Protists and Eukaryotic Diversity (Vocabulary Flashcards)

Protists: Definition and Diversity

• Protists: eukaryotic organisms that are not animals, plants, or fungi.
• Described diversity: >$100{,}000$ described living species; many more undescribed; potential diversity tied to host relationships.
• Major driver of diversity: endosymbiosis leading to mitochondria and plastids; plastids give rise to red and green algae.
• Six eukaryotic supergroups (monophyletic): Excavata, SAR, Archaeplastida, Unikonta, with the remainder forming a framework for protist and animal/plant/fungi evolution.

Endosymbiosis and Plastids

• Endosymbiosis as a key source of diversity:
  • Engulfed aerobic bacterium → mitochondrion.
  • Engulfed photosynthetic cyanobacterium → plastid.
• Plastid-bearing ancestors gave rise to red and green algae.
• Implication: plastids evolved in multiple lineages; secondary endosymbiosis occurred in some groups.

Protist Cell Structure and Variability

• Cells range from microscopic and unicellular to true multicellular forms; some colonies act like a coordinated group.
• Some protists are large multinucleate single cells; nuclei can be different sizes with distinct roles.

Cell Protection and Motility

• Protective coverings:
  • Animal-like membranes or plant-like cell walls.
  • Silica-based shells or pellicles of protein strips.
• Pellicle: flexible armor that preserves movement.
• Motility types:
  • Cilia (e.g., Paramecium)
  • Pseudopodia (Amoeba)
  • Flagellum (Euglena)

Metabolism and Nutrition

• Metabolic modes: aerobic or anaerobic.
• Photoautotrophs: chloroplast-containing; photosynthetic energy storage.
• Mixotrophs: utilize both photosynthesis and heterotrophy depending on conditions.
• Heterotrophs: acquire organic material from other organisms.
• Saprobes: absorb nutrients from dead matter or wastes.

Reproduction

• Asexual reproduction: binary fission (transverse or longitudinal); paramecia can exhibit this; some form of multiple fission.
• Budding: some protists produce small buds that grow to parental size.
• Sexual reproduction: meiosis and fertilization; many protists can switch from asexual to sexual modes.
• Cysts: resistant resting stages that aid survival under stress; enable dispersal.
• Paramecium example: macronucleus and micronucleus; macronucleus dissolves during sexual reproduction and is replaced by micronucleus.

Habitat and Ecology

• Mostly aquatic: freshwater, marine, damp soil, snow.
• Parasites infect animals or plants; some saprobe in dead matter.

Classification: Six Eukaryotic Supergroups

• Excavata, SAR, Archaeplastida, Unikonta, plus two others within SAR and Excavata groupings; supergroups believed monophyletic but subject to ongoing research.
• Purpose: organize protists and relate them to plants, fungi, and animals.

Excavata

• Characteristics: asymmetrical, feeding groove; diverse lifestyles (heterotrophic predators, photosynthetic, parasites).
• Subgroups: diplomonads, parabasalids, euglenozoans.
• Diplomonads: Giardia lamblia; mitosomes (mitochondrial remnants); anaerobic energy via glycolysis; two identical nuclei; multiple flagella.
• Euglenozoans: kinetoplastids (e.g., Trypanosoma) and euglenids (e.g., Euglena).
• Sleeping sickness: Trypanosoma brucei; antigenic variation to evade host immunity.

SAR Clade and Chromalveolates

• Chromalveolates: thought to arise from secondary endosymbiosis with red algal cells; may have lost plastids in some lineages; grouping is a hypothesis-based framework.
• Subgroups:
  • Alveolates: membrane-enclosed sacs (alveoli) under the plasma membrane; includes dinoflagellates, apicomplexans, ciliates.
  • Stramenopiles: hairy flagellum and often a second smooth flagellum; includes diatoms, golden algae, brown algae, oomycetes.

Alveolates

• Dinoflagellates: photosynthetic, heterotrophic, or mixotrophic; cell walls with interlocking cellulose plates; two flagella (one longitudinal, one encircling) for spinning movement.
• Bioluminescence: some dinoflagellates produce light; common in plankton.
• Ciliates: Paramecium, Tetrahymena; densely ciliated surface; pellicle; cilia coordinate movement and feeding; Paramecium sexual reproduction with macronucleus/micronucleus.
• Apicomplexans: includes Plasmodium (malaria parasite).

