What are Lichens? – Characteristics, Types, Structure, Reproduction

What is a Lichen?

• Lichens are fascinating symbiotic organisms. They consist of algae or cyanobacteria living among filaments of multiple fungi species. Embedded within this structure, there is often yeast in the cortex, or “skin,” forming a mutualistic relationship. This unique partnership results in organisms that have properties distinct from their individual components.
• Lichens play a crucial role in nutrient cycling. They act as producers, providing a food source for various higher trophic feeders, such as reindeer, gastropods, and mites. Despite their plant-like appearance, lichens are not plants. They exhibit a range of growth forms, including tiny, leafless branches (fruticose), flat leaf-like structures (foliose), and crust-like growths adhering tightly to surfaces (crustose).
• Macrolichens are either bush-like or leafy, while microlichens encompass all other forms. The terms “macro” and “micro” refer to growth form rather than size. Lichens are often mistakenly associated with moss due to similar appearances and common names like “reindeer moss,” but they are unrelated to mosses or any plant species.
• Lichens are incredibly adaptable, thriving in diverse environments from sea level to high alpine elevations. They can grow on various surfaces, including bark, leaves, rock, and even man-made structures like walls and roofs. Some lichens are resilient enough to survive in extreme conditions, such as arctic tundra or hot deserts.
• There are approximately 20,000 known species of lichens. Some species have lost the ability to reproduce sexually yet continue to speciate. Lichens can be seen as self-contained miniature ecosystems, where fungi, algae, or cyanobacteria interact with other microorganisms. They may evolve into more complex composite organisms over time.
• Lichens are notable for their longevity, with some species considered among the oldest living organisms. They are often pioneers, colonizing fresh rock surfaces after events like landslides. Their slow, regular growth rates are useful in dating geological events through lichenometry. As keystone species, lichens benefit many ecosystems, supporting trees and birds.

Habit and Habitat of Lichens

• General Distribution
  • Lichens are found globally, encompassing around 400 genera and 15,000 species.
  • They thrive in a variety of regions, from temperate woodlands to arctic tundras.
• Thalloid Structure
  • The plant body of a lichen is thalloid, meaning it lacks true roots, stems, and leaves.
  • This structure allows lichens to grow on diverse surfaces, such as tree bark, leaves, dead logs, and bare rocks.
• Preferred Habitats
  • Lichens grow abundantly in forest areas with minimal pollution and high moisture levels.
  • They are commonly found in regions with clean air, indicating their sensitivity to air quality.
• Extreme Environments
  • Certain species, like Cladonia rangiferina (reindeer moss), can survive in extremely cold conditions, such as the Arctic tundra and Antarctic regions.
  • These lichens demonstrate remarkable adaptability, thriving in some of the harshest climates on Earth.
• Regional Abundance
  • In India, lichens are particularly abundant in the Eastern Himalayan regions, where the environment supports their growth.
  • These areas provide the clean air and moisture lichens need to flourish.
• Pollution Sensitivity
  • Lichens do not grow well in highly polluted areas, such as industrial zones.
  • Their absence in such regions can be an indicator of poor air quality and environmental degradation.
• Growth Rate
  • Lichens have a slow growth rate, which allows them to survive in adverse conditions.
  • This slow and steady growth contributes to their longevity and resilience.
• Ecological Niches
  • Lichens occupy various ecological niches, playing essential roles in nutrient cycling and providing food for other organisms.
  • They can grow on multiple substrates, including rocks, soil, and even within the grains of solid rock (endolithic growth).
• Habitual Adaptations
  • Different lichen species have adapted to specific habitats based on their growth forms:
    • Corticolous: Grow on tree bark.
    • Foliicolous: Grow on leaf surfaces.
    • Saxicolous: Grow on rock surfaces.
    • Terricolous: Grow on soil.
    • Muscicolous: Grow on mosses.
• Environmental Indicators
  • Lichens serve as bioindicators, reflecting the health of their environments.
  • Their presence or absence can provide valuable insights into the air quality and ecological conditions of a region.

General Characteristics of Lichens

• Symbiotic Nature
  • Lichens are symbiotic organisms consisting of fungi and algae or cyanobacteria.
  • The fungal component, known as the mycobiont, provides structure and absorbs nutrients.
  • The algal component, or phycobiont, performs photosynthesis, producing food for both partners.
• Thallus Structure
  • The thallus is the main body of the lichen, resembling the vegetative portions of mosses and liverworts.
  • Lichens can be classified based on their thallus morphology into three main types: crustose (crust-like), foliose (leaf-like), and fruticose (shrubby).
• Habitat Adaptability
  • Lichens thrive in diverse habitats and climatic conditions, including extreme environments like arctic tundra and deserts.
  • They can grow on various surfaces, such as tree bark, leaves, rocks, and soil.
• Reproductive Methods
  • Lichens reproduce through vegetative, asexual, and sexual means.
    • Vegetative reproduction occurs via fragmentation or the formation of special structures like soredia and isidia.
    • Asexual reproduction involves the production of oidia.
    • Sexual reproduction involves the formation of fruiting bodies, producing ascospores or basidiospores depending on the fungal component (Ascolichen or Basidiolichen).
• Functional Integration
  • The fungal and algal partners lose their distinct identities, functioning as a single organism both morphologically and physiologically.
  • Lichens produce their own food through photosynthesis and absorb nutrients from their environment.
• Environmental Indicators
  • Lichens are sensitive to environmental changes, often serving as bioindicators of air quality and pollution levels.
  • They are among the first organisms to colonize newly exposed surfaces, such as after a landslide.
• Morphological Diversity
  • Lichens display a wide range of colors and forms, often appearing as grayish, greenish, or orange patches on surfaces.
  • Macrolichens (bushy or leafy) and microlichens (crust-like) are categorized based on their growth forms.
• Slow Growth and Longevity
  • Lichens grow slowly but can survive in harsh conditions, making them long-lived organisms.
  • Their slow growth rates can be used to date geological events through a method called lichenometry.
• Ecological Role
  • Lichens contribute to nutrient cycling and provide food for various higher trophic organisms.
  • They play a keystone role in many ecosystems, benefiting other species like trees and birds.
• Reproductive Structures
  • Sexual reproduction involves specialized structures like spermogonia (male) and carpogonia (female).
  • Ascolichen produces ascospores, while Basidiolichen produces basidiospores, each forming distinct fruiting bodies.

