© Springer Nature Singapore Pte Ltd. 2020

S. T. Sukumaran et al. (eds.)Plant Metabolites: Methods, Applications and Prospectshttps://doi.org/10.1007/978-981-15-5136-9_12

12. Bioactive Secondary Metabolites from Lichens

Sanjeeva Nayaka¹   and Biju Haridas²

(1)

Lichenology Laboratory, CSIR—National Botanical Research Institute, Lucknow, Uttar Pradesh, India

(2)

Microbiology Division, KSCSTE—Jawaharlal Nehru Tropical Botanic Garden and Research Institute, Thiruvananthapuram, Kerala, India

 

Abstract

Lichens are traditionally used as medicine since the medieval period. More than 1000 secondary metabolites have been identified in lichens. It is still unknown why the lichens produce such a plethora of secondary metabolites. However, scientists have successfully utilized them for taxonomy and bioprospecting. The extracts of lichens have exhibited wide range of biological activities, such as antimicrobial (antibacterial, antifungal, antiviral, anti-HIV), antioxidant, anti-inflammatory, antipyretic, analgesic, anti-ulcer, and anticancer activities. The lichen metabolites are also being assayed and found useful as hepatoprotective, cardiovascular protective, gastrointestinal protective, antidiabetic, and probiotic, which are considered as the lifestyle diseases of modern days. Polyketides are one of the major groups of secondary metabolites produced by lichens involving polyketide synthase genes. Interestingly among the 1000 secondary metabolites known from lichens only a few are isolated and tested for their biological activities, while in all remaining cases, activity is indirectly attributed to the presence of various metabolites. In the present chapter, a total of 35 secondary metabolites that are isolated from lichens were tested for biological activities and are listed along with their structure, substance class, and occurrence. Further scope for bioprospecting studies is also discussed.

Keywords

Lichenized fungiBiodiversityBiological activityLichen substancesLichen chemistry

12.1 Introduction

Lichens by definition are symbiotic plant-like organisms, usually composed of a fungal partner (mycobiont) and one or more photosynthetic partners (photobiont), most often either a green alga or cyanobacterium. They are an outstandingly successful group, exploiting a wide range of habitats throughout the world and dominating about 8% of terrestrial ecosystems (Nash 2008). In the world, about 20,000 species of lichens are known at present (Lücking et al. 2016) and among them India represents 2750 species (Sinha 2018; Mao and Dash 2019). The dual nature of the lichens is now widely recognized and the lichen products are widely used in traditional medicine for centuries (Nash 2008). During the medieval period, lichens figured prominently in the herbals used by medicinal practitioners (Hale 1983). Lichens have been used in traditional medicine since the time of the first Chinese and Egyptian civilizations. Pseudevernia furfuracea (L.) Zopf. found in an Egyptian antique vase from the 18th Dynasty (1700–1600 BC) is considered as a clear evidence in support of this (Llano 1948).

In India, reference to lichens as medicine can be traced back to Rigveda (6000–4000 BC) and Atharva veda (1500 BC) where it is called as “Shipal.” Later, Sanskrit literature referred lichens as “Shailaya” and “Shilapushp” in Sushruta Samhita (1000 BC), Charaka Samhita (300–200 BC), and several Nighantu (ancient dictionaries) (1100–1800 AD) (Kumar and Upreti 2001). About 150 lichens occurring in India are known to have medicinal value either in traditional medicine or in biological assays. Nayaka et al. (2010) reviewed in detail the ethnolichenological and traditional uses of lichens in India and listed their biological activities. It is revealed that a total of 36 species are used in traditional medicine either in India or elsewhere, 55 have been screened for antimicrobial activity, 57 for antioxidant property, while about 37 for anticancer and cytotoxicity. Some of the common macrolichens utilized as medicine are Cetraria islandica (L.) Ach.; Cladonia rangiferina (L.) Weber; Dolichousnea longissima (Ach.) Articus; Evernia prunastri (L.) Ach.; Hypogymnia physodes (L.) Ach.; Hypotrachyna cirrhata (Fr.) Divakar, A. Crespo, Sipman, Elix, and Lumbsch; Peltigera canina (L.) Willd. The medicinal values of various lichens are attributed to the presence of unique metabolites, which are discussed in this chapter.

12.2 Lichen Chemistry

Lichen produces a great number of organic compounds that attracted the researchers as early as 1830s. These compounds are usually referred as lichen substances. Vulpinic acid (Bebert 1831), picrolichenic acid (Alms 1832), and usnic acid (Knop 1844) are few of the lichen substances isolated from lichens in the early ages of lichen chemistry research. Gmelin (1858) published the first review on lichen substances. Zopf (1907) was one of the pioneers who first described over 150 lichen compounds. However, the structural elucidation of many of these compounds came from the painstaking work of Asahina and co-workers in Japan during 1930s (Asahina and Shibata 1954). The lichen substances can be categorized as primary and secondary metabolites. The primary metabolites are intracellular in nature and belong to classes of proteins, amino acids, polyols, carotenoids, polysaccharides, and vitamins. Both mycobiont and photobiont contribute to synthesizing these primary metabolites. Whereas, secondary metabolites are extracellular, stored either on the cortex or in medulla and mostly synthesized by mycobiont. Lichens are reported to produce more than 1000 secondary metabolites (Elix 2014) among them except for 50–60, remaining all are unique to lichens. The secondary metabolites in lichens are derived from the polyketide pathway (Elix 1996) through three pathways, shikimic acid pathway, mevalonic acid pathway, and acetate–malonate pathway. Polyketides are built primarily from combinations of acetate (acetyl-CoA) and malonate (malonyl-CoA). The shikimic acid pathway provides an alternative route to aromatic compounds, particularly the aromatic amino acids l-phenylalanine, l-tyrosine, and l-tryptophan. Mevalonic acid pathways, on the other hand, produce mainly terpenoids, which are derived from C5 isoprene units (Dewick 2002). Depsides, depsidones, and dibenzofurans are formed by the acetate–malonate pathway. The most important of these are the esters and the oxidative coupling products of simple phenolic units related to orcinol and p-orcinol. Most depsides and depsidones are colorless compounds that occur in the medulla of the lichen, however, usnic acids, yellow cortical compounds formed by the oxidative coupling of methylphloroacetophenone units are found in the cortex of many lichen species. Anthraquinones, xanthones, and chromones are all pigmented compounds which occur in the cortex, which are also produced by the acetate–malonate pathway by intramolecular condensation of long-folded polyketide units rather than the coupling of phenolic units. The shikimic acid biosynthetic pathway produces two major groups of pigmented compounds, which occur in the cortex, pulvinic acid derivatives, and terphenyl quinones. Terpenes and steroids are produced by the mevalonic acid pathway. Lichens can produce secondary metabolites as much as 20% of their thallus dry weight, but generally amount varies between 5 and 10%.

