Protists

Introduction

  • Protists: eukaryotes that are not classified in the plant, animal, or fungal kingdoms, though some protists are closely related to plants or animals or fungi
  • Two common characteristics   * Most abundant in moist habitats   * Most of them are microscopic in size

Classification

  • Classified by ecological role   * Three major groups:     * Algae: generally photoautotrophic     * Protozoa: heterotrophic     * Fungus-like: resemble fungi in body form and absorptive nutrition   * Terms lack taxonomic or evolutionary meaning
  • Classified by habitat   * Particularly common and diverse in oceans, lakes, wetlands and rivers   * Plankton: swimming or floating     * Phytoplankton: photosynthetic     * protozoan plankton: heterotrophic     * Occur primarily as single cells, colonies or short filaments
  • Classified by motility   * Swim using eukaryotic flagella     * Flagellates     * Some flagellated reproductive cells   * Cilia: shorter and more abundant than flagella     * Ciliates   * Amoeboid movement: using pseudopodia     * Amoebae   * Gliding on protein or carbohydrate slime

Evolution and Relationships

  • At one time, protists were in a single kingdom   * However, “protists” is not a monophyletic group
  • Evolutionary understanding is in flux   * Some relationships are uncertain or disputed   * New protists still being discovered
  • Classified into supergroups

Supergroup Excavata

  • Related to some of Earth’s earliest eukaryotes

  • Named for a feeding groove “excavated” into the cells of many representatives

  • Food particles are taken into cells by phagotrophy   * Endocytosis   * Evolutionary basis for endosymbiosis

  • Some are parasites

  • Trichomonas vaginalis and Giardia lamblia

  • Once thought to lack mitochondria

  • Possess highly modified mitochondria

  • Euglenozoa: protein strips under plasma membrane allow crawling   * Some are heterotrophic, but Euglena is photosynthetic

  • Kinetoplastids: named for unusually large mass of DNA (kinetoplast) in a single large mitochondrion   * Leishmania   * Trypansosoma brucei

Supergroup and Plants and Relatives

  • Supergroup that includes land plants also encompasses several algal phyla
  • Kingdom plantae (land plants) evolved from green algal ancestors
  • Phylum chlorophyta: green algae
  • Phylum rhodophyta: red algae
  • Green algae   * Phylum Chlorophyta    * Diverse structural types   * Occur in fresh waters, the ocean, and on land   * Most are photosynthetic   * Cells contain same type of plastids and photosynthetic pigments as in land plants
  • Red algae   * Most are multicellular marine macroalgae     * Red appearance due to distinctive photosynthetic pigments     * Lack flagella     * Unusually complex life cycles   * Cryptomonads     * Unicellular flagellates     * Most contain red, blue-green, or brown plastids from secondary endosymbiosis     * Photosynthetic   * Haptophytes     * Also unicellular photosynthesizers with secondary plastids     * Some known as coccolithophorids        * Have a covering of white calcium carbonate discs called coccoliths

Supergroup Alveolata

  • Ciliophora   * Ciliates - conjugation
  • Apicomplexa: medically important parasites   * Plasmodium
  • Dinozoa   * Dinoflagelllates - some photosynthetic, others not   * Red tide and mutualistic relationship with coral   * About half of dinoflagellates are heterotrophic   * Other half possess photosynthetic plastids of diverse types that originated by secondary or even tertiary endosymbiosis   * Tertiary plastids are obtained by tertiary endosymbiosis     * Acquisition by hosts of plastids from cells that already possessed secondary plastids
  • Named for saclike membranous vesicles (alveoli) present in cell periphery

Supergroup Stramenopila

  • Wide range of algae, protozoa, and fungus-like protists
  • Usually produce flagellate cells at some point
  • Named for distinctive strawlike hairs on the surface of flagella
  • Heterotrophic or photosynthetic   * Plastids from secondary endosymbiosis \n with red algae

Supergroup Rhizaria

  • Have thin, hairlike extensions of the cytoplasm called filose pseudopodia
  • Phylum Chlorarachniophyta
  • Phylum Radiolaria
  • Phylum Foraminifera

Supergroup Amoebozoa

  • Many types of amoebae
  • Move using pseudopodia
  • ex: Dictyostelium discoideum*, **slime mold    Model organism for understanding movement, cell communication, and development.   * In response to starvation, single amoebae aggregate into a multicellular “slug” that develops into a stalked structure containing spores   * Spores pop out and produce new amoebae

Supergroup Opisthokonta

  • Includes animal and fungal kingdoms and related protists

  • Named for single posterior flagellum on swimming cells

  • Choanoflagellate protists   * Feature distinctive collar surrounding flagella   * These are the modern protists most related to the common ancestor of animals

Nutritional and Defensive Adaptations

  • Phagotrophy: heterotrophs that ingest particles
  • Osmotrophy: heterotrophs that rely on uptake of small organic molecules
  • Photoautotrophy: photosynthetic
  • Mixotrophy: able to use autotrophy and phagotrophy or osmotrophy depending on conditions
  • Algal protists   * Variety of pigments     * Adapt photosystems to capture more light     * Water absorbs the longer red and yellow wavelengths more than the shorter blue and green wavelengths     * Accessory pigments absorb light and transfer it \n to chlorophyll a   * Variety of types of food storage molecules     * Starch, polysacchrides, and oil
  • Defense   * Slimy mucilage or cell walls defend against herbivores and pathogens     * Calcium carbonate, silica, iron, manganese armor   * Trichocysts: spear-shaped projectiles to discourage herbivores   * Bioluminescence: startles herbivores   * Toxins: inhibit animal physiology     * Ex: toxic dinoflagellate Pfiesteria       * Responsible for fish kills – “killer alga” or “the cell from hell”

Reproductive Adaptations

  • Asexual reproduction   * All protists can reproduce asexually   * Many produce cysts with thick, protective       walls that remain dormant in bad conditions    * Many protozoan pathogens spread from one  host to another via cysts
  • Sexual reproduction   * Eukaryotic sexual reproduction with gametes and zygotes arose among the protists   * Generally adaptive because it produces diverse genotypes   * Zygotic and sporic life cycles   * Zygotic life cycles     * Most unicellular sexually reproducing protists     * Haploid cells develop into gametes     * + and - mating strains     * Thick-walled diploid zygotes       * Survive like cysts   * Sporic life cycle     * Many multicellular green and brown seaweeds     * Also known as alternation of generations     * 2 types of multicellular organisms       * Haploid gametophyte produces gametes       * Diploid sporophyte produces spores by meiosis     * Red seaweed variation involves 3 distinct multicellular generations   * Gametic life cycle     * All cells except the gametes are diploid     * Gametes produced by meiosis     * Diatoms: one of few protists with this life cycle       * Asexual reproduction reduces the size of the daughter cells       * Sexual reproduction restores maximal size   * Ciliate sexual reproduction     * Most complex sexual process in protists     * Have 2 types of nuclei (single macronucleus and one or more micronuclei)     * Macronuclei are the source of the information for cell function     * 2 cell pairs and fuse -  conjugation     * Micronuclei undergo meiosis, exchange, fusion, and mitosis
  • Parasitic protist life cycle   * Parasitic protists often use more than one host organism, in which different life stages occur   * ex: Malarial parasite Plasmodium      * Alternates between humans and Anopheles mosquitoes   * Different stages in different hosts and host tissues

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