Types of Lichens

Lichens, fascinating organisms formed by the symbiotic relationship between fungi and algae, display a variety of forms and structures. These types are primarily categorized based on their morphology and growth forms.

1. Crustose Lichens
   • General Characteristics
     • Crustose lichens form a crust that is closely attached to the substrate, such as rocks, soil, or bark.
     • The thallus is a flat, dorsiventral structure.
     • Water loss is restricted to the upper exposed surface.
   • Sub-types
     • Leprose (Powdery): Simple, powdery thallus lacking distinct organization. Example: Lepraria incana.
     • Peltate: Found in hot, arid regions, often on soil or rock surfaces. Example: Peltula radicata.
     • Bullate: Forms inflated squamules. Example: Mobergia.
     • Suffruticose: Forms coralloid tufted cushions. Example: Peltula clavata.
     • Lobate: Radially striate thallus with marginal, partially raised lobes. Example: Caloplaca sp.
     • Endolithic: Grows inside solid rock, with only the fruiting bodies exposed. Example: Caloplaca sp.
     • Pulvinate: Thallus has swelling at the base. Example: Euopsis pulvinata.
     • Effigurate: Radially arranged prolonged marginal lobes. Example: Acarospora.
2. Foliose Lichens
   • General Characteristics
     • Foliose lichens have a leaf-like, flat thallus, partially attached to the substrate.
     • They exhibit dorsiventral organization with distinct upper and lower surfaces.
     • The thallus is divided into lobes with various degrees of branching, attached by rhizines.
   • Sub-types
     • Laciniate: Lobate and varying in size, with radially arranged or overlapping lobes. Example: Parmelia sp., Peltigera.
     • Umbilicate: Circular thallus with single or multi-lobate structure, attached by a central umbilicus. Example: Lasallia papulosa.
3. Fruticose Lichens
   • General Characteristics
     • Fruticose lichens have hair-like, strap-shaped, or shrubby lobes.
     • The lobes can be flat or cylindrical.
   • Sub-types
     • Shrubby: Bushy structure with cylindrical, twig-like branches. Example: Usnea.
     • Hanging: Long, hair-like strands that hang down, appearing like curtains. Example: Usnea.
     • Cylindrical: Cylindrical, often hollow structures resembling stems or rods. Example: Cladonia.
     • Coralloid: Branches resemble coral or fungi, with dense clusters. Example: Cladonia.
     • Leafy: Flattened, leaf-like appearance with a central holdfast. Example: Peltigera.
     • Hairy: Long, hair-like structures that can be straight or twisted. Example: Alectoria.
4. Additional Types
   • Gelatinous Lichens
     • Formed primarily of cyanobacteria with some fungal components.
     • Gelatinous appearance from mucilage on the exterior of blue-green algae cells. Example: Collema nigrescens.
   • Filamentous Lichens
     • Growth form resembles thin, stringy, non-branching hairs or filaments. Example: Ephebe.
   • Byssoid Lichens
     • Wispy growth form, similar to teased wool. Example: Coenogonium implexus.
   • Squamulose Lichens
     • Composed of overlapping scales called squamules. Example: Cladonia.
   • Placoid Lichens
     • Entire surface of the thallus is radially striate with raised marginal tissue. Example: Caloplaca.

Classification of Lichens based on different factors

Lichens, symbiotic associations between fungi and algae or cyanobacteria, are classified based on various factors. This classification helps in understanding their diverse forms and ecological roles.

1. Based on Growth Form
   • Crustose Lichens
     • Form a crust-like structure tightly attached to the substrate.
     • Example: Graphis scripta.
   • Foliose Lichens
     • Leaf-like, flat thallus partially attached to the substrate.
     • Example: Parmelia.
   • Squamulose Lichens
     • Composed of small, overlapping scale-like structures.
     • Example: Cladonia.
   • Fruticose Lichens
     • Branching, tube-like structures resembling small shrubs.
     • Example: Usnea.
   • Gelatinous Lichens
     • Form a jelly-like structure, often with cyanobacteria.
     • Example: Collema nigrescens.
   • Leprose Lichens
     • Powdery appearance, lacking distinct organization.
     • Example: Lepraria incana.
2. Based on Habitat
   • Lignicolous Lichens
     • Found on wood and decaying logs.
     • Example: Lecanora conizaeoides.
   • Corticolous Lichens
     • Grow on the bark of trees.
     • Example: Ramalina.
   • Saxicolous Lichens
     • Inhabit stones or rocks.
     • Example: Rhizocarpon geographicum.
   • Marine Lichens
     • Found on coastal rocks.
     • Example: Verrucaria maura.
   • Freshwater Lichens
     • Grow on rocks in freshwater environments.
     • Example: Hydropunctaria.
   • Terricolous Lichens
     • Grow on soil, also known as terrestrial lichens.
     • Example: Cladonia rangiferina.
3. Based on Internal Structure
   • Heteromerous Lichens
     • Algal cells are concentrated in a distinct layer within the thallus.
     • Example: Xanthoria parietina.
   • Homoiomerous Lichens
     • Algal cells are uniformly distributed throughout the thallus.
     • Example: Leptogium.
4. Based on Fungal Partner
   • Ascolichens
     • Lichens with fungi belonging to the Ascomycetes class.
     • Example: Physcia.
   • Basidiolichens
     • Lichens with fungi from the Basidiomycetes class.
     • Example: Dictyonema glabratum.
   • Hymenolichens
     • Lichens associated with fungi from the Hymenomycetes group.
     • Example: Cora pavonia.

Structure of Lichens

The structure of lichens is a complex interplay between two primary partners: the fungal component and the photosynthetic component. Understanding this structure involves examining the roles and organization of each component.