12.3 Lichen Chemistry in Taxonomy

It is not yet clear why lichens produce such a large number of secondary metabolites. But, scientists have wisely utilized these metabolites for taxonomy and bioprospecting. The application of chemical discriminators to lichen taxonomy began inadvertently when thallus color was accepted as a valid generic or specific character (Elix 2014). Generally, yellowish lichens have usnic acid, orange ones have anthroquinones, while grayish one would have atranorin. However, most lichen substances are colorless and can be detected only by indirect means. Nylander (1866) was the first lichenologist to conduct chemical tests on lichen thalli for taxonomic purposes. He detected the presence of various lichen substances by spotting reagents such as aqueous solution of potassium hydroxide, iodine solution, and calcium hypochlorite directly on the lichen thallus or medulla to produce characteristic color changes. Asahina developed an additional spot test reagent P or PD, an alcoholic solution of p-phenylenediamine. Further, he also invented a microcrystallization technique for more definitive recognition of individual lichen acids on a routine basis (Asahina and Shibata 1954). Subsequently, the techniques for identification of lichen substances for taxonomic purpose improvised. Wachtmeister (1952) introduced paper chromatography which is superseded by thin layer chromatography (TLC) introduced and improvised by Culberson and coworkers (Culberson 1972; Culberson and Ahmadjian, 1979; Culberson et al. 1981; Culberson and Johnson, 1982). In recent times, several more techniques are introduced such as high-performance thin layer chromatography, gas chromatography coupled with mass spectroscopy, and high-performance liquid chromatography. The major disadvantage of these techniques mostly is the expense of the equipment and purified solvents. Therefore, they are used only on special occasions and for routine lichen taxonomic work TLC still remains as the best technique. It can be noted that among hundreds of metabolites that lichen produces only about 200 are utilized for taxonomy purpose.

12.4 Lichen Chemistry in Bioprospecting

Production of secondary metabolites is costly to the organisms in terms of nutrient and energy, so one would expect that the plethora of metabolites produced by lichens would have biological significance to the organisms. Recent field and laboratory studies have shown that many of these compounds are indeed involved in important ecological roles (Bjerkea et al. 2005; Lawrey 1995; Rikkinen 1995). It is also now established that most of the lichens utilized in the traditional medicine have potential lichen substances, acting either independently or in combination with others. Some of the possible biological significance of lichen metabolites in nature can be summarized as antibiotic activities—provide protection against microorganisms; photoprotective activities—aromatic substances absorb UV light to protect algae (photobionts) against intensive irradiation; promote symbiotic equilibrium by affecting the cell wall permeability of photobionts; chelating agents—capture and supply important minerals from the substrate; antifeedant/antiherbivory activities—protect the lichens from insect and animal feedings; hydrophobic properties—prevent saturation of the medulla with water and allow continuous gas exchange; stress metabolites—metabolites secreted under extreme conditions. Biological assay has proven that lichen crude extracts or isolated compounds have antimicrobial (antibacterial, antifungal, antiviral, anti-HIV), antioxidant, anti-inflammatory, antipyretic, analgesic, anti-ulcer, and anticancer activities. The lichen metabolites are also being assayed and found useful as hepatoprotective, cardiovascular protective, gastrointestinal protective, antidiabetic, and probiotic, which are considered as the lifestyle disease of modern days. It is also becoming a trend in the recent days to produce silver and gold nanoparticles using lichen extract for improved applications under the flag of “green chemistry.” Therefore, it can be said that “sky is the limit” for bioprospecting lichens and their metabolites.

Burkholder and coworkers (Burkholder et al. 1944; Burkholder and Evans 1945) initially discovered that extracts of 52 species of lichens collected from eastern North America inhibited the growth of several bacteria. Their study resulted in a mad race among scientists throughout the world for testing lichens for antimicrobial activities. Hence, antimicrobial activity is the most common study that is attempted using lichens so far. However, initial studies were limited to experimenting with crude extracts and only in the recent times, identification and isolation of active metabolites are attempted. Several reviews are available summarizing the bioactive or pharmacological potential of lichens (Boustie and Grube, 2005; Molnár and Farkas 2010; Shukla et al. 2010; Mitrović et al. 2011; Zambare and Christopher, 2012; Shrestha and St. Clair 2013). These reviews are mostly categorized according to various biological activities exhibited by lichens. It can be noted that about 95% of the studies involved crude extract and indirect inference to the secondary metabolites they are possessing. In this communication, we are listing the secondary metabolites that are isolated from lichens for bioactivity testing.

12.5 Secondary Metabolites Isolated from Lichens

12.5.1 β-Alectoronic Acid (8′-O-Ethyl-β-Alectoronic Acid)

Substance class: Diphenyl ether, Molecular formula: C₃₀H₃₆O₉.

Occurrence: Asahinea chrysantha (Tuck.) W.L. Culb. and C.F. Culb.; Parmelia birulae Elenkin (Elix 2014) etc.

β-alectoronic acid (Fig. 12.1) isolated from Alectoria sarmentosa (Ach.) Ach. showed fairly good antimicrobial activity against Staphylococcus aureus, Mycobacterium smegmatis, and Candida albicans (Gollapudi et al. 1994).

Fig. 12.1

Structure of β-alectoronic acid

12.5.2 16-O-Acetylleucotylic Acid (16β-Acetoxy-22-Hydroxyhopane-4α-Oic Acid)

Substance Class: Terpenoids, Molecular formula: C₃₂H₅₂O₅.

Occurrence: Myelochroa entotheiochroa (Hue) Eilx and Hale (Elix 2014), M. aurulenta (Tuck.) Elix and Hale etc.

The acid isolated from M. aurulenta exhibited strong antiproliferative activity against human leukemia cell lines HL-60 with an EC₅₀ value of 21 μM, whereas leucotylic acid, a derivative of 16-O-acetly-leucotylic acid had a higher EC₅₀ value (72 μM) (Tokiwano et al. 2009) (Fig. 12.2).

Fig. 12.2

Structure of 16-O-acetly-leucotylic acid

12.5.3 Argopsin (1′-Chloropannarin)

Substance class: β-Orcinol Depsidones, Molecular formula: C₁₈H₁₄C_l2O₆.

Occurrence: Argopsis spp., Micarea lignaria (Ach.) Heldl., M. leprosula (Th. Fr.) Coppins & A. Fletcher (Elix 2014) etc.

