Symbiosis describes a close biological interaction between different species. When both organisms benefit, it is known as mutualism. A mutualistic partnership between fungi and algae forms a unique composite organism: a lichen.
The Symbiotic Partners: Fungi and Algae
Fungi are diverse eukaryotic organisms, distinct from plants and animals. They are heterotrophic, obtaining nutrients by absorbing organic compounds from their environment. Many function as decomposers, breaking down dead organic matter and recycling elements into ecosystems. Their bodies consist of filamentous hyphae, forming a network called a mycelium.
Algae, in contrast, are mostly photosynthetic organisms. They are autotrophic, producing their own food through photosynthesis using sunlight, carbon dioxide, and water. Algae are found in aquatic or moist environments, from oceans to damp soils. Unlike plants, algae lack specialized structures like roots, stems, and leaves, and do not possess vascular tissues.
The Mutualistic Exchange
In the lichen partnership, the algal component (photobiont) provides energy. This photosynthetic partner, typically a green alga or cyanobacterium, converts light into organic carbon compounds, primarily simple sugars. These carbohydrates are transferred to the fungal partner, supplying energy for its growth and survival.
The fungal partner, known as the mycobiont, provides a protective environment and facilitates nutrient acquisition for the algal cells. Fungal hyphae form the bulk of the lichen structure, shielding algal cells from harsh external conditions like desiccation and intense sunlight. The fungus absorbs water vapor and gathers minerals from the substrate, making these resources available to the photobiont. This support allows algal cells to survive in environments where they could not otherwise exist independently.
The Formed Organism: Lichens
The symbiotic relationship between fungi and algae forms a lichen. A lichen’s body, or thallus, is structurally distinct from either partner growing alone. Fungal hyphae constitute most of the thallus, forming a mesh that surrounds and incorporates algal cells. This arrangement often creates a stratified structure, with layers of fungal filaments, an algal zone, and a fungal medulla.
Lichens exhibit diverse physical forms, categorized into three main growth types. Crustose lichens adhere tightly to their substrate, appearing as flat, crust-like patches. Foliose lichens have a flattened, leaf-like appearance with distinct upper and lower surfaces, often attached to the substrate at multiple points. Fruticose lichens are typically shrubby, coral-like, or hair-like, branching outward and attaching to the substrate at a single point. These forms enable lichens to colonize a wide range of surfaces and habitats.
Ecological Roles of Lichens
Lichens are pioneer species, colonizing barren or newly exposed land where other organisms cannot survive. They play a role in primary succession, initiating soil formation by breaking down rocks through biological weathering. This releases acids that contribute to mineral dissolution. This activity creates a substrate for subsequent plant growth, transforming inhospitable environments.
These organisms thrive in extreme environments, including deserts, arctic tundras, and high mountain peaks, enduring harsh conditions like temperature fluctuations and prolonged desiccation. Their ability to absorb water vapor and tolerate drying allows them to persist in places with minimal moisture. Lichens are also bioindicators; their sensitivity to air pollution means their presence or absence can signal the health of an ecosystem. Beyond these roles, lichens contribute to biodiversity, serve as a food source for certain animals, and participate in nutrient cycling, including nitrogen fixation by those containing cyanobacteria.