1. Fungal Component (Mycobiont)
   • Structural Framework
     • The mycobiont, or fungal partner, constitutes the primary structural element of the lichen.
     • It consists of filamentous cells known as hyphae, which interweave to form the lichen’s thallus.
     • These hyphae create a dense, often intricate network that supports and protects the photobionts.
   • Reproductive Structures
     • The fungal partner produces reproductive structures, including fruiting bodies.
     • These fruiting bodies generate spores capable of developing into new lichen thalli under appropriate conditions.
   • Nutrient Absorption and Attachment
     • The fungal component absorbs water and essential nutrients from the environment, supporting the lichen’s growth.
     • Rhizines, which are outgrowths from the fungal body, anchor the lichen to its substrate, such as rocks or tree bark.
2. Photosynthetic Component (Photobiont)
   • Role in Photosynthesis
     • The photobiont is responsible for the photosynthesis process, converting light energy into chemical energy.
     • Algal photobionts are typically green algae, while cyanobacterial photobionts are blue-green bacteria capable of nitrogen fixation.
   • Integration with Fungal Hyphae
     • Photobionts are embedded within the fungal hyphae, often residing in specialized structures known as cephalodia or photobiont layers.
     • This integration allows for efficient nutrient exchange and mutual support between the fungal and photosynthetic partners.
3. Thallus
   • Main Body
     • The thallus is the primary structure of the lichen, composed of a network of fungal hyphae interspersed with photobionts.
     • It serves as the main functional and protective layer of the lichen.
   • Morphological Forms
     • The thallus can take various forms based on its morphology:
       • Crustose: Crust-like, tightly adhering to the substrate.
       • Foliose: Leafy or lobed, with a more loosely attached structure.
       • Fruticose: Shrubby, with branched and often cylindrical growths.

Structure of Thallus in Lichens

A. External Structure of Thallus

The thallus of lichens exhibits various structural forms, each with distinctive external characteristics. The classification of lichens based on their external structure helps in understanding their ecological roles and environmental adaptations. Below are the primary types and their detailed descriptions:

1. Leprose Lichens
   • Description
     • Leprose lichens are characterized by a powdery appearance.
     • The thallus consists of fungal mycelium enveloping either single or clusters of algal cells.
     • The algal cells are not fully covered by the fungal hyphae.
   • Characteristics
     • The thallus appears as a fine, dust-like mass on substrates.
     • Example: Lepraria incana.
2. Crustose Lichens
   • Description
     • Crustose lichens form a thin, flat layer or crust on their substrate.
     • The thallus is either wholly or partially embedded in the substrate.
   • Characteristics
     • They adhere tightly to surfaces such as barks, stones, and rocks.
     • Examples: Graphis, Lecanora, Ochrolechia.
   • Features
     • Often difficult to separate from the substrate.
     • Form a continuous, smooth layer.
3. Foliose Lichens
   • Description
     • Foliose lichens have a leaf-like appearance with a flat, spreading thallus.
     • The thallus is often lobed and attached to the substrate by rhizines, which are fungal outgrowths.
   • Characteristics
     • The thallus is loosely attached, allowing some parts to lift off the substrate.
     • Examples: Parmelia, Physcia, Peltigera.
   • Features
     • The thallus is generally more visible and distinct compared to crustose lichens.
4. Fruticose Lichens
   • Description
     • Fruticose lichens have a shrubby, cylindrical, and branched appearance.
     • The thallus can be either erect or hanging from the substrate.
   • Characteristics
     • They are attached to the substrate by a basal disc or holdfast.
     • Examples: Cladonia, Usnea, Letharia.
   • Features
     • The thallus structure resembles small shrubs or branches.
5. Filamentous Lichens
   • Description
     • In filamentous lichens, the algal components are filamentous and well-developed.
     • The algal filaments are covered by a sparse layer of fungal hyphae.
   • Characteristics
     • The algal member is the dominant partner in this type of lichen.
     • Examples: Racodium, Ephebe, Cystocoleus.
   • Features
     • The thallus has a filamentous or thread-like appearance.

B. Internal Structure of Thallus

The internal structure of lichen thalli varies based on the distribution of algal cells and the type of lichen. Lichens are categorized into two main types: homoisomerous and heteromerous. Each type has distinct internal layers that contribute to its overall function and structure.

1. Homoisomerous Thallus
   • Characteristics
     • Fungal hyphae and algal cells are distributed more or less uniformly throughout the thallus.
     • Algal members are typically cyanobacteria.
     • Commonly found in crustose lichens.
   • Structure
     • Both fungal and algal partners intermingle.
     • The arrangement forms a thin outer protective layer.
   • Examples
     • Leptogium, Collema.
2. Heteromerous Thallus
   • Characteristics
     • The thallus is differentiated into four distinct layers.
     • Algal cells are confined to a specific layer known as the algal zone.
     • This type is prevalent in foliose and fruticose lichens.
   • Structure
     • Upper Cortex
       • A thick, outermost protective layer.
       • Composed of densely packed, interwoven fungal hyphae oriented perpendicular to the thallus surface.
       • May contain gelatinous substances in intercellular spaces, if present.
     • Algal Zone
       • Located just below the upper cortex.
       • Contains loosely interwoven fungal hyphae that entangle algal cells.
       • Algal members can be cyanobacteria (e.g., Gloeocapsa, Nostoc) or green algae (e.g., Chlorella, Pleurococcus).
       • Functions primarily in photosynthesis.
     • Medulla
       • Positioned below the algal zone.
       • Composed of loosely interwoven, thick-walled fungal hyphae with large spaces between them.
     • Lower Cortex
       • The lowermost layer of the thallus.
       • Made up of compactly arranged hyphae, either perpendicular or parallel to the thallus surface.
       • Some hyphae extend downward to form rhizines, which aid in substrate anchorage.

