How Lichens Benefit Ecosystems and Why Students Should Learn Them
A lichen is a symbiosis of different organisms such as fungus and a cyanobacterium or algae. Cyanobacteria are also known as blue-green algae besides being distinct from algae. The non-fungal part is called a photobiont that consists of Chlorophyll. Many lichen partners including one photobiont and one mycobiont are not universal and lichens with more than one photobiont partner are also present in certain cases. The fungal partner is composed of filamentous cells, where every filament is called a hypha. The most common fungi in lichens are species of Ascomycetes or Basidiomycetes. The green algae Chlorophyta or Cyanophyceae family of blue-green bacteria are the most common algal partners. The fungal partners cannot survive without their phycobiont, whereas algae can survive alone in water or moist soil. The color of lichens ranges from yellow to greens and black hues.
Most of the lichens grow at a slower rate. The phycobiont is a blue-green bacterium that converts nitrogen gas into ammonia. The lichens that live in stressful environments such as the arctic tundra or alpine are said to reach the age of many centuries.
Lichens are Classified Into Three Types
Crustose.
Foliose.
Fruticose.
Crustose
Crustose is a lichen that sticks tight to the substance on which it is growing on. As the name suggests, it is crusty and even forms a crust on the substrate it is growing (such as rocks, trees, or dirt). This crust is difficult to remove.
Fruticose
Fruticose looks like a little branching shrub such as a miniature leafless tree. It looks similar to a bushy coral. It grows on rocks, trees, and soils. The word fruticose has a Latin origin which means a bush or shrub
Foliose
Foliose consists of a leafy shape structure. It can be easily removed from the surface on which it grows. Foliose gets its name as they look similar to foliage which is a plant leaf.
Classification Based on Their Habitat:
Lignicolous is a term used to describe a person who has these lichens can be found in the woods.
Corticolous lichens are those that live on the bark of trees.
Saxicolous lichens are those that live on stones or rocks.
The sea. These lichens can be found growing on siliceous rocks near the sea's edge.
Water that is not salty. These lichens are found growing on hard siliceous rocks, particularly near freshwater.
Terricolous- These lichens grow on the soil and are thus referred to as terrestrial lichens.
Classification Based on Their Internal Structure:
Heteromerous lichens
Homoiomerous lichens.
Classification Based on their Fungal Partner:
Ascolichens
Basidiolichens
Hymenolichens
Lichen Algae
The lichen fungus is classified as a fungus, and the fungal partners are classified as Ascomycota and Basidiomycota. Lichens can also be classified according to their morphology. There are three major types of lichens, though there are others. Crustose lichens are lichens that are tightly attached to the substrate, giving them a crusty appearance. Foliose lichens have leaf-like lobes, can only be attached at one point in the growth form, and have a second cortex below the medulla. Finally, fruticose lichens are known to have rounded structures and a branched appearance in general. Figure 2 depicts an example of each type of lichen.
General Characteristics of Lichens
Let’s discuss lichen characteristics:
In general, the major portion of the thallus is occupied by the fungal component and this fungal component produces its own reproductive structures.
The algal partner is basically responsible for making the food by the process of photosynthesis. The food diffuses out as well as it is absorbed by the fungal partner.
Owing to their symbiotic relationship, lichens can live in a variety of habitats as well as climatic conditions including extreme environments.
Lichens can be classified into the following types based on their growing medium.
Follicolous (grows on tree barks), Corticolous (grows on tree barks) (grows on leaves surfaces),
Saxicolous is a type of saxicolous plant (grows on rock surfaces)
Terribly impressive (grows on soil)
Musically inclined (grows on mosses)
Lichen growth forms are commonly seen on surfaces, forming greyish, greenish, or orange areas. They are divided into three major types based on their morphology and size, namely,
Crustose is a sugar (crust like)
Foliose is an abbreviation for Foliose (leaf like)
Fructose (shrubby)
Crustose lichens are known as microlichens whereas foliose and fruticose lichens are known as macrolichens.
The thallus is the main plant body of the lichen. The thallus is the vegetative portion, which is similar to that of mosses and liverworts.
Mycobionts (Ascomycete or Basidiomycete) form a close symbiotic relationship with phycobionts (green algae or blue-green algae). After association, both phycobionts and mycobionts lose their distinct identities and are referred to as lichens. Lichens now function as a single organism, both morphologically and physiologically.
