Structure and importance of Lichens

Structure of Lichens

The body of a lichen, known as the thallus, exhibits a distinct layered structure, though variations exist depending on the growth form. The primary layers include:

• Cortical Layer: The outermost protective layer, formed by fungal hyphae. It provides protection from desiccation, UV radiation, and physical damage.
• Algal Layer (Photobiont Layer): Located beneath the cortex, this layer contains the photosynthetic partner (alga or cyanobacterium) responsible for producing carbohydrates through photosynthesis.
• Medullary Layer: A loose, filamentous layer composed of fungal hyphae. It facilitates gas exchange and water storage.
• Rhizines: Root-like structures that anchor the lichen to the substrate. They are not true roots and primarily serve for attachment, not nutrient absorption.

Lichens exhibit three main growth forms based on their structure:

• Crustose: Forming a crust-like layer tightly adhered to the substrate (e.g., rocks, bark).
• Foliose: Leaf-like structures with distinct upper and lower surfaces, loosely attached to the substrate.
• Fruticose: Shrub-like or hair-like structures, often hanging from branches or rocks.

Importance of Lichens

Ecological Roles

Lichens play a vital role in various ecological processes:

• Bioindicators: Lichens are highly sensitive to air pollution, particularly sulfur dioxide. Their presence or absence, and their species composition, can indicate air quality levels. (Nash, 2008)
• Pioneer Species: They are often the first organisms to colonize bare rock or other inhospitable surfaces, initiating soil formation and paving the way for other plant life.
• Nutrient Cycling: Lichens contribute to nutrient cycling by fixing atmospheric nitrogen (in the case of cyanolichens) and releasing organic acids that weather rocks, releasing minerals.
• Food Source & Habitat: They serve as a food source for certain invertebrates and provide habitat for small organisms.

Economic and Other Uses

Beyond their ecological roles, lichens have several practical applications:

• Dye Production: Historically, lichens have been used to produce natural dyes for textiles (e.g., litmus from Roccella tinctoria).
• Medicinal Properties: Some lichens contain compounds with antibiotic, antiviral, and anticancer properties (e.g., Usnic acid).
• Perfume Industry: Certain lichen species are used in the perfume industry for their unique fragrance.
• Bioaccumulation studies: Lichens accumulate heavy metals and radioactive elements, making them useful in monitoring environmental contamination.

The symbiotic relationship in lichens is a remarkable example of mutualism, where both partners benefit. The fungus provides structure, protection, and mineral nutrients, while the alga/cyanobacterium provides carbohydrates through photosynthesis. This partnership allows lichens to thrive in environments where neither partner could survive alone.