Microbiology: Eukaryotic Microorganisms

Informal Groups: Microscopic eukaryotes are commonly categorized into key informal groups:
- Fungi: Heterotrophic organisms characterized by chitin in their cell walls.
- Algae: Simple autotrophs, meaning they are photosynthesizers.
- Protozoa: Microscopic heterotrophs that are distinct from fungi.
Kingdom Protista: This kingdom encompasses two sub-kingdoms, primarily Algae and Protozoa.
Cellularity: Microscopic eukaryotes exhibit varied cellularity:
- Always Unicellular: Protozoa.
- May be Unicellular or Multicellular: Fungi, Algae.
- Always Multicellular: Helminths (which are macroscopic but have microscopic unicellular egg or larval forms relevant to microbiology).

Eukaryotic microorganisms possess several key features that differentiate them from prokaryotes:

Presence of a true nucleus encapsulating genetic material.
Existence of various membrane-bound organelles (e.g., mitochondria, endoplasmic reticulum).
They may exist in haploid and/or diploid states.
Asexual reproduction primarily occurs via mitosis.
Sexual reproduction is accomplished through meiosis.

Mycology: The scientific study of fungi.
Microscopic Fungi: These include yeasts and molds, which are morphological designations rather than taxonomic classifications.
- Yeasts: Unicellular fungi.
- Molds: Filamentous fungi.

Yeasts: Are strictly unicellular; individual cells separate after cell division.
Filamentous Fungi (Molds): Most microscopic fungi have a filamentous structure, composed of individual threads called hyphae.
Mycelium: A visible mass of hyphae forms a mycelium.
Types of Hyphae:
- Submerged (vegetative) hyphae: Involved in nutrient absorption.
- Aerial (reproductive) hyphae: Extend into the air and produce spores.
Dimorphic Fungi: These fungi have the unique ability to grow as either single yeast cells or multicellular mycelia, depending on environmental conditions.
- Examples: Candida albicans, Histoplasma capsulatum.

Cell Walls: Primarily composed of chitin.
Cell Membranes: Typically contain ergosterol.
Exoenzymes: Fungi secrete powerful exoenzymes outside their cells to break down larger polymeric molecules like polysaccharides and proteins into smaller, absorbable units.
Decomposition: Alongside bacteria, fungi are the principal decomposers in ecosystems, capable of degrading tough substances like cellulose and lignin (found in wood).
Nutrient Cycling: Their activity releases carbon dioxide (CO_2) and various nitrogen compounds back into the soil, crucial for nutrient cycling.
Saprobes: Most fungi are saprobes, meaning they obtain nutrients primarily from dead or decaying organic matter.
Parasitism: Some fungi act as parasites of living tissues, though none are obligate parasites.
Infections: Few fungal species infect humans; however, plant infections caused by fungi are exceedingly common and economically significant.

Yeast Reproduction: Reproduce primarily by mitosis or through a process called budding.
Mold Reproduction: Produce hyphae that can be classified as:
- Septate: Possessing cross-walls.
- Non-septate (coenocytic): Lacking cross-walls, appearing as a continuous multinucleated tube.
Spore Production: Aerial hyphae produce new spores.
- Asexual Spores: Produced via mitosis.
- Sexual Spores: Produced via meiosis.
Sexual Reproduction: Sexual spores combine to form zygotes, a process that is often used as a basis for fungal categorization.

The classification of fungi (Eumycota) is currently undergoing updates and is often described as being