Among the species of the genus Cetraria, C. islandica (L.) Ach. has been the most widely used in traditional medicine. Its folk uses have been reported in a variety of documents, including handbooks, pharmacopoeias, compendia and pharmacognostical texts ^[1][12]. The main uses for C. islandica, commonly known as Iceland moss, are for the treatment of digestive and respiratory diseases. Herbal preparation varies from decoctions, tinctures and aqueous extracts to infusions. Hence, in Iceland, C. islandica has been used to the relief of both gastric and duodenal ulcers ^[2][72]. Moreover, decoctions of C. islandica were used to treat colds in Finland ^[3][73]. Furthermore, for centuries, C. islandica was famous as a laxative and antitussive in Central Europe. In addition to uses to treat respiratory and digestive conditions, C. islandica has been used in other countries for other medical purposes. This is the case in Sweden, where it has been used to treat nephritis and diabetes ^[4][74], and in Turkey, where it has been employed as a hemostatic and antihemorrhoidal agent ^[5][6][75,76]. Moreover, C. islandica has been used for tuberculosis in several countries including Spain, France, and Turkey ^[3][6][7][73,76,77]. These medicinal properties have been attributed mainly to its lichen acids, such as fumaroprotocetraric acid (2.6–11.5%), protocetraric acid (0.2–0.3%), protolichesterinic acid (0.1–1.5%) and usnic acid (0.04%) ^[8][9][78,79].

Based on the therapeutic benefits of C. islandica, comminuted herbal substances and soft extracts of this lichen in the form of syrup, oral gum, and lozenges have been marketed to relieve dry and irritating coughs and hoarseness. Moreover, combined commercial drug products have been developed that contain C. islandica together with other medicinal plants (Thymus vulgaris L., Hyssopus officinalis L., Saponaria officinalis L. and Marrubium vulgare L., among others) for the inflammatory processes of the upper airway, for the management of bronchial secretions and for the alleviation of coughs ^[1][12].

Other ethnobotanical studies have revealed traditional uses for other Cetraria species. Hence, in the Catalan Pyrenees (Spain), Cetraria cucullata (Bellardi) Ach. has been employed for asthma ^[7][77].

Many of the traditional uses of Cetraria  spp. have been validated, such as antidiabetic and anti-inflammatory, and other new pharmacological activities are being investigated, such as cytotoxic and genotoxic/antigenotoxic, which researcherswe will discuss later in the section of pharmacological activities.

In addition to the medicinal uses of C. islandica, its nutritional value is noteworthy. It is mainly consumed as tea. Moreover, in Italy, it is a food supplementation product valued for its digestive-facilitating properties ^[1][12]. In Northern Europe, during times of famine, it was used for bread, sometimes mixed with rice or flour ^[10][11][80,81]. Furthermore, C. islandica is approved as a flavoring for alcoholic beverages in United States ^[12][82]. In Iceland, C. islandica is used in a variety of recipes including soups, porridges and sausages, and is added to “skyr” (curd) ^[9][79], and it is also marketed as a bitter alcoholic beverage (38% alcohol content) called “Cetraria islandica schnapps” ^[11][81]. On the other hand, in Russia, during the years 1942–1943, C. islandica was used to industrially extract glucose because of the beet sugar scarcity ^[11][81]. Moreover, the lichen species Cetraria ericetorum Opiz was chopped up and added to soups for flavoring ^[13][83]. In addition to food uses in humans, the species C. islandica has been used as food for pigs and cows, especially during World War II ^[13][83]. Finally, it is also worth mentioning its uses in cosmetics. C. islandica is part of the composition of diversity cosmetic products, including shampoos and conditioners, deodorants, toothpastes, exfoliating and anti-cellulite creams, manicure and pedicure products, and aftershave lotions. Moreover, the lichen Cetraria nivalis (L.) Ach. is used in rejuvenating cream formulations ^[14][84].

