Diversity of Microbial Eukarya — Protists, Fungi, and Algal Lineages Explained | Chapter 18 from Brock Biology of Microorganisms

Microbial eukaryotes encompass a wide range of life forms that are foundational to both ecosystem function and evolutionary history. Chapter 18 of Brock Biology of Microorganisms delves into the origin, structure, classification, and ecological importance of microbial eukarya—including protists, fungi, and algae. With a focus on cellular complexity and evolutionary innovation, this chapter highlights how microbial eukaryotes continue to shape life on Earth.

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Origins and Evolution of Eukaryotic Microbes

Eukaryotic cells evolved through endosymbiotic events: mitochondria originated from α-proteobacteria, and chloroplasts from cyanobacteria. Some eukaryotes later acquired plastids through secondary endosymbiosis, leading to phototrophic lineages like euglenids and dinoflagellates. Eukaryotes are considered genetic chimeras, with both archaeal and bacterial contributions to their genomes.

The five major supergroups of eukaryotes include:

• Archaeplastida (green algae, red algae, land plants)
• SAR (Stramenopiles, Alveolata, Rhizaria)
• Excavates
• Amoebozoa
• Opisthokonta (fungi and animals)

Protists — The Diverse Eukaryotic Microbes

Protists are defined by exclusion—not plants, animals, or fungi—and exhibit remarkable variability in structure and lifestyle. Many are phototrophic, heterotrophic, or mixotrophic, with diverse reproductive strategies and ecological roles.

Excavates

• Include Giardia, Trypanosoma, Leishmania, Euglena
• Often anaerobic with reduced mitochondria (mitosomes or hydrogenosomes)
• Kinetoplastids contain kinetoplasts—DNA-rich organelles inside mitochondria

Alveolata

• Characterized by alveoli beneath the plasma membrane
• Include:
  • Ciliates like Paramecium (dual nuclei)
  • Apicomplexans like Plasmodium (apicoplast organelle)
  • Dinoflagellates like Gonyaulax (photosynthetic, some cause red tides)

Stramenopiles

• Include diatoms (silica frustules), golden and brown algae, and oomycetes (non-fungal water molds)
• Brown algae like kelp are macroscopic marine phototrophs

Rhizaria

• Foraminiferans, radiolarians, and chlorarachniophytes
• Use threadlike pseudopodia for movement and feeding
• Chlorarachniophytes retain a nucleomorph—a remnant algal nucleus from secondary endosymbiosis

Haptophytes and Coccolithophores

• Marine phototrophs with calcite plates (coccoliths)
• Significant in carbon cycling and chalk formation

Amoebozoa

• Use lobe-shaped pseudopodia
• Include:
  • Gymnamoebas like Amoeba
  • Entamoebas (e.g., Entamoeba histolytica, a human parasite)
  • Slime molds:
    • Plasmodial (e.g., Physarum) form large multinucleate masses
    • Cellular (e.g., Dictyostelium) aggregate into fruiting bodies under stress

Fungi — Nonphotosynthetic Eukaryotic Microbes

Fungi are key decomposers in terrestrial ecosystems. They form hyphae that aggregate into mycelia, and reproduce via spores. While some fungi are unicellular (yeasts), others are multicellular with complex life cycles.

Fungal Phyla

• Microsporidia: Obligate intracellular parasites; lack mitochondria
• Chytridiomycota: Aquatic fungi with flagellated spores
• Mucoromycota: Zygospore-forming fungi, often involved in food spoilage
• Glomeromycota: Arbuscular mycorrhizal fungi; symbionts of plant roots
• Ascomycota: Largest fungal group; includes Saccharomyces, Aspergillus
• Basidiomycota: Mushrooms, smuts, and rusts; produce spores on basidia

Archaeplastida — The Photosynthetic Microbial Eukaryotes

Unified by primary endosymbiosis of cyanobacteria-derived chloroplasts, Archaeplastida includes both microbial and multicellular phototrophs.

Red Algae (Rhodophytes)

• Marine phototrophs with phycobiliproteins
• Important for agar and carrageenan production
• Some species live in extreme environments

Green Algae (Chlorophytes)

• Use chlorophyll a and b; no phycobilins
• Range from unicellular (Chlamydomonas) to multicellular (Ulva)
• Chara and related charophyceans are the closest relatives to land plants
• Botryococcus is a potential source of biofuel hydrocarbons

Glossary of Key Terms

• Apicoplast: Remnant plastid in apicomplexan parasites
• Chlorosome: Light-harvesting organelle in some algae
• Coccolithophore: Marine organism with calcite exoskeleton
• Hypha/Mycelium: Fungal filaments and networks
• Kinetoplast: Unique DNA-containing organelle in kinetoplastids
• Mitosome/Hydrogenosome: Mitochondrial derivatives in anaerobic protists
• Nucleomorph: A remnant algal nucleus from endosymbiosis
• Plasmodium: Vegetative stage in plasmodial slime molds
• Slime Mold: Protists with fungal-like life stages
• Secondary Endosymbiosis: Uptake of one eukaryote by another to gain plastids

Conclusion: Microbial Eukaryotes as Innovators of Life

From diatoms to slime molds, microbial eukaryotes demonstrate the full range of cellular innovation and ecological function. Chapter 18 of Brock Biology of Microorganisms provides a critical understanding of how these organisms evolved, diversified, and contributed to Earth’s biosphere—laying the foundation for complex multicellular life.

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