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
    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
      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