Lichens

Lichens are unique organisms that represent a symbiotic association between a fungus (mycobiont) and a photosynthetic partner (photobiont), which can be either green algae or cyanobacteria. In this mutualistic relationship:

• The fungal partner provides structure, protection, and absorbs water and minerals from the environment.
• The photosynthetic partner produces organic nutrients through photosynthesis, which nourishes both partners.

1.0Characteristics of Lichens

Lichens exhibit a set of unique characteristics that differentiate them from other fungi and plants:

• Symbiotic Relationship: Lichens are mutualistic associations between fungi and photobionts.
• Non-parasitic: Neither partner harms the other; both benefit from the association.
• Slow Growth: Many lichens grow at rates of 1–5 mm per year, depending on the environment.
• Resilience: It can withstand extreme temperatures, desiccation, high UV radiation, and nutrient-poor conditions.
• Reproduction:  It can reproduce asexually (through soredia or isidia) and sexually (via fungal spores).
• Indicator Species: Sensitive to air pollution, lichens act as bioindicators of environmental quality.

2.0Classification of Lichens

Lichens are classified based on their morphology, growth form, and reproductive structures rather than taxonomy of the fungal partner alone. The primary growth forms include:

1. Crustose Lichens
2. • Form thin crust-like layers tightly attached to rocks, soil, or tree bark.
   • Example: Graphis, Lecanora
2. Foliose Lichens
3. • Leaf-like, with distinct upper and lower surfaces loosely attached to the substrate.
   • Example: Parmeliaceae, Xanthoria
3. Fruticose Lichens
4. • Shrub-like or hair-like structures, often upright or hanging.
   • Example: Usnea, Cladonia
4. Squamulose Lichens
5. • Small, scale-like lobes, usually forming intermediate structures between crustose and foliose forms.
   • Example: Cladonia squamosa

3.0Structure of Lichens

Lichen thallus (body) is typically composed of major layers:

1. Upper Cortex
   Protective layer made of tightly packed fungal hyphae.
   Shields the lichen from UV radiation, desiccation, and mechanical damage.
2. Algal Layer / Photobiont Layer
   Contains the photosynthetic partner (green algae or cyanobacteria).
   Responsible for photosynthesis, producing carbohydrates for the fungal partner.
3. Medulla
   Loosely arranged fungal hyphae below the algal layer.
   Provides support and water retention for the lichen thallus.
4. Lower Cortex (optional)
   Present in foliose lichens, providing additional protection.
   Often bears rhizines, which anchor the lichen to the substrate.

4.0Habitat and Distribution

Lichens are highly adaptable and can grow on a variety of substrates:

• Terrestrial: Rocks, soil, tree bark, and decaying wood.
• Aquatic: Some species grow on submerged rocks or in freshwater streams.
• Extreme Environments: Arctic tundra, deserts, high-altitude mountains, and exposed cliffs.

Because lichens absorb nutrients directly from the air and rainwater, they are highly sensitive to environmental changes, making them excellent indicators of air quality and pollution.

5.0Reproduction in Lichens

Lichens reproduce through both sexual and asexual methods:

1. Asexual Reproduction

• Soredia: Small clusters of fungal hyphae and photobiont cells dispersed by wind or water.
• Isidia: Tiny finger-like outgrowths containing both partners, which break off and grow into new lichens.
• Fragmentation: Parts of the lichen thallus break off and develop independently.

2. Sexual Reproduction

• Only the fungal partner reproduces sexually through ascospores or basidiospores (depending on fungal classification).
• The fungal spores must encounter a compatible photobiont to form a new lichen.
• Sexual reproduction is less common due to the difficulty of re-establishing the symbiotic association.

6.0Examples of Lichens

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