In the case of fruticose lichens like Usnea, the internal structure includes:

• Cortex: The outer protective layer.
• Medulla: Contains both algal cells and fungal mycelium.
• Central Chondroid Axis: Composed of compactly arranged fungal mycelium, providing structural support.

C. Specialised Structures of Thallus

Lichens exhibit several specialized structures on their thallus that contribute to their adaptation and functionality in various environments. These structures aid in processes such as gaseous exchange, aeration, and moisture retention.

1. Breathing Pores
   • Description
     • Found in some foliose lichens, such as Parmelia.
     • These are openings in the upper cortex.
   • Function
     • Facilitate gaseous exchange between the lichen and its environment.
     • Allow for the intake of carbon dioxide and release of oxygen, crucial for photosynthesis.
2. Cyphellae
   • Description
     • Present on the lower cortex of certain foliose lichens, such as Sticta.
     • Appear as small, cup-like depressions or white spots.
     • Some pits may form without definite borders and are referred to as pseudocyphellae.
   • Function
     • Aid in aeration by allowing the exchange of gases between the lichen and its environment.
     • Help in maintaining the lichen’s overall health and functionality by facilitating gas flow.
3. Cephalodium
   • Description
     • Small, warty outgrowths located on the upper surface of the thallus.
     • Contain fungal hyphae similar to those of the main thallus, but with different algal elements.
     • In some species, like Neproma, cephalodia are endotrophic.
   • Function
     • Likely assist in moisture retention, helping the lichen survive in dry conditions.
     • Provide additional habitat for the algal partners, enhancing their growth and function.

Vegetative Structure of Lichens

Lichens exhibit several specialized vegetative structures that play crucial roles in their adaptation, survival, and taxonomic classification. These structures include:

1. Breathing Pores
   • Description
     • Found in some lichen species, such as Oropogon sp.
     • Formed by interwoven hyphae in specific areas of the cortex.
   • Function
     • Facilitate gaseous exchange, allowing for the uptake of carbon dioxide and release of oxygen.
     • Crucial for the lichen’s photosynthetic processes and overall metabolism.
2. Cyphellae
   • Description
     • Large, circular depressions located in the lower cortex of foliose lichens.
   • Function
     • Aid in gaseous exchange similar to breathing pores.
     • Enhance the lichen’s ability to perform photosynthesis by improving air flow to the algal cells.
3. Pseudocyphellae
   • Description
     • Tiny pores found on the surface of various foliose and fruticose lichens.
     • These pores are covered by a network of short cells.
   • Function
     • Contribute to gaseous exchange, although less pronounced than cyphellae.
     • Facilitate the lichen’s interaction with its environment, particularly in terms of air flow and moisture.
4. Cephalodia
   • Description
     • Small, dark-colored structures containing cyanobacterial symbionts.
     • Can occur within the lichen’s tissue or on its surface.
   • Function
     • Assist in nitrogen fixation through the cyanobacteria, which benefits the lichen by providing essential nutrients.
     • May help in moisture retention, aiding the lichen’s survival in various environmental conditions.
5. Isidia
   • Description
     • Small, corticated outgrowths on the upper surface of the lichen thallus.
     • Composed of green algal cells and fungal hyphae.
   • Function
     • Provide a larger surface area for photosynthesis.
     • Serve as a means of vegetative propagation, allowing the lichen to spread and colonize new areas.
6. Soredia
   • Description
     • Small, non-corticated, bud-like outgrowths on the upper surface of the lichen thallus.
     • Consist of a few algal cells enveloped by fungal hyphae.
   • Function
     • Function as a method of vegetative reproduction.
     • Allow the lichen to disperse and establish new colonies without the need for sexual reproduction.

Reproduction in Lichens

A. Vegetative Reproduction in Lichens

Lichens utilize various methods for vegetative reproduction, allowing them to spread and colonize new habitats efficiently. The main methods include:

1. Fragmentation
   • Description
     • The thallus of the lichen breaks into small fragments.
     • Each fragment can develop into a new thallus under favorable conditions.
   • Example
     • Ramelia reticulata exhibits this method of reproduction.
   • Function
     • Allows rapid expansion and colonization of new areas.
     • Each fragment retains the genetic material of the original thallus, ensuring continuity of the lichen’s traits.
2. Isidia and Soredia
   • Description
     • Isidia: Small, corticated outgrowths on the thallus’s surface, formed from the algal and fungal components.
     • Soredia: Tiny, non-corticated, bud-like structures consisting of a few algal cells surrounded by fungal hyphae.
   • Formation
     • Both isidia and soredia arise from the algal zone beneath the upper cortex.
     • Algal cells divide and become enveloped by fungal hyphae, forming these structures.
   • Function
     • Soredia are lightweight and dispersed by wind or rain, facilitating widespread colonization.
     • Isidia provide an increased surface area for photosynthesis and also aid in vegetative propagation.
   • Example
     • Parmelia reproduces through soredia.
3. Phyllidia
   • Description
     • Leaf-like or scale-like, dorsiventral parts of the thallus.
   • Function
     • Serve as reproductive structures that can detach and grow into new individuals.
   • Example
     • Found in Peltigera sp.
4. Blastidia
   • Description
     • Yeast-like, small, fragmented branches that detach from the parent lichen.
   • Function
     • Each blastidium can grow into a new lichen.
   • Example
     • Physcia exhibits blastidia.
5. Schizidia
   • Description
     • Scale-like, split segments from the upper layer of the thallus.
   • Function
     • These segments can develop into new lichen individuals.
   • Example
     • Parmelia taylorensis reproduces using schizidia.
6. Hormocysts
   • Description
     • Clumps formed by chains of algal filaments and fungal hyphae.
   • Function
     • Each clump can develop into a new lichen.
   • Example
     • Found in Lempholema.
7. Goniocysts
   • Description
     • Structures where algal cells and derivatives are wrapped in fungal hyphae, resembling unsorralium-like formations.
   • Function
     • Facilitate vegetative reproduction and dispersal.
   • Example
     • Present in Goniocystagia.