Argopsin was discovered from the lichen Argopsis friesiana Müll. Arg. by Huneck and Lamb (1975) (Fig.12.3). Argopsin isolated from continental Antarctic lichens showed excellent concentration and time-dependent cytotoxicity in terms of intracellular lactate dehydrogenase release in rat hepatocytes. The IC₅₀ was >50 μg/mL, after incubation of 8 h and 27 μg/mL after 24 h (Correché et al. 2004). In another experiment, same compound from same lichen showed a stronger cytotoxicity in comparison to reference material colchicine against lymphocytes cell culture (Correche et al. 2002). Hidalgo et al. (1994) reported the antioxidant activity (AA) of Argopsin isolated from Erioderma chilense Mont., which inhibited rat brain homogenate auto-oxidation at concentration of 0.58 μM (AA 27) and β-carotene oxidation at 0.8 μM (AA 23). Fernández et al. (1998) reported the photoprotection properties of argopsin which inhibited photobinding to 8-Methoxypsoralen –human serum albumin (HAS) by 31.7% at a concentration of 10 μM and irradiated at 360 nm.

Fig. 12.3

Structure of argopsin

12.5.4 Alectosarmentin

Substance class: Dibenzofurans, Molecular formula: C₁₅H₁₀O₆.

Occurrence: Alectoria sarmentosa (Ach.) Ach., Cladonia strepsilis (Ach.) Grognot (Elix 2014) etc.

Alectosarmentin is a novel lichen metabolite isolated from A. sarmentosa by Gollapudi et al. (1994), which exhibited good antimicrobial activity against S. aureus, M. smegmatis, and C. albicans (Fig. 12.4).

Fig. 12.4

Structure of alectosarmentin

12.5.5 Atranorin

Substance class: β-Orcinol depsides, Molecular formula: C₁₉H₁₈O₈.

Occurrence: Very common in most of the lichens (Elix 2014).

Atranorin isolated from Cladonia foliacea (Huds.) Willd. and Physcia aipolia (Ehrh.) Fürnr. demonstrated a strong antimicrobial activity against pathogenic bacteria (Yılmaz et al. 2004; Ranković et al. 2008). Whereas, atranorin extracted from Parmotrema dilatatum (Vain.) Hale and Parmotrema tinctorum (Despr.) Hale exhibited weak inhibitory activity against Mycobacterium tuberculosis (Honda et al. 2010). Similarly, atranorin extracted from Lepraria lobificans Nyl. was found to be a weak antimicrobial agent (Kokubun et al. 2007). Commercially available atranorin exhibited effective anticancer activity, but at higher concentration (200 μm) it is capable of inducing a massive loss in mitochondrial membrane potential, along with caspase-3 activation in cell line HT-29 (human colon adenocarcinoma) and phosphatidylserine externalization in cell lines A2780 (human ovarian carcinoma) and HT-29 (Bačkorová et al. 2012). Atranorin isolated from Bacidia stipata I.M. Lamb showed a lower activity inhibiting the prostate androgen-sensitive (LNCaP) and androgen-insensitive (DU-145) human prostate cancer cells only at high concentrations (25 and 50 μM) (Russo 2012). Commercially available atranorin showed lowest treatment level of 50 μM as effective against cancer cell lines HL-60 cells 24 h after administration (Bačkorová et al. 2011). Atranorin isolated from continental Antarctic lichens showed poor, but concentration and time-dependent cytotoxicity in terms of intracellular lactate dehydrogenase release in rat hepatocytes. The IC₅₀ was 111 μg/mL after incubation of 8 h and 82 μg/mL after 24 h (Correché et al. 2004). Hidalgo et al. (1994) reported the antioxidant activity of atranorin isolated from Placopsis sp., which inhibited rat brain homogenate auto-oxidation at a concentration of 5 μM (AA 7.3) and β-carotene oxidation at 0.84 μM (AA 6.5). Fernández et al. (1998) reported the photoprotection properties of atranorin, which inhibited photobinding to 8-Methoxypsoralen–human serum albumin (HAS) by 20.1% at concentration 10 μM and irradiated at 360 nm (Fig. 12.5).

Fig. 12.5

Structure of atranorin

12.5.6 Barbatic Acid

Substance class: β-Orcinol depsides, Molecular formula: C₁₉H₂₀O₇.

Occurrence: Widespread in lichens species of Cladia, Cladonia, Usnea etc.

Martins et al. (2010) reported barbatic acid in the extract of Cladia aggregata (Sw.) Nyl. and proved that the crude or purified form of the same inhibited the growth of four multi-strain resistant strains of S. aureus (Fig. 12.6).

Fig. 12.6

Structure of barbatic acid

12.5.7 Epiphorellic Acid-1

Substance class: Diphenyl ethers, Molecular formula: C₂₆H₃₄O₈.

Occurrence: Coelopogon abraxas Brusse, C. epiphorellus (Nyl.) Brusse and Kärnefelt (Elix 2014) etc.

Epiphorellic acid-1 isolated from C. epiphorellus showed significant (P < 0.001) inhibitory effect on human prostate carcinoma DU-145 cells at the concentration (6–50 μmol/l) that is nontoxic to normal human prostatic epithelial cells (Russo 2006) (Fig. 12.7).

Fig. 12.7

Structure of epiphorellic acid-1

12.5.8 Diffractaic Acid

Substance class: β-Orcinol depsides, Molecular formula: C₂₀H₂₂O₇.

Occurrence: Cladia muelleri (Hampe) Parnmen, Elix and Lumbsch, Usnea subcavata Motyka, Dolichousnea diffracta (Vain.) Articus (Elix 2014) etc.

Diffractaic acid isolated from U. subcavata Motyka was found to be the most active compound with MIC value 15.6 μg/mL, 41.7 μM against tuberculosis bacteria M. tuberculosis (Honda et al. 2010). Diffractaic acid isolated from Protousnea magellanica (Mont.) Krog showed a good antiproliferative activity against HCT-116 (colon carcinoma) cells from 25 μM, with IC50 value of 42.2 μM, whereas a reduction of viability in MCF-7 (breast adenocarcinoma) and HeLa (cervix adenocarcinoma) cells was observed at concentrations higher than 50 μM (Brisdelli et al. 2012). Diffractaic acid isolated from P. magellanica showed a lower activity inhibiting the prostate androgen-sensitive (LNCaP) and androgen-insensitive DU-145 (human prostate) cancer cells only at high concentrations (25 and 50 μM) (Russo 2012). Diffractaic acid isolated from continental Antarctic lichens showed poor, concentration and time-dependent cytotoxicity in terms of intracellular lactate dehydrogenase release in rat hepatocytes. The IC₅₀ was 149 μg/mL after incubation of 8 h and 119 μg/mL after 24 h (Correché et al. 2004). Same acid isolated from parmelioid lichens reported as potent antiproliferative agent against leukotrienes-mediated inflammation with IC₅₀ value of 2.6 μM (Kumar and Müller 1999). Diffractaic acid isolated from D. diffracta (Vain.) Articus showed analgesic effect in mice in vitro (Okuyama 1995) (Fig. 12.8).

