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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