Diversification of Eukaryotes (Protists): Chapter 27 Study Notes

Diversification of Eukaryotes (Protists)

Chapter Overview

This chapter explores the diversity, ecological roles, evolutionary relationships, and key features of protists, a major group within the domain Eukarya. Protists are essential for understanding the evolution of eukaryotic life and their impact on health and ecosystems.

The Three Domains of Life

Classification of Life

All living organisms are classified into three domains based on cellular structure and evolutionary history:

• Domain Bacteria: Prokaryotic, unicellular, lack membrane-bound nucleus, found almost everywhere.

• Domain Archaea: Prokaryotic, unicellular, lack membrane-bound nucleus, often found in extreme environments, may represent the earliest cells.

• Domain Eukarya: Eukaryotic, unicellular or multicellular, possess a membrane-bound nucleus.

Domains and Kingdoms of Life

Each domain contains multiple kingdoms, representing major evolutionary lineages. The following table summarizes key features:

Domain	Example Organism	Key Features
Archaea	Methanosarcina mazei	Capable of living in extreme environments; methane-generating prokaryote.
Bacteria	Escherichia coli	Structurally simple but metabolically diverse; found in intestinal tracts.
Eukarya: Protists	Euglena	Diverse group of eukaryotes, many single-celled, with both plant and animal-like characteristics.
Eukarya: Plants	Pinus longaeva	Multicellular photosynthesizers; bristlecone pine is one of the oldest organisms.
Eukarya: Animals	Homo sapiens	Multicellular organisms that ingest food.

Taxonomy and Systematics

Organizing the Diversity of Life

Taxonomy is the discipline of naming and classifying organisms according to specific rules. Systematics classifies organisms based on presumed evolutionary relationships.

• Categories of Classification (from most to least inclusive):

  1. Domain

  2. Kingdom

  3. Phylum

  4. Class

  5. Order

  6. Family

  7. Genus

  8. Species

Example: Homo sapiens (humans) belong to the domain Eukarya, kingdom Animalia, phylum Chordata, class Mammalia, order Primates, family Hominidae, genus Homo, species sapiens.

Protists: Introduction and Importance

What are Protists?

Protists are any eukaryotic organisms that are not animals, plants, or fungi. They are highly diverse, ranging from unicellular to multicellular forms, and inhabit various environments.

• Most protists are free-living; some are parasitic.

• Reproduction can be sexual or asexual.

• Size varies from microscopic algae and protozoans to giant kelp.

Medical and Ecological Importance

Protists have significant impacts on human health and ecosystems:

• Human Health:

  • Some protists cause diseases (e.g., Plasmodium causes malaria).

  • Protists can affect crops (e.g., Phytophthora infestans caused the Irish potato famine).

• Ecological Roles:

  • Primary producers in aquatic environments (e.g., diatoms, dinoflagellates).

  • Key players in global carbon cycling and climate regulation.

Examples of Protist Impact

• Malaria: Caused by Plasmodium, transmitted by mosquitoes. Rapid evolution of drug resistance and difficulty in vaccine development.

• Algal Blooms: Rapid growth of toxin-producing protists (e.g., dinoflagellates) can poison humans via shellfish consumption.

Protists in Aquatic Food Chains and the Carbon Cycle

Role in Food Chains

Protists are often at the base of aquatic food chains, serving as primary producers that convert CO2 into organic matter via photosynthesis.

• Marine protists contribute significantly to global carbon fixation.

• Planktonic protists (e.g., diatoms) are abundant in oceans and lakes.

Role in the Carbon Cycle

The carbon cycle involves the movement of carbon among the atmosphere, land, and ocean. Protists act as carbon sinks, storing carbon in sedimentary rocks and petroleum.

• Photosynthesis by protists helps reduce atmospheric CO2.

• Dead protists contribute to long-term carbon storage.

Key Steps in the Carbon Cycle:

• Photosynthesis

• Decomposition

• Respiration

• Combustion

Evolutionary Relationships and Phylogeny

Protist Diversity and Classification

Protists are a paraphyletic group, meaning they do not share a single common ancestor exclusive to them. They lack synapomorphies (shared derived traits) that set them apart from other eukaryotes.

• Protists are ancestral to plants, fungi, and animals.

• Microscopy and direct sequencing help identify protist lineages.

Major Lineages of Protists

Protists are classified into several major groups based on morphology and genetics. Key lineages include:

• Amoebozoa: Lack cell walls, move via amoeboid motion, produce pseudopodia.

• Excavata: Some lack mitochondria, diverse feeding strategies.

• Plantae: Includes green algae, red algae; primary endosymbiosis with cyanobacteria.

• Rhizaria: Single-celled, often with elaborate shells, move via slender pseudopodia.

• Alveolata: Unicellular, diverse morphology, some bioluminescent (e.g., dinoflagellates).

• Stramenopila: Includes diatoms, brown algae; distinctive flagella with hair-like structures.

Cellular Innovations in Protists

Endosymbiosis and the Origin of Mitochondria

The endosymbiosis theory explains the origin of mitochondria as a result of a symbiotic relationship between a bacterial cell and a host eukaryotic cell.

• Mitochondria replicate by fission, have their own ribosomes and circular DNA.

• Double membranes suggest engulfing mechanism.

• Phylogenetic data links mitochondria to alpha-proteobacteria.

Key Terms:

• Symbiosis: Two species living in physical contact.

• Endosymbiosis: One species living inside the cells of another.

Structures for Support and Protection

Protists exhibit diverse structures for support and protection:

• Cell walls

• External shells

• Rigid internal structures

Multicellularity

Multicellularity evolved independently in several protist lineages. It involves cells sticking together after division and specializing for different functions.

• Not all cells express the same genes.

• Selection pressures favored diversification of multicellular forms.

Nutrition and Movement in Protists

How Protists Obtain Food

Protists have evolved various methods for obtaining nutrients:

• Ingestive Feeding: Eating bacteria, archaea, or other protists via phagocytosis.

• Absorptive Feeding: Taking up nutrients directly from the environment; includes decomposers and parasites.

• Photosynthesis: Autotrophic protists produce organic compounds using light energy.

Movement in Protists

Protists move to find food or light using:

• Amoeboid Motion: Sliding movement via pseudopodia, requires ATP.

• Flagella: Long, whip-like structures for swimming.

• Cilia: Short, numerous structures for movement and feeding.

Reproduction in Protists

Sexual and Asexual Reproduction

Protists can reproduce both sexually and asexually:

• Sexual Reproduction: Involves meiosis and fusion of gametes, resulting in genetic diversity.

• Asexual Reproduction: Involves mitosis and cell division, producing genetically identical offspring.

Adaptive Value: Sexual reproduction is favored in changing environments due to increased genetic variation, which helps resist parasites and pathogens.

Summary Table: Key Features of Major Protist Lineages

Lineage	Key Features	Example
Amoebozoa	Lack cell walls, move via pseudopodia, abundant in freshwater, some are parasites	Chaos carinensis
Excavata	Some lack mitochondria, diverse feeding, genes of mitochondrial origin in nuclear genome	Euglena
Plantae	Primary endosymbiosis with cyanobacteria, includes green and red algae	Chlamydomonas
Rhizaria	Single-celled, elaborate shells, move via slender pseudopodia	Foraminifera
Alveolata	Unicellular, diverse morphology, some bioluminescent	Dinoflagellates
Stramenopila	Distinctive flagella with hair-like structures, includes diatoms and brown algae	Brown algae

Additional info: These notes expand on the brief points in the slides, providing definitions, examples, and context for key biological concepts relevant to the study of protists and eukaryotic diversity.