1
Overview of Lichen

Ashoke Kumar Das1, Subrata Sarkar1, and Papori Devi2

1 Department of Botany, Abhayapuri College, Abhayapuri, Assam, India

2 Department of Botany, Arya Vidyapeeth College, Guwahati, Assam, India

1.1 Introduction

The term “lichen” was coined by Theophrastus (Father of Botany) more than two thousand years ago. Till the 19th century, lichens were thought to be individually recognized organisms. In 1869, only it was accepted that it was composed of two different organisms [1, 2]. Lichens are composed of different species of fungi (Mycobiont), algae, or cyanobacteria (Photobiont) [3–9], and some microorganisms like bacteria are also associated with them [10–14]. There is a long debate and study regarding the combination of association of different components of lichen symbiosis and their physiology [15–17]. Though lichen is composed of different components, they form morphologically constant forms that are designated as species [18]. They are frequently specified as the best example of mutualistic partnerships. Photobionts have the capacity to photosynthesize; they provide carbohydrates to the fungal partner, and the mycobiont creates a physical scaffold that encloses and supports the growth of photobionts [5]. According to Hawksworth, “a lichen is a stable self‐supporting association of a mycobiont and a photobiont in which mycobiont is the exhabitant” [4]. Through a process called lichenization, a fungus and a photosynthetic partner transformed into a lichen thallus, from a free‐living to a symbiotic state [19].

Lichens are distributed throughout the world and found to grow in almost all climatic conditions (“Lichens,” [20]). They are dominating the earth’s terrestrial ecosystem, particularly in the subarctic and arctic regions, covering about 8–10% of the total area [21]. About 13,500–20,000 species of lichens have been recognized globally so far [22–25]. According to the Botanical Survey of India, 19,500 species have been discovered so far (“Lichens,” [20]). Lucking et al. made a prediction, which is 26,000 lichen taxa [26]. Among approximately 20,000 species of lichen‐forming fungus known to exist worldwide, Ascomycetes make up 98% of all known lichenized fungi, followed by Deuteromycetes (1.6%), and Basidiomycetes (0.4%). On the other hand, there are only roughly 156 species of photobionts across 56 taxa. The majority of photobionts are either cyanobacteria (Cyanoprokaryota‐35 species, 22.3%) or green algae (Chlorophyta‐116 species, 73.9% of total photobiont diversity). The three most common photobiont genera in lichens are Trebouxia, Trentepohlia, and Nostoc [5].

Lichens are separated into two groups based on their size: macrolichens and microlichens. They are classified as corticolous, ramicolous, lignicolous, saxicolous, musicolous, terricolous, and foliicolous based on the substrate on which they develop. They are also separated into three groups based on their physical characteristics: foliose, fruticose, and crustose [27]. Anatomically, the majority of lichen thallus is stratified into the upper cortex, photobiont layer, medulla, and lower cortex. The cortex of many foliose and fruticose lichens is made up of pseudoparenchymatous or prosoplectenchymatous tissues of fungi. The medullary layer consists of loosely interwoven long‐celled hyphae having internal airspace. The photobiont layer is formed by the upper part of the medulla. Crystalline secondary products often encrusted the hyphal cell walls of the algal and medullary layers. The lower cortex is well‐developed in some typical foliose lichen groups like Parmeliaceae. Reproduction of lichen is mainly expressed by its fungal partner with sexual and asexual methods, while it is reduced in case of the algal partner in the lichenized state [28].

The significant role played by lichens includes natural soil formation and nutrient cycling [29]. For monitoring anthropogenic disturbances over time, such as air pollution, acid rain, nitrogen deposition, and several other environmental variables, lichens have been utilized as a crucial biological indicator [7, 30]. Since long time, a wide range of lichen species have been used as traditional medicine and in different folk cultures in countries like North America, Europe, India, Nepal, and China [31–35]. They are also good sources of human food and are also taken by various wild and domesticated animals as feed, containing a range of nutrients and biologically active compounds [36–38]. Some lichens are also used as raw materials in various industries like cosmetics and perfume, minerals, brewing, distilling, and essential oil [32, 38].

