Fungi and the Colonization of Land

Role of Fungi in Ecosystems

Fungi are essential organisms in terrestrial ecosystems, performing critical ecological functions that support nutrient cycling and plant health.

• Chemical Decomposers: Fungi break down complex organic matter, such as cellulose and lignin, returning inorganic minerals and carbon to the soil and atmosphere for use by primary producers.

• Evolutionary Symbionts: Fungi formed early symbiotic relationships with ancestral non-vascular plants, acting as extended root systems and facilitating the colonization of land.

• Population Pathogens: Some fungi act as parasites, controlling populations by causing diseases in plants and animals.

Nutritional Adaptations of Fungi

Fungi are absorptive heterotrophs, meaning they obtain nutrients by absorbing dissolved organic molecules from their environment.

• Absorptive Heterotrophy: Fungi secrete enzymes into their surroundings and absorb the resulting small organic molecules.

How Fungi Obtain Nutrients

• Secretion of Exoenzymes: Fungi release hydrolytic enzymes (exoenzymes) that break down complex macromolecules externally.

• External Digestion & Absorption: The breakdown products are absorbed across the fungal cell membrane.

Structure of Fungi: Hyphae and Mycelia

Fungi have unique structural adaptations that maximize nutrient absorption.

• Hyphae: Microscopic, tubular filaments that form the main body of multicellular fungi.

• Mycelia: A mass of interwoven hyphae that increases surface area for absorption.

• Chitin: Fungal cell walls are reinforced with chitin, a strong, flexible polysaccharide.

Fungal Relationships: Pathogenic, Beneficial, and Mutualistic

• Pathogenic: About 30% of fungi are parasites or pathogens, affecting plants (e.g., chestnut blight) and animals (e.g., chytridiomycosis in amphibians).

• Mutualistic:

  • Mycorrhizae: Symbiosis between fungal hyphae and plant roots, exchanging minerals for sugars.

  • Lichens: Symbiosis between fungi and photosynthetic microorganisms (algae or cyanobacteria).

Fungal Reproduction: Spores, Asexual and Sexual Cycles

Fungi reproduce by producing spores, either asexually or sexually.

• Asexual Reproduction: Molds produce haploid spores by mitosis; yeasts reproduce by cell division or budding.

• Sexual Reproduction: Involves three phases:

  1. Plasmogamy: Fusion of cytoplasm from two parent mycelia.

  2. Heterokaryotic Stage: Coexistence of two haploid nuclei (n + n) in the same cell.

  3. Karyogamy: Fusion of nuclei to form a diploid zygote (2n), which undergoes meiosis to produce spores.

Major Fungal Groups and Their Characteristics

Challenges for Plants Colonizing Land

• Desiccation: Risk of water loss in air.

• Structural Collapse: Need for support against gravity.

• Resource Separation: Light and CO2 in air; water and minerals in soil.

• Gamete Transport: Need for new fertilization strategies without water.

Derived Traits for Plant Success on Land

• Alternation of Generations: Life cycle alternates between multicellular haploid and diploid stages.

• Waxy Cuticle: Reduces water loss.

• Stomata: Pores for gas exchange, regulated to minimize water loss.

• Apical Meristems: Regions of growth at shoot and root tips.

Sporopollenin

• Sporopollenin: A tough, inert polymer that protects spores and zygotes from desiccation and UV radiation.

Characteristics of Green Algae, Bryophytes, and Vascular Plants

How Bryophytes Survive Without Vascular Tissue

• Remain short and close to moisture sources.

• Transport water and nutrients by diffusion and osmosis.

• Live in moist, shaded environments.

• Use rhizoids for anchoring, not absorption.

Major Groups of Vascular Plants

Strategies for Completing the Plant Life Cycle on Land

• Airborne Pollen: Male gametophyte travels through air, eliminating need for water in fertilization.

• Multicellular Seeds: Protect and nourish the developing embryo, allowing survival in dry conditions.

Alternation of Generations and Evolution of Fertilization/Dispersal

• Gametophyte (n): Produces gametes by mitosis.

• Sporophyte (2n): Produces spores by meiosis.

• Evolutionary Trend: Gametophyte reduced; sporophyte dominant.

Pollen and Seeds vs. Sperm and Spores

• Pollen: Multicellular male gametophyte; delivers sperm without water.

• Sperm: Single haploid cell; requires water for movement in non-seed plants.

• Seeds: Multicellular, diploid, with food supply and protective coat.

• Spores: Single-celled, haploid, must grow into gametophyte.

Lichen Anatomy and Symbiosis

• Anatomy: Layers of fungal hyphae surround a central zone of photosynthetic algae or cyanobacteria.

• Symbiosis: Algae/cyanobacteria provide sugars (and sometimes fixed nitrogen); fungus offers structure, protection, and mineral extraction.

Relationship Between Roots and Fungi

• Mycorrhizal Network: Fungal hyphae integrate with plant roots, forming mycorrhizae.

• Reciprocal Exchange: Fungi provide minerals and water; plants provide organic sugars.

Additional info: Mycorrhizal associations are ancient and fundamental to plant success on land.