Protists and the Diversity of Eukaryotes

CHAPTER 28: PROTISTS

Introduction to Protists

Protists are a diverse group of mostly unicellular eukaryotic organisms. They are found in nearly every environment that contains water and play essential roles in ecological systems. This chapter explores the classification, diversity, and significance of protists within the broader context of the three domains of life.

Three Domains of Life

Overview of the Domains

• Bacteria: Prokaryotic cells lacking a nucleus and membrane-bound organelles.

• Archaea: Prokaryotic cells, genetically distinct from bacteria, often found in extreme environments.

• Eukarya: Organisms with eukaryotic cells, which have a true nucleus and membrane-bound organelles. Includes protists, plants, fungi, and animals.

Key Point: The three domains of life are distinguished by fundamental differences in cell structure and genetics.

Monophyly and Paraphyly in Eukaryotes

• Monophyletic group: A group consisting of an ancestor and all its descendants (e.g., Eukarya).

• Paraphyletic group: A group that includes most, but not all, descendants of a common ancestor (e.g., "Kingdom Protista").

• "Protista" is not monophyletic because it excludes land plants, fungi, and animals, even though they share a common ancestor with protists.

• Multiple independent origins of multicellularity are present among eukaryotes (e.g., in plants, animals, red algae, brown algae).

Protists: Diversity and Classification

Defining Protists

• Protists are eukaryotes that are not classified as plants, animals, or fungi.

• They exhibit a wide range of forms and lifestyles, including autotrophs, heterotrophs, and mixotrophs.

• Protists are considered a "grade" rather than a true clade, sharing ancestral characteristics but not forming a single evolutionary lineage.

Major Eukaryotic Clades

Molecular phylogenetic analyses have identified six major clades of eukaryotes. These clades are based on DNA sequence data and cellular characteristics.

• SAR: Stramenopiles, Alveolates, and Rhizarians

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

• Excavata: Includes diplomonads, parabasalids, and euglenozoans

• Unikonta: Includes amoebozoans, fungi, and animals

• Other clades as identified by ongoing research

Note: It is difficult to generalize about these clades due to their diversity.

Key Groups of Protists

Excavata

• Mostly parasitic or free-living flagellates.

• Includes Giardia (causes giardiasis) and Trypanosoma (causes African sleeping sickness).

• Some are mixotrophs, such as Euglena.

SAR Clade

• Stramenopiles: Includes diatoms (silica cell walls), brown algae (kelp), and water molds.

• Alveolates: Includes ciliates (e.g., Paramecium), dinoflagellates (some cause red tides), and apicomplexans (e.g., Plasmodium, which causes malaria).

• Rhizarians: Includes foraminiferans and radiolarians, often with intricate mineral skeletons.

Archaeplastida

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

• All are photosynthetic, with primary plastids derived from cyanobacteria.

• Green algae can be unicellular, colonial, or multicellular.

Unikonta

• Includes amoebozoans (slime molds, lobose amoebas) and opisthokonts (fungi and animals).

• Amoebozoans move via pseudopodia and can form multicellular aggregates.

Evolutionary Trends in Eukaryotes

Repeated Evolution of Traits

• Some traits, such as chloroplasts (via primary endosymbiosis), have arisen only once.

• Other traits, such as multicellularity and secondary endosymbiosis (acquisition of plastids from other eukaryotes), have evolved multiple times independently (convergent evolution).

Life Cycles and Multicellularity

Overview

• Protists exhibit a variety of life cycles, including asexual and sexual reproduction.

• Multicellularity has evolved independently in several protist lineages, providing increased organization and specialization.

Ecological Roles of Protists

Producers and Mutualists

• Many protists are primary producers, especially in aquatic environments, contributing significantly to global photosynthesis.

• Some form important mutualistic relationships, such as dinoflagellates living in coral tissues or protists in termite guts aiding cellulose digestion.

Important Protist Pathogens

Human Diseases

• Giardia (Diplomonada): Causes giardiasis, a diarrheal disease often contracted from contaminated water.

• Trypanosoma (Euglenozoa): Causes African sleeping sickness, transmitted by tsetse flies.

• Plasmodium (Apicomplexa): Causes malaria, transmitted by mosquitoes, with a complex life cycle.

Summary Table: Major Protist Groups and Features

Group	Main Features	Examples	Ecological/Medical Importance
Excavata	Flagellated, often parasitic or mixotrophic	Giardia, Trypanosoma, Euglena	Human pathogens, photosynthetic protists
SAR	Diverse; includes photosynthetic and heterotrophic forms	Diatoms, brown algae, Paramecium, dinoflagellates, Plasmodium	Primary producers, red tides, malaria
Archaeplastida	Photosynthetic, primary plastids	Red algae, green algae, land plants	Oxygen production, food sources
Unikonta	Amoeboid movement, includes fungi and animals	Slime molds, amoebas	Decomposers, some pathogens

Key Terms

• Monophyletic: A group containing an ancestor and all its descendants.

• Paraphyletic: A group containing an ancestor and some, but not all, descendants.

• Endosymbiosis: The process by which one organism lives inside another, leading to the origin of organelles such as mitochondria and chloroplasts.

• Mixotroph: An organism capable of both photosynthesis and heterotrophy.

• Pseudopodia: Extensions of the cell used for movement and feeding in amoeboid protists.