginali Parabasalids that commonly inhabit the female vagina Flagella Figure 28.4 The parabasalid Trichomonas vaginalis (colorized SEM) Undulating membrane 5 µm
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Euglenozoans Euglenozoa is a diverse clade that includes predatory heterotrophs, photosynthetic autotrophs, and pathogenic parasites. The main feature distinguishing them as a clade is a spiral or crystalline rod of unknown function inside their flagella. This clade includes the kinetoplastids and euglenids.
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Euglenozoans Flagella 0.2 µm Crystalline rod inside flagella
Figure 28.5 Euglenozoan flagellum Crystalline rod inside flagella Ring of microtubules
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9 µm Trypannnosoma - the Kinetoplastid that causes Sleeping Sickness
Figure 28.6 Trypanosoma, the kinetoplastid that causes sleeping sickness 9 µm
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Euglenids can be both autotrophic and heterotrophic
Long flagellum Eyespot Short flagellum Light detector Adaptation for photosynthesis Contractile vacuole osmoregulation Nucleus Chloroplast Figure 28.7 Euglena, a euglenid commonly found in pond water Plasma membrane Pellicle Protein bands beneath plasma membrane Provide strength and Flexibility. Euglena (LM) 5 µm
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Dinoflagellates some are bioluminescent /cause “Red Tides”
Flagellum Alveoli - sacs under the plasma membrane Alveolate Figure 28.8 Alveoli 0.2 µm
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Two host life cycle of Plasmodium - causes malaria
Inside mosquito Inside human Merozoite Sporozoites (n) Liver Liver cell Oocyst Apex MEIOSIS Red blood cell Merozoite (n) 0.5 µm Zygote (2n) Red blood cells Figure The two-host cycle of Plasmodium, the apicomplexan that causes malaria FERTILIZATION Gametes Gametocytes (n) Key Haploid (n) Diploid (2n)
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Ciliates: Paramecium Ciliates, a large varied group of protists, are named for their use of cilia to move and feed. They have large macronuclei and small micronuclei - micronuclei function during conjugation, a sexual process that produces genetic variation. Conjugation is separate from reproduction, which generally occurs by binary fission.
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Paramecium Contractile vacuole Oral groove Cell mouth Cilia 50 µm Micronucleus Food vacuoles Macronucleus (a) Feeding, waste removal, and water balance MEIOSIS Haploid micronucleus Diploid micronucleus Compatible mates Figure Structure and function in the ciliate Paramecium caudatum The original macronucleus disintegrates. Diploid micronucleus MICRONUCLEAR FUSION Key ConjugationReproduction (b) Conjugation and reproduction
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Diatoms Diatoms are unicellular algae with a unique two-part, glass-like wall of hydrated silica. Diatoms usually reproduce asexually, and occasionally sexually. Diatoms are a major component of phytoplankton and are highly diverse. Fossilized diatom walls compose much of the sediments known as diatomaceous earth.
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3 µm Freshwater Diatom with glass-like wall of silica
Figure A freshwater diatom (colorized SEM) 3 µm
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Golden Algae Golden algae are named for their color, which results from yellow and brown carotenoids. The cells of golden algae are typically biflagellated, with both flagella near one end. All golden algae are photosynthetic, and some are also heterotrophic. Most are unicellular, but some are colonial.
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Flagellum Outer container Living cell 25 µm Colonial Golden Algae
Figure Dinobryon, a colonial golden alga found in fresh water (LM) 25 µm
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Brown Algae Brown algae are the largest and most complex algae. All are multicellular, and most are marine. Brown algae include many species commonly called “seaweeds.” Giant seaweeds called kelps live in deep parts of the ocean. Brown algae have the most complex multicellular anatomy of all algae. The algal body is plantlike but lacks true roots, stems, and leaves and is called a thallus. The rootlike holdfast anchors the stemlike stipe, which in turn supports the leaflike blades.
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Brown Algae Blade Stipe Holdfast
Figure Seaweeds: adapted to life at the ocean’s margins Holdfast
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Alternation of Generations
A variety of life cycles have evolved among the multicellular algae. The most complex life cycles include an alternation of generations, the alternation of multicellular haploid (gametophyte) and diploid forms (sporophyte). Heteromorphic generations are structurally different. Isomorphic generations look similar.
