TR

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