B. Asexual Reproduction in Lichens

Lichens reproduce asexually through several specialized structures, which facilitate their spread and establishment in new environments. The key structures involved in asexual reproduction include:

1. Soredia (singular: Soredium)
   • Description
     • Soredia are small, grayish-white, bud-like outgrowths located on the upper cortex of the thallus.
     • Each soredium consists of one or a few algal cells surrounded by a mass of fungal hyphae.
   • Function
     • These structures detach from the thallus due to rain or wind.
     • Upon landing on a suitable substrate, soredia germinate and develop into new thalli.
   • Formation
     • When soredia form in an organized manner within a special pustule-like region, they are termed Soralia.
   • Examples
     • Found in lichens such as Parmelia and Physcia.
2. Isidia (singular: Isidium)
   • Description
     • Isidia are small, stalked, simple or branched, grayish-black, coral-like outgrowths on the thallus’s upper surface.
     • They possess an outer cortical layer continuous with the upper cortex of the mother thallus and enclose the same algal and fungal elements.
   • Function
     • Besides aiding in asexual reproduction, isidia increase the photosynthetic surface area of the thallus.
     • Isidia are typically constricted at the base, allowing them to detach easily from the parent thallus.
   • Formation
     • Under favorable conditions, isidia germinate and form new thalli.
   • Examples
     • Varied shapes include coral-like in Peltigera, rod-like in Parmelia, cigar-like in Usnea, and scale-like in Collema.
3. Pycniospore
   • Description
     • Pycniospore or spermatium is produced inside a flask-shaped structure known as a pycnidium.
     • These structures typically function as gametes but can also germinate under certain conditions.
   • Function
     • When pycniospore germinates and comes into contact with the appropriate algal partner, it develops into a new lichen thallus.
   • Examples
     • Found in some lichens that form pycnidia.

C. Sexual Reproduction

Sexual reproduction in lichens involves complex interactions between fungal and algal partners. The process varies between Ascolichens and Basidiolichens, corresponding to the Ascomycetes and Basidiomycetes, respectively. Here is an overview of the mechanisms and structures involved:

1. Ascolichens
   • Reproductive Structures
     • Spermagonium
       • Flask-shaped structures embedded in the upper thallus.
       • Opens to the outside through a small pore called an ostiole.
       • Contains fertile hyphae that produce minute rounded cells, known as spermatia.
       • Pycnia-like structures may also function as spermagonia.
     • Carpogonium
       • Consists of two parts:
         • Ascogonium: Lower coiled, multicellular portion located deep in the medullary layer.
         • Trichogyne: Upper, elongated, thread-like portion that extends out of the thallus and receives spermatia.
   • Fertilization and Development
     • Spermatia adhere to the trichogyne and fuse with the carpogonium’s egg cell.
     • Following fertilization, the ascogonium produces branched hyphae.
     • These hyphae form asci at their tips, surrounded by sterile hyphae, creating the fruiting body.
     • Fruit bodies may be either apothecia or perithecia.
   • Apothecium and Perithecium
     • Apothecium: Cup-shaped structure with three distinct layers—hymenium (fertile zone), epitheca (upper sterile zone), and hypotheca (lower sterile zone). Example lichens: Parmelia.
     • Perithecium: Flask-shaped structure housing the asci. Example lichens: Verrucaria.
     • Ascospore Development
       • Ascospores are released from asci and germinate to form new fungal hyphae.
       • The new hyphae, upon contacting the proper algal partner, develop into new thalli.
2. Basidiolichens
   • Reproductive Structures
     • Basidiolichens form basidiospores on basidia, similar to other Basidiomycetes.
     • The fungal member (often belonging to Thelephoraceae) interacts with an algal partner to form the lichen thallus.
   • Thallus Structure
     • On soil, the thallus produces a hypothallus without rhizines.
     • On tree trunks, it forms a bracket-like structure with a differentiated internal structure including an upper cortex, algal layer, medulla, and lower fertile region.
   • Basidiospore Development
     • Basidiospores are produced on basidia and contribute to the propagation of the lichen.

What is The Lichen Symbiosis?

Lichens are unique organisms formed through the close interaction between fungi and one or more photobionts, which may include algae or cyanobacteria. This symbiotic relationship is fundamental to the lichen’s structure and function. Here, we explore the components and complexities of this symbiosis.

• Components of Lichen Symbiosis
  • Fungi
    • The fungal component of lichens is typically a member of the Ascomycota or Basidiomycota. These fungi are heterotrophic, meaning they rely on external sources for nutrients. They form the structural framework of the lichen thallus, creating an environment suitable for the photobionts.
  • Photobionts
    • Photobionts, which can be algae or cyanobacteria, are autotrophic organisms capable of photosynthesis. They produce simple sugars that serve as a carbon source for the fungi. In return, the fungi provide a stable habitat and often enhance the photobionts’ access to essential nutrients.
• Nature of the Symbiotic Relationship
  • Mutualistic Aspect
    • Evidence suggests that the lichen symbiosis is mutualistic. Fungi obtain carbon sources from the photobionts, while the photobionts benefit from optimal living conditions within the lichen. This environment often supports larger populations of photobionts compared to their numbers outside lichens.
    • The fungal matrix may also facilitate better access to mineral nutrients for the photobionts, as fungal digestion processes release nutrients that the photobionts can utilize.
    • Additionally, lichens produce complex secondary metabolites that protect both partners from UV radiation, desiccation, and herbivory. These compounds are unique to lichens and play a crucial role in their survival.
  • Controversies and Alternative Views
    • Despite the mutualistic view, some evidence suggests a parasitic aspect to the relationship. For instance, up to 50% of the carbon fixed by photobionts is converted into fungal sugars, which are not available to the photobionts.
    • In laboratory settings, certain lichens exhibit parasitic interactions with non-host algae, which indicates a potential for controlled parasitism in some contexts.
    • Moreover, lichen fungi can be parasitic on other lichens, known as lichenicolous lichens. These fungi may evolve from lichenized forms into specialized parasites or saprophytes, competing for resources within the lichen thallus.
• Complexity of the Symbiosis
  • Additional Symbionts
    • Recent research has identified yeasts embedded within the fungal cortex of ascomycete macrolichens. The presence of these yeasts correlates with variations in lichen phenotype, suggesting they may play a role in the lichen’s biology.
    • Non-photosynthetic bacteria are also consistently found within lichen thalli. Although their exact role is not yet fully understood, their presence is significant. It has been observed that these bacteria facilitate the relichenization process when fungi and algae are cultured separately in the lab.

