What Two Organisms Make Up A Lichen

Lichens, those fascinating splashes of color and texture you often see on rocks, trees, and even soil, are far more complex than they appear. They represent a remarkable partnership, a symbiosis, between two completely different organisms: a fungus and an alga (or, in some cases, a cyanobacterium). This union creates a unique entity with properties and characteristics distinct from either of its individual components. Understanding the nature of this partnership is key to appreciating the ecological significance and sheer ingenuity of lichens.

The Two Partners: Fungus and Alga (or Cyanobacterium)

At its core, a lichen is a composite organism formed through a symbiotic relationship between a fungus (the mycobiont) and an alga or cyanobacterium (the photobiont). Let's delve deeper into the role each partner plays:

1. The Fungus (Mycobiont): The Architect and Protector

• The fungal partner, or mycobiont, typically makes up the bulk of the lichen structure, sometimes accounting for as much as 95% of its mass.
• In the vast majority of lichens, the mycobiont belongs to the Ascomycota phylum (sac fungi). More rarely, it can be a Basidiomycota (club fungi). This fungal component determines the overall shape, size, and structure of the lichen, often referred to as the thallus.
• Role and Function:
  • Structure: The mycobiont provides the physical framework for the lichen. It forms a protective outer layer called the cortex and a more loosely packed inner layer called the medulla. These layers shield the photobiont from excessive sunlight and desiccation.
  • Attachment: The fungus anchors the lichen to its substrate, whether it's a rock, tree bark, or soil. It does this using specialized structures called rhizines, which are root-like hyphae that penetrate the surface.
  • Water and Nutrient Absorption: The mycobiont is highly efficient at absorbing water and nutrients from the environment, including rainwater, atmospheric moisture, and dissolved minerals. These are then transferred to the photobiont.
  • Reproduction: The fungus is primarily responsible for sexual reproduction in lichens. It produces spores within specialized structures, such as apothecia (cup-shaped) or perithecia (flask-shaped), which are visible on the lichen surface.

2. The Alga or Cyanobacterium (Photobiont): The Food Producer

• The photobiont, whether an alga or a cyanobacterium, resides within the protective structure created by the mycobiont.
• Algae are eukaryotic organisms containing chloroplasts, enabling them to perform photosynthesis. Common algal photobionts include species from the genera Trebouxia, Pseudotrebouxia, and Trentepohlia.
• Cyanobacteria, also known as blue-green algae, are prokaryotic organisms that also perform photosynthesis. Common cyanobacterial photobionts include species from the genera Nostoc, Gloeocapsa, and Scytonema.
• Role and Function:
  • Photosynthesis: The photobiont is responsible for producing food for the lichen through photosynthesis. Using sunlight, water, and carbon dioxide, it synthesizes carbohydrates (sugars) that provide energy for both the alga/cyanobacterium and the fungus.
  • Nitrogen Fixation (Cyanobacteria Only): Some cyanobacterial photobionts can fix atmospheric nitrogen, converting it into a usable form (ammonia) that benefits both partners. This is particularly important in nutrient-poor environments.

The Symbiotic Relationship: A Partnership of Give and Take

The relationship between the mycobiont and the photobiont is a classic example of symbiosis, specifically a type often described as mutualism (although the exact nature is debated). This means that both partners benefit from the association.

• What the Fungus Gets: The fungus receives a constant supply of carbohydrates produced by the alga or cyanobacterium through photosynthesis. This provides the fungus with the energy it needs to grow and reproduce.
• What the Alga/Cyanobacterium Gets: The alga or cyanobacterium receives a protected environment within the fungal thallus. The fungus provides shelter from harsh environmental conditions, such as excessive sunlight, desiccation, and extreme temperatures. It also provides the photobiont with water and essential minerals.

Is it Mutualism, Parasitism, or Controlled Parasitism?

While often described as mutualism, the lichen symbiosis is more complex than a simple win-win situation. There is evidence suggesting that the fungal partner may exert a degree of control or even parasitism over the photobiont.

• Controlled Parasitism: Some researchers argue that the fungus may actively control the growth and reproduction of the alga or cyanobacterium, extracting resources without providing equal benefits. In some cases, the alga may even be damaged or killed by the fungus.
• The Balance of Power: The exact nature of the relationship likely varies depending on the specific lichen species and environmental conditions. It's probably best described as a dynamic balance, where the fungus benefits more, but the alga/cyanobacterium still gains a significant advantage compared to living independently.

The Lichen Thallus: A Unique Structure

The lichen thallus is the body of the lichen, a structure that is distinctly different from either the fungus or the alga/cyanobacterium living alone. It is the result of the close interaction between the two partners and exhibits several unique features.

Types of Lichen Thalli:

Lichens are broadly classified based on the morphology of their thalli:

• Crustose: These lichens form a crust-like layer that is tightly attached to the substrate. They are often difficult to remove without damaging the surface they grow on.
• Foliose: These lichens have a leaf-like structure with distinct upper and lower surfaces. They are attached to the substrate by rhizines and can be easily detached.
• Fruticose: These lichens are shrub-like or hair-like, with a three-dimensional structure. They are attached to the substrate at a single point and can be erect or pendulous.
• Squamulose: These lichens consist of small, scale-like units called squamules, which are clustered together.

