Lichens are composite organisms, often mistaken for mosses or simple plants. They result from a close, long-term association between two or more distinct biological partners. This partnership forms a single, stable body, known as a thallus, which functions as a unified organism. The composite nature allows lichens to colonize environments, such as polar regions and deserts, where neither partner could survive alone. This mutualistic relationship ensures both participants benefit from the presence of the other.
The Dual Nature of the Partnership
The structural body of the lichen is predominantly built by the fungal partner, termed the mycobiont. In most lichens, this partner is a sac fungus (Ascomycota), which constructs the protective tissue layers and the bulk of the thallus. The mycobiont provides physical shelter, anchoring the organism and creating a chamber that helps retain moisture. This fungal framework offers a stable microenvironment, shielding the photobiont from intense sunlight and desiccation.
The second participant is the photosynthetic partner, called the photobiont, typically either a green alga or a cyanobacterium. The photobiont converts light energy into chemical energy through photosynthesis, producing carbohydrates for the entire organism. Green algae often transfer ribitol to the fungus, while cyanobacteria commonly provide glucose. This nutritional exchange drives the symbiosis, with the fungus receiving sugars and the photobiont gaining protection and water collected by the fungal structure.
In some lichens, the cyanobacterium photobiont can fix atmospheric nitrogen. This process converts inert nitrogen gas into biologically usable compounds, enriching the local environment and providing nutrients for both partners. The mycobiont physically envelops the photobiont cells to facilitate the continuous transfer of these vital organic molecules.
The Layered Internal Structure
The internal anatomy of most lichens is highly organized into distinct layers, a structure referred to as a heteromerous thallus. This organization maximizes the function of each partner within the unified body.
The Upper Cortex
The outermost layer is the upper cortex, a dense protective sheath composed of tightly packed, thick-walled fungal filaments, or hyphae. This compact arrangement functions like a skin, reducing the rate of water loss and filtering ultraviolet radiation. The upper cortex often contains pigments that give the lichen its characteristic color, further contributing to light regulation and protection.
The Photobiont Layer
Directly beneath this protective layer lies the photobiont layer, which is the engine of the lichen’s metabolism. This thin zone contains the majority of the algal or cyanobacterial cells, interspersed among loosely woven fungal hyphae. The photobiont layer is positioned immediately below the cortex to ensure maximum exposure to sunlight for photosynthesis. The fungal hyphae in this layer often form specialized contact points, sometimes penetrating the algal cell wall, to facilitate the efficient transfer of photosynthetically produced sugars.
The Medulla
The largest portion of the lichen body is the medulla, a thick, cottony layer situated below the photobiont zone. This layer consists of very loosely packed fungal hyphae, creating numerous air spaces that function primarily for gas exchange and water storage. The medulla acts as a reservoir, absorbing and holding moisture, which is especially important for lichens surviving in arid or exposed habitats. Its loose structure also aids in insulation and providing bulk to the thallus.
The Lower Cortex and Rhizines
In many lichens, particularly the leaf-like foliose forms, the bottom surface features a lower cortex, which is structurally similar to the upper cortex but often less dense. This layer anchors the lichen to its substrate through specialized, root-like fungal bundles called rhizines. These rhizines serve a purely mechanical function of attachment to surfaces like rock, soil, or tree bark. In crustose lichens, which adhere tightly to their substrate, a distinct lower cortex is typically absent, with the medulla being in direct contact with the surface.
How Lichens Reproduce as a Unit
To propagate successfully, lichens must maintain the fungal and photosynthetic partnership in the offspring. The most reliable strategy is vegetative (asexual) reproduction, where both partners are dispersed simultaneously within a single package. These methods allow the lichen to colonize new areas without the risk of the individual components failing to re-establish the symbiosis.
Soredia
One common method involves soredia, minute, powdery fragments released from the thallus surface. Each soredium is a tiny cluster of photobiont cells tightly wrapped by fungal hyphae. Soredia are light and easily dispersed by wind or water, growing into a new thallus upon landing on a suitable substrate.
Isidia
Another specialized structure is the isidium, which appears as a small, finger-like or cylindrical outgrowth on the upper surface. Isidia are larger than soredia and possess a protective outer cortex, making them more robust. When an isidium breaks off, it carries both partners, ready to establish a new colony.
Sexual Reproduction
The fungal partner is also capable of sexual reproduction by producing spores within fruiting bodies, such as cup-shaped apothecia. These fungal spores contain only the fungus’s genetic material and must independently encounter a compatible, free-living photobiont cell to form a new thallus. Since the chances of a fungal spore finding the correct partner are low, vegetative methods remain the most effective means for widespread propagation.