Lichens — Revision Notes

⚡ 30-Second Revision

Definition: — Mutualistic symbiosis of fungus (mycobiont) + alga/cyanobacterium (phycobiont).
Mycobiont: — Fungal partner, provides structure, protection, water, minerals. Mostly Ascomycetes.
Phycobiont: — Algal/cyanobacterial partner, performs photosynthesis (food). Cyanobacteria also fix $N_2$.
Morphological Types:

* Crustose: Flat, crust-like, tightly adherent (e.g., *Graphis*). * Foliose: Leaf-like, lobed, attached by rhizines (e.g., *Parmelia*). * Fruticose: Shrub-like, branched, attached at single point (e.g., *Usnea*).

Reproduction: — Primarily asexual via soredia (powdery clusters) and isidia (finger-like outgrowths), containing both partners.
Ecological Role:

* Pioneer species: Initiate primary succession, weather rocks, form soil. * Bioindicators: Highly sensitive to air pollution, especially $SO_2$. Absence indicates pollution.

2-Minute Revision

Lichens are fascinating composite organisms, representing a classic example of mutualistic symbiosis. They consist of a fungal partner, the mycobiont (typically an Ascomycete), and a photosynthetic partner, the phycobiont, which can be a green alga (e.

g., *Trebouxia*) or a cyanobacterium (e.g., *Nostoc*). The mycobiont provides structure, protection from desiccation and UV light, and absorbs water and minerals. In return, the phycobiont produces carbohydrates through photosynthesis, feeding both partners.

If the phycobiont is a cyanobacterium, it also fixes atmospheric nitrogen. Lichens are classified into three main morphological types: crustose (crust-like, e.g., *Graphis*), foliose (leaf-like, e.g., *Parmelia*), and fruticose (shrub-like, e.

g., *Usnea*). They primarily reproduce asexually through specialized propagules like soredia (powdery clusters of fungal hyphae and algal cells) and isidia (finger-like outgrowths), ensuring the dispersal of both partners.

Ecologically, lichens are vital pioneer species in primary succession, weathering rocks and contributing to soil formation. Crucially for NEET, they are excellent bioindicators of air pollution, particularly sulfur dioxide ($SO_2$), due to their direct atmospheric absorption and lack of protective layers; their presence or absence reflects air quality.

5-Minute Revision

Lichens are not single organisms but a remarkable mutualistic symbiotic association between a fungus (mycobiont) and a photosynthetic partner (phycobiont). The mycobiont, usually an ascomycete, forms the structural body (thallus), providing protection, anchorage, and absorbing water and minerals.

The phycobiont, either a green alga like *Trebouxia* or a cyanobacterium like *Nostoc*, performs photosynthesis, supplying carbohydrates to the fungus. Cyanobacterial phycobionts also contribute fixed nitrogen.

This partnership allows lichens to thrive in harsh, nutrient-poor environments where neither partner could survive alone.

Their morphology dictates classification into three main types:

1
Crustose: — Flat, tightly adhering to the substrate, like a painted crust. Example: *Graphis scripta*.
2
Foliose: — Leaf-like, lobed, attached at a few points by rhizines. Example: *Parmelia sulcata*.
3
Fruticose: — Shrub-like, branched, or pendulous, attached at a single point. Example: *Usnea* (old man's beard).

Reproduction is predominantly asexual, ensuring the co-dispersal of both partners. Soredia are powdery clusters of algal cells enveloped by fungal hyphae, easily detached and wind-dispersed. Isidia are small, corticated, finger-like outgrowths from the thallus surface, also containing both partners, which break off and disperse.

Ecologically, lichens are critical. They are pioneer species in primary succession, colonizing bare rocks and initiating soil formation through biological weathering (secreting acids that break down rock) and adding organic matter.

Most importantly for NEET, lichens are highly sensitive bioindicators of air pollution, especially sulfur dioxide ($SO_2$). They absorb pollutants directly from the atmosphere due to their lack of a cuticle and stomata.

Different species have varying tolerances, allowing their presence, absence, and diversity to signal air quality. For instance, fruticose lichens are generally more sensitive than crustose forms. Understanding these specific roles and characteristics is key to mastering lichen-related questions in NEET.

Prelims Revision Notes

Lichens: NEET UG Revision Notes

1. Definition & Symbiosis:

* Composite organisms: Mutualistic association between a fungus (mycobiont) and a photosynthetic partner (phycobiont). * Mycobiont: Fungal component. Mostly Ascomycetes (98%), some Basidiomycetes.

Provides structure, protection, absorbs water & minerals. * Phycobiont: Algal/Cyanobacterial component. Green algae (*Trebouxia*, *Trentepohlia*) or Cyanobacteria (*Nostoc*, *Scytonema*). Performs photosynthesis (food).

Cyanobacteria also fix atmospheric nitrogen. * Mutualism: Both partners benefit. Fungus gets food; alga gets protection, water, minerals, stable environment.

2. Structure (Thallus):

* Undifferentiated body, but layered internally: * Upper Cortex: Dense fungal hyphae, protective. * Algal Layer: Phycobiont cells interspersed among fungal hyphae, beneath upper cortex. * Medulla: Loosely packed fungal hyphae, main bulk. * Lower Cortex: Fungal hyphae. * Rhizines: Fungal structures for attachment, not primary nutrient absorption.

3. Morphological Types (Growth Forms):

* Crustose: Flat, crust-like, tightly adherent to substrate. Difficult to remove. E.g., *Graphis*, *Rhizocarpon*. * Foliose: Leaf-like, flattened, lobed, attached at few points by rhizines. E.g., *Parmelia*, *Physcia*. * Fruticose: Shrub-like, branched, pendulous, attached at a single point. E.g., *Usnea* (old man's beard), *Cladonia* (reindeer moss).

4. Reproduction:

* Asexual (Vegetative): Most common and effective, disperses both partners together. * Soredia: Microscopic, powdery clusters of algal cells + fungal hyphae. Formed in soralia. * Isidia: Small, finger-like/coral-like outgrowths from thallus surface, containing both partners. * Fragmentation: Breaking off of thallus pieces. * Sexual: Only by mycobiont (fungal spores, e.g., ascospores). Spore must find compatible phycobiont to form new lichen.

5. Ecological Significance:

* Pioneer Species: First colonizers of barren environments (e.g., bare rock). Initiate primary succession. * Biological Weathering: Secrete organic acids (e.g., oxalic acid) that break down rocks, contributing to soil formation.

* Bioindicators of Air Pollution: Highly sensitive to atmospheric pollutants, especially sulfur dioxide ($SO_2$). Lack cuticle/stomata, absorb directly from air. Different species have varying tolerances.

Absence/low diversity indicates pollution. * Food Source: For some animals (e.g., reindeer feed on *Cladonia*). * Nitrogen Fixation: Lichens with cyanobacterial phycobionts contribute to nitrogen cycling.

6. Common Misconceptions:

* Not a single organism, not a plant, not parasitic, rhizines are for attachment, not roots for nutrient absorption.

Vyyuha Quick Recall

Living In Cooperation, Helping Each New Species:

Lichens are Layered Indicators of pollution Crustose, Foliose, Fruticose (CFF) Helping soil formation (Pioneer species) Everyone benefits (Mutualism) Nitrogen fixation (if cyanobacteria) Soredia and Isidia for spreading