Chapter 28

Chapter 28: Protists

Key Concepts

  • Most eukaryotes are single-celled organisms.

  • Red algae and green algae are the closest relatives of land plants.

  • Unikonts include protists closely related to fungi and animals.

  • The relationships of some protists to other eukaryotes remain uncertain.

  • Protists play key roles in ecological communities.

Paramecium bursaria

  • Paramecium bursaria is a non-photosynthetic ciliate hosting green algae in its cytoplasm.

  • Mutualistic benefits: algae provide photosynthate, while paramecium supplies inorganic nutrients.

Diversity of Protists

  • Protists are a diverse group of eukaryotes spanning all four supergroups: Excavata, SAR, Archaeplastida, and Unikonta.

  • Eukaryotic cells have organelles and are more complex than prokaryotic cells.

  • Most protists are unicellular but exhibit structural and functional diversity.

Nutritional Diversity

  • Photoautotrophs: Contain chloroplasts.

  • Heterotrophs: Absorb organic molecules or ingest food particles.

  • Mixotrophs: Combine photosynthesis and heterotrophic nutrition.

  • Reproduction can be asexual or sexual, involving meiosis and fertilization.

Four Supergroups of Eukaryotes

  • Eukaryotes, including protists, are divided into four supergroups:

    • Excavata: Includes parabasalids, diplomonads, and euglenozoans.

    • SAR Clade: Comprises stramenopiles, alveolates, and rhizarians.

    • Archaeplastida: Includes red algae, green algae, and land plants.

    • Unikonta: Incorporates animals, fungi, and certain protists.

Excavates

  • Characterized by cytoskeleton and an excavated feeding groove.

  • Include modified mitochondria and flagella that differ from other organisms.

  • Includes parasites (Giardia) and many predatory/photosynthetic species.

SAR Clade

  • Diverse group identified by DNA similarities.

  • Includes stramenopiles (diatoms, brown algae, oomycetes), alveolates (dinoflagellates, apicomplexans, ciliates), and rhizarians (forams, cercozoans, radiolarians).

Stramenopiles

  • Important photosynthetic organisms including diatoms and brown algae.

  • Stramenopiles feature a combination of hairy and smooth flagella.

Diatoms

  • Unicellular algae with silicon dioxide walls, major components of phytoplankton.

  • Provide ecological benefits by removing carbon dioxide from the atmosphere when they bloom and die.

Brown Algae

  • The largest and most complex algae, primarily marine; includes seaweeds.

  • Exhibit structures analogous to plants (holdfast, stipe, blades).

Alternation of Generations

  • Multicellular algae display life cycles alternating between multicellular haploid and diploid forms.

  • Diverse structures: Heteromorphic (different) and isomorphic (similar) generations are noted.

Oomycetes

  • Once classified as fungi; differ due to cellulose cell walls instead of chitin.

  • Include water molds and downy mildews.

Alveolates

  • Characterized by membrane-enclosed sacs (alveoli).

  • Includes dinoflagellates, apicomplexans, and ciliates.

Dinoflagellates

  • Display two flagella, reinforced by cellulose plates; significant in marine environments.

  • Some species cause toxic red tides.

Apicomplexans

  • Are animal parasites, e.g., Plasmodium (malaria); require multiple hosts for their life cycles.

  • Approximately 900,000 deaths annually due to malaria.

Ciliates

  • Large group using cilia for movement and feeding; notable for genetic variability via conjugation.

Rhizarians

  • Often amoebas with threadlike pseudopodia; includes radiolarians, forams, and cercozoans.

Foraminiferans (Forams)

  • Characterized by porous shells (tests), significant contributors to the fossil record of marine sediments.

Red and Green Algae

  • Closest relatives of land plants, included in the Archaeplastida subgroup.

Red Algae

  • Multicellular; color results from phycoerythrin pigment. Significant in tropical coastal waters.

Green Algae

  • Share chloroplast characteristics with land plants and include two main groups: chlorophytes and charophyceans.

  • Complex life cycles that involve both sexual and asexual reproduction.

Unikonts

  • Comprised of animals, fungi, and some protists, specifically amoebozoans and opisthokonts.

  • Amoebozoans have lobe- or tube-shaped pseudopodia; include slime molds.

Slime Molds

  • Resemble fungi due to convergent evolution; can be plasmodial or cellular.

Ecological Roles of Protists

  • Protists are vital as producers and symbionts in ecological communities.

  • Photosynthetic protists are primary producers in aquatic environments; their rates of growth and biomass are responsive to nutrient availability.

Impact in Changing Oceans

  • Rising sea temperatures due to global warming can dramatically affect marine ecosystems, impacting fishery yields and the global carbon cycle.