What is Lichen Fungi?

Lichen fungi are a diverse and integral part of the lichen symbiosis, comprising a wide range of fungal species that interact with photosynthetic partners to form lichens. Understanding these fungi involves exploring their evolution, diversity, and classification.

• Evolution and Classification
  • Historical Background
    • Initially, lichens were not recognized as a combination of fungi and photobionts, leading to their exclusion from mainstream mycological research. This perspective persisted until molecular biology revealed that lichen fungi share evolutionary roots with non-lichen fungi, confirming their place in the Kingdom Fungi.
    • Today, it is understood that lichen fungi have evolved from various lineages within the fungal tree of life, with many originating from the Ascomycota (cup fungi) and some from the Basidiomycota (mushroom-forming fungi).
  • Diversity and Lineages
    • Approximately 98% of lichens are derived from Ascomycota, particularly the cup fungi. These fungi produce the majority of lichen types, including crustose, foliose, and fruticose forms.
    • Estimates suggest that about 20% of all known fungi and 50% of Ascomycota are lichenized, encompassing approximately 28,000 species globally. The greatest diversity of lichen fungi is found in tropical regions, though they remain less studied compared to temperate zones.
• Morphological and Functional Characteristics
  • Spore-Producing Structures
    • Lichen fungi are classified based on the structures they produce for spore dispersal. Cup fungi, or ascomycetes, are known for their open, cup-shaped structures (apothecia) that bear sexual spores. However, not all ascomycetes have these cup-shaped structures; some have flask-shaped spore-bearing structures (perithecia).
    • The bulk of lichen diversity belongs to the class Lecanoromycetes, which includes genera such as Lecanora, Cladonia, Parmelia, and Peltigera. These fungi typically produce spores in open or cup-shaped structures.
  • Evolutionary Insights
    • The Lecanora-group, which includes many well-known lichen genera, is thought to have evolved during the Carboniferous period. These early lichens likely predate the emergence of land plants, with most early biodiversity concentrated in marine environments.
    • Conversely, other lichen groups, such as those in the Arthonia-, Trypethelium-, and Pyrenula-groups, display distinct morphological and molecular characteristics. These groups may have different evolutionary origins within the Ascomycete tree of life.
• Identification and Challenges
  • Morphological Complexity
    • Identifying lichen fungi can be challenging due to the limited and sometimes ambiguous morphological characteristics. Many fungi exhibit convergent evolution, where similar traits develop independently in unrelated lineages.
    • For instance, the fruticose growth form has evolved multiple times within both the Lecanora-group and the distantly related Arthonia-group. This convergence complicates the identification process, as similar morphologies can arise from different evolutionary backgrounds.
  • Practical Observations
    • To study lichen fungi effectively, it is helpful to observe them in their natural habitats. For example, in tropical regions, the Arthonia-, Trypethelium-, and Pyrenula-groups can be seen forming colorful crusts. In temperate regions like Britain, smooth-barked trees in western districts may harbor species from the Arthonia and Pyrenula genera.

What are Lichen Photobionts?

Lichen photobionts are crucial components of lichen symbiosis, serving as the photosynthetic partners in this mutualistic relationship. They provide essential nutrients to the fungal partner, contributing significantly to the lichen’s overall functioning.

• Types of Photobionts
  • Green Algae (Chlorophyta)
    • Most lichens contain green-algal photobionts. These are predominantly unicellular green algae, though small colonial and filamentous forms are also present.
    • Genera such as Trebouxia and Trentepohlia are the most common green-algal photobionts found in lichens.
  • Cyanobacteria
    • About 10% of lichens host cyanobacteria. These bacteria can fix atmospheric nitrogen, adding a vital nutrient that is often limiting in heavily leached environments.
    • Genera like Nostoc and Scytonema represent the cyanobacterial photobionts.
• Functional Roles
  • Carbon Provision
    • The primary role of photobionts is to provide carbon in the form of simple sugars through photosynthesis. This carbon is essential for the growth, maintenance, and reproduction of lichen fungi.
    • Lichen fungi can capture and assimilate a significant proportion of the carbon fixed by their photobionts, converting it into forms not usable by the photobionts.
  • Nitrogen Fixation
    • In lichens containing both green algae and cyanobacteria, the cyanobacteria contribute fixed nitrogen. This provides an additional nutrient source, especially beneficial in nutrient-poor conditions.
• Diversity and Identification
  • Species and Genera
    • Approximately 100 species of photobionts are commonly found in lichens, belonging to four main genera. Most are from the genus Trebouxia, followed by Trentepohlia (green algae), and Nostoc and Scytonema (cyanobacteria).
    • The morphology of photobionts within the lichen thallus often differs from their form in isolation, complicating identification.
  • Identification Techniques
    • Traditional microscopic methods are often inadequate for identifying photobionts due to their altered morphology. Molecular methods and culturing studies are more effective in distinguishing between different strains.
    • Jelly-lichens, containing Nostoc, are an exception, as their distinct chain-of-pearls structure is clearly visible under the microscope.
• Ecological and Evolutionary Insights
  • Ecological Adaptation
    • Lichen fungi may associate with different photobionts based on ecological needs. A single fungal species might partner with various photobionts in different environments, adapting to local conditions.
    • Some lichens can harbor multiple photobiont strains simultaneously, similar to how other symbiotic organisms regulate their symbionts to optimize performance.
  • Regulation and Adaptation
    • There is a hypothesis that lichens, like corals and plants, might regulate their photobionts to maximize photosynthetic efficiency in response to environmental changes. This potential regulatory mechanism is an area of ongoing research.

