Model Answer
0 min readIntroduction
Lichens represent a fascinating symbiotic partnership between a fungus (the mycobiont) and one or more photosynthetic partners, usually an alga or cyanobacterium (the photobiont). This unique association allows lichens to colonize diverse and often harsh environments where neither partner could survive alone. They are not a single organism, but rather a composite organism exhibiting characteristics distinct from both its components. Lichens are globally distributed, playing crucial roles in various ecosystems, from pioneering new substrates to serving as bioindicators of air quality. Understanding their salient features is vital for appreciating their ecological significance and evolutionary adaptations.
Thallus Structure
The body of a lichen, known as the thallus, is highly organized and differs significantly from the structures of the individual fungal and algal components. It typically consists of three layers:
- Cortex: The outermost protective layer, formed by fungal hyphae, providing protection from desiccation, UV radiation, and physical damage.
- Algal Layer (Photobiont Layer): Located beneath the cortex, this layer contains the photosynthetic partner(s) responsible for carbohydrate production through photosynthesis.
- Medulla: The innermost, loosest layer, composed of fungal hyphae interspersed with algal cells. It facilitates gas exchange and water absorption.
Lichens exhibit diverse growth forms, categorized as:
- Crustose: Forming a crust-like layer tightly adhered to the substrate (e.g., rocks, bark).
- Foliose: Leaf-like, loosely attached to the substrate with lobes or scales.
- Fruticose: Shrub-like or hair-like, branching and often hanging from substrates.
Reproduction
Lichens reproduce through both sexual and asexual means. The fungal component can reproduce sexually via ascospores produced in apothecia (disc-like structures) or perithecia (flask-like structures). However, the algal component typically reproduces asexually. Asexual reproduction in lichens is more common and occurs through:
- Soredia: Small, powdery granules containing both fungal hyphae and algal cells, dispersed by wind or water.
- Isidia: Small, cylindrical or coral-like outgrowths containing both partners, breaking off and establishing new lichens.
- Fragmentation: The thallus itself can break into fragments, each capable of forming a new lichen.
Physiological Adaptations
Lichens exhibit remarkable physiological adaptations enabling them to survive in extreme environments:
- Poikilohydry: The ability to tolerate extreme desiccation and rehydrate rapidly upon water availability.
- Photosynthesis: The algal/cyanobacterial component performs photosynthesis, providing carbohydrates to the fungus.
- Nutrient Acquisition: Lichens can obtain nutrients directly from the atmosphere (nitrogen fixation by cyanobacteria), rainwater, and substrate.
- Secondary Metabolites: Production of unique chemical compounds (lichen acids) providing protection against herbivores, UV radiation, and pathogens.
Ecological Roles
Lichens play significant ecological roles:
- Pioneer Species: Colonizing bare rock and initiating soil formation.
- Bioindicators: Sensitive to air pollution, particularly sulfur dioxide, making them valuable indicators of environmental quality.
- Food Source: Providing food for various animals, including reindeer and invertebrates.
- Habitat Provision: Offering shelter and microhabitats for small invertebrates.
Conclusion
In conclusion, lichens are remarkable symbiotic organisms characterized by a unique thallus structure, diverse reproductive strategies, and exceptional physiological adaptations. Their ability to thrive in harsh environments and their crucial ecological roles highlight their importance in various ecosystems. Continued research into lichen biology is essential for understanding their responses to environmental changes and for utilizing their potential as bioindicators and sources of novel compounds.
Answer Length
This is a comprehensive model answer for learning purposes and may exceed the word limit. In the exam, always adhere to the prescribed word count.