Lichens are unique biological entities found across the planet, often appearing as colorful crusts or growths on trees and rocks. They are not plants, mosses, or simple fungi, but instead represent complex, composite organisms. Their ability to thrive in extremely harsh environments, from scorching deserts to the arctic tundra, is a testament to their resilience. Lichens possess an unusual structure that allows them to colonize substrates where other life forms cannot survive, making them ubiquitous components of nearly every ecosystem.
The Symbiotic Partnership
The entire existence of a lichen is founded upon a stable, intimate relationship known as a symbiosis. The primary partners are a fungus, called the mycobiont, and a photosynthetic organism, known as the photobiont. This association is so successful that the combined entity, the lichen, develops properties and a physical form entirely different from either partner growing alone.
The mycobiont, typically an Ascomycete fungus, provides the entire physical structure, or thallus, of the lichen. The fungal filaments, or hyphae, create a protective layer that shelters the other partner from harsh light, desiccation, and physical damage. The fungus also absorbs water and minerals directly from the atmosphere and the surface they grow on, acting as the lichen’s anchor and defense system.
The photobiont is usually a green alga or a cyanobacterium, and its role is to produce food through photosynthesis. The sugars generated by the photobiont are absorbed by the mycobiont, providing the energy necessary for the fungus to grow and maintain the structure. When a cyanobacterium is the photobiont, it adds the ability to fix atmospheric nitrogen, converting it into a biologically usable form for both partners.
This unique interdependence enables lichens to colonize barren environments like bare rock or exposed soil. The fungal partner is rarely found living outside of the lichen association, though the algal partner can often live independently in a moist environment. Recent research has also revealed that many lichens contain a third partner, a Basidiomycete yeast, suggesting that the complexity of this relationship is even greater than previously thought.
Structural Diversity
The physical appearance of a lichen, known as its growth form, is determined primarily by the fungal partner and is the most common way to categorize them. Lichens are broadly classified into three main types based on the morphology of their body, or thallus. The internal structure of the thallus is layered, with the fungal hyphae forming a dense protective outer layer called the cortex, and the photobiont cells situated beneath it in a distinct layer.
The crustose lichens form a thin, flat crust that adheres tightly to the substrate, making it impossible to separate without damaging the surface. They often look like a patch of paint sprayed onto rocks or tree bark. These forms are typically the first to colonize a new, exposed surface and are highly resistant to environmental stress.
Foliose lichens have a flattened, leaf-like appearance with lobes that partially peel away from the surface. They are attached to the substrate at various points by root-like fungal structures called rhizines, which allows for some air circulation beneath the thallus. This form features a complex internal structure, typically possessing a distinct upper cortex, an algal layer, a loosely packed medulla, and a lower cortex.
Fruticose lichens are the most complex in structure, often appearing shrubby, bushy, or hair-like, growing either upright or hanging down. Their branches are typically cylindrical or ribbon-shaped, and unlike the other forms, they lack a distinct upper and lower surface. The impressive, hanging strands often referred to as “old man’s beard” are examples of this growth form.
Environmental Roles
Lichens play multiple roles in the environment, many of which are disproportionate to their small size and slow growth rate. They function as pioneer species, initiating ecological succession on barren landscapes. By settling on bare rock, they secrete mild acids that slowly break down the surface. This process, known as biological weathering, is the first step in creating new soil.
Their physiology makes them exceptional natural monitors of air quality, known as bioindicators. Because lichens lack roots and absorb all their water and nutrients directly from the atmosphere, they are highly sensitive to airborne pollutants. The presence, absence, or health of certain species can indicate the level of contaminants, particularly sulfur dioxide and nitrogen compounds, in the air.
Lichens also serve as a foundational food source in specific ecosystems, providing forage for a range of herbivores. Species like Cladonia rangiferina, commonly known as reindeer moss, are a substantial part of the winter diet for caribou and reindeer in arctic and subarctic regions. They are also used by various bird species as nesting material and by insects for camouflage.
In specific species where the photobiont is a cyanobacterium, lichens contribute significantly to nutrient cycling by performing nitrogen fixation. This process converts inert atmospheric nitrogen gas into biologically available compounds like ammonia, which enriches the soil and supports the growth of other plants. Their ability to absorb heavy metals and other pollutants also means they act as long-term repositories, effectively cleaning the air and providing data for environmental studies.