Lichens reproduce in a way that is completely different from that of fungi and algae. Lichens reproduce vegetatively by the formation of special propagules known as diaspores. Soredia and isidia are the most common lichen diaspores.
The lichens' fungal partner reproduces sexually. In lichens, sexual reproduction begins with the formation of fruiting bodies, which is followed by the formation of spores known as ascospores. Ascospores are dormant spores that can survive in harsh environments.
FAQs on Lichens: Meaning, Classification, and Characteristics
1. What are lichens in biology?
Lichens are composite organisms that arise from a symbiotic relationship between a fungus (the mycobiont) and an alga or cyanobacterium (the phycobiont). The fungus provides the structural framework, protection from the elements, and absorbs water and minerals, while the photosynthetic partner produces carbohydrates (food) for both organisms. This unique partnership allows them to survive in harsh environments where neither organism could live alone.
2. What are the three main types of lichens based on their structure?
Lichens are primarily classified into three main forms based on their thallus (body) structure:
- Crustose: These lichens form a thin, flat crust that is tightly attached to the substrate, like paint on a rock or bark. It is very difficult to remove them without damaging the substrate.
- Foliose: These are leaf-like lichens with a more complex structure, featuring upper and lower surfaces. They are attached to the substrate at one or more points but can be peeled off more easily than crustose lichens.
- Fruticose: These lichens are shrub-like, hair-like, or bushy in appearance. They are often branched and are attached to the substrate at a single point, growing upright or hanging down.
3. How do lichens obtain nutrition to survive?
Lichens obtain nutrition through a process of mutualism. The algal or cyanobacterial partner (phycobiont) contains chlorophyll and performs photosynthesis, converting sunlight, water, and carbon dioxide into sugars. These sugars serve as the primary food source for both the phycobiont and the fungal partner (mycobiont). In return, the fungus provides a stable environment, moisture, and essential minerals it absorbs from the air, rain, and the surface it grows on.
4. Why are lichens considered excellent indicators of air pollution?
Lichens are highly sensitive to atmospheric pollutants, especially sulfur dioxide (SO₂). Because they lack roots and a protective outer layer (cuticle), they absorb water, nutrients, and any present toxins directly from the atmosphere. High levels of pollution can destroy the chlorophyll in the algal partner, disrupting photosynthesis and ultimately killing the lichen. Therefore, the absence of lichens in an area, particularly in forests and cities, is a strong indicator of poor air quality.
5. What is the ecological importance of lichens?
Lichens play several crucial roles in ecosystems. They are often pioneer species, being the first organisms to colonise bare rock and soil. By slowly breaking down rock surfaces and trapping organic matter, they contribute to soil formation, paving the way for other plants. They also serve as a food source for animals like reindeer and insects, provide nesting material for birds, and contribute to the nitrogen cycle when the phycobiont is a nitrogen-fixing cyanobacterium.
6. How is the symbiotic relationship in lichens different from parasitism?
The relationship in lichens is a classic example of mutualism, not parasitism. In a mutualistic relationship, both partners benefit. The fungus gets food from the alga, and the alga gets protection, water, and minerals from the fungus. In contrast, parasitism is a relationship where one organism (the parasite) benefits at the direct expense of the other (the host). In lichens, both organisms thrive together in a cooperative partnership that allows them to colonise new habitats.
7. Why is a lichen not considered a true plant?
A lichen is not a true plant because it is a composite organism formed by members of two different biological kingdoms: Fungi and Protista (algae) or Monera (cyanobacteria). Plants belong to the Kingdom Plantae and are characterised by having differentiated organs like true roots, stems, and leaves. Lichens lack these complex structures and represent a stable symbiotic association rather than a single, unified organism with a shared genetic lineage.
8. Why don't lichens have a single, formal classification in the taxonomic system?
Lichens do not have a single formal taxonomic rank because they are not a monophyletic group (a group descending from a single common ancestor). They are a biological partnership between genetically distant organisms. Since modern taxonomy is based on evolutionary lineage, a composite organism cannot be placed in a single branch. By convention, lichenised fungi are classified based on the fungal partner (the mycobiont), as the same fungus can sometimes form a lichen with different photobionts.
