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An Overview on Lichen's Morphology, Chemistry of Derived Products, and Current Scenario

Mohd Yusuf

Department of Chemistry, YMD College, M. D. University, Nuh, Haryana, India

Abstract

Lichens, the mutual symbionts of a fungus with microalga and/or a cyanobacterium, are important constituents of many ecosystems. A large number of natural products and secondary metabolites have been obtained from lichens, especially, dyes, drugs, and other constituents. In the present scenario, these natural products/secondary metabolites have shown impressive range of biological activities such as antimicrobial, anticancer, antifungal, antioxidant, antiviral, anti-inflammatory, analgesic, antipyretic activities, and many more. Traditionally, lichens as whole and their extracts were used to treat various diseases and ailments around the world. This chapter deals with the discussion on lichen's morphology and chemistry of derived products.

Keywords: Lichen, morphology, biological activity, antibacterial, antifungal, dyes

1.1 Introduction

Lichens are an inevitable segment of all ecosystems due to their symbiotic nature not only fixation of essential nutrients from the air to the soil but also production of fresh oxygen into air. Lichens are slowest growing, unique in nature and shape, stable, and self-supporting symbiotic organisms propagate on living organisms such as shrubs, tortoise's carapaces, trunks and woods of trees, etc., and mounted surfaces like soil, bricks, and many more, in an association of a fungi with algal isolate [1, 2]. It is estimated that lichens are survived with approximately 25,000 species in which 98% are Ascomycotes fungal partners and remaining 2% are cyanobacteria partners [3]. Old documentations reveal the uses of lichens and their extracts/obtained products have been utilized, as one of the best traditional medicines for the treatment of several human (wounds, burns, fissures, impotency, etc.), plants (rots and worts), and animal diseases (disfertility, wounds, skin issues) [4, 5]. Lichen found to exhibit manifold biological activities in various screenings, for example, antimicrobial, anticancer, antifungal, antioxidant, antiviral, anti-inflammatory, analgesic, antipyretic, growth inhibitory, and enzyme inhibitory activities, etc. [1-8]. Extracts of many lichen species of the genus Usnea were found usable as analgesic remedy in several countries of Asia, Europe, Africa, and Argentina [9].

The plant-like portion of the lichens is morphologically multifarious and generally often called as thallus. Primarily, the main three categories of lichens were recognized already such as crustose, foliose, and fruticose [1]. Furthermore, lichenologists, chiefly Baron, Dobson, Smith, and their co-workers described comprehensive diversities of thallus types (Table 1.1). Lichens inherently synthesize various types of secondary metabolites. There are more than 1,000 lichen-derived substances/biomolecules that have been identified through analytical and experimental methods [2]. The discussion on lichen's thallus morphology and the chemistry of their derived products with their considerable applications is highlighted herein this chapter.

1.2 The Lichen's Body: Thallus Morphology

Lichens are found growing on high peaks from the sea to the mountains such as wood, trunks, barks of trees, crustacean to the soil, rocks, walls, and many more places either in the sunlight or in the dark grooves. Abundantly, they propagate in clean environment like, forest ecosystem, and in a wide range of locations and climates from the Polar Regions to the tropics. Thus, they act as environmental indicator. Lichen thallus is the vegetative part and morphologically diversified in nature with branched, proliferated, tabulate, leafy, fibrous, tiny plates, layers of powdery granules, or without any specified thallus [10]. The main three categories of lichens were recognized already such as crustose, foliose, and fruticose [9]. Furthermore, lichenologists, chiefly Baron, Dobson, Smith, and their co-workers described comprehensive diversities of thallus types (Table 1.1) [5-10].

Table 1.1 Morphological diversities in lichen thalli.

1.2.1 Foliose

Foliose lichens mostly grow up in stratum with variable upper and lower surfaces. The thallus are somewhat flat but are convoluted or in leafy form and also can stand in erect mode away from the substrate like a swarm with multiple branches. Examples are Evernia prunastri, Pseudevernia furfuracea, Evernia prunastri, Peltigera membranacea, and Parmotrema perlatum.

1.2.2 Fruiticose

Fruiticose lichens are tickering in air like pendulum and have either a flat or a branched thallus covered by a solo outer cortex. For example, flattened branches-Ramalina sp., and with branched aerial lobes such as Evernia sp. Other examples are Usnea subfloridana and Roccella phycopsis.

1.2.3 Squamulose Lichens

The squamulose lichens are specific arrangements of squamules (over-lapping scales) which are heavy in weight, oblate, and succinct in size. Anatomically, squamulose lichens indicate variation to the smaller foliose lichens in which the presence of lower cortex is either not and or is highly differentiated. In case of Catapyrenium and Placidium species, medulla part may be sometimes colored. More examples are Squamarina cartilaginea, Cladonia subcervicornis, and Vahliella leucophaea.

1.2.4 Crustose

Crustose lichens mostly grow and spread over the crust such as soil, bark, and trunks of trees, and rocks with much variation like yellow, red, and grey-green in color-appearance, but found abundantly in grey-green color. Such lichens are firmly attached to the surface and are sighted in a bag-like texture. Examples are Ophioparma ventosa, Diploschistes scruposus, and Caloplaca ochracea.

1.2.5 Placodioid/Crustose-Placodioid/Squamulose-Placodioid

This is a true variant of crustose and squamulose that alternatively became placodioid. Some placodioid species are Caloplaca thallincola, Placopsis lambii, and Lecanora muralis. This type of thallus does not attach too much with the base and have a lower cortex. Also, the lobe margin extends into narrow and spread after.

1.2.6 Leprose

The surface of this type of thallus made up of lumps or granules like structure in which algal and fungal hyphae are present. Examples are Lepraria incana, Caloplaca chrysodeta, and Chrysothrix candelaris.

1.2.7 Filamentous Lichens

Filamentous lichens are also known as fibrous lichens owe to its shape. The morphology of the lichen generally seems dark green in appearance. At inner side algal cells like fiber are situated and the fungal hypae around the fiber form an outer sheath, for example, Trentepohlia or trichome-forming cyanobacteria. Ephebe lanata is a type of cyanolichens, which is blackish green in color and seen as smooth shrub or bear. Other common examples given are Cystocoleus ebeneus and Polychidium muscicola.

1.2.8 Without any Definite Shape or Lichens Without any Visible Thallus

Rarely, in this type of Lichens, no thallus are definitely visible. It can be understood in two ways, either it disappears very quickly or it gets immersed in the substrate in which both algae cells and fungal hyphae are present together. Examples of this group are Acrocordia conoidea and Collemopsidium foveolatum.

1.3 Chemistry of Lichen-Derived...