Lichens

Lichens, also known as lichen trees or lichen plants, are symbiotic associations between fungi and algae or cyanobacteria. The lichen types include – lichen crustose, lichen foliose, and lichen fruticose. Lichen is an example of mutualism.

The fungal component provides structure and protection, while the algal or cyanobacterial component contributes through photosynthesis. Lichens are highly adaptable and thrive in various environments, from arctic tundras to deserts.

Lichens play ecological roles such as soil stabilization, nitrogen fixation, and contributing to primary succession on bare substrates. In this article, we will read about Lichens, their general characteristics, structure, types, classification, and significance of lichen.

What are Lichens?

Definition of Lichens: Lichens are symbiotic association between a fungus and an algae. The fungal partner helps in absorption of nutrients and in reproduction whereas algal partner helps in photosynthesis.

Lichens meaning a group of plant-like organisms formed when a symbiotic relationship is established between a fungus and a photosynthetic partner, typically algae or cyanobacteria that contain chlorophyll. Symbiosis is a process in which two organisms associate with each other and mutually benefit from the association. Lichens are model organisms that show symbiotic association.

This association creates a composite organism where the lichen fungus provides structural support and protection while the photosynthetic partner produces energy through photosynthesis. Usually, the fungal part is the dominant partner that gives the lichens most of its morphological and reproductive characteristics.

Lichens Characteristics

Some characteristics of Lichens are listed below:

• Lichens are symbiotic organisms formed by a fungus and a photosynthetic partner (algae or cyanobacterium).
• The body of the lichen do not have a distinct or identifiable root, shoot, or leaf. Hence termed as thallus.
• Lichens exhibit various forms, including crustose, foliose, and fruticose.
• Lichens can tolerate extreme environmental conditions and are found in diverse habitats.
• Reproduction occurs through both sexual (spores) and asexual (soredia) means.
• Some lichen species are bioindicators, reflecting environmental conditions.

Diagram of Lichens

The labeled diagram of lichens is given below:

Structure of Lichens

The overall structure of lichen can be described by looking at its associated partners.

Fungal Component (Mycobiont)

The fungal partner, or mycobiont, forms the structural framework of the lichen. The lichen fungus body comprises filamentous cells called hyphae that form the thalli. The fungal portions also produce fruiting bodies. These reproductive structures produce spores, which can germinate to form new lichen thalli under suitable environmental conditions.

The fungal partner plays a crucial role in absorbing water and nutrients from the environment. Also, an outgrowth from the fungal body called rhizines is responsible for attachment to the substrate.

Photosynthetic Component (Photobiont)

The photosynthetic partner, or photobiont, provides the lichen with the ability to produce energy through photosynthesis. Algae photobionts are typically green algae, while cyanobacterial photobionts are blue-green bacteria capable of nitrogen fixation. These organisms are embedded within the fungal hyphae, often in specialized structures called cephalodia or photobiont layers.

Thallus

The thallus refers to the main body or structure of the lichen, composed of intertwined fungal hyphae and photobionts. The thallus can exhibit various forms, including crustose (crusty), foliose (leafy), and fruticose (shrubby), depending on the type of lichen.

Types of Lichens

Lichens can be classified into different types based on their physical appearance and attachment to their substrate.

• Lichens Crustose appear as crust-like structures that tightly adhere to their substrate. These usually grow on new habitats like rocks and hard substrate. Examples include species like Caloplaca, Rhizocarpon, and Lecanora.
• Lichens Foliose have a leafy or lobed appearance, with thalli more loosely attached to the substrate than crustose lichens. These are mainly found on tree branches and moist substrates. Examples include species like Parmelia, Physcia, and Lobaria.
• Lichens Fruticose have a three-dimensional, branching, or shrubby growth form, often resembling miniature trees or bushes. Examples include species like Usnea, Cladonia, and Ramalina.

Some more lichen types are listed below.

• Leprose lichens have small thalli that have a powder-like appearance.
• Squamulose lichens have a scale-like appearance.
• Gelatinous or jelly lichens are formed with a transparent jelly-like appearance.

Lichens Classification

Lichens are a group of symbiotic organisms that can be broadly classified in two ways: one based on their growth form and the other based on their growth medium.

Based on their Growth

Lichens can be classified according to their growth form and physical appearance. The physical appearance of a lichen depends on its thallus, which is mainly composed of its fungal counterpart. These include:

• Foliose Lichens: These lichens have a leaf-like appearance.
• Fructose Lichens: These lichens have a branching appearance.
• Crustose Lichens: These lichens have flattened crust-like structures.
• Leprose Lichens: These lichens are powdery.
• Jelly Lichens: These lichens are characterized by a gelatinous appearance.
• Squamulose Lichens: These lichens often have a scaly appearance.