2. Therapeutic Potential

2.1. Antibacterial, Antifungal and Antitrypanosomal Activities

The antibacterial and antifungal activities of species of the genus Cetraria have been investigated for Cetraria pinastri, Cetraria aculeata and Cetraria islandica. These lichen species have shown antibacterial activity against a broad group of both Gram-positive and Gram-negative bacteria. Hence, the Gram-positive microorganisms sensitive to the methanol extract of Cetraria pinastri were Enterococcus fecalis (Minimum Inhibitory Concentration (MIC) 0.23 mg/mL), Micrococcus lysodeikticus (MIC 0.46 mg/mL) and Staphylococcus aereus (MIC 0.94 mg/mL). The only Gram-negative bacteria that was sensitive to the methanol extract of Cetraria pinastri was Escherichia coli, with an MIC value of 1.87 mg/mL ^[15][107]. In another study, the antimicrobial activity of the ethanol, acetone and diethylether extracts of Cetraria acuelata against 12 different Gram-positive (Bacillus cereus, Bacillus subtilis, Listeria monocytogenes, Staphylococcus aureus and Streptococcus faecalis) and Gram-negative (Escherichia coli, Klebsiella pneumoniae, Pseudomonas aeruginosa, Pseudomonas syringae, Proteus vulgaris, Aeromonas hydrophila and Yersinia enterocolitica) bacteria was investigated. The three extracts tested showed antimicrobial activity against all bacteria species, except for the Gram-negative bacteria Pseudomonas syringae, Klebsiella pneumoniae and Yersinia enterocolitica. The most potent of the three tested Cetraria aculeata extracts was the diethylether extract, being especially active against the Gram-positive bacteria Bacillus cereus, Bacillus subtilis, Listeria monocytogenes and Streptococcus faecalis, and against the Gram-negative bacteria Pseudomonas aeruginosa and Proteus vulgaris, with MIC values of 8460 µg/mL ^[16][13]. Regarding the antimicrobial activity of Cetraria islandica, the extracts of methanol, acetone, water, and light petrolatum were assayed against Helicobacter pylori by using the Kirby and Bauery disk diffusion test. The light petrolatum extract of Cetraria islandica was the one that showed the greatest inhibitory capacity, followed by the acetone extract. However, the methanol and water extracts showed no activity. In addition, it was demonstrated that the compound responsible for the antimicrobial activity in the light petrolatum extract was the protolichesterinic acid ^[17][108]. In another study, the antimicrobial activity of the methanol extract of Cetraria islandica against Gram-positive species such as Staphylococcus aureus, Bacillus subtilis and Bacillus cereus, and against Gram-negative species such as Escherichia coli and Proteus mirabilis, was evaluated. The results show that the methanol extract of Cetraria islandica was more active against Escherichia coli with an MIC value of 2.5 mg/mL, followed by Staphylococcus aureus (MIC 1.25 mg/mL), Proteus mirabilis (MIC 1.25 mg/mL), Bacillus subtilis (MIC 0.625 mg/mL) and Bacillus cereus (MIC 0.312 mg/mL) ^[18][15]. In addition to studies evaluating the antibacterial activity of the lichen extracts from Cetraria species, the antimicrobial action of the compound protolichesterinic acid, isolated from Cetraria, Cetraria aculeata and Cetraria islandica, was investigated against different types of Gram-positive and Gram-negative bacteria. Hence, in one study, the protolichesterinic acid was active against Escherichia coli (MIC 7341 µg/mL), Bacillus subtilis (MIC 7341 µg/mL), Pseudomonas aeruginosa (MIC 7341 µg/mL) and Listeria monocytogenes (MIC 3670 µg/mL) ^[16][13]. In another study, the inhibitory capacity of the protolichesterinic acid was evaluated against a total of 35 Helicobacter pylori strains randomly selected from human biopsy samples, obtaining MIC values ranging from 16 to 64 µg/mL ^[17][108]. Apart from their antibacterial activity, Cetraria species, specifically Cetraria aculeata, Cetraria pinastri and Cetraria islandica, have been found to be active against pathogenic fungi. Hence, the methanol extract of Cetraria pinastri showed antifungal activity against Acremonium chrysogenum (MIC 3.75 mg/mL), Alternaria alternata (MIC 1.87 mg/mL), Aspergillus flavus (MIC 7.5 mg/mL), Aspergillus niger (MIC3.75 mg/mL), Candida albicans (MIC 1.87 mg/mL), Cladosporium cladosporioides (MIC 0.94 mg/mL), Fusarium oxysporum (MIC 7.5 mg/mL), Mucor mucedo (MIC 7.5 mg/mL), Paecilomyces variotii (MIC 15 mg/mL), Penicillium verrucosum (MIC 15 mg/mL) and Trichoderma harsianum (MIC 3.75 mg/mL). In another study, the ethanol, acetone, and diethyl ether extracts of Cetraria aculeata were assayed against the fungal species Penicillum sp., Cladosporium sp., Fusarium oxysporum, Fusarium culmorum, Rhizopus sp., Fusarium moniliforme, Fusarium solani and Aspergillus sp. The results show that none of the extracts of Cetraria aculeata had antifungal activity ^[15][16][13,107]. Finally, the methanol extract of Cetraria islandica was investigated on Aspergillus flavus, Candida albicans, Fusarium oxysporum, Penicillium purpurescens and Trichoderma harsianum species. Cetraria islandica showed a high antifungal activity against Aspergillus flavus and Penicillium purpurescens, with MIC values of 5 mg/mL, followed by its activity against Fusarium oxysporum and Trichoderma harsianum, with MIC values of 2.5 mg/mL. The lowest antifungal activity of the methanol extract of Cetraria islandica was against Candida albicans (MIC 1.25 mg/mL) ^[18][15]. In addition to the antifungal and antibacterial activity studies, the antitrypanosomal activity against Trypanosoma brucei brucei of bioactive compounds isolated from Cetraria islandica species was also evaluated. Protolichesterinic acid, fumarprotocetraric acid, lichesterinic acid and protocetraric acid were isolated. It was observed that only protolichesterinic acid (MIC 12.5 µM) and lichesterinic acid (MIC 6.30 µM) showed antitrypanosomal activity, with protolichesterinic acid being more effective ^[19][109].