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Brown Algae Alternation of Generations
Sporangia 10 cm MEIOSIS Sporophyte (2n) Zoospore Female Developing sporophyte Figure The life cycle of the brown alga Laminaria: an example of alternation of generations Gametophytes (n) Zygote (2n) Mature female gametophyte (n) Male Egg FERTILIZATION Key Sperm Haploid (n) Diploid (2n)
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Oomycetes (Water Molds and Their Relatives)
Oomycetes include water molds, white rusts, and downy mildews… Fungus-like. They were once considered fungi based on morphological studies. Most oomycetes are decomposers or parasites They have filaments (hyphae) that facilitate nutrient uptake. Their ecological impact can be great, as in Phytophthora infestans causing potato blight.
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Life Cycle of Oomycetes = Water Mold
Oogonium Germ tube Egg nucleus (n) Cyst Antheridial hypha with sperm nuclei (n) MEIOSIS Hyphae ASEXUAL REPRODUCTION Zoospore (2n) FERTILIZATION Zygote germination Zygotes (oospores) (2n) SEXUAL REPRODUCTION Zoosporangium (2n) Key Haploid (n) Diploid (2n) Figure The life cycle of a water mold
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Concept 28.4: Rhizarians are a diverse group of protists defined by DNA similarities
DNA evidence supports Rhizaria as a monophyletic clade. Amoebas move and feed by pseudopodia; some but not all belong to the clade Rhizaria. Rhizarians include forams and radiolarians.
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Protists: Forams and Radiolarians
Foraminiferans, or forams, are named for porous, generally multichambered shells, called tests. Pseudopodia extend through the pores in the test. Foram tests in marine sediments form an extensive fossil record. Radiolarians have tests fused into one delicate piece, made of silica. Radiolarians use their pseudopodia to engulf microorganisms by phagocytosis.
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Radiolarian Protist Figure A radiolarian Pseudopodia 200 µm
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Concept 28.5: Red algae and green algae are the closest relatives of land plants
Over a billion years ago, a heterotrophic protist acquired a cyanobacterial endosymbiont. The photosynthetic descendants of this ancient protist evolved into red algae and green algae. Land plants are descended from the green algae. Archaeplastida is a supergroup used by some scientists and includes red algae, green algae, and land plants.
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Red Algae Red algae are reddish in color due to an accessory pigment call phycoerythrin, which masks the green of chlorophyll. The color varies from greenish-red in shallow water to dark red or almost black in deep water. Red algae are the most abundant large algae in coastal waters of the tropics. Red algae are usually multicellular; the largest are seaweeds.
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Bonnemaisonia hamifera 8 mm
Red Algae : This species has a delicate filamentous form Bonnemaisonia hamifera Figure Red algae 8 mm
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Dulse (Palmaria palmata)
Red Algae: This leafy species is edible 20 cm Figure Red algae Dulse (Palmaria palmata)
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Nori. The red alga Porphyra is the
source of a traditional Japanese food. The seaweed is grown on nets in shallow coastal waters. The harvested seaweed is spread on bamboo screens to dry. Figure Red algae Paper-thin, glossy sheets of nori make a mineral-rich wrap for rice, seafood, and vegetables in sushi.
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Green Algae Green algae are named for their grass-green chloroplasts.
Plants are descended from the green algae. The two main groups are chlorophytes and charophyceans. Most chlorophytes live in fresh water, although many are marine. Other chlorophytes live in damp soil, as symbionts in lichens, or in snow. Chlorophytes include unicellular, colonial, and multicellular forms.
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Green Algae Ulva, or sea lettuce edible seaweed 2 cm (b) Caulerpa, an
Figure Multicellular chlorophytes (b) Caulerpa, an intertidal chloro- phyte
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Most chlorophytes have complex life cycles with both sexual and asexual reproductive stages:
Flagella – 1 µm Cell wall + Gamete (n) – + Nucleus Zoospore Mature cell (n) FERTILIZATION ASEXUAL REPRODUCTION SEXUAL REPRODUCTION Zygote (2n) Cross section of cup-shaped chloroplast Figure The life cycle of Chlamydomonas, a unicellular chlorophyte Key MEIOSIS Haploid (n) Diploid (2n)
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Concept 28.6: Unikonts include protists that are closely related to fungi and animals
The supergroup Unikonta includes animals, fungi, and some protists. This group includes two clades: the amoebozoans and the opisthokonts (animals, fungi, and related protists) The root of the eukaryotic tree remains controversial.