The globally declining trends of abundance and diversity of lichens have been documented by various authors across the world. Pollution due to industrialization, large‐scale modern agriculture, urbanization, deforestation, habitat loss, overexploitation, global warming, and climate change are the main causes of their decline [22, 39].

1.2 Distribution

Lichens have some surprising capacity of ecological resilience and adaptability. They can absorb and retain moisture from various sources, due to which they can easily grow in exposed substratum like leaves and barks of trees, rocks, etc., and can survive in extreme conditions like hot deserts, barren rocky cliffs, and frozen environments of the polar regions. They can also grow in the marble of old buildings and monuments [5, 25].

Lichens are an old group of fungi that may be traced back to 400 MA to the Early Devonian in the Rhynie chert deposits in Scotland and to 600 MA in marine phosphorite of the Doushantuo Formation at Weng’an in South China. The earliest ascomycetes may symbiotically associate with the already available algae and cyanobacteria, which formed the hypothetical Protolichen group. Eighteen (18) patterns of lichen distributions are described by Galloway, which are Cosmopolitan, Endemic, Austral, Bipolar, Paleotropical, Neotropical, Pantropical, Australasian, Circum‐pacific, Atlantic, Eastern North America–western European (amphi‐Atlantic), Western North American–western European, Mediterranean, American–Asian, South American–African, Southern xeric and Boreal arctic–alpine taxa [40]. The cosmopolitan taxa are widespread in their occurrence and found in all land masses and various oceanic islands. The family Parmeliaceae has a worldwide distribution having species like Parmelia sulcata, Flavoparmelia caperata, Hypotrachyna sinuosa, Parmotrema perlatum, etc. The endemic lichen taxa are present in some particular geographical areas with limited distribution. Lichen flora of India comprises over 2900 species, among which 18% (540 species) are endemic. (“Lichens,” [20]). In New Zealand, 23% of their lichen flora is endemic. Some of the endemic lichen species of New Zealand are Austrella brunnea, Caloplaca erecta, Lobaria asperula, Umbilicaria murihikuana, etc. A high percentage of endemic lichen flora is found in South Georgia (24%) to continental Antarctica (50%). The Austral taxa are represented by the southern hemisphere land masses, and the lichen flora is divided into Paleoaustral and Neoaustral lichens. The Paleoaustral lichens are represented by the primitive Gondwanan groups, which are poorly adapted for long‐distance dispersal; examples are—Bartlettiella fragilis, Brigantiaea phaeomma, Bryoria austromontana, Caloplaca cribrosa, etc. The Neoaustral lichens are dispersed after Gondwanaland fragmentation, which takes place between the post‐Oligocene and the present. Some examples of Neoaustral lichens are—Caloplaca cirrochrooides, Leifidium tenerum, Parmelia cunninghamii, etc. The lichens which are distributed in high latitudes of both the northern and southern hemispheres are the bipolar taxa. Some examples of bipolar lichen taxa are Cladonia ecmocyna, Caloplaca tornoensis, Bellemera alpine, etc. The lichen flora found in Africa, the Indian subcontinent, the Arabian peninsula, the Malesian Archipelago, and some islands of the Pacific Ocean are called Paleotropical taxa. Bactrospora metabola, Cladia aggregate, Parmelinopsis swinscowii, etc., are examples of Paleotropical taxa. The lichen species represented in some regions of South America and the Caribbean islands are known as Neotropical taxa. Some of the Neotropical lichen taxa are species of Erioderma, Leptogium, and Peltigera. Pantropical taxa are found in most of the tropical regions and show affinities with warm temperate characteristics, for example—species of Parmotrema, Glyphis, Graphis, etc. Australasian lichen taxa have similar characteristics to the lichen flora represented in Australia and New Zealand. Species of the genus Nothofagus, Placopsis, are included in the Australasian taxa. The Western Pacific lichen taxa are found in the extent northwards to Japan, westwards to India, and in some parts of Africa of Australia. Calopadia subcoerulescens, Parmelia erumpens, and Rinodina reagens are some examples of Western Pacific lichen...