Life on Earth is characterized by organisms living in close, long-term associations, known as symbiosis. When the interaction provides a clear benefit to both participants, it is categorized as mutualism. The unique partnership formed between a fungus and a photosynthetic organism, such as an alga or cyanobacterium, is a widely recognized example of this mutualistic living arrangement. This collaboration allows both partners to thrive in environments where neither could survive independently.
The Lichen Structure
The physical manifestation of this fungal-algal partnership is an organism known as a lichen, which is a stable, composite association. The fungal partner, called the mycobiont, constructs the entire body of the lichen, known as the thallus. This thallus is a highly organized structure that provides a protected habitat for the photosynthetic partner.
The internal structure of the thallus typically consists of distinct layers. The outer layers form a protective cortex of tightly packed fungal filaments. Directly beneath this upper cortex lies the algal layer, where the photosynthetic cells are strategically located to receive sunlight. Below the algal layer is the medulla, a looser mass of fungal hyphae that provides bulk and helps with gas exchange.
The overall appearance of the lichen thallus is determined by the dominant fungal component, leading to several recognized growth forms. Crustose lichens are flat and tightly bound to the substrate. Foliose lichens are leaf-like, flattened with distinct upper and lower surfaces, and are attached more loosely by root-like structures called rhizines. Fruticose lichens are three-dimensional, taking on a shrubby or hair-like form that is often branched and attached at only a single point.
Defining the Functional Roles of the Partners
The relationship between the fungus and the alga is defined by a highly specialized exchange of resources. The mycobiont is responsible for the physical architecture of the organism, a complex network of fungal filaments. This structure offers the photobiont crucial protection from environmental stressors, including ultraviolet radiation and the risk of desiccation.
The fungal hyphae absorb and retain water and various minerals from the environment, which are then made available to the photosynthetic cells. In return, the photobiont, which is either green algae or cyanobacteria, performs photosynthesis to produce organic carbon compounds. These compounds, primarily simple sugars, are then transferred from the photobiont to the fungus.
This transfer of carbohydrates provides the mycobiont with its sole source of metabolic energy, as the fungus is heterotrophic. This association is considered an obligate mutualism for the fungus, because it is generally unable to survive in nature without the photobiont. Conversely, the photobiont often exhibits a more facultative relationship, meaning the species can sometimes be found living independently in the environment.
When the photobiont is a cyanobacterium, the mutualism gains an added dimension through nitrogen fixation. Cyanobacteria convert atmospheric nitrogen gas into biologically usable forms, such as ammonia. This fixed nitrogen is then shared with the mycobiont, enriching the entire organism in nutrient-poor habitats.
Environmental Importance
Lichens play a significant role in various ecosystems due to their ability to survive in harsh, exposed environments. They are often considered pioneer species, being among the first life forms to colonize barren substrates like newly exposed rock surfaces. As they grow, they release organic acids that contribute to the weathering and breakdown of rock, initiating soil formation.
Their unique biology involves absorbing all necessary nutrients directly from the atmosphere, making them acutely sensitive to air quality. This sensitivity establishes lichens as reliable biological indicators, or bioindicators, of environmental health. Specific lichen species can be monitored to track the presence and concentration of atmospheric pollutants, such as sulfur dioxide and various nitrogen compounds.
The presence of certain nitrogen-sensitive species indicates relatively clean air, while a shift toward more tolerant species suggests increased nitrogen deposition from industrial or agricultural sources. Furthermore, lichens that contain cyanobacteria directly enrich nutrient-poor environments by fixing atmospheric nitrogen. This process introduces usable nitrogen compounds into the ecosystem, making the habitat more viable for other plant life.