Diversification of Eukaryotes and Protist Biology

Chapter 27: Diversification of Eukaryotes

Introduction to Eukaryotes

Eukaryotes represent the third domain of life, encompassing a vast range of organisms from single-celled protists to multicellular giants like sequoia trees and blue whales. Despite their diversity, eukaryotes share several fundamental features that distinguish them from bacteria and archaea:

• Large cell size and increased organelle complexity

• Extensive cytoskeleton

• Nuclear envelope

• Multicellularity evolved multiple times

• Asexual and sexual reproduction

Protists include all eukaryotes except land plants, fungi, and animals. They do not form a unified group and are typically found in aquatic environments.

Why Do Biologists Study Protists?

Protists are studied for their medical, ecological, and evolutionary importance:

• Medical relevance: Some protists cause diseases in humans and crops (e.g., malaria, Irish potato famine).

• Ecological importance: Protists are abundant in aquatic environments and play key roles in food chains.

• Evolutionary significance: Understanding protists is critical for tracing the evolution of plants, fungi, and animals.

Impacts on Human Health and Welfare

Several protists are pathogenic:

• Phytophthora infestans caused the Irish potato famine.

• Plasmodium species cause malaria, a major chronic infectious disease.

• Other protists cause diseases such as amoebic dysentery, sleeping sickness, and Chagas disease.

Species	Disease
Plasmodium (P. falciparum, P. vivax)	Malaria
Naegleria fowleri	Meningoencephalitis (brain-eating amoeba)
Toxoplasma gondii	Toxoplasmosis
Dinoflagellates	Shellfish poisoning (harmful algal blooms)
Trypanosoma gambiense, T. rhodesiense	Sleeping sickness
Trypanosoma cruzi	Chagas disease
Entamoeba histolytica	Amoebic dysentery

Harmful Algal Blooms

Rapid population growth of toxin-producing protists (dinoflagellates) can cause harmful algal blooms. Toxins accumulate in shellfish, posing risks to humans who consume them.

Protists in Aquatic Food Chains

Photosynthetic protists are primary producers, fixing carbon dioxide to form sugars. Marine protists contribute to nearly half of Earth's carbon fixation. Plankton, including diatoms, are foundational to aquatic food chains.

Protists and Global Climate Change

Protists play a role in the global carbon cycle by acting as carbon sinks. Their shells contribute to sedimentary rocks and petroleum formation. Ocean fertilization with iron can increase protist populations and enhance carbon sequestration.

Studying Protist Diversity

Microscopy and Cell Structure

Protists are grouped by cell structure and organelles. Light and electron microscopy reveal diagnostic features, such as the distinctive flagella of Stramenopila.

Major Eukaryotic Lineages

Seven major groups of eukaryotes are identified by morphological synapomorphies:

Lineage	Distinguishing Features
Amoebozoa	No cell walls, large lobes for movement
Opisthokonta	Single flagellum, flat mitochondrial cristae
Excavata	Feeding groove, lack typical mitochondria
Plantae	Chloroplasts with double membrane
Rhizaria	No cell walls, threadlike pseudopodia
Alveolata	Alveoli under plasma membrane
Stramenopila	Flagella with hairlike projections

Direct Sequencing

Direct sequencing of environmental samples allows placement of unknown protists on phylogenetic trees, revealing new lineages such as picoplankton.

Themes in Protist Diversification

Morphological Innovations

Early eukaryotes were likely single-celled with mitochondria, nucleus, endomembrane system, cytoskeleton, and no cell wall. They moved using flagella.

Endosymbiosis Theory

Mitochondria originated via endosymbiosis, where a bacterial cell was engulfed by another cell. Evidence includes:

• Similar size to bacteria

• Replication by fission

• Own ribosomes and proteins

• Double membranes

• Circular genome

Phylogenetic data show mitochondrial genes are closely related to bacterial genes.

Origin of Chloroplasts

Chloroplasts in photosynthetic protists originated when a protist engulfed a cyanobacterium. Evidence includes:

• Bacteria-like characteristics

• Circular DNA

• Peptidoglycan in cell wall

• Extant endosymbiotic cyanobacteria

Secondary endosymbiosis led to chloroplasts with more than two membranes in some protists.

