A lichenized fungus is a composite organism formed from a partnership between a fungus and a photosynthetic partner, either a green alga or a cyanobacterium. This association creates a unique structure called a thallus, which looks and behaves differently than either partner would alone. Despite their plant-like appearance, lichens are classified within the Fungus Kingdom because the fungus dictates their physical form. This symbiotic lifestyle is remarkably successful, with estimates suggesting that about 20% of all known fungal species engage in this type of partnership.
The Two Partners in Symbiosis
Every lichen results from a symbiotic relationship between a fungus (the mycobiont) and a photosynthetic partner (the photobiont). The mycobiont, often a species of Ascomycetes, provides the bulk of the lichen’s body and creates a protective environment. Its fungal filaments, or hyphae, surround the photosynthetic cells, shielding them from intense sunlight and dehydration. The fungus is also responsible for anchoring the lichen to its substrate and absorbing water and essential nutrients from the surroundings.
The photobiont’s role is to perform photosynthesis, converting sunlight, carbon dioxide, and water into energy-rich sugars. These sugars nourish both partners, as the fungus cannot produce its own food. This exchange allows lichens to thrive in extreme habitats, from arid deserts to frozen arctic tundra, where neither organism could survive alone. The relationship is so integrated that the fungus can even control the photobiont’s cell division to ensure a stable system.
Common Forms of Lichens
Lichens exhibit various growth forms, categorized into three main types. One of the most common is crustose, which appears as a thin, flat crust tightly fused to a substrate like rock or tree bark. These lichens can look like a layer of dried paint and are so firmly attached that they cannot be removed without taking part of the underlying surface with them.
Another prevalent type is the foliose lichen, characterized by a leafy, flattened structure. Unlike crustose lichens, foliose lichens are more loosely attached to their substrate, often by small root-like structures called rhizines. This form gives them a lobe-like appearance, with a distinct upper and lower surface, resembling small, flat leaves.
The third form is fruticose, which has a shrubby, hair-like, or beard-like appearance with branching structures. They are often attached to their substrate at a single point, growing upright or hanging from tree branches. This three-dimensional growth maximizes their surface area for sunlight and moisture.
Ecological Functions
Lichenized fungi are pioneer species, often the first to colonize bare rock and other barren landscapes. They begin the slow process of soil formation by secreting acids that chemically weather the rock surface. This process gradually breaks the rock down into smaller mineral particles.
The lichen’s physical structure also builds soil by trapping windblown dust and organic debris. As older parts of the lichen die and decompose, they add organic matter to the accumulating mineral particles. This mixture of mineral and organic material forms a primitive soil that can support the germination and growth of other organisms, such as mosses and eventually vascular plants.
Lichens contribute to the food web, with animals like caribou and reindeer relying on them as a primary winter food source. In lichens with a cyanobacterial partner, a process called nitrogen fixation occurs. These cyanobacteria convert nitrogen gas from the atmosphere into compounds that plants can use, enriching the soil for the entire community.
Significance as Bioindicators
Lichens are sensitive to atmospheric pollution because they absorb water and nutrients directly from the air. They lack the protective outer layer, or cuticle, that vascular plants have, meaning they cannot filter what they absorb. This direct exposure makes them susceptible to airborne pollutants like sulfur dioxide, nitrogen compounds, and heavy metals.
The health and diversity of local lichen populations serve as a reliable indicator of air quality. In regions with clean air, a wide variety of species, including the more sensitive fruticose and foliose types, will be abundant. In areas with high air pollution, these sensitive species die off, leaving only the most tolerant crustose species or no lichens at all.
Scientists use lichen surveys as a low-cost method for monitoring long-term changes in air quality. By observing the types of lichens present and their health, researchers can map pollution zones and assess the environmental impact of industrial activities. The disappearance of lichens from a forest can be an early warning that air quality is declining.