Fig. 12.8

Structure of diffractaic acid

12.5.9 Divaric Acid (2,4-Dihydoxy-6-Propylbezoic Acid; Olivetol Carboxylic Acid)

Substance class: Monocyclic aromatic compounds, Molecular formula: C₁₀H₁₂O₄.

Occurrence: Cladonia macaronesica Ahti (Elix 2014), Evernia divaricata (L.) Ach and other lichens.

Divaric acid is quite a rare secondary metabolite in lichens. Yuan et al. (2010) for the first time reported the antibacterial properties of Divaric acid from E. divaricata (L.) Ach., which showed very potent inhibitory activity against bacteria B. subtilis, S. aureus, E. coli, and P. aeruginosa. Divaricatic acid isolated from Protousnea malacea (Stirt.) Krog showed a lower activity inhibiting the prostate androgen-sensitive (LNCaP) and androgen-insensitive (DU-145) human prostate cancer cells only at high concentrations (25 and 50 μM) (Russo 2012) (Fig. 12.9).

Fig. 12.9

Structure of divaric acid

12.5.10 Divaricatic Acid

Substance class: Orcinol depsides, Molecular formula: C₂₁H₂₄O₇.

Occurrence: Canoparmelia texana (Tuck.) Elix and Hale, Evernia divaricata (L.) Ach. (Elix 2014) etc.

Divaricatic acid isolated from continental Antarctic lichens showed moderate, but concentration and time-dependent cytotoxicity in terms of intracellular lactate dehydrogenase release in rat hepatocytes. The IC₅₀ was ≥50 μg/mL after incubation of 8 h and 32 μg/mL after 24 h (Correché et al. 2004). Hidalgo et al. (1994) reported the antioxidant activity of divaricatic acid isolated from P. malacea, which inhibited rat brain homogenate auto-oxidation at concentration of 5 μM (10) and β-carotene oxidation concentration of 0.8 μM (AA 8.6) (Fig. 12.10).

Fig. 12.10

Structure of divaricatic acid

12.5.11 Evernic Acid

Substance class: Orcinol depsides, Molecular formula: C₁₇H₁₆O₇.

Occurrence: Evernia prunastri (L.) Ach.

Evernic acid extracted from the lichen E. prunastri exhibited weak inhibitory action against multidrug-resistant bacteria S. aureus (Kokubun et al. 2007) (Fig. 12.11).

Fig. 12.11

Structure of evernic acid

12.5.12 Fumarprotocetraric Acid

Substance class: β-Orcinol depsidones, Molecular formula: C₂₂H₁₆O₁₂.

Occurrence: Cladonia phyllophora Ehrh. C. foliacea, and C. furcata (Elix 2014) and other lichens.

Fumarprotocetraric acid isolated from C. foliacea and C. furcata demonstrated potential antimicrobial activity against several bacterial strains (Ranković and Misić 2008; Yılmaz et al. 2004), whereas fumarprotocetraric acid isolated from continental Antarctic lichens showed poor, but concentration and time-dependent cytotoxicity in terms of intracellular lactate dehydrogenase release in rat hepatocytes. The IC₅₀ was >150 μg/mL after incubation of 8 h and > 150 μg/mL after 24 h (Correché et al. 2004) (Fig. 12.12).

Fig. 12.12

Structure of fumarprotocetraric acid

12.5.13 Gyrophoric Acid

Substance class: Orcinol tridepsides, Molecular formula: C₂₄H₂₀O₁₀.

Occurrence: Punctelia borreri (Turner) Krog, Umbilicaria polyphylla (L.) Baumg. and Xanthoparmelia pokornyi (Körb.) O. Blanco, A. Crespo, Elix, D. Hawksw. and Lumbsch, (Elix 2014), Umbilicaria hirsuta (Sw.) Ach and other lichens.

Gyrophoric acid extracted from U. polyphylla (L.) Baumg. and X. pokornyi (Körb.) O. Blanco, A. Crespo, Elix, D. Hawksw. and Lumbsch demonstrated a strong antimicrobial activity against pathogenic bacteria (Ranković et al 2008; Candan et al. 2006). Gyrophoric acid isolated from U. hirsuta (Sw.) Ach. exhibited good anticancer activity against cell lines A2780 and HT-29 at higher concentration (200 μM), however, its effect is much lesser than the usnic acid and atranorin (Bačkorová et al. 2012). Though it is ineffective at lowest concentrations, concentrations at 100 μM gyrophoric acid induced a strong effect in HL-60 cells as quickly as only 24 h after incubation (Bačkorová et al. 2011). In another experiment, gyrophoric acid isolated from Lasallia pustulata (L.) Mérat in combination with usnic acid showed strong wound closure effects on HaCaT cells (Burlando et al. 2009). Gyrophoric acid isolated from continental Antarctic lichens showed poor, but concentration and time-dependent cytotoxicity in terms of intracellular lactate dehydrogenase release in rat hepatocytes. The IC₅₀ was >150 μg/mL after incubation of 8 h and 61 μg/mL after 24 h (Correché et al. 2004). Gyrophoric acid isolated from parmelioid lichens was reported as potent antiproliferative agent against leukotrienes-mediated inflammation with IC₅₀ value of 1.7 μM (Kumar and Müller, 1999) (Fig. 12.13).

Fig. 12.13

Structure of gyrophoric acid

12.5.14 Hypostictic Acid

Substance class: β-Orcinol depsidones, Molecular formula: C₁₉H₁₆O₈.

Occurrence: Xanthoparmelia quintaria (Hale) Hale (Elix 2014) and other lichens.

Hypostictic acid (Fig. 12.14) extracted from Psuedoparmelia sphaerospora (Nyl.) Hale exhibited middle range of antimicrobial activity against M. tuberculosis (MIC value 94.0 μg/ml, 251 μM) (Honda et al. 2010).

Fig. 12.14

Structure of hypostictic acid

12.5.15 Hybocarpone

Substance class: Naphthaquinones, Molecular formula: C₂₆H₂₄O₁₃.

