Lichen is an interesting organism in the environment. It looks like a single organism. But it is a symbiotic relationship between two organisms helping each other in nutrition: fungi partner, mycobiont and photosynthetic partner, photobiont, the relationship with algae or cyanobacteria (blue- green algae) with fungi Ascomycota (cup fungi).
According to research by an international team, an individual lichen may contain three different fungi or more than single algae. The interaction of lichen is a most complex, beautiful, tiny and colourful organism.
Algae and cyanobacteria are photosynthetic organisms, sharing their synthetic carbohydrate with fungi and fungi supply water by absorbing water vapour in the air to algae or cyanobacteria to maintain symbiotic relationship with both of them. The fungus cannot do photosynthesis, but can give protection by sheltering photobiont and help the other organisms with chemicals that it produces as well as by physical protection.
Symbiotic relationship
There is a disagreement whether relationship of lichen is mutualistic (symbiosis which is beneficial to both organisms involved) or it is controlled parasitism (living as a parasite on or with another animal or organism). Fungus and algae seem to be in a naturalistic relationship because they are helping each other by controlling ecological condition and no partner harms others. Lichen has a diverse community of microbiological organisms inside them.
Lichen symbiotic relationship is a successful relationship in ecosystem. They can live in almost all terrestrial habitats as well as in the air. This helps lichen grow adapted to a miniature ecosystem. They can be found from tropical to polar regions, including desserts. Lichens are different in colour, size and appearance. Some are like plants, but have no plant features, only plant like appearance, such as tiny leafless branches, leaf like structures, flakes that lie on the surface and powder-like appearance.
They like to lie on tree bark, leaf, branches, rock surfaces and ground if there is sunlight, air and water. Lichens prefer to grow like mosses. Sometime, millions of lichens live together to form different forms. They do not have root to absorb nutrient or water, but have root like structures.
The body of a lichen is called a thallus that consists of cells of mycobiont and photobiont. The individual lichen is one thallus. Every lichen has a thallus different from each other. A lichen consists of four layers: upper cortex, algae layer, medulla and lower cortex.
The upper cortex of a lichen is the outermost layer made up of densely packed fungal cells. It contains pigments to protect from radiation. The algae layer is made up of concentrated algae cells or cyanobacteria. The medulla layer is made up of loosely packed fungal hyphae (thread-like fungal cells). The root-like structure of some lichen or rhizines is made for anchoring to a substrate in the lower cortex.
Reproduction of lichen by vegetative; part of lichen breaks from the thallus and fall away to being new growth nearby and fungi Ascomycota have sexual reproduction (spores). They have a long life span and a slow regular growth rate. The vegetative structure of a lichen has developed complexity with different colours and shapes. Lichen has no taxonomic basics to be categorised, but they can be categorised by the growth form and structure. Thallus size can be divided into micro lichens and macro lichens. Foliose, crustose, fruticose, and squamulose are morphologically different fungi.
Foliose lichens are the most common type of lichen that has a leaf-like structure growing more or less like real plants. This lichen grows on the trunk of trees and on a rock in the shady woods. They are usually gray-green and form circular colonies. Lichen thallus is flattened and spread horizontally and simple dichotomously or irregularly divided and grow circular outside. Rhizines are present on the lower cortex.
Structure
Crustose lichens that are growing flat crust closely adhere to the substrate. These are colourful shade of yellow, green, orange, or black. The thallus is thin and has no lower cortex because the medulla directly contacts the substrate and commonly grows to some extent. The surface of the thallus is smooth or lumpy and sometimes the surface is areolate (eg. Arthonia sp.).
The thallus is branched and the branches are covered by a single cortex which extends to both sides of the branch. The thallus is attached to a single point called holdfast. There is no upper or lower surface, photobiont found below the cortex and the medulla lies deeper. Radiated fruticose and stratose –fruticose are two types of fruticose lichen (eg. Ramalina sp.). Squamulose lichen, the thallus is small, flat and the lobes are overlapping. The thallus is clustered and slightly flattened pebbles like units. They differ anatomically from foliose lichen, which has not lower cortex like in crustose (eg. Cladonia sp.).
Except these main four categories, leprose is powdery like lichen, gelatinous is jelly-like lichen, filamentous is a string or matted hair-like lichen and byssoid is wispy, teased wool-like lichen. Lichen flora can be classified according to a substrate which they are growing on: bark- corticolous, on soil- terricolous, on rock- saxicolous, on wood- lignicolous and on leaves- foliicolous.
Sri Lanka is a tropical country in the Indian Ocean that has different climate changes because of the island’s geographical and vegetation types. Geography affects lichen. Altitude, humidity, rainfall, temperature and wind are the other main environmental factors that affect lichen flora.
Endemic species
Diverse and endemic plant and animal species are prominent in Sri Lanka because it is an island which has different climate and geographical changes.
Nearly 60 lichen species are endemic to Sri Lanka (30 species are newly recorded in the subcontinent of India and eight are new for Asia). Nearly 20,000 lichen species have been identified in the world. According to scientists, about six percent of the earth’s surface has been covered by lichens.
When considering lichen flora in Sri Lanka, there are 1,500 species observed (according to investigations of G. Weerakoon and A.Aptroot, 2016). Lichen diversity of forest sites of Sri Lanka was further investigated according to the different geographical regions. Researchers say there are nearly 2,000 or more lichen species in Sri Lanka.
Crustose lichen is the most prominent and diverse type of lichen in the country. Letrouitia parabola, Lacanora tropica, Malmedia leproloma, Briganatiaea leucoxantha, Cresponea proximata Astrothelium conjugatum, Porina monilisidiata, Lecanactis minutissima and Megalotremis cylindrica are some lichen species in the country. Lichens are an environmental indicator. Lichens have a long lifespan and because of that, they mix with environmental changes. Lichen can be used to identify air pollution, metal contamination and the condition of the ecosystem and carry out primary succession in a terrestrial environment. It prevents drying of places and keeps moisture in the substrate and helps reduce carbon dioxide in the air.