Protists and the Domain Eukarya

Introduction to Eukaryotes and Protists

Protists are a diverse group of eukaryotic organisms that are neither plants, fungi, nor animals. They exhibit a wide range of cellular structures, reproductive strategies, and ecological roles. Eukaryotes are characterized by their nuclear envelope, complex organelles, and cytoskeleton, distinguishing them from bacteria and archaea.

• Protists include all eukaryotes except land plants, fungi, and animals.

• They are not a monophyletic group and lack unique defining features.

• Most protists inhabit aquatic environments.

Medical and Ecological Importance of Protists

Protists in Human Health

Protists are responsible for several significant human diseases and ecological phenomena.

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

• Other diseases include amoebic dysentery (Entamoeba histolytica), sleeping sickness (Trypanosoma), and toxoplasmosis (Toxoplasma gondii).

Protists and Plant Disease

Some protists cause devastating plant diseases, such as late blight in potatoes and tomatoes, which led to the Irish potato famine.

• Phytophthora infestans is a water mold responsible for late blight.

• Damage affects both foliage and tuber storage quality.

Harmful Algal Blooms

Unicellular, photosynthetic, toxin-producing protists such as dinoflagellates can cause harmful algal blooms, leading to shellfish poisoning in humans.

• Shellfish accumulate toxins but are not harmed themselves.

• Blooms can result in closures of shellfish harvesting areas.

Protist Diversity and Cell Structure

Cellular Features and Lineages

Protists are grouped based on cell structure and distinctive organelles. Innovations such as unusual flagella, cell walls, and internal support structures have contributed to their diversification.

• Stramenopiles have flagella covered with hollow, hairlike structures.

• Protists display a variety of cell coverings, including glass-like walls (diatoms), cellulose plates (dinoflagellates), and calcium carbonate shells (foraminiferans).

Distinguishing Features of Major Lineages

Each major protist lineage has unique morphological characteristics.

• Amoebozoa: Flexible membranes, amoeboid movement.

• Excavata: Excavated feeding groove, flagella.

• Plantae: Chloroplasts, cellulose cell walls.

• Rhizaria: Shell-like coverings, slender pseudopodia.

• Alveolata: Alveoli under plasma membrane, cilia or flagella.

• Stramenopila: Flagella with hollow hairs.

Endosymbiosis and Organelle Evolution

Origin of Mitochondria

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

• Mitochondria are similar in size to α-proteobacteria, replicate by fission, and have their own genomes.

• Phylogenetic data supports their bacterial ancestry.

Origin of Chloroplasts

Chloroplasts originated when a protist engulfed a cyanobacterium, leading to photosynthetic eukaryotes.

• Chloroplasts have bacteria-like characteristics and circular DNA.

• Secondary endosymbiosis occurred when a predatory protist engulfed a photosynthetic protist, resulting in chloroplasts with four membranes.

Multicellularity and Feeding Strategies

Multicellularity

Mutations leading to multicellularity allowed cells to stick together after division, eventually resulting in specialized cell functions and complex organisms.

• Multicellularity evolved independently in several eukaryotic lineages.

Feeding Strategies

Protists obtain food by ingesting packets of food, absorbing organic molecules, or performing photosynthesis.

• Ingestive feeding involves phagocytosis or sweeping food particles with cilia.

• Absorptive feeding involves uptake of nutrients across the plasma membrane; some are decomposers or parasites.

• Photosynthetic protists are primary producers in aquatic ecosystems.

Locomotion in Protists

Movement Mechanisms

Protists move using amoeboid motion, flagella, or cilia.

• Amoeboid motion involves streaming of pseudopodia and is possible in species lacking cell walls.

• Flagella are long and usually found alone or in pairs; cilia are short and numerous.

Reproduction and Life Cycles

Asexual and Sexual Reproduction

Protists reproduce both asexually (mitosis) and sexually (meiosis and gamete fusion). Sexual reproduction increases genetic diversity, which is adaptive in changing environments.

Life Cycle Variability

Protist life cycles vary widely, with distinctions between haploid and diploid phases.

• Alternation of generations involves multicellular haploid (gametophyte) and diploid (sporophyte) forms.

Key Lineages of Eukaryotes

Amoebozoa

Amoebozoa lack cell walls, move via amoeboid motion, and are abundant in freshwater and wet soils. Major subgroups include amoebae, cellular slime molds, and plasmodial slime molds.

Excavata

Excavata have an excavated feeding groove and swim via flagella. Subgroups include parabasalids, diplomonads, and euglenids.

Plantae

Plantae are a monophyletic group with chloroplasts and cellulose cell walls. Subgroups include glaucophyte algae, red algae, green algae, and land plants.

Rhizaria

Rhizaria are single-celled, lack cell walls, and move by slender pseudopodia. Some have elaborate shell-like coverings.

Alveolata

Alveolata have flattened vesicles (alveoli) under their plasma membrane and move by cilia or flagella. Subgroups include ciliates, dinoflagellates, and apicomplexans.

Stramenopila

Stramenopila have flagella with distinctive hollow "hairs" and include water molds, diatoms, and brown algae.

Summary Table: Distinguishing Features of Major Protist Lineages