2.2. Antioxidant Activity

Oxidative stress is characterized by an imbalance between the production of reactive species (ROS) and the antioxidant defense activity. This pathological state has been associated with many chronic and/or degenerative diseases, such as diabetes, Alzheimer’s disease, and cardiovascular disease. The use of exogenous antioxidants that act as scavengers or that modulate the endogenous antioxidant system is one of the most promising therapies used to deal with oxidative stress. In this context, phenolic compounds have turned out to be powerful antioxidants. The efficacy is directly related to the number of hydroxyl groups in the phenolic structure ^[20][110]. Lichens produce unique phenolic compounds as secondary metabolites that have aroused great research interest due to their antioxidant capacity ^[21][111]. Ranković B et al. evaluated the antioxidant activity of the methanol extract of Cetraria pinastri species by measuring the oxidation products of linoleic acid. The results have revealed that this lichen species was able to inhibit linoleic acid oxidation by 48.8%. This activity is related to its high polyphenol content of 32.9 mg/g in the dry extract ^[15][107]. Likewise, antioxidant activity studies have been carried out with the methanol extract and the ethyl acetate extract of Cetraria aculeata. In general, the antioxidant capacity of the methanol extract was higher than that of the ethyl acetate extract. Hence, the methanol extract showed an IC₅₀ value of 51.65 µg/mL in the 2,2-diphenyl-1-picrylhydrazyl (DPPH) assay, a value of 45.55 µg/mL for lipid peroxidation inhibition capacity, a value of 50.43 µg/mL for ferrous ion chelating capacity and a value of 90.1 µg/mL for hydroxyl radical scavenging activity. Regarding antioxidant enzymes, the methanol extract of C. aculeata increased superoxide dismutase (SOD) and glutathione peroxidase (GPx) enzymes’ activities. On the contrary, a slight increase in malondialdehyde (MDA) levels and a decrease in reduced glutathione (GSH) levels were also observed at low doses ^[22][112]. On the other hand, the lichen in which the antioxidant activity has been the most studied is Cetraria islandica. Hence, the methanol extract of Cetraria islandica exhibited a high reduction capability and powerful free radical scavenging, as shown in the DPPH assay (IC₅₀ 678.38 μg/mL), superoxide anion assay (IC₅₀ 792.48 μg/mL) and reducing power assay (from 0.0512 μg/mL to 0.4562 μg/mL) ^[18][15]. In another study, Kotan E. et al. investigated the antioxidant ability of the methanol extract of Cetraria islandica (5 and 10 µg/mL) in an aflatoxin B1 induced oxidative stress model in blood lymphocytes from healthy non-smoking volunteers. The methanol extracts of this lichen species increased SOD and glutathione peroxidase (GPx) enzyme activities, and decreased MDA levels. The most protective concentration of Cetraria islandica was 5 µg/mL ^[23][113]. In addition to the methanol extract, the aqueous extract of Cetraria islandica has shown antioxidant activity using the thiocyanate method and the reducing antioxidant power, superoxide anion radical and DPPH assays. Different concentrations of the aqueous extract of Cetraria islandica (50, 100 and 250 µg) inhibited the peroxidation of linoleic acid by 96 to 100%. Moreover, this extract reduced iron from its ferric state to its ferrous state in a significant and concentration-dependent manner, this activity being higher than in the reference compound BHT. Moreover, the aqueous extract of Cetraria islandica at a concentration of 100 μg showed a higher superoxide radical scavenging activity than the reference compounds hydroxybutylanisole (BHA), butylated hydroxytoluene (BHT), and quercetin ^[24][114]. Furthermore, the ethanol extracts of Cetraria islandica (96%, 70% and 40%) showed antioxidant activity. The DPPH assay showed that the ethanol extract 70% and ethanol extract 40% had IC₅₀ values of 2.40 mg/mL and 2.45 mg/mL; the Ferric Reducing Antioxidant Power (FRAP) assay revealed that the ethanol extract 96% had the highest value of 486 μmol/L, and the 2, 2′-Azinobis-3-ethyl-benzo-thiazoline-6-sulphonic acid (ABTS) assay demonstrated that the ethanol extract 40% of this lichen was the most active ^[25][115]. In another study, Kosanic M. et al. compared the antioxidant activities of different extracts (methanol, water, and acetone) of Cetraria islandica by using the DPPH assay, reducing antioxidant power method and superoxide anion radical assay. This study revealed that the methanol extract had the highest activity in all these methods, followed by the acetone extract and aqueous extract ^[26][116]. Recently, it has been observed that melanin extracted from Cetraria islandica also possesses free radical scavenging and reducing capacity in the DPPH assay, with an IC₅₀ of 405 μg/mL ^[27][105]. Finally, based on the antioxidant properties and considering the implications of oxidative processes in diabetes and neurodegenerative diseases, there are several studies focused on the protective effect of Cetraria islandica. Hence, the aqueous extract of this lichen species increased the activity of the antioxidant enzymes SOD, catalase (CAT), and GPx, and reduced the levels of the lipid peroxidation biomarker MDA in human erythrocytes with type 1 diabetes mellitus ^[28][117]. Moreover, the aqueous extract of Cetraria islandica decreased the total oxidative stress (TOS) and increased the total antioxidant capacity (TAC) in streptozotocin-induced Diabetes Mellitus type 1 Sprague-Dawley rats ^[29][118]. Using the same model, the aqueous extract of Cetraria islandica (250–500 mg/kg/day) increased the levels of SOD, CAT and GSH, and reduced the levels of MDA ^[30][31][32][16,119,120]. Regarding neuroprotection studies, the protective role of Cetraria islandica and its isolated secondary metabolite fumarprotocetraric acid has been demonstrated in a hydrogen peroxide-induced oxidative stress model on human U373MG astrocytoma cells and human SH-SY5Y neuroblastoma cells. The methanol extract of Cetraria islandica at 10 µg/mL concentration increased cell viability, reduced intracellular ROS production and MDA levels, and increased the ratio of reduced/oxidized glutathione (GSH/GSSG). The depsidone fumarprotocetraric acid at 1 μg/mL in the neurons model and 25 μg/mL in the astrocytes decreased lipid peroxidation levels and intracellular ROS production and increased the ratio GSH/GSSG. Moreover, this compound ameliorated the H₂O₂-induced mitochondrial dysfunction and alterations in calcium homeostasis and inhibited apoptotic cell death. Its neuroprotective activity is related, at least in part, to its ability to activate the Nrf2 pathway that regulates antioxidant enzymes. Furthermore, both the lichen extract and fumarprotocetraric acid showed scavenging activities in the oxygen radical absorbance capacity (ORAC) assay (value of 3.06 µmol TE/mg and 5. 07 μmol TE/mg, respectively) and in the DPPH assay (IC₅₀ value of 1183.55 µg/mL and 1393.83 μg/mL, respectively) ^[33][34][121,122].

