5.5 Lichens

Lichens

Lichens are composite organisms formed by a stable symbiotic association between a fungus and a photosynthetic partner (either algae or cyanobacteria). They're one of the clearest real-world examples of mutualistic symbiosis you'll encounter in microbiology, and they thrive in environments where neither partner could survive alone.

Relevance of Lichens in Microbiology

Lichens are a classic example of symbiosis, which is a close, long-term interaction between two or more different biological species. Specifically, lichens demonstrate mutualistic symbiosis, a relationship where both organisms benefit from the interaction.

Why does this matter for microbiology? Studying lichens helps you understand how microorganisms (fungi, algae, cyanobacteria) form complex partnerships. These aren't just lab curiosities. Lichens colonize an enormous range of ecosystems, from arctic tundra to deserts to temperate forests, showing just how adaptable microbial partnerships can be.

Symbiosis in Lichen Structures

A lichen is composed of two main partners:

• The mycobiont (fungal partner) provides a protective structure called the thallus. The thallus shields the photosynthetic partner from environmental stressors like UV radiation and desiccation (drying out). The fungal partner also absorbs minerals (nitrogen, phosphorus) and water from the environment, which the photosynthetic partner needs.
• The photobiont (photosynthetic partner) can be either green algae (commonly Trebouxia) or cyanobacteria (commonly Nostoc). The photobiont performs photosynthesis, producing organic compounds like glucose and other sugars that serve as the food source for the fungal partner.

So the trade-off is straightforward: the fungus provides structure, protection, and mineral access, while the photobiont provides food through photosynthesis. This mutualistic exchange is what allows lichens to colonize harsh surfaces like bare rock and tree bark where most organisms can't survive.

Lichen Morphology and Reproduction

Lichens come in three major growth forms:

• Foliose lichens have a leaf-like appearance with distinct upper and lower surfaces. They're loosely attached to their substrate.
• Crustose lichens form a thin, crust-like layer tightly adhered to the substrate. They can be difficult to remove without damaging the surface beneath.
• Fruticose lichens have a shrub-like or hanging structure, often branching outward from their point of attachment.

Some lichens also develop podetia, which are hollow, upright structures that often bear reproductive structures at their tips.

Lichens reproduce through several methods:

• Soredia are small clusters of algal cells wrapped in fungal hyphae. They break off and disperse, carrying both partners together for asexual reproduction.
• Isidia are small, coral-like outgrowths on the lichen surface that also serve as asexual propagules containing both partners.
• The fungal component can reproduce sexually on its own. In most lichens, the mycobiont belongs to the phylum Ascomycetes, which produce spores in sac-like structures called asci. However, these spores contain only the fungus; a new lichen forms only if the spore encounters a compatible photobiont.

Ecological Impact of Lichens

Bioindicators of air quality. Lichens are highly sensitive to air pollution, particularly sulfur dioxide and nitrogen oxides from fossil fuel combustion. The presence or absence of certain species in an area can indicate pollution levels. For example, Hypogymnia physodes is moderately pollution-tolerant, while Lecanora conizaeoides is notably resistant to sulfur dioxide. Areas with high lichen diversity generally have cleaner air.

Nutrient cycling and soil formation. Lichens contribute to the weathering of rocks by secreting organic acids (lichen acids) that slowly break down mineral surfaces. Over time, this process helps create soil. Lichens containing cyanobacteria as their photobiont can also fix atmospheric nitrogen, converting it into forms usable by other organisms. This is especially important in nutrient-poor environments.

Food source and habitat. Various animals depend on lichens. Invertebrates like mites and springtails live among lichen structures, while larger mammals like reindeer and caribou rely on lichens (particularly Cladonia rangiferina, or reindeer lichen) as a major food source during winter months.

Medicinal and industrial uses. Some lichen species have been used in traditional medicine for antimicrobial and anti-inflammatory properties. Usnea barbata (old man's beard) and Cetraria islandica (Iceland moss) are two well-known examples.

Bioremediation potential. Lichens can accumulate heavy metals such as lead and cadmium from their surroundings. This ability makes them useful both as pollution monitors and as potential tools for cleaning up contaminated sites like abandoned mines and industrial areas.