Lecture 25: Protists

Overview

• Protists are mostly unicellular eukaryotes, but some are colonial and multicellular.
• Classification has evolved significantly; protists form a polyphyletic group and are no longer a valid kingdom.
• Protists exhibit extensive structural and functional diversity.

Nutritional Diversity

• Photoautotrophs: contain chloroplasts.
• Heterotrophs: absorb organic molecules or ingest larger food particles.
• Mixotrophs: combine photosynthesis and heterotrophic nutrition.
• Reproduction varies: asexual, sexual, or via meiosis and fertilization.

Endosymbiosis and Evolution

• Much protist diversity originates from endosymbiosis.
• Mitochondria and plastids evolved from engulfed prokaryotes.
• Endosymbiotic events: mitochondria from alpha proteobacterium; plastids from cyanobacteria.
• Secondary endosymbiosis: red and green algae were ingested by heterotrophic eukaryotes.
  • Ex: chlorarachniophytes evolved from engulfing green alga.

Protist Classification by Movement

• Ingestive (animal-like): protozoa.
• Photosynthetic (plant-like): algae.
• Absorptive (fungus-like).
• Movement methods:
  • Sarcodinians: pseudopods, e.g., amebas.
  • Zooflagellates: flagella, e.g., trichonympha.
  • Ciliophorans: cilia, e.g., paramecium.
  • Sporozoans (Apicomplexans): lack movement structures; e.g., plasmodium causes malaria.

Apicomplexans and Malaria

• Caused by Plasmodium species, carried by Anopheles mosquito.
• Millions affected annually; preventable and treatable.
• Life-cycle involves multiple hosts (mosquitoes and humans).

Protist Classification by Structure

• Unicellular: e.g., dinoflagellates, diatoms, euglenoids.
• Multicellular: classified by pigments and storage compounds, e.g., green, red, and brown algae.

Slime Molds and Water Molds

• Plasmodial Slime Molds: have feeding and reproductive stages.
• Cellular Slime Molds: single cells form pseudoplasmodium for reproduction.
• Water Molds (Oomycetes): decomposers, some are parasitic.

Eukaryotic Supergroups

• Excavata: modified mitochondria, unique flagella; includes diplomonads, parabasalids, euglenozoans.
• SAR Clade: stramenopiles (diatoms, algae), alveolates, rhizarians.
• Archaeplastida: red algae, green algae, land plants.
• Unikonta: includes amoebozoans (slime molds) and opisthokonts.

Protists' Ecological Roles

• Protists are symbionts and producers.
• Symbiotic relationships include coral reef building and termite digestion.
• Parasites like Plasmodium cause diseases such as malaria.
• Many protists are primary producers in ecological food webs.