5.5 Lichens

Lichens are fascinating examples of symbiosis in nature. These organisms, formed by fungi and algae or cyanobacteria living together, showcase the power of cooperation in the microbial world.

Lichens demonstrate how different species can work together to thrive in harsh environments. They play important roles in ecosystems, from helping create soil to cleaning the air, and even have potential uses in medicine and environmental cleanup.

Lichens

Relevance of lichens in microbiology

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  Lichens and algae [explored] | Lichens – Xanthoria parietina… | Flickr View original

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  Frontiers | Complex Interaction Networks Among Cyanolichens of a Tropical Biodiversity Hotspot View original

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  Lichens and algae [explored] | Lichens – Xanthoria parietina… | Flickr View original

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  Frontiers | Complex Interaction Networks Among Cyanolichens of a Tropical Biodiversity Hotspot View original

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• Lichens classic example of symbiosis, key concept in microbiology
  • Symbiosis close, long-term interaction between two or more different biological species (fungi and algae or cyanobacteria)
• Lichens demonstrate principles of mutualistic symbiosis
  • Mutualism type of symbiotic relationship in which both organisms benefit from interaction (protection and nutrients)
• Studying lichens helps understand complex interactions between microorganisms (fungi, algae, cyanobacteria)
• Lichens showcase adaptability and diversity of microorganisms in various ecosystems (arctic tundra, deserts, forests)

Symbiosis in lichen structures

• Lichens composed of two main components: fungal partner (mycobiont) and photosynthetic partner (photobiont)
  • Photobiont can be either algae (Trebouxia) or cyanobacteria (Nostoc)
• Fungal partner provides protective structure called thallus
  • Thallus shields photobiont from environmental stressors like UV radiation and desiccation (drying out)
• Photobiont, either algae or cyanobacteria, performs photosynthesis
  • Photosynthesis produces organic compounds that serve as food source for fungal partner (glucose, other sugars)
• Fungal partner absorbs minerals (nitrogen, phosphorus) and water from environment, which benefits photobiont
• Mutualistic relationship allows lichens to thrive in diverse and often harsh environments (rock surfaces, tree bark)

Lichen morphology and reproduction

• Lichens exhibit various growth forms:
  • Foliose lichens have a leaf-like appearance with distinct upper and lower surfaces
  • Crustose lichens form a crust-like layer tightly adhered to the substrate
  • Fruticose lichens have a shrub-like or hanging structure
• Some lichens develop specialized structures:
  • Podetia are hollow, upright structures found in certain lichens, often bearing reproductive structures
• Lichens reproduce through various methods:
  • Soredia are small clusters of algal cells wrapped in fungal hyphae, used for asexual reproduction
  • Isidia are small, coral-like outgrowths on the lichen surface, also used for asexual reproduction
• The fungal component of most lichens belongs to the phylum Ascomycetes, which produce spores in sac-like structures

Ecological impact of lichens

• Lichens important bioindicators of air quality
  • Sensitive to air pollution, particularly sulfur dioxide and nitrogen oxides (from fossil fuel combustion)
  • Presence or absence of certain lichen species can indicate level of air pollution in an area (Hypogymnia physodes, Lecanora conizaeoides)
• Lichens play crucial role in nutrient cycling in ecosystems
  • Contribute to weathering of rocks, aiding in soil formation (through secretion of lichen acids)
  • Lichens fix atmospheric nitrogen, making it available for other organisms (especially in nutrient-poor environments)
• Lichens serve as food source and habitat for various animals, including invertebrates (mites, springtails) and mammals (reindeer, caribou)
• Some lichen species used in traditional medicine for their antimicrobial and anti-inflammatory properties (Usnea barbata, Cetraria islandica)
• Lichens have potential applications in bioremediation
  • Can accumulate heavy metals (lead, cadmium) and other pollutants, helping to clean up contaminated sites (abandoned mines, industrial areas)