Occurrence: Heterodermia hybocarponica Elix, Lecanora conizaeoides Nyl. (Elix 2014) and other lichens.

Hybocarpone (Fig. 12.15) isolated from Lecanora conizaeoides Nyl. exhibited the strongest activity of all strains of S. aureus tested with MIC ranging from 4 to 8 μ/mL (813–16.3 μM). This is an important finding as it suggests that these compounds are opaque to the efflux mechanisms expressed by the strains S. aureus tested (Kokubun et al. 2007).

Fig. 12.15

Structure of hybocarpone

12.5.16 Lecanoric Acid

Substance class: Orcinol depsides, Molecular formula: C₁₆H₁₄O₇.

Occurrence: Parmotrema tinctorum (Despr.) Hale (Elix 2014) and other lichens.

Lecanoric acid (Fig. 12.16) isolated from Ochrolechia androgyan (Hoffm.) Arnold showed relatively strong antimicrobial activity against several bacterial strains (Ranković and Misić 2008). Cytotoxicity assay was carried out in in vitro for lecanoric acid isolated from P. tinctorum and its orsellinates derivatives obtained by structural modification with sulforhodamine B (SRB) using HEp-2 larynx carcinoma, MCF7 breast carcinoma, 786-0 kidney carcinoma, and B16-F10 murine melanoma cell lines, in addition to a normal (Vero) cell line. It is found that n-butyl orsellinate was the most active compound, with IC₅₀ values ranging from 7.2 to 14.0 μg/mL, against all the cell lines tested. The compound was more active (IC₅₀ = 11.4 μg/mL) against B16-F10 cells. However, lecanoric acid and methyl orsellinate were less active against all cell lines, having an IC₅₀ value higher than 50 μg/mL (Bogo et al. 2010).

Fig. 12.16

Structure of lecanoric acid

12.5.17 Lobaric Acid

Substance class: Orcinol depsidones, Molecular formula: C₂₅H₂₈O₈.

Occurrence: Protoparmelia badia (Hoffm.) Hafellner (Elix 2014).

Lobaric acid extracted from Sterocaulon dactylophyllum Flörke found to be an encouraging metabolite which displayed MIC of 8 μg/mL (17.5 μM) against S. aureus (Kokubun et al. 2007). Lobaric acid isolated from Streocaulong alpinum Laurer ex Funck exhibited antiproliferative activity at the higher concentrations only on HeLa (cervix adenocarcinoma) and HCT-116 cells (colon carcinoma) (Brisdelli et al. 2012). Lobaric acid isolated from continental Antarctic lichens showed moderate, but concentration and time-dependent cytotoxicity in terms of intracellular lactate dehydrogenase release in rat hepatocytes. The IC₅₀ was >100 μg/mL, after incubation of 8 h and 43 μg/mL after 24 h (Correché et al. 2004). Lobaric acid isolated from S. alpinum caused a significant reduction in DNA synthesis as measured by thymidine uptake, in three malignant cell lines (T-47D and ZR-75-1—breast carcinoma; K-562—erythroleukemia), the dose inducing 50% of maximum inhibition (ED₅₀) was between 14.5 and 44.7 μg/mL. The proliferative response of mitogen-stimulated lymphocytes was inhibited with mean ED₅₀ of 24.5 μg/mL. Significant cell death occurred in the three malignant cell lines at concentrations above 30 μg/mL and up to 38% cell death was observed at 15 mg/mL lobaric acid in mitogen-stimulated lymphocytes (Ögmundsdóttir et al. 1998) (Fig. 12.17).

Fig. 12.17

Structure of lobaric acid

12.5.18 Methyl β-Orsellinate (Methyl β-Orcinolcarboxylate; Atranol)

Substance class: Monocyclic aromatic derivatives, Molecular formula: C₈H₈O₃.

Occurrence: E. prunastri (Elix 2014), H. cirrhata, P. furfuracea, and other lichens.

Methyl β-orsellinate isolated from H. cirrhata found to be excellent antimicrobial agent against azole-resistant strains of C. albicans and S. cerevisiae in the range of 10–400 μg/mL. Further, it also exhibited anticancer activity against liver, colon, ova, or mouth (oral) cancer cells of humans in the range 1–10 μg/mL (Khanuja et al. 2007). Earlier, atranol was found to be a major bioactive molecule in the crude extract of P. furfuracea which was highly active against bacteria like B. subtilis, E. coli, P. digiratum, and S. cerevisiae (Caccamese et al. 1985) (Fig. 12.18).

Fig. 12.18

Structure of methyl β-orsellinate (atranol)

12.5.19 Norstictic Acid

Substance class: β-Orcinol depsidones, Molecular formula: C₁₈H₁₂O₉.

Occurrence: Xanthoparmelia substrigosa (Hale) Hale (Elix 2014) and other lichens.

Norstictic acid extracted from Ramalina sp. showed significant inhibition of growth of M. tuberculosis with MIC value of value 62.5 μg/mL, 168 μM) (Honda et al. 2010) (Fig. 12.19).

Fig. 12.19

Structure of norstictic acid

12.5.20 Olivetoric Acid

Substance class: Orcinol depsides, Molecular formula: C₂₆H₃₂O₈.

Occurrence: Cetrelia olivetorum (Elix 2014), P. furfuracea etc.

Olivetoric acid isolated from P. furfuracea displayed dose-dependent antiangiogenic activities, inhibited cell proliferation, and disrupted endothelial tube formation in adipose tissue. Olivetoric acid also inhibited the formation of actin stress fibers in a dose-dependent manner, which may be due to the decrease in tube formation (Koparal and Ulus 2010) (Fig. 12.20).

Fig. 12.20

Structure of olivertoric acid

12.5.21 Pannarin

Substance class: β-Orcinol depsidones, Molecular formula: C₁₈H₁₅ClO₆.

Occurrence: Pannaria conoplea (Ach.) Bory (Elix 2014), Psoroma pallidum Nyl, Psoroma spp. etc.

