Ch. 29 – Fungi: Diversity, Structure, Function, and Ecological Roles

Ch. 29 – Fungi

Introduction to Fungi

Fungi are a diverse group of eukaryotic organisms that play critical roles in ecosystems as decomposers, symbionts, and pathogens. They are distinct from plants, animals, and protists, with unique strategies for acquiring nutrients and reproducing.

• Species Diversity: Estimated ~100,000 described species (possibly up to 1.5 million).

• Time of Origin: Approximately 1.0 billion years ago.

• Major Lineages: Ascomycetes (e.g., morels, yeasts), Basidiomycetes (e.g., mushrooms), and several lesser-known groups (e.g., Microsporidia).

Fungal Phylogeny and Evolution

Fungi are more closely related to animals than to plants, sharing a common ancestor with animals after diverging from plants. Phylogenetic relationships within fungi are increasingly well-resolved, revealing patterns of character evolution and lineage diversification.

• Phylogenetic Tree: Shows fungi as a sister group to animals within the Eukaryota.

• Major Clades: Ascomycetes and Basidiomycetes account for ~95% of described fungal species.

Key Traits and Evolutionary Innovations

Fungi exhibit a range of morphological and biochemical traits that have evolved multiple times, allowing them to occupy diverse ecological niches.

• Multicellularity: Evolved multiple times; some fungi are unicellular (e.g., yeasts), others multicellular with hyphae.

• Fruiting Body Development: Complex structures (e.g., mushrooms) for spore dispersal.

• Dimorphism: Some fungi can switch between unicellular and multicellular forms, especially in pathogenic species.

• Secondary Metabolism: Production of diverse chemicals, including antibiotics and toxins.

• Wood Decay: Ability to digest lignin and cellulose, critical for decomposition.

• Mycorrhizae: Symbiotic associations with plant roots for nutrient exchange.

Fungal Structure

Vegetative Structures

• Unicellular Fungi: Yeasts such as Saccharomyces cerevisiae are single-celled and important in baking and brewing.

• Multicellular Fungi: Composed of hyphae—long, threadlike filaments that form a mycelium, providing a large surface area for nutrient absorption.

Reproductive Structures

• Swimming Gametes and Spores: Motile reproductive cells in some primitive fungi.

• Zygosporangia: Spore-producing structures formed when hyphae of different mating types fuse.

• Basidia: Club-shaped cells where meiosis occurs, producing four spores (Basidiomycetes).

• Asci: Sac-like cells where meiosis and mitosis occur, producing eight spores (Ascomycetes).

Fungal Nutrition and Ecology

Fungi are heterotrophic, acquiring nutrients by absorbing organic matter from their environment. They use external digestion, secreting enzymes to break down complex molecules.

• Saprotrophic: Decompose dead organic material.

• Biotrophic: Obtain nutrients from living hosts (can be pathogenic).

• Mycelium: Extensive network of hyphae increases surface area for absorption.

• Diverse Enzyme Biochemistry: Enables digestion of tough substances like lignin and cellulose.

Ecological Interactions

• Consumption: Fungi digest living or dead organic matter, acting as decomposers or pathogens.

• Mutualism: Mycorrhizal fungi form mutualistic relationships with plant roots, exchanging nutrients and water for sugars.

• Lichens: Symbiotic associations between fungi and photosynthetic partners (algae or cyanobacteria), enabling colonization of harsh environments.

Major Ecological Importance

• Decomposition and Nutrient Recycling: Fungi are essential for breaking down organic matter and recycling nutrients in ecosystems.

• Symbiotic Relationships: Mycorrhizae are critical for plant health and ecosystem productivity.

• Pathogens and Parasites: Fungi can cause diseases in plants and animals, with significant impacts on health and agriculture.

Fungal Life Cycle

Fungi can reproduce both sexually and asexually, often involving the production of spores.

• Asexual Reproduction: Spores produced by mitosis are genetically identical to the parent.

• Sexual Reproduction: Involves plasmogamy (fusion of cytoplasm), karyogamy (fusion of nuclei), and meiosis, resulting in genetically diverse spores.

Comparative Table: Protists, Plants, Fungi, and Animals

Trait	Protists	Plants	Fungi	Animals
Age / origin	~2 billion yrs ago	~500 million yrs ago	~1 billion yrs ago	Additional info: ~600 million yrs ago
# of species (mostly underestimated)	~100,000	~300,000	~100,000	Additional info: ~1.5 million
Uni or Multicellular	Unicellular	Multicellular	Both	Multicellular
Mitochondria	Yes	Yes	Yes	Yes
Chloroplast	Some, but not all	Yes	No	No
Cell Wall	Some, but not all	Yes, cellulose	Yes, chitin	No
Auto- or Heterotrophic	Both	Autotrophic (some parasites)	Heterotrophic	Heterotrophic

Summary of Major Evolutionary Innovations

• Morphological and Biochemical Diversity: Enables fungi to exploit a wide range of ecological niches.

• Symbioses: Multiple types of mycorrhizal and lichen relationships, often with multiple evolutionary origins.

• Decomposition: Fungi are highly efficient decomposers due to their large surface area, diverse enzyme biochemistry, and extracellular digestion.

Example: Saccharomyces cerevisiae (Baker's Yeast)

• Unicellular ascomycete fungus.

• Used in bread making (aerobic respiration produces CO2 for rising) and alcohol production (anaerobic fermentation).

Formulas and Equations

• Photosynthesis (for comparison with plants):

• $6CO_2 + 6H_2O + light \rightarrow C_6H_{12}O_6 + 6O_2$

• Aerobic Respiration (as in yeast):

• $C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + \text{energy}$

• Anaerobic Fermentation (yeast):

• $C_6H_{12}O_6 \rightarrow 2C_2H_5OH + 2CO_2 + \text{energy}$

Additional info:

• Fungal diseases are a growing concern for both human health and agriculture, with over 1.6 million deaths annually and more than a billion people affected worldwide.

• Environmental changes may be contributing to the rise in fungal diseases.