Lecture 3.2 - Protists

Learning Objectives

  • Describe some of the key characteristics of protists.
  • Give 3 examples of protists.
  • Explain the key roles protists play in the environment.

What Are Protists?

  • Diversity: Protists are a very diverse group of organisms.
  • Defining Characteristics:
    • They are eukaryotes, meaning they have a nucleus.
    • This group includes all eukaryotes that are not land plants, fungi, or animals.
    • Most protists are unicellular and very small.

Discovery of Protists

  • Historical Perspective:
    • Discovered by Antonie van Leeuwenhoek, who noted the abundance of living creatures in a drop of water.
    • Examples include Euglena, a common protist found in pond water.

Types of Protists

  • Cell Structure:
    • Most protists are unicellular, but some form colonies or simple multicellular structures.
  • Nutritional Diversity:
    • Photoautotrophs: Contain chloroplasts (e.g., algae).
    • Heterotrophs: Absorb organic molecules or ingest larger particles.
    • Mixotrophs: Combine photosynthesis with heterotrophic nutrition.
  • Reproductive Strategies:
    • Can reproduce sexually or asexually (e.g., via fission).

Protist Phylogeny

  • Still a work in progress with four main clades:
    • Excavata
    • SAR (Stramenopiles, Alveolates, Rhizarians)
    • Archaeplastida
    • Unikonta

Clade Excavata

  • Characteristics:
    • Monophyletic group, some have an excavated feeding groove.
    • Nutritional modes: predatory, photosynthetic, mixotrophic, and parasitic.
  • Example: Giardia intestinalis
    • A flagellated unicellular eukaryote that is a parasite in the human intestine.
  • Transmission:
    • Spread through contaminated water and is resistant to chlorine.

Symptoms and Prevention of Giardia

  • Symptoms may appear 1-3 weeks post-infection, lasting 1-2 weeks or more:
    • Diarrhea
    • Gas
    • Greasy, floating stools
    • Abdominal cramps, nausea/vomiting, dehydration.
  • Prevention includes avoiding contaminated water.

Clade SAR

  • Diversity:
    • Monophyletic group characterized by DNA similarities.
    • Includes diatoms, dinoflagellates, brown algae, and more.

Stramenopiles

  • Key Organisms:
    • Includes diatoms, brown algae, and important pathogens.
  • Diatoms:
    • Unicellular photosynthetic algae with a glass-like silica wall.
    • Important in marine productivity and carbon fixation (~20% globally).

Brown Algae

  • Characteristics:
    • Multicellular and primarily marine; largest and most complex algae.
    • Have chlorophyll-a, chlorophyll-c, and carotenoid pigments; store carbohydrates as laminarin.
Brown Algae Lifecycle: Alternation of Generations
  1. Sporophyte (2n) grows from zygote.
  2. Spores produced by meiosis develop into gametophytes (n).
  3. Male gametophytes release sperm; female gametophytes produce eggs, enhancing fertilization chances.

Alveolates

  • Defined by: Membrane-enclosed sacs (alveoli) beneath the plasma membrane.
  • Examples:
    • Dinoflagellates: Marine photosynthetic organisms; important in phytoplankton; can produce toxic blooms (red tides).
    • Apicomplexans: Nearly all are animal parasites, e.g., Plasmodium (causes malaria).

Malaria and Plasmodium

  • In 2015, 210 million cases worldwide, with significant mortality in sub-Saharan Africa.
  • Symptoms include fever, headache, and chills; serious if untreated.
  • Control strategies include mosquito nets and spraying.

Clade Archaeplastida

  • Contains red and green algae, and the ancestors of land plants.

Rhodophyta (Red Algae)

  • Mostly marine; possess chlorophyll-a and phycoerythrin.
  • Used in sushi and food stabilizers.

Chlorophyta (Green Algae)

  • Most plant-like algae, commonly found in freshwater; studied for biotechnology.

Clade Unikonta

  • Includes protists closely related to fungi and animals, alongside fungi and animals themselves.

Amoebozoans

  • Example: Slime molds, can be plasmodial or cellular.
  • Plasmodial slime molds form a multinucleate mass and produce spores in stressful conditions.

Role of Protists in the Environment

  • Symbiotic Relationships:
    • Mutualistic relationships benefit both species.
    • Example: Coral relies on zooxanthellae (a dinoflagellate) for energy while offering protection.
  • Photosynthesis:
    • Phytoplankton contribute ~30% of global photosynthesis.
  • Parasitic Roles:
    • Malaria and other diseases demonstrate the impact of parasitic protists on human health.
    • Pathogens like Phytophthora significantly affect agriculture and ecosystem health.

Summary of Learning

  • Protists are a diverse group including various types of eukaryotes.
  • Phylogenetically categorized into four main clades:
    • Excavata, SAR, Archaeplastida, and Unikonta.
  • Their ecological roles encompass symbiosis, photosynthesis, and parasitism, highlighting their importance in various ecosystems and environments.

Recommended Readings

  • Campbell's Biology, 12th Ed., Chapter 28: Protists.