Origin of Nuclear Envelope

The nuclear envelope likely originated from infoldings of the plasma membrane, which also gave rise to the endoplasmic reticulum. This separation of transcription and translation allowed novel gene regulation.

Structures for Support and Protection

Protists exhibit diverse structures for support and protection, including cell walls (silicon dioxide in diatoms, cellulose plates in dinoflagellates), hard external shells, and rigid internal structures.

Multicellularity

Multicellularity evolved independently in many eukaryotic lineages. Mutations caused cells to stick together after division, leading to specialization and diversification.

Protist Nutrition and Movement

Feeding Strategies

• Ingestive feeding: Eating live or dead organisms via phagocytosis (possible in protists lacking cell walls).

• Absorptive feeding: Uptake of nutrients across the plasma membrane; includes decomposers and parasites.

• Photosynthesis: Autotrophic protists produce organic compounds using CO2 and sunlight.

Movement

• Amoeboid motion: Sliding movement using pseudopodia, requiring ATP.

• Swimming: Using flagella (long, few) or cilia (short, numerous).

Protist Reproduction and Life Cycles

Asexual and Sexual Reproduction

• Asexual reproduction: Mitosis and cell division produce genetically identical offspring.

• Sexual reproduction: Meiosis and gamete fusion produce genetically diverse offspring.

Sexual reproduction is adaptive for resisting rapidly evolving parasites and pathogens.

Life Cycles: Haploid vs. Diploid Dominance

Protist life cycles vary widely, with both haploid and diploid phases. Fertilization produces a diploid zygote, which undergoes further development.

Alternation of Generations

Many multicellular protists exhibit alternation of generations:

• Gametophyte: Multicellular haploid form producing gametes by mitosis.

• Sporophyte: Multicellular diploid form producing spores by meiosis.

Key Lineages of Eukaryotes

Amoebozoa

Amoebozoa lack cell walls, move via amoeboid motion, and feed by engulfing food. Subgroups include amoebae, cellular slime molds, and plasmodial slime molds. They influence nutrient cycling and are used as model organisms.

Opisthokonta

Includes fungi, animals, and choanoflagellates. Fungi are decomposers; animals are multicellular heterotrophs. Choanoflagellates are the closest living relatives to animals.

Excavata

Characterized by an excavated feeding groove. Some lack mitochondria but have mitochondrial genes. Subgroups include parabasalids, diplomonads, and euglenids. Some are pathogenic (e.g., Giardia).

Plantae

Monophyletic group descended from a cyanobacterium-engulfing ancestor. Includes glaucophyte algae, red algae, green algae, and land plants. Primary producers in most ecosystems.

Rhizaria

Single-celled, lack cell walls, some have shell-like coverings. Move by slender pseudopodia. Shells of dead foraminiferans form sediments and limestone.

Alveolata

Contain alveoli under plasma membrane. Includes ciliates, dinoflagellates (some bioluminescent), and apicomplexans (parasites).

Stramenopila (Heterokonta)

Flagella with hollow "hairs" are a synapomorphy. Includes water molds, diatoms, and brown algae. Diatoms are primary producers; brown algae form kelp forests.

Summary Table: Major Eukaryotic Lineages

Lineage	Morphology & Movement	Reproduction	Relevance
Amoebozoa	No cell walls, amoeboid movement	Asexual & sexual; slime molds haploid/diploid dominant	Nutrient cycling, model organism
Opisthokonta	Single/multicellular, motile cells	Asexual & sexual	Decomposers, consumers, animal evolution
Excavata	Feeding groove, flagella	Primarily asexual	Pathogens, freshwater plankton
Plantae	Chloroplasts, cell walls	Asexual & sexual, alternation of generations	Primary producers, agar, coral reefs
Rhizaria	No cell walls, shell-like covering	Asexual & sexual, alternation of generations	Limestone formation, petroleum exploration
Alveolata	Alveoli, cilia/flagella	Asexual & sexual	Photosynthesis, bioluminescence, disease
Stramenopila	Flagella with hairs, chloroplasts	Asexual & sexual, diploid dominant	Primary producers, kelp forests, Irish potato famine