Significance of Lichens/Importance of Lichens

Lichens hold considerable importance due to their diverse roles in ecosystems and their practical applications for humans. Their significance can be categorized into several key areas:

• Environmental Indicators
  • Pollution Monitoring
    • Lichens are highly sensitive to air pollutants, such as sulfur dioxide and heavy metals.
    • Their presence or absence in an area serves as an indicator of environmental health and air quality.
  • Ecosystem Health
    • Due to their sensitivity, lichens can reveal changes in ecosystem conditions and contribute to ecological monitoring efforts.
• Soil Formation and Stability
  • Soil Development
    • Lichens aid in soil formation by breaking down rocks through physical and chemical weathering.
    • This process converts rocky substrates into soil, which supports the growth of other plant species.
  • Surface Stabilization
    • By colonizing barren habitats, lichens stabilize surfaces and prevent erosion, facilitating the establishment of vegetation.
• Pioneer Organisms
  • Ecological Succession
    • Lichens are often the first organisms to colonize bare or disturbed environments.
    • They play a crucial role in ecological succession by preparing the substrate for more complex plant communities.
• Food Source
  • Wildlife Nutrition
    • Lichens provide food for various wildlife, including insects, birds, and small mammals like reindeer.
    • Some human populations also consume lichens, especially during famines or in traditional diets.
• Medicinal Uses
  • Pharmaceutical Applications
    • Certain lichens produce bioactive compounds used in medicine.
    • These compounds have applications in treating skin diseases, respiratory ailments, and other health issues.
• Industrial Applications
  • Dye Production
    • Lichens are sources of natural dyes used in textiles and other materials.
    • For example, extracts from lichens are used to produce colors for wool and silk fabrics.

Economic Importance of Lichens

Lichens, though often overlooked, play significant roles in various economic sectors. Their applications span food, medicine, and industrial uses, reflecting their diverse value to humans. The following points detail the economic importance of lichens:

1. As Food and Fodder
   • Human Consumption
     • Several lichen species are utilized as food in different regions worldwide. For example, Parmelia species are used as curry powder in India, while Endocarpon miniatum is consumed as a vegetable in Japan.
     • Evernia prunastri is used in bread-making in Egypt, and Cetraria islandica (Iceland moss) is a traditional food source in Iceland. In France, lichens contribute to the production of chocolates and pastries.
     • Additionally, lichens such as Umbillicaria, Parmelia, and Lecanora species are incorporated into local diets globally.
   • Animal Fodder
     • Lichens also serve as fodder for various animals. Species like Ramalina traxinea, R. fastigiata, Evernia prunastri, and Lobaria pulmonaria are rich in polysaccharides like lichenin, making them valuable food sources.
     • In the Tundra region, Cladonia rangiferina (reindeer moss) is a primary food source for reindeer and muskoxen. Dried lichens are also fed to horses and other domesticated animals.
2. As Medicine
   • Traditional and Modern Uses
     • Lichens have been employed in medicine since ancient times due to their bioactive compounds. They contain lichenin, as well as various bitter and astringent substances, which are useful in treating various ailments.
     • Cetraria islandica and Lobaria pulmonaria are used for treating tuberculosis and other lung diseases. Parmelia sexatilis is used for epilepsy, while Parmelia perlata is utilized for dyspepsia.
     • Other medicinal applications include Cladonia pyxidata for whooping cough, Xanthoria parietina for jaundice, and several species of Pertusaria, Cladonia, and Cetraria islandica for intermittent fever.
     • Usnic acid, derived from species such as Usnea and Cladonia, functions as a broad-spectrum antibiotic. Usnea and Evernia furfuracea are employed as astringents for hemorrhages. Lichens are also integral in the formulation of antiseptic creams due to their spasmolytic and tumor-inhibiting properties.
3. Industrial Uses
   • Tanning Industry
     • Lichens like Lobaria pulmonaria and Cetraria islandica are utilized in the tanning of leather, where their chemical properties aid in the preservation and treatment of hides.
   • Brewery and Distillation
     • In the brewing and distillation industries, lichens such as Lobaria pulmonaria, Usnea florida, Cladonia rangiferina, and Ramalina fraxinea are used. These lichens are rich in lichenin, a carbohydrate beneficial in alcohol production.
   • Preparation of Dye
     • Historically, lichens have been employed to produce dyes of various colors, including brown, red, purple, and blue. For instance, Parmelia omphalodes yields a brown dye for wool and silk fabrics, while Ochrolechia androgyna and O. tartaria provide red and purple dyes.
     • The blue dye “Orchil,” obtained from Cetraria islandica and other species, is used for dyeing woolen goods. Orcein, derived from orchil dye, is widely used in laboratories for histological studies and coir dyeing.
     • Roccella tinctoria, R. montagnei, and Lasallia pustulata are sources of litmus, an essential acid-base indicator dye.
   • Cosmetics and Perfumery
     • Aromatic compounds extracted from lichens are used in cosmetics and perfumes. Essential oils from Ramalina and Evernia species contribute to the manufacture of cosmetic soaps.
     • Ramalina calicaris is utilized to whiten hair in wigs, while species of Usnea are known for their scent retention and are commercially used in perfumery. Evernia prunastri and Pseudevernia furfuracea are also prominent in the fragrance industry.

