Chapter+28-+Sections+28.5+and+28.6

Chapter 28: Protists

Section 28.5: Unikonts

  • Unikonts include protists closely related to fungi and animals.

Supergroup Unikonta

  • Comprises animals, fungi, and some protists.

  • Includes two clades:

    • Amoebozoans

    • Opisthokonts (animals, fungi, and related protists).

Eukaryotic Tree Controversy

  • The root of the eukaryotic tree is controversial.

  • One hypothesis is that unikonts were the first to diverge from other eukaryote groups.

Amoebozoans

  • Characterized by lobe- or tube-shaped pseudopodia rather than threadlike.

  • Includes:

    • Tubulinids

    • Slime molds

    • Entamoebas.

Tubulinids

  • Diverse group of amoebozoans with lobe- or tube-shaped pseudopodia.

  • Common in soil and aquatic environments (freshwater and marine).

  • Most are heterotrophic, preying on bacteria and other protists.

  • Example: Amoeba proteus.

Slime Molds (Mycetozoans)

  • Once categorized as fungi due to spore-producing fruiting bodies.

  • Resemblance to fungi is a result of convergent evolution.

  • Two lineages of slime molds:

    • Plasmodial slime molds

    • Cellular slime molds.

Plasmodial Slime Molds

  • Often brightly pigmented (yellow or orange).

  • Form a large feeding mass called a plasmodium during one life stage (not to be confused with malarial Plasmodium).

Plasmodium Structure

  • A single "super cell" containing multiple diploid nuclei, not divided by membranes.

  • Forms a fruiting body for sexual reproduction under unfavorable conditions.

Cellular Slime Molds

  • Form multicellular aggregates with cells separated by membranes.

  • Solitary cells unite to form a slug-like aggregate when conditions are poor.

  • This aggregation leads to the formation of a fruiting body.

Model Organism

  • Dictyostelium discoideum is used to study the evolution of multicellularity.

Entamoebas

  • Comprises Entamoeba species, which are parasites of vertebrates and invertebrates.

  • Humans host at least six Entamoeba species; E. histolytica is pathogenic, causing amoebic dysentery, a leading cause of death by eukaryotic parasites.

Opisthokonts

  • Group includes animals, fungi, and several protist groups.

Section 28.6: Ecological Roles of Protists

Habitat Diversity

  • Protists live in various aquatic and moist terrestrial environments.

  • Play vital roles as symbionts and producers.

Symbiotic Protists

  • Some protist symbionts benefit hosts:

    • Dinoflagellates nourish reef-building corals.

    • Some inhabit the guts of termites, aiding in wood digestion.

  • Some protist symbionts are parasites:

    • Plasmodium causes malaria in humans.

    • Pfiesteria shumwayae feeds on skin of fish.

    • Phytophthora ramorum causes sudden oak death.

Photosynthetic Protists

  • Crucial producers that convert CO2 to organic compounds using light or inorganic energy.

  • In aquatic ecosystems, they and prokaryotes are primary producers.

  • Other organisms (consumers) depend on them for food.

Ecological Implications

  • Photosynthetic protists are nutrient-limited; nutrient addition can lead to population explosions, causing marine "dead zones."

  • Biomass declines with increasing sea surface temperatures, impacting phytoplankton growth.

  • Warm surface waters can inhibit the upwelling of nutrients essential for phytoplankton.

Global Warming Effects

  • Continued warming may have significant impacts on marine ecosystems, fisheries, and the global carbon cycle.

Overall Groups in Eukaryotes

Supergroup Overview

  • Excavata: Includes diplomonads, parabasalids, and euglenozoans.

  • SAR: Includes stramenopiles and alveolates.

  • Archaeplastida: Includes red algae and green algae.

  • Unikonta: Includes amoebozoans and opisthokonts (animals and fungi).