Lichens

Introduction

Lichens are leafy organisms that colonise any hard surface like those of rocks, tree barks, cave walls and soil. Until the advent of microscopy, Lichens were considered to be single organisms. Lichens are the combination of a special type of algae called cyanobacteria and fungi, specifically ascomycetes and basidiomycetes. These complex organisms have been food for humans for a long time, but they also serve as one of the main food sources for Caribous and Reindeers. 

Lichens are complex, composite organisms. The lichens come under a single fungi species called ascomycete (in most cases). Only one of the partners in this symbiotic relationship produces food or performs photosynthesis. With more research, scientists have found another classification of fungi that inhabits the core of the organism called basidiomycete yeast. Therefore, the lichens cannot be called a simple algae-fungi organism but a community of organisms.

Structure of Lichens

The whole body of a lichen is called a Thallus which is composed of algae and fungi. The whole organism is rooted on a substrate with fibrous growths known as Rhizines. The different body types of lichens mount these Rhizines.

Types of Lichens

There are about 17000 types of lichens that are components of the various species of Lichens. There are several reasons based on which these classifications are assumed. The different types of lichens are classified based on the following:

1. Growth

Based on the growths that are observed in Lichens, they can be classified as follows:

•   Crustose

These lichens attach themselves to hard substances that occur in natural surroundings like tree trunks and barks, rocks, stones etc. They have a flat, leafy structure with no lobes. Some examples of the crustose lichens are Haematoma puniceum and Graphic scripta

•   Foliose

These are flat, broad-leafed and smooth lichens that grow ornamentally on foliage barks, trees and rocks. The examples of this type are Cluiudhuria, Xanthuria, Parmelia and Cetraria

•   Fruticose

This type of lichen is more of flat and broad-leafed structures. The most common examples of which are Ramaline, Usnea and Cladonia.

2. Habitat

•   Lignicolous

These lichens grow on wood.

•   Corticolous

They are prominently seen on tree barks and trunks.

•   Saxicolous

These colonise stones and rocks.

•   Marine

There is a habitat underwater on siliceous rocks found near the seashores.

•   Freshwater

Hard siliceous rocks are found near freshwater bodies, and these types of lichens are found on these rocks.

•   Terricolous

These are also called terrestrial lichens because they grow on soil.

Difference between Mosses and Lichens

Mosses and lichens usually have the same appearance, habitat and size, so a distinction between the two is important to be established. Some of these differences can be seen as follows:

Importance of Lichens in the Ecosystems

The Lichens are the “canaries in a coal mine” for the ecosystem, as lichens are sensitive to any changes in the atmosphere. They serve a wide array of purposes in the ecosystem. We can observe some of these factors as follows:

·   Bio-Indicators

Lichens are organisms that do not have roots, but they derive their nutrition in small amounts from the atmosphere around them. So even the slightest change in the composition of the atmosphere makes them susceptible to change and degradation. Lichens are sensitive to pollutants such as Nitrogen(N). Nitrogen is a gas that causes an overload of nutrients in the lichens. Too many minerals can cause the algae to fall out as chlorophyll in the algae dies out because of it. Lichens are sensitive to the presence of SO2 in the atmosphere also. There is a stark change in the structure of Lichens when they are subjected to any atmospheric change. This helps scientists study the indications of the decline of ecosystems.

·   Food

Lichens serve as a primary food source for animals like reindeers and caribous. However, in conditions such as famines, they have to serve humans as a nutritive food source to survive. Icelanders use lichens for cooking porridges, bread, puddings and soups.

·   Aesthetic Appeal

Lichens come in a wide variety of colours such as yellow, grey, green and blue. These varieties grace the landscapes of national parks like the Yosemite National Park. The fruticose lichens are used for the modelling of railroads. They also have a mysterious touch to the forests.

·   Lichenometry

The ages of rocks can be determined by measuring the length of thalli on a lichen covering the rock. This technique is called lichenometry.

·   As Dyes

Since time immemorial, humans have been using pigmented Lichens as dyes. The wide range of colours of the lichens makes for a wide variety of natural dyes. The fact that these dyes are organic and chemical-free appeals most to people now. 

The pH indicator called Litmus is a pigment extracted from lichens called Roccella tinctoria. In Scotland, clothes are dyed with the help of lichens called ‘crottles.’

•  Medicinal Properties

Lobaria Pulmonaria is a type of lichen that resembles the shape of lungs and is used to treat diseases related to the lungs. They are collected in large quantities as ‘Lungwort’. Metabolites are produced by Lichens which have therapeutic and medicinal value. Research is still going on for its medicinal properties.

Conclusion

The dual nature of Lichens, of being both algae and lichens, was first suggested by a Swiss botanist called Simon Schwendener. However, Schwendener’s theory lacked research, and this theory was rejected by Lichenologists like James Crombie and Nylander. This idea was later related to the host and parasitic relationships that are seen in other organisms also.

Lichens form a very important part of the ecosystem now, and it is credited to its properties that it serves purposes such as that of bio-indicators.