Week 4

PROTISTS

  • Size reference: 20 μm

PROTIST EVOLUTION AND DIVERSITY

General Biology of Protists

  • Classified in the domain Eukarya and the kingdom Protista

  • Protists are eukaryotic organisms that cannot be classified as animal, plant, or fungi

  • Many protists are unicellular but highly complex

Phylogeny of Protists

  • Diversity and relationships among various protist groups

  • Taxonomy presents challenges due to the complex evolutionary history of protists

GENERAL BIOLOGY OF PROTISTS

  • Protists are classified under the kingdom Protista, which itself is part of the domain Eukarya.

  • They cannot be classified under the other kingdoms (Animalia, Plantae, Fungi) because:

    • They exhibit diverse characteristics that do not conform to those categories.

    • Many are unicellular organisms with complex internal structures.

TREE OF LIFE

  • A detailed classification tree displaying major domains and groups, including:

    • Bacteria

    • Archaea

    • Eukaryotes

  • The tree illustrates various phylogenetic branches including several bacterial phyla and their evolutionary relationships.

IMPACT ON HEALTH AND HUMAN WELFARE

Dinoflagellates

  • Produce neurotoxins that can be harmful to humans and marine life.

  • Associated with harmful algal blooms known as “red tides.”

  • Notably, species of Phytophthora infestans are detrimental to crops like nightshade plants.

  • Size reference: 20 μm

Plasmodium Species

  • Responsible for malaria; five species account for approximately 600,000 deaths annually.

  • Demonstrated co-evolution between apicomplexan parasites and medical interventions, as well as mosquitoes and pesticides.

  • Carrier: Anopheles mosquito (CDC).

ECOLOGICAL IMPORTANCE OF PROTISTS

  • Protists play a crucial role in the ecosystem:

    • Photoautotrophic forms:

    • Produce oxygen, thus contributing to atmospheric balance.

    • Serve as primary producers in freshwater and marine ecosystems.

    • Form an essential component of plankton.

ENDOSYMBIOTIC THEORY

Overview

  • Proposed by Lynn Margulis

  • Explanation of the evolutionary origin of eukaryotic cells:

    • One cell engulfed another leading to mutual benefits in survival (endosymbiosis).

    • Primary endosymbiosis:

    • Aerobic bacteria evolved into mitochondria.

    • Cyanobacteria evolved into chloroplasts.

Steps in Endosymbiotic Theory

  1. Infoldings in plasma membrane of an ancestral prokaryote led to the formation of endomembrane components, including a nucleus and endoplasmic reticulum.

  2. The ancestral eukaryote consumed aerobic bacteria; these bacteria evolved into mitochondria.

  3. A second symbiotic event occurred where the early eukaryote engulfed photosynthetic bacteria that became chloroplasts.

Supporting Evidence

  • Mitochondria replicate by fission and possess their own ribosomes and genomes, indicating a bacterial ancestry.

  • Metabolic organelles have genes for energy processes closely related to Bacteria.

SYNAPOMORPHY

  • Definition: Shared, derived traits present in the most recent common ancestor.

    • Examples:

    • Skull, limbs, hair, lactation (in mammals)

    • Synapomorphies shared by various lineages, including lungfish, lizards, dogs, and humans.

TAXONOMY OF PROTISTS

  • Complexity and diversity make the classification of protists challenging.

    • The Kingdom Protista is NOT monophyletic; different protist groups do not share a common evolutionary lineage.

    • Evidence exists for convergent evolution among species in this kingdom.

MOLECULAR PHYLOGENIES

  • Eukaryotic Ancestors exhibit notable diversity including the following lineages:

    • Excavata, Rhizaria, Chromalveolata, Archaeplastids, Unikonta.

SYNAPOMORPHIES: SUPPORT AND PROTECTION

  • Different synapomorphies present in various groups:

    • Diatoms: Silicon dioxide cell walls.

    • Dinoflagellates: Cellulose cell walls (part of Alveolates).

    • Water molds: Cellulose (Stramenopiles).

    • Green plants: Similar structures found in Archaeplastids.

SUPPORT AND PROTECTION

  • Additional synapomorphies include:

    • Shells or tests made of calcium carbonate found in Foraminifera.

    • Euglena, an Excavata type, possess a protein pellicle.

    • Internal support structures consist of microtubule networks in Parabasalids.

AUTOTROPHIC NUTRITION

  • Photosynthesis as a primary means of nutrient acquisition.

  • Example Organisms: Spirogyra, Volvox

HETEROTROPHIC NUTRITION

  • Mechanisms include:

    • Phagocytosis for ingestion of food.

    • Absorption of food from the surrounding environment.

    • Methods of parasitism and saprophytism as forms of nutrient acquisition.

    • Example organisms depicted include Oomycetes.

MOTILITY

  • Types of motility observed in protists:

    • Amoeboid motion via pseudopodia (e.g., Chaos carolinensis).

    • Swimming motion via flagella.

    • Swimming motion via cilia.

LIFE CYCLES

  • Reproductive strategies include:

    • Asexual reproduction via mitotic division is common.

    • Sexual reproduction, which involves meiotic division, is exclusive to eukaryotes.

