Unicellular Eukaryotes (Protozoans)

Unicellular Eukaryotes

Origins of Eukaryotes

  • First evidence for life approximately 3.5 billion years ago (bya)
  • Initial organisms identified as prokaryotes
  • Eukaryotes arose by symbiogenesis (also known as endosymbiosis)
    • Process whereby one cell engulfs another cell
    • Alpha-protobacterium: Derived energy from carbon compounds and became mitochondria
    • Cyanobacterium: Derived energy from sunlight and became plastid, specifically chloroplast

Protozoan Definition

  • Protozoan: Literally means "first animal-like organisms in time"
    • Although not classified as animals, protozoans exhibit animal-like features
    • Characteristic features include:
    • Lack of a cell wall
    • At least one motile stage present in their life cycle

Diversity of Protozoans

  • Approximately 64,000 named species (Hickman et al. 2017)
  • Estimates suggest there could be as many as 250,000 species
  • The difficulties in classification arise from revisions and the concepts of species.

General Characteristics of Protozoans

  • Many species exhibit symbiotic relationships with other species:
    • Can be mutualistic, commensalistic, or parasitic
  • Movement:
    • Previously used as a classification method, now outdated
    • Modes of movement include:
    • Flagella: Generally few in number, long, and used to propel water parallel to the axis of the flagella
    • Cilia: Present in large numbers, short, used to propel water parallel to the surface on which they are attached
    • Pseudopodia: Extensions of the cytoplasm utilized for movement
  • Protozoans do not possess germ layers, tissues, or organs.

Microstructure of Cilia and Flagella

  • Structure characterized by a 9+2 arrangement of microtubules (known as axoneme)
    • Where the axoneme enters the body, there is a structure called the kinetosome, which consists of 9 triplets of microtubules.
    • Kinetosome structures are identical to centrioles that organize the mitotic spindle during cell division.

Functional Components of Protozoans

  • Nucleus: Contains genetic material organized as chromosomes; communicates with other cellular components via pores; can include nucleoli; ciliates may have two nuclei (macro and micro)
  • Mitochondria: Function to recover energy from carbon bonds
  • Golgi complex: Involved in intracellular digestion
  • Plastids: Involved in photosynthesis

Nutrition in Protozoans

  • Phagocytosis:
    • Mechanism where a food particle is enveloped by the cell, forming a food vacuole
    • Lysosomes fuse with the vacuole to secrete digestive enzymes
    • Final products of digestion are absorbed across the vacuole membrane
    • In certain groups, the site of phagocytosis may be fixed, while in others it is not.
  • Saprozoic feeding: Involves absorption of nutrients from the environment.

Homeostatic Functions and Structures

  • Protozoans do not possess a circulatory, respiratory, or excretory system
  • All necessary exchanges occur via diffusion
  • Maintain no specific temperature regulation
  • Osmoregulation is managed through contractile vacuoles.

Reproduction in Protozoans

  • Mostly undergo sexual reproduction, without embryological development
    • Mechanisms of sexual reproduction include syngamy and autogamy
  • Asexual reproduction is also common:
    • Binary fission: A single organism divides into two
    • Schizogony: Multiple fissions occur simultaneously
    • Budding: A new organism develops from an outgrowth of the parent.

Phylum Parabasala

  • Comprises approximately 400 species
    • Diagnosis based on the presence of the axostyle
    • Characterized by many flagella
    • Includes medically and veterinary important species such as:
    • Trichomonas vaginalis: Causes sexually transmitted disease (STD) in humans
    • Tritrichomonas foetus: Causes infertility and spontaneous abortion in cattle
    • Trichomonas gallinae: Affects pigeons/doves and other bird species
    • Infection by Trichomonas gallinae can be fatal to young raptors.

Phylum Euglenozoa

  • Presence of a pellicle
  • Divided into two subphyla:
    • Euglenida: Free-living organisms; possess chloroplasts that contain chlorophyll b; inhabit freshwater; generally have a single long flagellum; often exhibit a red stigma (eyespot)
    • Typically autotrophic but can switch to saprozoic if deprived of light
    • Kinetoplasta: Parasitic organisms
    • Use flagella and an undulating membrane for locomotion
    • Notable parasites include:
      • Trypanosoma brucei: Causes African sleeping sickness, transmitted by the tsetse fly
      • Trypanosoma cruzi: Causes Chagas disease, transmitted by Triatoma
      • Leishmania spp.: Transmitted by sand flies, causes leishmaniasis

Phylum Ciliophora

  • Many species are entirely covered in cilia, while others have limited cilia around the oral region
  • Generally larger than other protozoans
  • Varied relationships include free-living, commensal, or parasitic lifestyles
  • Display great variation in body form
  • Always possess multiple nuclei (macro and micronucleus)
  • May have specialized structures such as trichocysts or toxocysts for defense or offense.

Phylum Dinoflagellata

  • Important primary producers, especially in marine environments; about half are autotrophic
  • Characterized by two flagella located in grooves across their body
  • Body typically covered in armored plates
  • Engage in mutualism with corals as zooxanthellae; corals depend on this relationship for health

Bioluminescent and Toxin Production

  • Some dinoflagellates display bioluminescence
  • Others produce potent toxins leading to ecological phenomena such as red tides, which can be detrimental to fish populations.

Phylum Apicomplexa

  • Almost exclusively endoparasitic
  • Characterized by the presence of a unique structure known as the apical complex, hypothesized to assist in penetrating host cells and tissues
  • Less is known about locomotor organelles in this phylum; developing pseudopodia present in certain reproductive stages
  • Plasmodium spp.: Notable genus responsible for malaria in humans.
    • Infection cycle stages include: ( merozoite, exo-erythrocytic cycle, and erythrocytic cycle ).

Phylum Foraminifera

  • Composed of testate amoebas, with pseudopodia known as reticulopodia
  • Most species inhabit benthic environments
  • Tests (shells) are often chambered and primarily composed of calcium carbonate
  • Dead individuals contribute significantly to ocean floor ooze, commonly forming limestone and chalk deposits.

Phylum "Radiolaria"

  • Testate amoebas with pseudopodia referred to as axopodia
  • Commonly inhabit surface waters
  • Tests may be made of silica, forming into chert (a type of flint).

Phylum Viridiplantae

  • Belongs to the clade Plantae, encompassing green plants and algae
  • Can be unicellular, colonial, or multicellular
    • Multicellular forms imply cellular division of labor
  • All members are photosynthetic
  • Use flagella for movement; cells may be either zooids or daughter colonies.

Phylum Amoebozoa

  • Includes both naked and testate (shelled) forms
  • Important disease-causing species include:
    • Acanthamoeba castellanii: Can be lethal to corneal cells, often transmitted via improperly disinfected contact lenses
    • Entamoeba histolytica: Causes amoebic dysentery
    • Naegleria fowleri: Known as the brain-eating amoeba, highly pathogenic to humans.

Phylum Opisthokonta

  • This clade includes animals, fungi, and certain unicellular eukaryotes
  • Choanoflagellates: Can be solitary or colonial; their morphology is similar to the collar cells (choanocytes) found in sponges
    • This similarity leads to the hypothesis that choanoflagellates are closely related to animals, possibly acting as a sister group.