Stramenopiles

• Key feature: textured or “hairy” flagellum; often another smooth flagellum.
• Range: single-celled diatoms to large multicellular brown algae (kelp).

Diatoms

• Unicellular photosynthetic protists with silica cell walls (glass-like). Planktonic in freshwater and marine environments.
• Carbon pump role: diatom blooms lead to carbon sequestration when they die and sink.
• Biological carbon pump: diatoms help remove CO₂ from atmosphere by sinking carbon to the sea floor.

Golden Algae and Brown Algae

• Golden algae: mostly unicellular; carotenoids give gold color; found in freshwater and marine plankton.
• Brown algae: mostly marine, multicellular; giant kelp; tissues resemble plant organs (holdfast, stipe, blades); life cycles can be complex with alternation of generations.

Brown Algae Life Cycle

• Alternation of generations example: sporophyte (2n) → meiosis → sporangia → zoospores (1n) → gametophytes (n) → fertilization → new sporophyte.

Oomycetes (Water Molds)

• Cell walls cellulose-based; filamentous nutrient uptake network; diploid spores with two flagella (hairy and smooth); nonphotosynthetic; saprobes and parasites; aquatic and some plant parasites.

Rhizaria

• Amoebas with threadlike or needle-like pseudopodia; movement and feeding via cytoplasmic extension.
• Foraminiferans: porous tests (calcium carbonate); may host photosynthetic algae; pseudopodia extend through pores.
• Radiolarians: silica-rich exteriors; axially structured pseudopodia supported by microtubules for capturing prey.

Archaeplastida

• Red algae and green algae; land plants evolved from a common ancestor; all Archaeplastida descended from endosymbiosis with a cyanobacterium.
• Red algae: mostly multicellular; lack flagella in many species; phycoerythrin pigments give red color; alternation of generations in many species.
• Green algae: chloroplasts similar to plants; includes chlorophytes and charophytes; Charophytes are closest relatives to land plants; Chara is an example.

Chlorophytes (Green Algae) and Diversity

• Habitat: freshwater and damp soil; common plankton components.
• Chlamydomonas: simple unicellular with two anterior flagella and eyespot for phototaxis.
• Volvox: colonial organism; 500 to $60{,}000$ cells; hollow sphere; cells with two flagella; some cells specialize for reproduction.
• Ulva (sea lettuce): true multicellularity among chlorophytes.

Amoebozoans and Opisthokonts

• Amoebozoans: amoebas with lobose or tube-shaped pseudopodia; include tubulinids and slime molds; some are parasites.
• Plasmodial slime molds: plasmodium is a giant, multinucleate single cell that moves and consumes bacteria, fungi, and decaying matter.
• Opisthokonts: choanoflagellates resemble the last common ancestor of animals and fungi; show a single apical flagellum surrounded by a collar of microvilli for filter feeding.

Protists as Producers, Symbionts, and Decomposers

• Primary producers: protists (including plankton) contribute substantially to global photosynthesis; approx. $\frac{1}{4}$ of the world’s photosynthesis.
• Symbionts: zooxanthellae (photosynthetic dinoflagellates) fix carbon and provide nutrients to coral polyps.
• Decomposers: saprobic protists recycle nutrients by breaking down dead matter and wastes; essential for nutrient cycling.

Slime Molds and Pathogens

• Slime molds: some species form plasmodia (giant multinucleate cells) that move and consume.
• Malaria: Plasmodium falciparum is a major cause of malaria in tropical regions; contributes significantly to mortality.
• African sleeping sickness: Trypanosoma brucei disrupts immune response via antigenic variation.

Summary: Key Relationships and Concepts

• Endosymbiosis explains mitochondria and plastids; plastids led to red/green algae and the broader Archaeplastida.
• Protists occupy diverse ecological roles: primary producers, symbionts, and decomposers; critical for global ecosystems and climate regulation.
• Classification into six supergroups is a working framework helping to organize protist diversity and evolutionary relationships.

Quick Practice Check

• What roles do protists play in Earth's climate and ecosystems (production, carbon cycling, and nutrient release through saprobic activity)?
• How does endosymbiosis explain the origin of mitochondria and plastids in protists?