Protists: Diversity, Evolution, and Biological Roles

CHAPTER 28: PROTISTS

Introduction

Protists are a diverse group of mostly unicellular eukaryotic organisms that play crucial roles in ecological systems and evolutionary history. This chapter explores their classification, evolutionary relationships, biological complexity, and ecological significance.

Three Domains of Life

Overview of Domains

Life is classified into three domains: Bacteria, Archaea, and Eukarya.

• Bacteria and Archaea are prokaryotic, lacking a nucleus and membrane-bound organelles.

• Eukarya includes all eukaryotic organisms, characterized by cells with a nucleus and organelles.

• The common ancestor of Eukarya gave rise to a monophyletic group containing single- and multicellular organisms.

Kingdom Protista

• Historically, 'Kingdom Protista' included all eukaryotes that are not land plants, fungi, or animals.

• Protists are not a monophyletic clade; they represent a grade of complexity rather than a true evolutionary lineage.

• Multicellularity has arisen independently in several eukaryotic lineages.

Protists: A "Grade", Not a Clade

Characteristics

• Unicellularity is ancestral for Eukarya and does not inform evolutionary relationships.

• Protists are generally more complex than prokaryotes, with some trending toward multicellularity.

• Eukaryotes arose about 2.1 billion years ago; multicellularity appeared later (~1.4 billion years ago).

Protists Are Complex and Diverse

Cellular Features

• Most protists are single-celled, but some are colonial or multicellular.

• Eukaryotic cells with organelles (nucleus, mitochondria, endoplasmic reticulum, etc.).

• Protists exhibit various modes of nutrition:

  • Autotrophs: photosynthetic, use light energy to fix inorganic CO2.

  • Heterotrophs: ingest other organisms or absorb dissolved molecules.

  • Mixotrophs: combine photosynthesis and heterotrophy.

Six Main Eukaryote Lineages

Major Clades

• Excavata

• Stramenopiles + Alveolates + Rhizaria = SAR

• Archaeplastida (includes plants)

• Unikonta (includes fungi and animals)

Molecular evidence (especially DNA analysis) is crucial for understanding eukaryotic evolution and relationships.

Excavates Are Mostly Parasitic

Groups and Features

• Diplomonads and Parabasalids: reduced mitochondria, often parasitic (e.g., Giardia, Trichomonas).

• Euglenozoans: unique flagella; includes free-living mixotrophs (Euglena) and parasitic kinetoplastids (Trypanosoma).

Alveolates Share Similar Cytoskeletons

Subgroups

• Ciliates: covered in cilia, mostly free-living (e.g., Paramecium).

• Apicomplexans: parasites with complex life cycles (e.g., Plasmodium, causes malaria).

• Dinoflagellates: covered in cellulose plates, some cause harmful algal blooms and neurotoxic shellfish poisoning (Red Tide).

Stramenopiles Have Characteristic Flagella

Major Lineages

• Diatoms: photosynthetic algae with silica cell walls.

• Oomycetes: lost photosynthetic plastids, resemble fungi, decomposers and parasites.

• Brown algae: multicellular 'seaweeds' and kelp.

Rhizarians: Amoeboid Protists

Groups

• Foraminiferans: calcium carbonate shells, planktonic.

• Radiolarians: silica shells, planktonic.

• Cercozoans: mostly heterotrophs, some mixotrophic, common in marine, freshwater, and soil environments.

Archaeplastida: Green Algae, Land Plants, and Red Algae

Key Groups

• Red algae: multicellular, accessory pigments.

• Chlorophytes: unicellular, colonial, and multicellular green algae.

• Charophyceans and Land Plants: closely related, covered in Chapter 29.

Unikonta: Amoeba, Fungi, and Animals

Major Groups

• Amoebozoans: amoebas with lobe-shaped pseudopodia.

• Opisthokonts: includes fungi and animals.

Trends in Eukaryote Evolution

Repeated Traits

• Some traits arose once (e.g., chloroplasts via primary endosymbiosis).

• Others arose multiple times (convergent evolution):

  • Secondary endosymbiosis (photosynthesis)

  • Multicellularity

  • Parasitism

Complex Life Cycles & Multicellularity

Life Cycle Features

• Alternation of generations: complex cycles with sexual and asexual phases.

• Multicellularity evolved independently in several protist groups.

• Cell differentiation and specialization are key for multicellular organization.

Ecological Roles of Protists

Importance in Ecosystems

• Producers: photosynthetic protists contribute ~25% of Earth's photosynthesis.

• Key in marine environments and mutualistic relationships (e.g., dinoflagellates in corals, symbionts in termite guts).

• Types of symbioses: mutualism (+/+), parasitism (+/-).

Important Protist Pathogens

Human Diseases

• Giardia (Diplomonad): causes giardiasis.

• Trypanosoma (Euglenozoan): causes African Sleeping Sickness.

• Plasmodium (Apicomplexan): causes malaria.

• Protist infections are difficult to treat due to cellular machinery similar to human cells.

Summary Table: Major Protist Groups and Features

Group	Main Features	Examples
Excavata	Flagellated, often parasitic, reduced mitochondria	Giardia, Trichomonas, Euglena
Alveolates	Alveoli under membrane, diverse nutrition	Paramecium, Plasmodium, Dinoflagellates
Stramenopiles	Hairy flagella, photosynthetic or heterotrophic	Diatoms, Brown algae, Oomycetes
Rhizarians	Amoeboid, shells of CaCO3 or silica	Foraminiferans, Radiolarians
Archaeplastida	Photosynthetic, includes land plants	Red algae, Green algae, Land plants
Unikonta	Amoebas, fungi, animals	Amoebozoans, Opisthokonts

Additional info:

• Protists are essential for understanding the evolution of eukaryotes and the origins of multicellularity.

• They play vital roles in global ecological cycles and human health.