Protists

Introduction to Protists

  • Protists are lesser-known eukaryotes characterized by their diversity and ecological significance.

Phylogeny of Protists

  • Protists have an exceptionally diverse phylogeny.

  • Important question: Do they form a clade?

    • Protists are often viewed as leftovers that do not fit into the kingdoms of plants, animals, and fungi.

History and Characteristics of Protists

  • Multicellularity and Horizontal Gene Transfer:

    • Exploration of the history of multicellularity in the context of eukaryotic evolution.

    • Critical consideration of whether these traits represent homologous traits or convergent evolution.

Types of Protists

  • Acknowledgment of diversity with examples.

  • Encouragement for active recall by listing known protists.

  • Common examples include:

    • Paramecium

    • Amoeba

    • Volvox (green algae)

    • Kelp (giant seaweed)

    • Plasmodium (malaria-causing parasite)

Classification of Protists

  • Protists can be classified based on:

    • Cellularity:

    • Unicellular (e.g., Paramecium, Amoeba, Plasmodium)

    • Multicellular (e.g., Kelp)

    • Colonial (e.g., Volvox)

    • Modes of Nutrition:

    • Autotrophic (photosynthetic like Volvox)

    • Heterotrophic (e.g., Plasmodium)

      • Some protists are predators.

  • Reproductive Modes:

    • Protists conduct both sexual and asexual reproduction, or both.

Habitat of Protists

  • Protists inhabit various environments, including:

    • Freshwater

    • Marine habitats

    • Soil

    • Snow (e.g., Chlamydomonas associated with 'watermelon snow')

Taxonomic Considerations

  • Previous classification of protists as a kingdom needs reevaluation.

  • Protists do not fit as a true clade based on phylogenetic analysis.

Complexities of Determining Evolutionary Relationships

  • Convergent Evolution:

    • Results in analogous structures that are superficially similar due to independent evolution.

  • Endosymbiosis/Horizontally Gene Transfer:

    • Genes entering organisms from entirely different sources, complicating phylogenetic relationships.

Multicellularity in Eukaryotic Evolution

  • Multicellularity evolved independently multiple times (around 25 times) across eukaryotic lineages.

  • Notable multicellular groups:

    • Land plants

    • Fungi

    • Animals

    • Brown Algae

    • Specifically highlights the seaweed.

Understanding Horizontal Gene Transfer Events

  • Key events include:

    • Primary Endosymbiosis:

    • Mitochondria evolution via engulfing aerobic bacteria.

    • Chloroplast acquisition via engulfing cyanobacteria in Archaeplastida.

    • Secondary Endosymbiosis:

    • Red algae integrated into groups like Chromalveolata, Euglenophyta, and others.

  • Understanding this process highlights how different eukaryotes acquired photosynthetic abilities.

Spotlight on Specific Protists

1. Slime Molds (Amoebozoa)

  • Characteristics:

    • Heterotrophic.

    • Unique life cycles and forms, including multinucleated cells.

  • Life Cycle Overview:

    • Mitosis: Results in a multinucleated feeding structure (plasmodium).

    • Fertilization: Forms diploid zygote, undergoes mitosis without cytokinesis, leading to a plasmodium with multiple nuclei.

    • Sporangium Formation: Produces haploid spores after meiosis.

2. Dinoflagellates (Chromalveolata)

  • Characteristics:

    • Unicellular, photosynthetic.

    • Form symbiotic relationships with coral (zooxanthellae).

  • Ecological Role:

    • Contributes to coral health; coral reefs depend on these dinoflagellates for additional nutritional support through photosynthesis.

    • Coral bleaching occurs when symbiosis is disrupted, leading to coral death.

3. Diatoms (Chromalveolata)

  • Characteristics:

    • Unicellular with silica dioxide walls.

    • Key players in oceanic photosynthesis; photosynthetic through secondary endosymbiosis of red algae.

  • Role in Carbon Cycle:

    • Capture carbon dioxide and can sequester it via sedimentation on ocean floors.

    • Associated with iron fertilization experiments aimed at increasing phytoplankton growth for carbon sequestration.

    • Historically, led to unfavourable results and cessation of such experiments out of concerns for efficiency and ecological cost.

Conclusion

  • Summary of discussed topics:

    • Diversity within protists and lack of clade formation.

    • Evolution of multicellularity as a convergent characteristic.

    • Understanding eukaryotic evolution through horizontal gene transfer and unique examples of slime molds, dinoflagellates, and diatoms.