Lichens Honor

Nature Study

Requirements

1. What are lichens? Do they fit into any of the kingdoms of living things we know? Why?

   Answer: Lichens are organisms resulting from the symbiosis between fungi (the main component, the mycobiont) and algae or cyanobacteria (the photobiont). They do not belong to a single kingdom, being considered composite organisms. Traditionally classified as fungi, but they represent a stable partnership of two or more organisms living together as a single functional ecological unit. — More than 20,000 species. Fungi of the division Ascomycota (98%) or Basidiomycota (2%). Algae such as Trebouxia (green) or cyanobacteria such as Nostoc (blue-green) provide photosynthesis. The fungus shelters and protects; the alga produces food. An obligate mutualistic symbiosis. The current system classifies lichens within the fungal species (the scientific name of the fungus). Pioneers in colonizing bare rocks and diverse extreme environments.

2. Know the life cycle of a lichen.

   Answer: Reproduction can be sexual (fungal spores) or asexual (soredia and isidia, propagules with fungus+alga). The fungal spores need to find a compatible alga to form a new lichen. Soredia are tiny balls of powder dispersed by wind or water. Growth is very slow (1-10 mm/year). Life can last from decades to centuries under stable conditions. — Sexual reproduction: ascomata/basidiomata on the fungus produce spores. Asexual reproduction (more common): soredia (greenish powder) and isidia (erect structures). Both contain fungus+alga and disperse. Slow growth allows lichens to be used to date surfaces (lichenometry). Lichens in the Andes can be thousands of years old. Sensitive to pollution, they are environmental bioindicators. Extreme resistance to temperatures and desiccation is characteristic.

3. Find, identify, and photograph fruticose, crustose, and foliose lichens.

   Answer: Crustose: adhere completely to the substrate (rock, bark), looking like a painted stain (e.g., Caloplaca). Foliose: have flattened, leaf-like lobes that can partially detach (e.g., Parmelia, Xanthoria). Fruticose: look like shrubs or hang from trees (e.g., Cladonia, Usnea). Photograph in close-up, with a scale (ruler), in focus, under diffuse natural light to preserve colors. — For identification: observe the form, color, substrate, and environment. Crustose ones on rocks and bark; foliose ones on trees; fruticose ones on trunks and branches (in humid environments). Use a magnifying glass for details. Photography: tripod, natural light, a visual scale. Usnea (old man's beard) is a hanging fruticose. Parmelia on bark is foliose. Caloplaca on rock is crustose. Each type has clearly distinct and specific visual characteristics.

4. What is the usefulness of lichens in nature?

   Answer: Lichens are pioneers in colonization: they help break down rocks and form soil. They serve as food for animals (reindeer, slugs, snails). They are bioindicators of pollution (sensitive to SO2 and heavy metals). They provide nesting and shelter for invertebrates and birds. They fix nitrogen from the air via cyanobacteria. They have medicinal and cosmetic uses and serve as a natural dye in various products. — In ecosystems, lichens initiate ecological succession on bare rocks and recent volcanic lava. Reindeer feed on Cladonia rangiferina in the Arctic. Bioindicators: their absence indicates polluted air. Birds' nests use lichens. Cyanolichens fix atmospheric N2. Usnea barbata in medicine (a natural antibiotic). Cetraria islandica in syrups. Roccella used in litmus pH dye. Multiple important ecological and economic uses.

5. What determines the coloration of a lichen?

   Answer: The coloration depends on pigments from the fungus (lichen substances such as usnic acid, lecanoric acid) and from the partner alga or cyanobacterium. Green ones have active chlorophyll; orange ones have pigments such as parietin; black ones may have melanin-rich fungi or cyanobacteria. Moisture alters the tone: dry lichens look paler and grayer, and damp ones recover their vividness. — Lichen substances are unique secondary compounds: usnic acid (yellow), parietin (orange), atranorin (white/gray), fumarprotocetraric acid. Green algae (Trebouxia) give a green tone. Cyanobacteria (Nostoc) give a dark blue-green. Melanin protects from UV. Pigments protect against solar radiation and pathogens. Chemical identification uses tests (KOH, K). More than 800 substances are documented in different lichens.

6. What is the relationship between the organisms that make up a lichen?

   Answer: A lichen is a mutualistic symbiosis: the fungus provides structure, protection, and absorption of water/minerals; the alga or cyanobacterium provides food via photosynthesis. Both benefit and neither normally survives well in isolation. — The symbiotic relationship of lichens was controversial for decades until Toby Spribille's research in 2016 (University of Montana) revealed the presence of a tertiary yeast in many lichens, further complicating the classic traditional symbiosis.

7. What are the main groups of fungi that associate with algae?

   Answer: The main groups are Ascomycota (98% of lichens, with the ascus as the reproductive structure) and Basidiomycota (2%, with basidia). Ascomycetes are globally dominant across all terrestrial climatic zones. — The taxonomy of lichens follows Erik Acharius (1803) and was refined by James Lawrey and Paul Diederich in 2003 with a modern DNA-based key, identifying today about 18,500 known lichen species worldwide.

8. Explain what symbiosis and mutualism are.

   Answer: Symbiosis is any close association between species (parasitism, commensalism, or mutualism). Mutualism is a type of symbiosis in which both partners benefit, as in lichens and in pollination between plants and bees. — The terms were coined by Heinrich Anton de Bary in 1879 ('Die Erscheinung der Symbiose'), considered the father of symbiosis as a biological concept. The endosymbiotic theory explains the origin of eukaryotic cells, scientifically accepted today.

9. Cite at least 2 benefits of lichens for humans and for the environment.

   Answer: Lichens provide natural dyes (orcin), antibiotics (usnic acid), cosmetics (Evernia perfume), environmental bioindication, food for herbivores, soil fixation in hostile environments, and nutrient cycling. — Usnic acid produced by lichens has a proven antibiotic action against Mycobacterium tuberculosis (USP studies from 2018), and is currently being investigated as an alternative for combating multidrug-resistant tuberculosis in pharmaceutical research.

10. What is the relationship between the lichen species Lecanora esculeta and the manna?

    Answer: Lecanora esculenta (also called Aspicilia esculenta) is an edible lichen of the Middle Eastern deserts, North Africa, and the steppes of Asia. It grows loose on the ground and, when dry, can be blown by the wind in large quantities, being gathered and ground into flour. For this reason, several scholars associate it with the 'manna' that fed the Israelites in the desert (Exodus 16): a food that appeared on the ground in the morning and could be collected. The relationship, therefore, is the hypothesis that this edible lichen was (or inspired the description of) the biblical manna. — Lecanora esculenta forms small wind-blown aggregates that mysteriously appear in the morning, a characteristic that coincides with the biblical description in Exodus 16, being a popular scientific hypothesis although the subject remains open and unproven.