Ecological Importance of Lichens

Lichens play significant roles in various ecosystems, demonstrating remarkable adaptability and interactions with their environment. Their ecological importance is evident in several key areas:

1. Pioneer of Rock Vegetation
   • Colonization of Harsh Environments
     • Lichens are among the first organisms to colonize bare rock surfaces. Their ability to thrive with minimal nutrients and water allows them to establish themselves in these inhospitable conditions.
     • Crustose lichens, which form a crust-like layer, are particularly effective at this. They grow luxuriantly on rocks and contribute to their weathering.
   • Soil Formation
     • Lichens secrete organic acids that chemically break down rock surfaces. This process disintegrates the rocks into smaller particles.
     • Upon death, lichens mix with these particles, forming a thin layer of soil. This soil layer supports the growth of pioneer plants such as mosses.
     • Over time, vascular plants can also establish themselves. For example, Lecanora saxicola and Crtmmia pulvinata are among the initial colonizers, followed by Poa compressa, which grows on the mosses’ remains.
2. Accumulation of Radioactive Substances
   • Absorption of Pollutants
     • Lichens are efficient at absorbing various substances from their environment. Species such as Cladonia rangiferina (reindeer moss) and Cetraria islandica (Iceland moss) are commonly found in tundra regions.
     • These lichens absorb radioactive isotopes like strontium-90 and cesium-137 from atmospheric fallout, often originating from nuclear activities.
   • Impact on Food Chains
     • The absorbed radioactive substances are incorporated into the lichen biomass. As lichens are consumed by herbivores such as caribou and reindeer, these pollutants enter the food chain.
     • Consequently, the radioactive substances can be passed on to humans who consume these animals, affecting local populations such as the Lapps and Eskimos.
3. Sensitivity to Air Pollutants
   • Indicators of Air Quality
     • Lichens are highly sensitive to air pollutants, including sulfur dioxide (SO₂), carbon monoxide (CO), and carbon dioxide (CO₂). Their presence and abundance can indicate the level of air pollution.
     • In polluted areas, the number of lichen thalli tends to decrease, and in extreme cases, lichens may be completely absent.
     • Crustose lichens are more tolerant of pollution compared to foliose and fruticose types. Consequently, lichens serve as effective bioindicators of air quality and environmental health.
   • Urban and Industrial Areas
     • Due to their sensitivity, lichens are notably scarce in urban and industrial zones. Their absence in these areas is a sign of high pollution levels.

Harmful Activities of Lichens

Despite their ecological benefits, lichens can also have detrimental effects on their surroundings. Their harmful activities are significant in certain contexts. The following points outline these adverse impacts:

1. Parasitism on Mosses
   • Destruction of Moss Colonies
     • Some lichens, such as Amphiloma and Cladonia, are parasitic to mosses. These lichens attach to moss colonies and disrupt their growth.
     • They invade and overtake the mosses, leading to the complete destruction of these plant communities. This parasitism can severely impact local ecosystems by eliminating a crucial component of the ground cover.
2. Damage to Plant Tissues
   • Invasion and Tissue Destruction
     • Lichens like Usnea possess holdfast hyphae that penetrate deeply into plant tissues.
     • These hyphae can reach and damage the middle lamella and inner cell content, resulting in the total destruction of the plant tissue. This form of damage can significantly affect the health and survival of the host plants.
3. Damage to Architectural Surfaces
   • Corrosion of Building Materials
     • Crustose lichens are known to cause substantial harm to building materials. They can damage window glasses and marble stones, especially in older structures.
     • The lichens secrete acids that corrode these materials over time, leading to aesthetic and structural degradation. The presence of lichens on historic buildings often requires costly restoration and maintenance.
4. Toxicity to Animals and Humans
   • Poisonous Compounds
     • Certain lichens, such as Letharia vulpina (wolf moss), are highly toxic. They contain vulpinic acid, which is harmful if ingested.
     • The toxicity of these lichens poses risks to herbivores and humans who might come into contact with them. The presence of such lichens in an area can be a health hazard and may require precautionary measures to avoid poisoning.

Examples of Lichens

Lichens represent a diverse group of symbiotic organisms, combining fungal and photosynthetic partners. They vary greatly in form, habitat, and ecological roles. Here are several examples of lichens, illustrating their diversity:

• Crustose Lichens
  • Lecanora
    • Description: Form thin, crust-like patches tightly attached to surfaces such as rocks and tree bark.
    • Habitat: Common on rocky surfaces and in arid environments.
    • Function: Contribute to the weathering of rocks and soil formation.
  • Graphis
    • Description: Known for its distinct, black, linear, or cracked appearance on surfaces.
    • Habitat: Often found on tree bark in tropical and subtropical forests.
    • Function: Plays a role in nutrient cycling within forest ecosystems.
• Foliose Lichens
  • Parmelia
    • Description: Forms leaf-like structures that are loosely attached to surfaces, with lobes that can be irregular or rounded.
    • Habitat: Frequently found on tree bark and rocks in a variety of environments.
    • Function: Act as indicators of air quality due to their sensitivity to pollution.
  • Xanthoria
    • Description: Characterized by bright yellow to orange thalli with a foliose form.
    • Habitat: Common on exposed, sunny surfaces like rocks and walls.
    • Function: Often used in studies of UV radiation and environmental monitoring.
• Fruticose Lichens
  • Usnea
    • Description: Resemble miniature shrubs or “beard-like” structures, with a central strand from which branching occurs.
    • Habitat: Typically found hanging from trees in forests and woodlands.
    • Function: Important in the forest canopy, contributing to habitat complexity and serving as food for various wildlife.
  • Cladonia
    • Description: Exhibits a variety of forms, including branching structures that resemble small cups or antlers.
    • Habitat: Found in a range of environments from forests to tundra.
    • Function: Plays a role in soil formation and stabilization in various ecosystems.
• Jelly-Lichens
  • Collema
    • Description: Forms gelatinous, often translucent thalli that can appear “jelly-like” and are typically found in moist environments.
    • Habitat: Common in moist, shaded areas like forest floors and on rocks.
    • Function: Contributes to nutrient cycling in damp ecosystems.
  • Leptogium
    • Description: Resembles Collema but often forms smaller, more delicate structures.
    • Habitat: Prefers moist environments and can be found on tree bark and rocks.
    • Function: Acts as a moisture indicator and contributes to nutrient recycling.
• Crustose-Foliose Transition Lichens
  • Lobaria
    • Description: Exhibits characteristics of both crustose and foliose lichens, with a leafy structure that can be somewhat attached.
    • Habitat: Typically found on tree bark in moist, shaded environments.
    • Function: Important in nutrient cycling and as a habitat for small invertebrates.

FAQ

References

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