Diversification of Eukaryotes: Protists and Major Eukaryotic Lineages

Chapter 27: Diversification of Eukaryotes

Introduction to Eukaryotes

Eukaryotes, classified under Domain Eukarya, represent a diverse group of organisms ranging from single-celled protists to complex multicellular forms such as plants and animals. They are distinguished from bacteria and archaea by several key features:

• Larger cell size and increased complexity

• Presence of membrane-bound organelles (e.g., nucleus, mitochondria)

• Extensive cytoskeleton

• Nuclear envelope

• Multicellularity evolved multiple times

• Both asexual and sexual reproduction are observed

Protists: Definition and Characteristics

Protists include all eukaryotes except land plants, fungi, and animals. They do not form a monophyletic group and lack unique defining features. Protists are primarily aquatic and are highly diverse in form and function.

Why Do Biologists Study Protists?

• Medical importance: 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 as primary producers and in food chains.

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

Impacts on Human Health and Welfare

• Irish potato famine (1845): Caused by Phytophthora infestans, a water mold protist.

• Malaria: Caused by Plasmodium species, transmitted by mosquitoes. Each stage of the life cycle is specialized for infecting specific host cells.

Table: Human Health Problems Caused by Protists

Species	Disease
Plasmodium spp.	Malaria
Naegleria fowleri	Meningoencephalitis (brain-eating amoeba)
Toxoplasma gondii	Toxoplasmosis
Dinoflagellates	Shellfish poisoning via algal blooms
Trypanosoma spp.	Sleeping sickness, Chagas disease
Entamoeba histolytica	Amoebic dysentery

Harmful Algal Blooms

Unicellular, toxin-producing protists called dinoflagellates can cause harmful algal blooms. Toxins accumulate in shellfish, which can poison humans who consume them.

Protists in Aquatic Food Chains

Photosynthetic protists are primary producers that fix carbon dioxide into sugars, forming the base of aquatic food chains. Plankton (e.g., diatoms) are especially important in open oceans and lakes.

Protists and the Global Carbon Cycle

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

Studying Protist Diversity

Microscopy and Cell Structure

Protists are grouped based on cell structure and organelles. For example, Stramenopila have flagella with hollow, hairlike structures.

Major Eukaryotic Lineages

Seven major groups of eukaryotes are identified by morphological and molecular data. Protists are a paraphyletic group (not monophyletic). The main lineages are:

• Amoebozoa

• Opisthokonta

• Excavata

• Plantae

• Rhizaria

• Alveolata

• Stramenopila

Table: Distinguishing Features of Major Eukaryotic Lineages

Lineage	Distinguishing Features
Amoebozoa	Lack cell walls; large lobes for movement
Opisthokonta	Single flagellum at base; flat mitochondrial cristae
Excavata	Feeding groove; reduced or absent mitochondria
Plantae	Chloroplasts with double membrane
Rhizaria	Lack cell walls; threadlike pseudopodia
Alveolata	Alveoli (sac-like structures) under plasma membrane
Stramenopila	Flagella with hairlike projections

Direct Sequencing

Direct sequencing of environmental DNA has revealed many new protist lineages, including diverse picoplankton.

Themes in Protist Diversification

Morphological Innovations

• Earliest eukaryotes were single-celled with mitochondria, nucleus, cytoskeleton, and no cell wall.

• Flagella enabled motility.

Endosymbiosis Theory

Origin of Mitochondria

Mitochondria originated via endosymbiosis when a bacterial cell was engulfed by a host cell. Evidence includes:

• Mitochondria are similar in size to bacteria

• Replicate by fission

• Have their own ribosomes and circular DNA

• Double membrane structure

Origin of Chloroplasts

Chloroplasts originated when a eukaryote engulfed a cyanobacterium. Some protists acquired chloroplasts via secondary endosymbiosis, resulting in organelles with more than two membranes.

Origin of the Nuclear Envelope

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

Structures for Support and Protection

• Protists exhibit diverse support structures: cell walls (e.g., diatoms with silica), cellulose plates (dinoflagellates), hard shells, and internal rigid structures.

Multicellularity

Multicellularity evolved independently in several eukaryotic lineages. It began with cells sticking together after division, followed by specialization of cells for different functions.

Protist Nutrition and Movement

How Do Protists Obtain Food?

• Ingestive feeding: Engulfing prey or organic debris (phagocytosis)

• Absorptive feeding: Uptake of nutrients directly from the environment; includes decomposers and parasites

• Photosynthesis: Autotrophic protists use light energy to produce organic compounds

How Do Protists Move?

• Amoeboid motion: Sliding movement using pseudopodia (requires ATP)

• Flagella and cilia: Swimming via long (flagella) or short (cilia) appendages

Protist Reproduction and Life Cycles

Asexual Reproduction

• Based on mitosis and cell division

• Produces genetically identical offspring

Sexual Reproduction

• Based on meiosis and fusion of gametes

• Produces genetically diverse offspring

• Sexual reproduction is considered an adaptation to fight disease due to increased genetic variation

Life Cycles: Haploid vs. Diploid Dominance

• Protist life cycles vary widely; some are haploid-dominant, others diploid-dominant

• Alternation of generations: Multicellular haploid (gametophyte) and diploid (sporophyte) forms alternate

Key Lineages of Eukaryotes

Amoebozoa

• Lack cell walls; move via large, lobe-like pseudopodia

• Include amoebae, cellular slime molds, and plasmodial slime molds

• Important in nutrient cycling and as model organisms

Opisthokonta

• Includes fungi, animals, and choanoflagellates

• Fungi are decomposers; animals are multicellular heterotrophs

• Choanoflagellates are closest living relatives to animals

Excavata

• Have an excavated feeding groove

• Some lack mitochondria or have vestigial mitochondria

• Includes parabasalids, diplomonads, and euglenids

Plantae

• Monophyletic group including glaucophyte algae, red algae, green algae, and land plants

• All have chloroplasts derived from primary endosymbiosis

Rhizaria

• Single-celled, lack cell walls, some with elaborate shells

• Move by slender pseudopodia

• Important in sediment formation and geological dating

Alveolata

• Have alveoli (membrane-bound sacs) under plasma membrane

• Include ciliates, dinoflagellates, and apicomplexans

• Some are bioluminescent or parasitic

Stramenopila (Heterokonta)

• Flagella with hollow projections

• Includes water molds, diatoms, and brown algae

• Important primary producers and habitat formers (e.g., kelp forests)