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