Based on their Habitat

Lichens can also be classified into subgroups based on their habitat or the medium they grow on. These include

• Terriculous Lichens: These lichens grow on terrestrial regions with soil.
• Aqueous Lichens: These lichens grow around areas near or under water. These are further divided into freshwater and marine lichens that grow on salicaceous rocks at the bottom or near the shores of the aqueous environment.
• Ligniculous Lichens: These lichens use dead wood as their substrate, such as twigs, branches, or stems. These woods have the bark stripped from it, and lichens utilize lignin as their growth medium
• Saxicolous Lichens: These lichens generally grow on hard substrates like rocks and stones.
• Corticulous Lichens: These lichens on tree bark attach and grow on trees.

Based on their Internal Structure

Lichens can also be classified into two types based on their internal structure and layers formed by their fungal and algae counterpart. There are

• Heteromerous Lichens: These are lichens in which the thallus is divided into four layers: three composed of fungi called upper cortex, lower cortex, and medulla, and one is made of algae.
• Homoimerous Lichens: Homoimerous lichens do not have distinct layers, and the algae are loosely arranged between the fungal components.

Based on their Fungal Partners

Lichens are composed of fungal and algal components and can be classified based on lichen fungal partners. These are

• Ascolichens: The fungal partner is of the fungal group Ascomycetes
• Basidiolichens: The fungal partner is of the fungal group Basidiomycetes.
• Deuterolichens: The fungal partner is of the fungal group Deuteromycetes.

Significance of Lichens

Lichens play a crucial role in ecosystems, providing numerous environmental and human benefits. These include:

• Indicator of Environmental Health: Lichens are highly sensitive to environmental changes. Lichens are good indicators of air pollution, making them valuable for ecological monitoring.
• Soil Formation and Stability: Lichens contribute to soil formation by breaking down rocks and other substrates through physical and chemical weathering processes. Their ability to colonize barren habitats helps initiate soil development and stabilize surfaces, paving the way for the establishment of other plant species.
• Pioneer Organisms: Lichens are often good pioneer organisms in bare habitats and play an important role in ecological succession.
• Food Source: Lichens are food for various wildlife species, including insects, birds, and small mammals like reindeer. Humans can also eat some species.
• Medicinal Uses: Certain lichen species contain bioactive compounds with potential pharmaceutical applications.
• Industrial Use: Some lichens are also used to extract dyes.

Conclusion – Lichens

Lichen symbiotic relationship consists between a fungus and a photosynthetic partner, usually algae or cyanobacteria. This unique lichen symbiotic relationship is called mutualism because both partners benefit from the association. Lichens or lichen plants are known for their ability to colonize diverse and sometimes harsh environments, including rocks, trees, soil, and even surfaces like gravestones or rooftops. They come in a wide variety of forms, shapes, and colors. Lichens are important for ecological processes, contributing to soil formation, nutrient cycling, and providing habitat for microorganisms and small invertebrates.

FAQs on Lichens

Where do Lichens Grow?

Lichens can grow on several substrates, including bare rock surfaces, tree barks, or concrete walls.

Are Lichens Safe to Touch?

Not all lichens are dangerous. Some species are poisonous, while humans can also eat some species.

Are Lichens Algae or Fungus?

Lichens are a mutual association between algae and fungus, with the fungal partner being the dominant partner providing structure and the algae doing the photosynthesis.

What are Lichens?

Lichens are a community of plant-like organisms produced by the symbiotic association between a fungus and a photosynthesis partner, such as algae or cyanobacteria.

What are the Characteristics of Lichens?

The structure of their fungal partner characterizes lichens. The main body of a lichen called the thallus, can be fructose, crustose, or lobulose.

Is Lichen a Plant?

Lichen is not a plant; it is an organism consisting of a fungus and an alga or cyanobacterium living in a symbiotic relationship.

Where are Lichen Found?

Lichen are found in various habitats worldwide, including forests, deserts, mountains, tundras, and even urban environments, where they can grow on rocks, soil, trees, and other surfaces.

Is a Lichen a Moss or Fungi?

Lichen is neither a moss nor a fungi; it’s a symbiotic association between a fungus and an alga or cyanobacterium, commonly found in various environments worldwide.

How do Lichens Respond to Environmental Stress?

Lichens respond to environmental stress by adjusting their metabolism and morphology to adapt to adverse conditions, such as pollution, drought, extreme temperatures, and changes in light intensity.

Is Lichen Edible?

While some cultures historically used certain lichens as food, many species contain toxins (eg. wolf lichen) and are not safe for human consumption. Therefore, precuation should be taken before consuming lichens.

What is Lichen Usnea?

Usnea is a lichen genus characterized by its beard-like appearance, commonly found hanging from tree branches or growing on rocks in humid environments. It is often used in traditional medicine for its antimicrobial properties.