Chapter 12

The Eukaryotes: Fungi, Algae, Protozoa, and Helminths

Fungi

Fungi are a diverse group of eukaryotic organisms with over 100,000 known species. They play crucial roles in ecosystems, industry, and medicine, both as pathogens and beneficial organisms.

• Mycology: The study of fungi.

• Chemoheterotrophs: Fungi obtain energy by decomposing organic matter.

• Respiration: Most fungi are aerobic, but some are facultative anaerobes.

• Ecological Role: Decompose dead plants and recycle elements in the food chain.

• Industrial Importance: Used in food and antibiotic production.

Comparison of Fungi and Bacteria

Fungi and bacteria differ in cell structure, metabolism, and reproduction. The following table summarizes key differences:

Feature	Fungi	Bacteria
Cell Type	Eukaryotic	Prokaryotic
Cell Membrane	Sterols present	Sterols absent (except Mycoplasma)
Cell Wall	Glucans, mannans, chitin (no peptidoglycan)	Peptidoglycan
Spores	Sexual and asexual reproductive spores	Endospores (not for reproduction); some asexual reproductive spores
Metabolism	Heterotrophic; aerobic, facultatively anaerobic	Heterotrophic, autotrophic; aerobic, facultatively anaerobic, anaerobic

Environmental and Nutritional Adaptations

• Grow best at pH 5

• Most molds are aerobic; most yeasts are facultative anaerobes

• Can grow in high sugar and salt concentrations; resistant to osmotic pressure

• Can grow in low moisture environments

• Require less nitrogen than bacteria

• Can metabolize complex carbohydrates

Medically Important Fungi

• Mucoromycota: Includes Rhizopus stolonifer (black bread mold)

• Microsporidia: Obligate intracellular parasites, lack mitochondria, cause diseases such as chronic diarrhea and keratoconjunctivitis

• Ascomycota and Basidiomycota: Other major fungal phyla

Fungal Diseases (Mycoses)

• Systemic mycoses: Deep infections affecting multiple tissues/organs

• Subcutaneous mycoses: Beneath the skin

• Cutaneous mycoses: Affect hair, skin, nails (e.g., ringworm, athlete’s foot)

• Superficial mycoses: Localized, e.g., hair shafts

• Opportunistic mycoses: Pathogenic in compromised hosts (e.g., yeast infection, thrush)

Economic Effects of Fungi

• Aspergillus niger: Production of citric acid

• Aspergillus terreus: Statins (cholesterol-lowering drugs)

• Saccharomyces cerevisiae: Bread, wine, hepatitis B vaccine

• Trichoderma: Cellulase production

• Taxomyces: Taxol (anticancer drug)

• Tolypocladium inflatum: Cyclosporine (immunosuppressant)

• Coniothyrium minitans: Biological pesticide

• Paecilomyces: Biological control of termites

Algae

Algae are a diverse group of mostly aquatic, photosynthetic organisms. They are not a formal taxonomic group but are important in ecological and industrial contexts.

• Structure: Unicellular or filamentous photoautotrophs; lack roots, stems, and leaves

• Habitat: Mostly aquatic; water is essential for growth and reproduction

Selected Phyla of Algae

• Brown algae (kelp): Multicellular, macroscopic; cell walls contain cellulose and alginic acid; produce algin (food thickener and industrial uses)

• Red algae: Mostly multicellular; live at greater depths; harvested for agar and carrageenan (food thickeners); some produce toxins

• Green algae: Cellulose cell walls; unicellular or multicellular; contain chlorophyll a and b; store starch; ancestral to terrestrial plants

• Diatoms: Unicellular or filamentous; cell walls of pectin and silica; store oil; produce domoic acid (neurotoxin)

• Dinoflagellates: Unicellular; cellulose in plasma membrane; component of plankton; produce neurotoxins (saxitoxins) causing paralytic shellfish poisoning

Roles of Algae in Nature

• Fix CO2 into organic molecules

• Produce 80% of Earth's O2

• Symbionts of animals

Protozoa

Protozoa are unicellular eukaryotes found in water and soil. They exhibit complex life cycles and diverse nutritional strategies.

• Over 50,000 species; some are normal microbiota, others cause disease

• Plasmodium species: Cause malaria

• Animal-like nutrition

• Complex life cycles

Medically Important Protozoa

• Euglenozoa: Photoautotrophs or facultative chemotrophs; hemoflagellates transmitted by blood-feeding insects; cause African sleeping sickness (Trypanosoma brucei) and Chagas disease (Trypanosoma cruzi)

• Apicomplexa: Nonmotile, obligate intracellular parasites; complex life cycles; Toxoplasma gondii (transmitted by cats, causes fetal infections), Cryptosporidium (waterborne illness)

Helminths

Helminths are multicellular parasitic worms specialized for life in hosts. They are divided into two main phyla:

• Platyhelminthes: Flatworms

• Nematoda: Roundworms

Characteristics of Helminths

• Multicellular eukaryotic animals

• Specialized for parasitism: may lack digestive system, reduced nervous system, reduced locomotion, complex reproductive system

Arthropods as Vectors

Arthropods are animals with segmented bodies, hard exoskeletons, and jointed legs. Some act as vectors, transmitting pathogenic microorganisms.

• Arachnida: Eight legs (spiders, mites, ticks)

• Crustacea: Four antennae (crabs, crayfish)

• Insecta: Six legs (bees, flies, mosquitoes, lice)

Transmission Mechanisms

• Mechanical transmission: Pathogen is carried on the vector's body

• Biological transmission: Pathogen multiplies within the vector

• Definitive host: Microbe's sexual reproduction occurs in the vector

Additional info: The notes have been expanded to provide academic context and definitions for key terms, as well as examples and applications relevant to microbiology students.