Pannarin isolated from Psoroma spp. showed a significant inhibitory effect on M14 (human melanoma) cells at a concentration of 12.5–50 μM. It also induced apoptotic cell death substantiated by DNA fragmentation and increased caspase-3 activity. It also inhibited superoxide anion formation (Russo 2008). In another experiment, pannarin isolated from same lichen at concentration 6–50 μmol/L, which is nontoxic to normal human prostatic epithelial cells showed significant (P < 0.001) inhibitory effect on human prostate carcinoma DU-145 cells (Russo 2006). Pannarin isolated from continental Antarctic lichens showed moderate, but concentration and time-dependent cytotoxicity in terms of intracellular lactate dehydrogenase release in rat hepatocytes. The IC₅₀ was 86 μg/mL after incubation of 8 h and 12 μg/mL after 24 h (Correché et al. 2004). In another experiment, same compound from same lichen showed a stronger cytotoxicity in comparison to reference material colchicine against lymphocytes cell culture (Correché et al. 2002). Hidalgo et al. (1994) reported the antioxidant activity of pannarin isolated from P. pallidum Nyl., which inhibited rat brain homogenate auto-oxidation at concentration of 0.57 μM (AA 13) and β-carotene oxidation at 0.88 μM (AA 23). Fernández et al. (1998) reported the photo protection properties of pannarin which inhibited photobinding to 8-Methoxypsoralen–human serum albumin (HAS) by 40.4% at concentrations of 10 μM when irradiated at 360 nm (Fig. 12.21).

Fig. 12.21

Structure of pannarin

12.5.22 Parietin (Physcoin)

Substance class: Anthraquinones, Molecular formula: C₁₆H₁₂O₅.

Occurrence: Xanthoria parietina (L.) Th. Fr. (Elix 2014).

Parietin isolated from X. parietina proved as a significant anticancer agent in some cell lines (A2780, Jurkat, or HT-29) at concentrations of 50 μM (Bačkorová et al. 2011) (Fig. 12.22).

Fig. 12.22

Structure of parietin

12.5.23 Physodic Acid

Substance class: Orcinol depsidones, Molecular formula: C₂₆H₃₀O₈.

Occurrence: H. physodes (L.) Nyl. (Elix 2014) and other lichens.

Physodic acid isolated from H. physodes is a weak (in comparison to usnic acid and streptomycin) antimicrobial agent inhibited the tested microorganisms but at high concentrations (1 μg/mL) (Ranković et al 2008). Türk et al. (2006) also showed similar results with physodic acid from P. furfuracea. However, Physodic acid isolated by Kokubun et al. (2007) from H. physodes showed good antibacterial activity against multidrug-resistant bacteria S. aureus with MIC of 32 μg/mL (68.0 μM). Similarly, physodic acid isolated from A. sarmentosa showed fairly good antimicrobial activity against S. aureus, M. smegmatis, and C. albicans (Gollapudi et al. 1994) (Figs. 12.22 and 12.23).

Fig. 12.23

Structure of physodic acid

12.5.24 Protocetraric Acid

Substance class: β-Orcinol depsidones, Molecular formula: C₁₈H₁₄O₉.

Occurrence: Flavoparmelia caperata (Elix 2014).

Procetraric acid extracted from P. dilatatum showed middle range of antimicrobial activity against M. tuberculosis (MIC value 125 μg/mL, 334 μM) (Honda et al. 2010). Protocetraric acid isolated from F. caperata showed strong antimicrobial activity against several bacterial strains (Ranković and Misić 2008) (Fig. 12.24).

Fig. 12.24

Structure of protocetraric acid

12.5.25 Protolichesterinic Acid

Substance class: Aliphatic acids, Molecular formula: C₁₉H₃₂O₄.

Occurrence: C. islandica (Elix 2014).

Protolichesterinic acid isolated from Cornicularia aculeata (Schreb.) Ach. exhibited as stronger cytotoxic effect against three human cancer cell lines, MCF-7 (breast adenocarcinoma), HeLa (cervix adenocarcinoma), and HCT-116 (colon carcinoma). The cell survival dropped significantly after treatment with concentrations higher than 25 μM (Brisdelli et al. 2012). Protolichesterinic acid isolated from R. melanophthalma (DC.) Leuckert showed dose-dependent relationship in the range of 6.25–50 μM concentrations and inhibited activity of androgen-sensitive (LNCaP) and androgen-insensitive (DU-145) human prostate cancer cells (Russo et al. 2012). Protolichesterinic acid isolated from C. islandica caused a significant reduction in DNA synthesis as measured by thymidine uptake, in three malignant cell lines (T-47D and ZR-75-1—breast carcinoma; K-562—erythroleukemia), the dose inducing 50% of maximum inhibition (ED₅₀) was between 1.1 and 24.6 μg/mL. The proliferative response of mitogen-stimulated lymphocytes was inhibited with mean ED₅₀ of 8.4 μg/mL. Significant cell death occurred in the three malignant cell-lines at concentrations above 20 μg/mL and up to 38% cell death was observed at 20 μg/mL lobaric acid in mitogen-stimulated lymphocytes (Ögmundsdóttir et al. 1998) (Fig. 12.25).

Fig. 12.25

Structure of protolichesterinic acid

12.5.26 Psoromic Acid

Substance class: β-Orcinol depsidones, Molecular formula: C₁₈H₁₄O₈.

Occurrence: Usnea inermis Motyka (Elix 2014).

Psoromic acid isolated from continental Antarctic lichens showed moderate, but concentration and time-dependent cytotoxicity in terms of intracellular lactate dehydrogenase release in rat hepatocytes. The IC₅₀ was 76 μg/mL after incubation for 8 h and 11 μg/mL after 24 h (Correché et al. 2004) (Fig. 12.26).

Fig. 12.26

Structure of psoromic acid

12.5.27 Retigeric Acid B

Substance class: Terpenoids, Molecular formula: C₃₀H₄₆O₆.

Occurrence: Lobaria retigera (Borry) Trevis. (Elix 2014).

Retigeric acid B, a naturally occurring pentacyclic triterpenic acid isolated from Lobaria kurokawae, inhibited prostate cancer cell proliferation and induced cell death in a dose-dependent manner, but exerted very little inhibitory effect on noncancerous prostate epithelial cell viability (Liu et al. 2010) (Fig. 12.27).

Fig. 12.27

Structure of retigeric acid

12.5.28 Rhizocarpic Acid

Substance class: Pulvinic acid derivatives, Molecular formula: C₂₈H₂₃NO₆.

Occurrence: Rhizocarpon geographicum (L.) DC. (Elix 2014) and other lichens.

Rhizocarpic acid extracted from Psilolechia lucida (Ach.) M. Choisy was reported to be a promising antimicrobial agent with MICs 32 to 64 μg/mL at 68.2 to 136 μM. (Kokubun et al. 2007) (Fig. 12.28).

Fig. 12.28

Structure of rhizocarpic acid

12.5.29 Salazinic Acid

Substance class: β-Orcinol depsidones, Molecular formula: C₁₈H₁₂O₁₀.

Occurrence: Xanthoparmelia tasmanica (Hook. f. Taylor) Hale (Elix 2014) and other lichens.

