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Protists

Week 3: Protists

Learning Goals

  • Develop an understanding of the Domain Eukarya.

  • Investigate the diversity in morphology, life cycles, and ecological roles of organisms in this group of protists.

Learning Objectives

  • Recognize and define in your own words the terms in bold found throughout this exercise.

  • Apply the terms in bold found throughout this exercise to specimens in the lab.

  • Compare and contrast the characteristics of prokaryotes and eukaryotes.

  • Identify specimens in lab based on morphological characteristics typical of the group to which they belong.

  • Illustrate and label the key structures of the specimens seen in lab.

  • Examine live protists from samples of a local water body.

Introduction

  • Protists are defined as a group consisting of all eukaryotes NOT classified as animals, plants, or fungi.

  • Important note: Although they are grouped together, many protists are more closely related to animals, plants, or fungi than to each other, indicating that protists are not a monophyletic group.

  • Informal classifications can be made based on ecological roles, habitats, or modes of motility.

    • Major Classes of Protists:

    • Ciliates: Use cilia for movement and feeding.

    • Flagellates: Use flagella for movement.

    • Sarcodinids: Move using pseudopodia (false feet).

  • This lab follows Campbell Biology (10th edition) and utilizes a formal classification scheme of eukaryotic supergroups.

  • Focus will be on protist representatives from four supergroups.

Eukaryotes

  • Definition: Eukaryotes are organisms from the domain Eukarya that possess complex cells, with their genetic material organized into a membrane-bound nucleus or nuclei.

  • Eukaryotic cells feature a nuclear membrane surrounding the nucleus, where well-defined chromosomes are located.

Endosymbiosis

  • Definition: Endosymbiosis is a hypothesized process by which prokaryotes gave rise to the first eukaryotic cells.

    • Primary Endosymbiosis: Involves the engulfment of a bacterium by another free-living organism.

    • Secondary Endosymbiosis: Occurs when the product of primary endosymbiosis is engulfed and retained by another free-living eukaryote. This has happened several times, leading to diverse groups of algae and other eukaryotes.

Supergroup Excavata

  • Members of Excavata possess a feeding groove that appears excavated from the cell surface. The group includes:

    • Euglenoids:

    • Description: Mostly freshwater flagellates that possess unique protein ribbons beneath their membranes, enhancing flexibility.

    • Mode of Nutrition: Many are heterotrophic; some perform photosynthesis, having acquired plastids through secondary endosymbiosis.

    • Kinetoplastids:

    • Definition: Possess a large mitochondrion with a kinetoplast (the mitochondrial genome copied multiple times). They lack chloroplasts and other photosynthetic plastids.

Euglena sp.

  • Euglena sp. can be photosynthetic, saprophytic (absorbing organic nutrients from the environment), or heterotrophic (ingesting food particles via phagocytosis).

  • Observation: Euglena sp. changes shape while swimming due to its pellicle (cell surface composed of plasma membrane and protein strips).

    • Labelled Structures:

    • Eyespot (stigma) near the flagellum

    • Flagellum

    • Nucleus

    • Chloroplast

    • Contractile vacuole

Trypanosoma sp.

  • A genus of kinetoplastids consisting of many parasitic species, with some causing sleeping sickness in humans and animals.

  • Observation: Noted that Trypanosoma appears eel-like in prepared blood slide, often among erythrocytes with an undulating membrane aiding in movement.

Giardia lambia

  • Parasitic excavate found in feces-contaminated water, causing giardiasis, which presents symptoms like watery diarrhea, fever, cramps, and vomiting.

Supergroup SAR Clade

  • This supergroup includes the clades:

    • Stramenopila

    • Alveolata

    • Rhizaria

Stramenopila

  • Characterized by straw-like hairs on flagella, enhancing swimming efficiency.

Diatoms

  • Unicellular algae containing pigments (chlorophyll a and c, and xanthophyll).

  • Habitat: Found nearly everywhere water exists, including damp soil.

  • Importance: Diatoms contribute approximately 25% of all organic carbon fixation and are crucial in providing atmospheric oxygen.

    • Observation: Diatoms can be pennate (long and thin) or centric (disc-like) and can form colonies.

    • Movement: They drift or float in water, sometimes using a raphe for gliding locomotion.

Brown Algae (Phylum Phaeophyta)

  • Examples include multicellular seaweeds like giant kelps.

  • Specialized structures include:

    • Stipe: Stalk-like structure.

    • Holdfast: Root-like structure.

    • Air bladders: Provide buoyancy.

    • Fronds: Leaf-like structures.

  • Example: Sargassum, a species causing ecological nuisance with rapid growth.

Alveolata

  • Characterized by sac-like vesicles called alveoli beneath the plasma membrane, the function of which is uncertain.

Dinoflagellates

  • Marine protists exhibiting two perpendicular flagella and often have cellulose plates for protection.

  • Key roles include being primary producers, endosymbionts of coral, and toxin producers that can cause red tides.

Rhizaria

  • Planktonic heterotrophic marine organisms with narrow pseudopodia, supported by microtubules, extruded through pore shells of silica or calcium carbonate.

Supergroup Unikonta

  • Include amoebozoans and opisthokonts (animals, fungi).

Phylum Amoebozoa

  • Characterized by locomotion via cytoplasmic streaming and phagocytosis.

  • Example: Amoeba sp. moves using pseudopods.

    • Structures: Contractile vacuoles maintain water balance and collect/excrete excess water.

Clade Mycetozoa (Slime Molds)

  • Previously classified as fungi but are actually quite distantly related to them.

  • Types:

    • Plasmodial Slime Molds: Form a plasmodium (mass of cytoplasm with multiple nuclei) but are not multicellular.

    • Cellular Slime Molds: Function as individually isolated cells but can aggregate when food is scarce.

Supergroup Archaeplastida

  • Includes primarily red and green algae, evolutionary lineage leading to land plants.

  • Organisms have plastids acquired through primary endosymbiosis, where heterotrophic cells captured cyanobacteria as photosynthetic endosymbionts.

Phylum Chlorophyta (Green Algae)

  • Comprised of diverse types including:

    • Unicellular: Chlamydomonas sp.

    • Typically haploid and can reproduce asexually or produce gametes under stress.

    • Filamentous: Spirogyra sp.

    • Colonial: Volvox sp., with differentiated cells for reproduction.

    • Multicellular: Caulerpa sp., resembling marine flora attached to substrates.

Phylum Rhodophyta (Red Algae)

  • Among the oldest groups of eukaryotic algae, characterized by the presence of phycoerythrin pigment, playing a significant role in coral reef formation.

  • Observation: Red algae categorized as important food sources in ecosystems.