Lichens are unique composite organisms formed from a symbiotic relationship, typically a fungus and a photosynthetic partner. They are widespread, thriving in diverse environments from polar regions to deserts, and can be found on rocks, trees, and soil. Their prevalence highlights their adaptability and significant role in various ecosystems, contributing to processes like soil formation and nutrient cycling.
The Symbiotic Partners
A lichen consists of two main components: the mycobiont (fungal partner) and the photobiont (photosynthetic partner). The mycobiont constitutes the larger portion of the lichen’s body, called the thallus, and provides its structural framework. The photobiont is a green alga (Chlorophyta) or a cyanobacterium (Cyanophyta), performing photosynthesis to produce sugars that nourish both partners.
The fungal partner protects photosynthetic cells by retaining water and serving as a larger surface area for collecting mineral nutrients from rain and dust. It can also provide minerals from the substrate. If a cyanobacterium is present, it can fix atmospheric nitrogen, supplementing the green alga’s activities. This intricate partnership allows both organisms to thrive in conditions where neither could survive independently.
External Morphology
Lichens display various growth forms, known as thallus types, determined by the fungal partner. Crustose lichens appear as a thin crust tightly adhering to the substrate, often on rocks or bark. They are difficult to remove without damaging the surface and blend seamlessly with their surroundings.
Foliose lichens have a flattened, leaf-like appearance, often with lobes somewhat detached from the substrate. They attach to surfaces using root-like structures called rhizines. Fruticose lichens exhibit a shrub-like or hair-like structure, often branching upright or hanging down. Other forms include squamulose lichens, with small, scale-like lobes, and leprose lichens, which appear as a powdery, undifferentiated layer.
Internal Structure
The internal organization of a lichen thallus is layered, with distinct functions for each part. The uppermost layer is the upper cortex, a protective outer covering of densely packed fungal hyphae. This layer can vary in thickness, sometimes reaching several hundred micrometers.
Beneath the upper cortex is the photobiont layer, where algal or cyanobacterial cells are located. These photosynthetic cells are enveloped by fungal hyphae, which may penetrate the algal cell walls, forming haustoria. This arrangement maximizes nutrient exchange between partners.
Below the photobiont layer is the medulla, a loosely interwoven layer of fungal hyphae that lacks photosynthetic cells. It plays a role in gas exchange and nutrient storage. In many lichens, a lower cortex is present beneath the medulla, mirroring the upper cortex in its dense fungal hyphae and providing attachment to the substrate through rhizines.
Reproductive Structures
Lichens employ both asexual and sexual methods for reproduction. Asexual reproduction allows for the dispersal of both symbiotic partners simultaneously. Soredia are minute, powdery bundles of fungal hyphae enveloping algal cells. They are released from the thallus surface and dispersed by wind or water to establish new lichens.
Isidia are finger-like or coral-like outgrowths from the thallus surface. These structures contain both fungal and algal components, and they can break off to develop into new lichen individuals when they land on a suitable substrate. Both soredia and isidia ensure the propagation of the complete symbiotic unit.
Sexual reproduction in lichens is carried out solely by the fungal partner, which forms fruiting bodies. Common types are apothecia (cup-shaped or disc-shaped structures) and perithecia (flask-shaped and partially embedded within the thallus). Within these fruiting bodies, the fungus produces spores through meiosis. Upon dispersal, these fungal spores must encounter a compatible photobiont to re-establish the symbiotic relationship and form a new lichen thallus.