Lichen is a plantlike autotrophic existence which is the result of a symbiotic cohabitation between fungi and either green algae or cyanobacteria. They are slow growing and grow in a colonized manner; and lichen examples can be found throughout the Earth in diverse environments.
The fungal portion of the symbiosis is called mycobiont and the algal part is known as phycobiont.
They are commonly known as Reindeer moss or Caribou moss. It is a fruticose type of lichen, extensively branched and greyish in color. They are generally found in taiga pine forests and low alpine forests. Reindeer and caribou like animals use them as winter forage.
These corticolous,foliose lichens are commonly called Hammered shield lichen or Wax paper lichen. Due to their high tolerance of pollutants, they are found throughout the world even in metropolitan areas.
These are a type of foliose lichen, native to North America and Europe.
These fruticose lichens are commonly known as Iceland moss. It can be used as food and even as traditional folk remedies.
They are fruticose lichens, commonly called as Beard moss or Old man’s beard. They grow on dying trees and are very sensitive towards air pollution. They can be used as bioindicators.
These light-green, fruticose lichens are known as Witch’s hair lichen and are found in temperate zones with high rainfall.
They are mostly saxicolous lichens attached loosely to the substratum surface. These lichens are native to Australia.
They are saxicolous lichens, commonly called Velvet moss. They are native to the northern North America.
These gelatinous lichens grow on moist, shady, calcareous substratum and are of foliose type.
These crustose type lichens are found in tropical areas throughout the world.
They are crustose type lichen, commonly called as Dust lichen.
These crustose type lichens are commonly called as Script lichen or secret writing lichen due to their unique growth pattern.
- Lichens are symbiotic existence between fungi, which is the more dominant part of the symbiont, acts as a host organism for either green algae or cyanobacteria and even sometimes for both.
- Mycobiont forms the body of lichen, which is known as thallus and it protects the phycobionts inside.
- A typical lichen structure can be divided into three parts, which are outer cortex or thallus, inner medulla and the basal attachment or holdfast.
- Outer cortex, is made of densely packed filamentous fungal cells forming a thick crusty cover.
- Inner medulla, is a striated structure made of alternating layers of phycobiont and mycobiont.
- The basal attachment, is a hair like fungal structure, known as rhizines. It acts an attachment organ that helps lichen to adhere to surface.
- Based on the ratio of phycobionts and mycobionts, thalli of lichens are of two types;
- Heteromerous thalli, is where fungal cells are predominant.
- Homoeomerous thalli, is where algal and fungal cells are uniformly distributed.
- Lichens reproduce vegetatively by propagating through broken thallus. The fungal part of lichens can reproduce with the help of spore formation.
- Lichens that have cyanobacteria as their photobionts acts as a nitrogen fixator.
- Lichens grow in various interesting shape and coloring from grey-green to bright red-orange.
Significances of symbiosis in lichen-
Phycobionts in lichen are capable of the process of photosynthesis, I.e., autotroph; because of the presence of chloroplasts. But the mycobionts cannot perform photosynthesis and thus are unable to sustain itself for long on its own. So mycobionts actively seek out the algal part through chemical reaction.
They form a mutualistic existence where the phycobionts provides the mycobionts with carbohydrate and the mycobionts offer protection from environmental elements.
Based on the surface they grow on, lichens are divided into four categories,
- Terricolous, are lichens that grows on soil. e.g., Caldonia.
- Corticolous, grows on the tree barks. e.g., Parmelia.
- Lignicolous, are lichens found in the woods. e.g., Cyphellum.
- Saxicolous, lichens grow on rocks or stones. e.g., Peltigera.
Based on the growth forms of thallus lichens are classified into six main types;
- Crustose- they have 2-dimensional, crusty appearance and stick tightly to the substrate they are growing on. e.g., Graphis.
- Foliose- they have flat-leaf like appearance and grows loosely on the substratum with one or multiple points of attachment. e.g., Parmelia.
- Fruticose– it has filamentous, branched, 3-dimensional structure; standing erect or hanging loosely on the substratum by help of basal disc. e.g., Usnea.
- Leprose– they have a powdery appearance, attached superficially on the substrate. e.g., Lepraria.
- Squamulose- these lichens have a closely clustered leaf-like crustose attached to the surface and erect at the tip. e.g., Cladonia.
- Byssoid- they have a wispy, teased wool-like appearance. e.g., Coenogonium.
There are also gelatinous lichens like Collema.
Lichens are classified as their mycobiont part, as in scientific name of a lichen is same as their fungal part.
The mycobiont part consist of fungi from either phylum Ascomycota or phylum Basidiomycota.
Difference between moss and lichen-
In-spite of being called non-vascular plants, moss and lichens are very different.
Firstly, only mosses are actually plants under the phylum Bryophyta and have roots, leaves and stems likes structure with intended actions. They are considered to be the ancestor of all land plants.
Whereas, lichens are not true plants and without roots, leaves or any stems.
Ecological importance of lichen-
Lichens are of great ecological significance. Such as,
- They act as nitrogen fixator, entrapping atmospheric nitrogen.
- Mycobiont part of lichens produce chemicals that are capable of breaking down rocks and releasing the minerals in nature. This process is called biological weathering and is essential for soil formation.
- Terricolous lichens prevents soil erosion.
- Lichens can be used a food source by various animals.
- They can colonize barren surfaces, acting as a “Pioneer species”.
- They also act as bioindicators by absorbing and trapping various heavy metals, carbon or sulfur pollutants into their thallus.
For evolutionary advantage, the symbiosis between fungi and algae, I.e., lichens are not only important for ecosystem and wildlife, they also act as a bioindicator for anthropogenic activities that cause pollution.