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Eukaryotes Choanoflagellates Animals Unikonta Fungi Common ancestor
of all eukaryotes Amoebozoans Diplomonads Excavata Euglenozoans Alveolates Chromalveolata Stramenopiles Figure What is the root of the eukaryotic tree? DHFR-TS gene fusion Rhizarians Rhizaria Red algae Green algae Archaeplastida Plants
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Amoebozoans Amoebozoans are amoeba that have lobe- or tube-shaped, rather than threadlike, pseudopodia. They include gymnamoebas, entamoebas, and slime molds.
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Slime Molds > fungus-like
Slime molds, or mycetozoans, were once thought to be fungi. Molecular systematics places slime molds in the clade Amoebozoa. Many species of plasmodial slime molds are brightly pigmented, usually yellow or orange. At one point in the life cycle, plasmodial slime molds form a mass called a plasmodium (not to be confused with malarial Plasmodium). The plasmodium is undivided by membranes and contains many diploid nuclei. It extends pseudopodia through decomposing material, engulfing food by phagocytosis.
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Life Cycle of Plasmodial Slime Mold
4 cm FERTILIZATION Zygote (2n) Feeding plasmodium Mature plasmodium (preparing to fruit) Flagellated cells (n) Young sporangium Amoeboid cells (n) Mature sporangium Germinating spore Spores (n) Figure The life cycle of a plasmodial slime mold MEIOSIS 1 mm Stalk Key Haploid (n) Diploid (2n)
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Cellular slime molds form multicellular aggregates in which cells are separated by their membranes.
Cells feed individually, but can aggregate to form a fruiting body. Dictyostelium discoideum is an experimental model for studying the evolution of multicellularity.
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Life Cycle of Cellular Slime Mold
Spores (n) FERTILIZATION Emerging amoeba (n) Zygote (2n) SEXUAL REPRODUCTION Solitary amoebas (feeding stage) (n) 600 µm MEIOSIS Fruiting bodies (n) ASEXUAL REPRODUCTION Amoebas (n) Aggregated amoebas Migrating aggregate Figure The life cycle of Dictyostelium, a cellular slime mold Key Haploid (n) Diploid (2n) 200 µm
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Protists play key roles in ecological relationships
Protists are found in diverse aquatic environments. Protists often play the role of symbiont or producer. Some protist symbionts ++ benefit their hosts Dinoflagellates nourish coral polyps that build reefs. Hypermastigotes digest cellulose in the gut of termites.
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Protist Symbiont ++ Figure A protist symbiont 10 µm
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Some protists are parasitic + - symbionts
Plasmodium causes malaria. Pfesteria shumwayae is a dinoflagellate that causes fish kills… toxic red tides .. Phytophthora ramorum causes sudden oak death.
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Nurseries with P. ramorum infections (2004) on
Risk Map for the Parasitic Protist Sudden Oak Death in United States Key Figure Risk map for sudden oak death in the contiguous United States High risk Moderate risk Low risk Nurseries with P. ramorum infections (2004) on other host plants (such as rhododendron).
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Photosynthetic Protists
Many protists are important producers that obtain energy from the sun. In aquatic environments, photosynthetic protists and prokaryotes are the main producers. The availability of nutrients can affect the concentration of protists.
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Protists are Key Producers in Aquatic Communities
Other consumers Herbivorous plankton Carnivorous plankton Bacteria absorbed by Figure Protists are key producers in aquatic communities Soluble organic matter Protistan producers secrete
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Review
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You should now be able to:
Explain why the kingdom Protista is no longer considered a legitimate taxon. Explain the process of endosymbiosis and state what living organisms are likely relatives of mitochondria and plastids. Distinguish between endosymbiosis and secondary endosymbiosis. Name the five supergroups, list their key characteristics, and describe some representative taxa.
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