Salazinic acid extracted from Parmotrema lichexanthonicum Eliasaro & Adler exhibited weak antimicrobial activity against M. tuberculosis (MIC value 4250 μg/mL, 643 μM) (Honda et al. 2010). Salazinic acid isolated from continental Antarctic lichens showed poor, but concentration and time-dependent cytotoxicity in terms of intracellular lactate dehydrogenase release in rat hepatocytes. The IC₅₀ was 154 μg/mL after incubation of 8 h and 18 μg/mL after 24 h (Correché et al. 2004) (Fig. 12.29).

Fig. 12.29

Structure of salazinic acid

12.5.30 Sphaerophorin

Substance class: Orcinol depsides, Molecular formula: C₂₃H₂₈O₇.

Occurrence: Sphaerophorus fragilis (L.) Pers. (Elix 2014) and other lichens.

Sphaerophorin isolated from S. globosus (Huds.) Vain. showed a significant inhibitory effect on M14 (human melanoma) cells at concentration 12.5–50 μM. It also induced apoptotic cell death substantiated by DNA fragmentation and increased caspase-3 activity. It also inhibited superoxide anion formation (Russo 2008). In another experiment, phaerophorin isolated from same lichen showed at concentrations ranging from 6 to 50 μmol/L that is nontoxic to normal human prostatic epithelial cells showed significant (P < 0.001) inhibitory effect on human prostate carcinoma DU-145 cells (Russo 2006). Sphaerophorin isolated from continental Antarctic lichens showed excellent concentration and time-dependent cytotoxicity in terms of intracellular lactate dehydrogenase release in rat hepatocytes. The IC₅₀ was 30 μg/mL after incubation of 8 h and 3 μg/mL after 24 h (Correché et al. 2004). In another experiment, same compound from same lichen showed a stronger cytotoxicity in comparison to reference material colchicine against lymphocytes cell culture (Correché et al. 2002) (Fig. 12.30).

Fig. 12.30

Structure of sphaerophorin

12.5.31 Stictic Acid

Substance class: β-Orcinol depsidones, Molecular formula: C₁₉H₁₄O₉.

Occurrence: Xanthoparmelia conspersa (Ehrh.) Hale (Elix 2014).

Stictic acid isolated from X. conspersa showed weak (in comparison to usnic acid and streptomycin) antimicrobial activity against several strains of bacteria (Ranković and Misić 2008). Stictic acid isolated from continental Antarctic lichens showed moderate, but concentration and time-dependent cytotoxicity in terms of intracellular lactate dehydrogenase release in rat hepatocytes. The IC₅₀ was 88 μg/mL after incubation of 8 h and 5 μg/mL after 24 h (Correché et al. 2004) (Fig. 12.31).

Fig. 12.31

Structure of stictic acid

12.5.32 Usnic Acid

Substance class: Usnic acid derivatives, Molecular formula: C₁₈H₁₆O₇.

Occurrence: Usnea spp. (Elix 2014) and other lichens.

It is one of the common and abundant secondary metabolite found in large amount in lichens such as Cladonia, Evernia, Ramalina, and Usnea. Lichens produce usnic acid up to 8% of their dry weight of the thalli, but undergo seasonal variation, reaching maximum in late spring and early summer, and low levels in autumn and winter. Also, usnic acid contents depend on geographic locality, insolation, and other ecological conditions (Bjerkea et al. 2005). As mentioned earlier, it was first isolated in 1844 by Knop. It is known to be present in three forms, i.e. (+)-usnic acid, (−)-usnic acid, and isousnic acid (Shibata and Taguchi 1967). Both (+) and (−)-usnic acid are biologically important. Probably it is the only lichen metabolite commercialized as pharmaceutical. It is used in antiseptic creams including “Usno” and “Evosin” and is sometimes believed to be more effective than penicillin salves in the treatment of external wounds and burns. It is also used in the treatment of tuberculosis (Nash 2008) and is a very effective drug used in antifeedant products, mouth rinses and dentifrices (Durazo et al. 2004), and cosmetics (Najdenova et al. 2001) (Fig. 12.32).

Fig. 12.32

Structure of usnic acid

Several reviews are already available describing the biological potential of usnic acid (Cocchietto et al. 2002; Ingolfsdottir, 2002; Luzina and Salakhutdinov 2018). A large portion of research on usnic acid mostly deals with antimicrobial (antibacterial, antiviral, fungicidal, antiprotozoal) activities and hence can be employed as antitubercular, antimalarial, and anti-influenza agents. These studies prove beyond doubt that usnic acid is a potential antimicrobial agent. The antibiotic action of usnic acid is due to the inhibition of oxidative phosphorylation, an effect similar to that shown by dinitrophenol (Abo-Khatwa et al. 1996). Usnic acid also acts as a potential antioxidant, insecticidal, larvicidal, analgesic, and anti-inflammatory agent. Usnic acid has also shown anticancer properties (Mayer et al. 2005) and it is antineoplastic (Takai et al. 1979). The antioxidant action of usnic acid is related to the phenolic fragment, responsible for quenching free radicals, while antitumor activity is due to apoptosis (Luzina and Salakhutdinov 2018). In one of the studies, usnic acid was reported to inhibit the osteoclast differentiation (Lee 2015), which can lead to hypogenesis, fracture, osteoporosis, rheumatoid arthritis, periodontal disease, Paget’s disease, transformative bone cancer, etc. Usnic acid can inhibit cellular differentiation of follicular cells, thereby inhibiting hair growth (Chao et al. 2015). Commercially available usnic acid exhibited effective anticancer activity, capable of inducing a massive loss in mitochondrial membrane potential, along with caspase-3 activation in cell line HT-29 and phosphatidylserine externalization in cell lines A2780 and HT29 (Bačkorová et al. 2012). Nanoparticles of usnic acid displayed very good water solubility, cell permeability, and bioavailability (Qu et al. 2015). The ecological role of usnic acid can be attributed to screening of excess of light (Rundel 1978) as it is a cortical substance. Therefore, lichens growing on high altitudes could be potential source of usnic acid for light screening product development.

Usnic acid isolated from continental Antarctic lichens showed excellent, but concentration and time-dependent cytotoxicity in terms of intracellular lactate dehydrogenase release in rat hepatocytes. The IC₅₀ was 25 μg/mL after incubation of 8 h and 21 μg/mL after 24 h (Correché et al. 2004). (+)-usnic acid isolated from parmelioid lichens reported as potent antiproliferative agent against leukotrienes-mediated inflammation with IC₅₀ value of 2.1 μM (Kumar and Müller 1999). Usnic acid also a gastro-protective compound, since it reduces oxidative damage and inhibits neutrophil infiltration in indomethacin-induced gastric ulcers in rats (Odabasoglu et al. 2006). It also exhibited strong larvicidal activity against the third and fourth instar larvae of the house mosquito (Culex pipiens), and larval mortality was dose-dependent (Cetin et al. 2008).