Reproductive Forms

  • Gametophyte:

    • A multicellular haploid form producing haploid gametes through mitosis.

  • Sporophyte:

    • A multicellular diploid form producing haploid spores via meiosis.

ALTERNATION OF GENERATIONS

  • Meiosis occurs in specialized structures:

    • Male and female gametophytes are separate.

  • Cycle represents the transition between haploid (n) and diploid (2n) forms:

    • Spores (n) → Gametophytes (n) → Fertilization → Zygote (2n).

TAXONOMY OF PROTISTS: EXCAVATA

  • Characterized by an “excavated” feeding groove.

  • Diplomonads:

    • Endosymbionts of animals that rely on fermentation for ATP.

    • Example: Giardia lamblia, which causes severe diarrhea.

EXCAVATA: EUGLENOIDS

  • Primarily freshwater unicellular organisms with:

    • Flexible pellicle instead of a rigid cell wall.

    • Chloroplasts surrounded by three membranes, unlike two in other eukaryotes.

TAXONOMY OF PROTISTS: RHIZARIA

  • Comprises foraminiferans and radiolarians.

    • Both possess a skeleton known as a test with pores for pseudopodia.

RHIZARIA: FORAMNIFERA AND RADIOLARIA

  • Example: Ammonia tepida – a foraminiferan known for threadlike pseudopodia.

  • Fossilized radiolarian shells contribute to geological records.

CHROMALVEOLATA: CILIATES

  • Among the most complex protozoans characterized by:

    • Hundreds of cilia that function in coordinated movement.

    • Sexual reproduction through conjugation involving exchange of haploid nuclei.

CHROMALVEOLATA: DINOFLAGELLATES

  • Dinoflagellates are significant for their symbiotic relationships and impacts on marine life:

    • Example: Zooxanthellae live in corals providing nutrients.

    • “Red tides” resulting in mass fish kills due to neurotoxins produced by blooms.

SUPERGROUP CHROMALVEOLATA: APICOMPLEXANS

  • Notable for being significant human parasites

    • Plasmodium spp. is responsible for malaria, transmitted via Anopheles mosquitoes.

    • Toxoplasma linked to birth defects and neurodevelopmental issues.

MALARIAL INFECTION CYCLE

  1. Plasmodium cell type is present in mosquito saliva when it bites a human host.

  2. The life cycle includes multiple stages:

    • Infection of liver (diploid stage).

    • Production of male and female gametes leading to fertilization.

    • Development of zygote and subsequent life stages within mosquito hosts, complete with mitotic and meiotic divisions.

SUPERGROUP CHROMALVEOLATA: STRAMENOPILA

  • Characterized by:

    • Morphological diversity including parasitic, saprophytic, and photosynthetic groups.

    • Flagella with tiny hollow “hairs.”

STRAMENOPILA: DETAILS

  • Brown algae found predominantly in warm waters, some thriving at depths exceeding 70 m.

  • Water molds, most being saprophytic, with some acting as parasites (e.g., Saprolegnia, Phytophthora).

  • Diatoms deemed essential marine primary producers with silica shells utilized in diatomaceous earth.

TAXONOMY OF PROTISTS: ARCHAEPLASTIDS

  • Distinguished by the presence of chloroplasts with two membranes.

  • Include:

    • Red algae: contain phycoerythrin which absorbs blue light.

    • Green algae: serve as precursors to terrestrial plants.

  • Land plants are descendants of green algae.

TAXONOMY OF PROTISTS: UNIKONTA

  • Includes groups such as:

    • Amoebozoa

    • Choanoflagellates

    • Kingdom Fungi

    • Kingdom Animalia

UNIKONTA: CHOANOFLAGELLATES

  • Unicellular organisms closely related to sponges.

  • Known for structures called choanocytes.

  • Examples include Codonosiga and Proterospongia.

UNIKONTA: AMOEBOZOA

  • Pseudopods are formed through cytoplasmic streaming, facilitating movement and food ingestion via phagocytosis.

  • Significant species include:

    • Entamoeba histolytica

    • Amoeba proteus

SLIME MOLDS

  • Plasmodial Slime Molds:

    • Characterized by a body in the form of a plasmodium, a multinucleated cell.

    • Eventually produces sporangium for spore production.

  • Cellular Slime Molds:

    • Exist as individual amoeboid cells.

LEARNING OBJECTIVES

  • Explain the endosymbiotic theory.

  • Describe cellular structures found in protists.

  • Summarize metabolic and locomotive diversity within protists.

  • Describe protists' life cycles.

  • Identify the five current supergroups of eukaryotes with representative protists in each group.

  • Recognize distinguishing features of protists across the six supergroups.

  • Describe the roles of pathogenic protists and their ecological importance.

ATTRIBUTIONS

  • Presentation licensed by Tamara Muldrow, Germanna Community College, under Creative Commons license CC BY NC SA.

  • Figures referenced from OpenStax Biology2e, CC BY NC SA 4.0, Mary Ann Clark, Matthew Douglas, Jung Choi; (https://openstax.org/details/books/biology-2e?Book%20details)

  • Additional images and examples acquired from respective sources including the CDC.