Lecture Notes on Protists and Their Classification

The Protists

Overview of Protists

• Protists are a diverse group of eukaryotic organisms, which can be either unicellular or multicellular.

• The eukaryotic domains are structured as follows:

  • Animalia (Multicellular, eukaryotic)

  • Plantae (Multicellular, eukaryotic)

  • Fungi (Multicellular, eukaryotic)

  • Protista (Eukaryotic, unicellular and multicellular)

  • Archaeabacteria (Unicellular, prokaryotic)

  • Eubacteria (Unicellular, prokaryotic)

Classification of Protists

Group 1: Excavata - The Most Ancient Protists

• Characteristics:

  • Unicellular

  • Heterotrophic

  • Possess flagella and pellicle

  • Many members are parasitic

I. Diplomonads

• Features:

  • Modified mitochondria known as mitosomes for anaerobic metabolism

  • Each cell has two equal-sized nuclei

  • Presence of multiple flagella facilitating movement

• Example:

  • Giardia lamblia, which causes giardiasis in animals.

II. Parabasalids

• Features:

  • Reduced mitochondria known as hydrogenosomes for anaerobic metabolism

• Example:

  • Trichomonas vaginalis, which causes trichomoniasis in animals (sexually transmitted disease).

III. Euglenozoans

• Habitat: Found in freshwater ponds and lakes.

• Special Features:

  • Contractile vacuole for osmoregulation

  • Eyespot, one of the first sensory organs developed in protists.

  • Approximately half are autotrophs and half are heterotrophs.

Structure of a Euglenoid

• Euglenoids display a more advanced cellular structure compared to the earliest Excavata.

• Key cellular components include:

  • Long flagellum for movement.

  • Chloroplasts for photosynthesis.

  • Contractile vacuole for expelling excess water.

  • Mitochondrion for ATP production.

  • Eyespot for light detection.

  • Pellicle providing structural support.

  • Endoplasmic reticulum and Golgi body involved in protein synthesis and processing.

Group 2: Chromalveolata

First Branch: Alveolates

• Common Characteristic:

  • All possess tiny membrane-bound sacs called alveoli beneath their outer membranes, which help stabilize the cell surface.

I. Ciliates

• Habitat: Found in freshwater, saltwater, or soil ecosystems.

• Locomotion: Use cilia for movement.

• Feeding: Primarily prey on bacteria and algae.

• Example: Paramecium.

• Nuclear Structure:

  • Ciliates have two types of nuclei:

  • Micronucleus: germ line nucleus that does not express its genes.

  • Macronucleus: provides the nuclear RNA for vegetative growth.

• Defense Mechanism:

  • Paramecia possess trichocysts, which are long, thread-like structures that can be rapidly discharged as a defensive or predatory response to stimuli, immobilizing potential threats.

II. Dinoflagellates

• Ecological Role: Serve as key primary producers in freshwater and marine habitats.

• Feeding Habits: Some dinoflagellates are heterotrophic and can consume bacteria or algae; others are parasitic.

• Bioluminescence: Some types exhibit bioluminescence.

• Cell Structure:

  • Reinforced by cellulose plates and feature two flagella allowing a spinning motion.

• Eutrophication:

  • Can lead to runaway populations, resulting in algal blooms, primarily due to excess nutrient availability in water.

• Impact of Algal Blooms:

  • Can cause "red tides" that lead to reduced oxygen levels, potentially suffocating marine animals and producing toxins (brevotoxins) that pose health risks to various species including fish and marine mammals.

III. Apicomplexans

• Characteristics:

  • Parasitic alveolates that possess a unique microtubular device designed for attachment to and penetration of host cells.

• Example: Plasmodium species which are responsible for malaria.

Malaria

• Global Prevalence:

  • Approximately 40% of the world resides in malaria-prone areas.

  • Kills about 2 individuals every minute, with over 80% of victims being children under the age of 5 in Africa.

Distribution of Malaria

• Current & Future Distribution:

  • Current maps indicate the extent of Plasmodium falciparum's distribution. Projections for 2050 indicate possible expansions influenced by climate suitability.

• Source: Rogers, Randolph. "The Global Spread of Malaria in a Future, Warmer World." Science.

Group 3: Rhizaria

• Subgroups:

  • I. Cercozoans: Some amoebae types;

  • II. Radiolarians: These organisms have internal skeletons made of silica;

  • III. Foraminifera: Found in marine and freshwater environments, characterized by porous shells.

Group 4: Archaeplastida

I. Rhodophyta (Red Algae)

• Diversity: Nearly 6,000 species found primarily in warm marine currents and tropical seas.

• Key Characteristics:

  • Possess red accessory pigments (phycoerythrin).

  • Most species are multicellular and show branching growth patterns.

  • Lack true tissues.

II. Chlorophyta and III. Charophyta (Green Algae)

• Similarity to Land Plants:

  • Resemble land plants closely.

  • Store sugars as starch and have cell walls containing cellulose and are photosynthetic.

• Evolutionary Consideration: Some green algae are more closely related to land plants than to each other.

Group 5: Fungi-Like Protists - Amoebozoans

• Habitat: Typically found in temperate regions.

• Feeding: Heterotrophic lifestyle is common.

• Adaptative Features:

  • Exhibit unique life cycles, including forms similar to slime molds.

  • Move via pseudopodia and are closely related to fungi.

• Life Cycle: Fascinating adaptations to environmental conditions make them a focus of study.