Few studies have shown that extensive uses of usnic acid as food supplement caused intoxication, severe hepatotoxicity leading to liver failure, weight loss, and allergy (Guo et al. 2008). These studies warn usage of usnic acid in pharmacological agent. On the other hand, Cladonia spp., lichens containing usnic acid are eaten in large quantity by wild animals, such as reindeers, indicating it does not cause toxicity. Further, pharmacokinetic studies in rabbits (Krishna and Venkataramana 1992) have shown that (+)-usnic acid is absorbed very well after oral administration. Thus, if usnic acid is used for therapy, it can be administered orally (Elo et al. 2007). Usnic acid being a wonder molecule for pharmacopeia, there is a need for derivatization to create a new biologically active compound with reduced toxicity (Fig. 12.32).

12.5.33 Variolaric Acid

Substance class: Orcinol depsidones, Molecular formula: C₁₆H₁₀O₇.

Occurrence: Ochrolechia parella (L.) A. Massal. (Elix 2014).

Variolaric acid isolated from continental Antarctic lichens showed moderate, but concentration and time-dependent cytotoxicity in terms of intracellular lactate dehydrogenase release in rat hepatocytes. The IC₅₀ was 95 μg/mL after incubation of 8 h and 64 μg/mL after 24 h (Correché et al. 2004) (Fig. 12.33).

Fig. 12.33

Structure of variolaric acid

12.5.34 Vicanicin

Substance class: β-Orcinol depsidones, Molecular formula: C₁₈H₁₆Cl₂O₅.

Occurrence: Teloschistes flavicans (Sw.) Norman (Elix 2014) and other lichens.

Vicanicin (Fig. 12.34) isolated from P. pallidum Nyl., P. pulchrum Malme induced a significant loss of viability in a dose-dependent manner in HeLa (cervix adenocarcinoma) and HCT-116 colon carcinoma) cells with IC₅₀ values of 67 μM and 40.5 μM, respectively, but did not show any cytotoxic effect on MCF-7 cells (breast adenocarcinoma) (Brisdelli et al. 2012). Vicanicin isolated from Psoroma dimorphum Malme showed a clear dose–response relationship in the range of 6.25–50 μM concentrations and inhibited activity of androgen-sensitive (LNCaP) and androgen-insensitive (DU-145) human prostate cancer cells (Russo 2012). Vicanicin isolated from continental Antarctic lichens showed poor, but concentration and time-dependent cytotoxicity in terms of intracellular lactate dehydrogenase release in rat hepatocytes. The IC₅₀ was >150 μg/mL after incubation of 8 h and > 75 μg/mL after 24 h (Correché et al. 2004).

Fig. 12.34

Structure of vicanicin

12.5.35 Vulpinic Acid

Substance class: Pulvinic acid derivatives, Molecular formula: C₁₉H₁₄O₅.

Occurrence: Letharia vulpina (L.) Hue (Elix 2014).

Vulpinic acid (Fig. 12.35) isolated from L. vulpina exhibited fairly good antimicrobial activity against several microbes with MIC ranging between 16 and 32 μg/mL, but it is less when compared to usnic acid in the same study (2–16 μg/mL) (Lauterwein et al. 1995). Vulpinic acid isolated from L. vulpina induced a stimulation of cell proliferation at lower concentration of (0.1–1.0 μM), however, EC₅₀ values were significantly higher. It was more toxic on HaCaT than on tumor cells, except for the Neutral Red assay on A431 (Burlando et al. 2009).

Fig. 12.35

Structure of vulpinic acid

12.6 Conclusions

Lichens are not always beneficial. Woodcutter’s eczema among forestry and horticultural workers (and in lichen collectors) is a good example of dermatitis allergy caused due to lichen spores, which contain lichen compounds (Aalto-Korte et al. 2005). Along with usnic acid, Molnár and Farkas (2010) listed 11 other compounds that were reported to be allergic in various literature. Interestingly the list includes atranorin, diffractaic acid, and physodic acid.

It can be noted that among 1000 secondary metabolites known from the lichens relatively few substances have been screened in detail for biological activity and therapeutic potential. Sometimes the activities of crude extracts were indirectly attributed to the secondary metabolites present in them. Further, the exact molecular mechanisms of the action of lichen secondary metabolites are almost entirely unknown. Limited bioactive experimentation in lichens is mostly owing to their slow growth and nonavailability in bulk quantity. Overexploitation of the useful lichen would lead to loss of biodiversity. Sometimes the quantity of the purified compound is insufficient for structural elucidation and pharmacological testing. Therefore, culture of lichens in bioreactors and mycobiont culture seems to be ideal alternative methods. In lichen tissue culture, the mycobiont synthesizes required secondary metabolites only if suitable conditions are provided. These conditions include carbon sources similar to the carbohydrate supplied by the photobiont; precursors of the targeted end products and mimicking the conditions to which lichens are exposed in the wild. However, mycobiont in culture can yield novel products than from their original thallus, which is proven to be bioactive. For example, axenic cultures of Bunodophoron patagonicum isolated either from spores or from thallus fragments formed two chemosyndromes (suites of chemotypes) of depsides and dibenzofurans synchronically. Physconia distorta grown on nutrient-rich media produced mainly oleic acid, linoleic acid, stearic acid, and their triglyceride derivatives. An area that needs to be explored in future for lichenology research is the whole genome sequencing, identification, and characterization of genes responsible for the synthesis of lichen metabolites. Further prospects include transferring these genes in fast-growing microfungi and producing required metabolite at industrial scale. So far, no such successful microfungi system has been identified for mass production of lichen metabolites and hence there is a lot of scope in this area. Another possible area that needs to be explored is derivatization of lichen compounds. According to a study among all 1562 newly approved drugs during the period 1981–2014, the portion comprised of unaltered natural product was only 4%, whereas portions of natural product derivatives, synthetic drugs with natural product pharmacophores, or mimics of natural products accounted for 21% and 31%, respectively. This fact confirms the high significance of the derivatization of natural compounds as a tool for searching for more active agents or compounds with newly discovered biological properties.

Acknowledgment

We thank Director, CSIR-NBRI and KSCSTE-JNTBGRI for providing infrastructural facilities, Dr. Siljo Joseph and members of Lichenology laboratory for their cooperation during the study.

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