Anatomy of a Typical Foliose Lichen Thallus:

A typical foliose lichen thallus exhibits the following layers:

• Upper Cortex: A protective outer layer composed of tightly packed fungal hyphae. It provides physical protection and reduces water loss.
• Algal Layer (Photobiont Layer): A layer beneath the upper cortex where the algal or cyanobacterial cells are located. This is the zone where photosynthesis occurs.
• Medulla: A loosely packed layer of fungal hyphae that provides air space and facilitates gas exchange.
• Lower Cortex: A protective lower layer similar to the upper cortex.
• Rhizines: Root-like hyphae that extend from the lower cortex and anchor the lichen to the substrate.

Reproduction in Lichens

Lichens reproduce through both sexual and asexual means. The fungal partner is primarily responsible for sexual reproduction, while asexual reproduction involves the dispersal of fragments containing both the fungus and the alga/cyanobacterium.

Sexual Reproduction:

• The fungal partner produces spores within specialized structures called apothecia or perithecia, which are often visible on the surface of the lichen thallus.
• When the spores are released, they must find a compatible algal or cyanobacterial partner in order to form a new lichen. This is a challenging process, and the success rate is relatively low.

Asexual Reproduction:

• Lichens can reproduce asexually through fragmentation, where small pieces of the thallus break off and are dispersed. If these fragments land in a suitable environment, they can grow into new lichens.
• Some lichens produce specialized asexual propagules called soredia or isidia.
  • Soredia: Small clusters of algal cells surrounded by fungal hyphae. They are released from the lichen thallus through openings called soralia.
  • Isidia: Small, cylindrical outgrowths from the upper cortex that contain both fungal and algal cells. They break off easily and can be dispersed by wind or water.

The Ecological Significance of Lichens

Lichens play important roles in various ecosystems:

• Pioneer Species: They are often the first organisms to colonize bare rock or other harsh environments, contributing to soil formation through weathering and the accumulation of organic matter.
• Bioindicators: Lichens are highly sensitive to air pollution and can be used as indicators of environmental quality. The presence or absence of certain lichen species can provide valuable information about the levels of pollutants in the air.
• Food Source: Lichens serve as a food source for various animals, including reindeer, caribou, and snails.
• Habitat: They provide habitat for small invertebrates, such as mites and springtails.
• Nitrogen Fixation: Lichens with cyanobacterial photobionts contribute to nitrogen fixation, making nitrogen available to other organisms in the ecosystem.
• Medicinal Uses: Some lichen species have been used in traditional medicine for their antibiotic, anti-inflammatory, and anticancer properties.
• Dyes and Perfumes: Certain lichens are used to produce natural dyes and perfumes.

Examples of Common Lichens

Here are a few examples of common lichens you might encounter:

• Cladonia rangiferina (Reindeer Lichen): A fruticose lichen that is a major food source for reindeer and caribou in arctic and subarctic regions.
• Usnea spp. (Beard Lichens): Fruticose lichens that hang from trees, resembling beards. They are sensitive to air pollution and are often used as bioindicators.
• Xanthoria parietina (Maritime Sunburst Lichen): A foliose lichen with a bright orange color, commonly found on rocks and walls near the coast.
• Parmelia spp. (Shield Lichens): Foliose lichens with a broad, leaf-like thallus. They are common on trees and rocks in various habitats.
• Crustose lichens: These are very common and appear as a crust on rocks, bark, or soil. Examples include species from the genera Lecanora and Rhizocarpon.

The Ongoing Mystery of Lichens

Despite decades of research, many aspects of the lichen symbiosis remain a mystery. Scientists are still investigating the precise mechanisms by which the fungus and alga/cyanobacterium interact, the genetic basis of the symbiosis, and the factors that determine the distribution and abundance of different lichen species.

• The Hunt for the Third Partner: Recent research has revealed the presence of a third fungal partner, a basidiomycete yeast, in some lichens. The role of this yeast is still being investigated, but it may contribute to the structure and stability of the lichen thallus.
• Understanding the Communication: Scientists are exploring the chemical signals and other forms of communication that occur between the fungus and the alga/cyanobacterium. Understanding these interactions could provide insights into the evolution and maintenance of the symbiosis.
• The Impact of Climate Change: Climate change is affecting lichen populations around the world. Changes in temperature, precipitation, and air pollution can alter the distribution and abundance of different lichen species.

Frequently Asked Questions (FAQ)

• Are lichens plants? No, lichens are not plants. They are composite organisms formed by a symbiotic relationship between a fungus and an alga or cyanobacterium.
• Are lichens harmful to trees? Generally, lichens are not harmful to trees. They are epiphytes, meaning they grow on the surface of trees but do not extract nutrients from them. However, in some cases, heavy lichen growth can shade out leaves and reduce photosynthesis.
• Can lichens grow on anything? Lichens can grow on a variety of substrates, including rocks, trees, soil, and even artificial surfaces like buildings and monuments.
• How long do lichens live? Lichens can live for a very long time, some species for hundreds or even thousands of years.
• Are lichens edible? Some lichen species are edible, but others are toxic. It is important to properly identify a lichen before consuming it.

Conclusion

Lichens are a testament to the power of symbiosis in nature. The partnership between a fungus and an alga or cyanobacterium creates a unique organism with remarkable adaptations and ecological significance. From their role as pioneer species to their use as bioindicators, lichens play a vital role in many ecosystems. While much remains to be discovered about these fascinating organisms, what we already know highlights their ingenuity and the intricate connections that bind the natural world. Understanding the nature of this partnership not only expands our knowledge of the biological world but also encourages a deeper appreciation for the interconnectedness of life on Earth. The next time you see a lichen, take a moment to consider the complex and fascinating relationship that makes it possible.