Eukaryotic Microorganisms: Fungi, Algae, and Protists

Eukaryotic Microorganisms

Introduction

Eukaryotic microorganisms include fungi, algae, and protists. These organisms are characterized by complex cellular organization and play essential roles in ecosystems, industry, and disease. This guide summarizes their classification, structure, life cycles, and ecological significance.

Cellular Organization and Classification

Prokaryotes vs. Eukaryotes

• Prokaryotes are organisms without a nucleus or membrane-bound organelles. Examples: Bacteria and Archaea.

• Eukaryotes possess a nucleus and membrane-bound organelles. Examples: Protista, Plantae, Fungi, Animalia.

• Living organisms are classified into six kingdoms based on cellular structure and evolutionary relationships.

• Prokaryotes appeared approximately 3.5 billion years ago; eukaryotes evolved about 1.5 billion years ago.

Universal Phylogenetic Tree

The universal phylogenetic tree is constructed using 16S/18S rRNA gene sequences, illustrating evolutionary relationships among Bacteria, Archaea, and Eukarya.

Origin of Eukarya

Evolutionary Background

• The origin of eukaryotic cells is still under investigation.

• Eukaryotes are thought to have evolved from ancestral prokaryotic lineages, possibly through endosymbiotic events.

• Key feature: membrane-bound nucleus (from Greek eu, "true" + karyon, "nut").

Fungi

General Characteristics

• Ubiquitous and non-photosynthetic.

• Majority are saprophytes: use dead or decaying organic matter for nutrition.

• Some are parasites or symbionts (e.g., lichens: fungus + algae association).

Unicellular Fungi: Yeasts

• Yeasts are unicellular fungi, e.g., Saccharomyces cerevisiae.

• Single, oval cells reproduce by budding.

• Some yeasts can form filaments (dimorphic).

• Filamentous growth allows invasion of tissues (e.g., Candida albicans).

Multicellular Fungi: Hyphae and Mycelium

• Most fungi have a thread-like body made up of hyphae.

• Hyphae collectively form the mycelium, which absorbs nutrients.

• Hyphae also make up the structure of fruiting bodies (e.g., mushrooms).

Fungal Cell Wall

• Composed of 90% polysaccharide, proteins, and some lipids.

• Main structural components: chitin (in Ascomycetes, Basidiomycetes) and sometimes cellulose (in Oomycetes).

• Chitin is a straight-chain polymer of N-acetyl glucosamine units joined by β-1,4 linkages.

Chitin Structure:

$\text{Chitin: (C_8H_{13}O_5N)_n}$

Fungal Reproduction

Asexual Reproduction

• Occurs via budding (yeasts) or production of asexual spores (e.g., conidia, sporangiospores).

Sexual Reproduction

• Involves fusion of two haploid mycelia.

• Key stages: plasmogamy (fusion of cytoplasm), karyogamy (fusion of nuclei), and meiosis.

• In some fungi, karyogamy is delayed, resulting in a dikaryon stage (two nuclei per cell).

Life Cycle Example: Ascomycete (Saccharomyces)

• Alternates between haploid and diploid stages.

• Sexual spores (ascospores) are produced after meiosis.

Classification and Phylogeny of Fungi

Major Groups

• Classified based on morphology and molecular data (e.g., 18S rRNA sequences).

• Main groups: Ascomycota, Basidiomycota, Zygomycota, Oomycota, Myxomycota.

Ecological and Industrial Roles of Fungi

Ecological Roles

• Decomposition of organic matter.

• Symbiotic relationships (e.g., mycorrhizae, lichens).

• Pathogens of plants and animals.

Industrial Uses

• Production of antibiotics (e.g., penicillin), enzymes, and fermented foods (e.g., bread, beer).

• Biotechnological applications.

Fungi and Disease

• Cause diseases in plants (e.g., rusts, smuts) and humans (e.g., candidiasis).

• Produce mycotoxins and secondary metabolites.

• Responsible for spoilage of food and materials.

Algae and Protists

Algal Reproduction

• Can reproduce sexually or asexually.

• Methods include fragmentation, simple cell division, and spore formation.

Protists (Protozoa)

• Single-celled eukaryotes, diverse in form and function.

• Play roles in nutrient cycling, especially the carbon cycle.

• Some are pathogens; others are important in aquatic food webs.

Summary Table: Key Features of Eukaryotic Microorganisms

Group	Cell Type	Reproduction	Ecological Role	Industrial/Medical Importance
Fungi	Unicellular (yeasts), Multicellular (hyphae/mycelium)	Asexual (budding, spores), Sexual (fusion, spores)	Decomposers, symbionts, pathogens	Antibiotics, food production, disease
Algae	Unicellular or multicellular	Asexual (fragmentation, division), Sexual	Primary producers, oxygen generation	Biofuels, food additives
Protists	Unicellular	Asexual, Sexual	Nutrient cycling, pathogens	Medical relevance (e.g., malaria)

Key Terms

• Hyphae: Thread-like filaments forming the body of multicellular fungi.

• Mycelium: Mass of hyphae, main vegetative structure of fungi.

• Chitin: Polysaccharide forming fungal cell walls.

• Plasmogamy: Fusion of cytoplasm from two parent mycelia.

• Karyogamy: Fusion of nuclei from two parent cells.

• Dikaryon: Fungal cell with two genetically distinct nuclei.

• Saprophyte: Organism that feeds on dead organic matter.

• Symbiont: Organism living in close association with another, often mutually beneficial.

Additional info: The notes have been expanded to include definitions, examples, and a summary table for clarity and completeness.