2.3. Immunomodulatory and Anti-Inflammatory Activities

The immunomodulatory activity has been investigated on Cetraria islandica aqueous extracts and the isolated compounds fumarprotocetraric acid and protolichesterinic acid, and the polysaccharides lichenan and isolichenan on the maturation of dendritic cells. This study showed that the aqueous extract and the polysaccharide lichenan reduced the IL-12p40/IL-10 ratio and CD209 expression and increased CD86 expression. Moreover, this lichen extract showed anti-inflammatory properties at a dose of 2.5 mg/kg on a BSA-induced arthritis rat model, as evidenced in the reduction in the diameter between the right and left knees ^[35][90]. It is also noteworthy that the study on the polysaccharide α-1,3/1,4-D-Glucan (Ci3) isolated from Cetraria islandica (100 ug/mL) showed an increase in granulocytic phagocytosis and a decrease in complement-induced hemolysis ^[36][123].

2.4. Cytotoxic, Genotoxic and Antigenotoxic Activities

The cytotoxic activities of the lichen species Cetraria acuelata and Cetraria islandica and their isolated bioactive compounds have been evaluated on different types of malignant cells. Hence, the methanol extract of Cetraria islandica reduced the cell viability of the human breast cancer cells (MCF-7) (IC₅₀ value of 19.51 µg/mL), the human liver cancer cells (HepG2) (IC₅₀ value of 181.05 µg/mL), human melanoma cells (FemX) (IC₅₀ value of 22.68 μg/mL) and human colon carcinoma cells (LS174) (IC₅₀ value of 33.74 μg/mL) ^[18][33][15,121]. On the other hand, the ethanol extract of Cetraria islandica was able to reduce cell viability in MCF-7 cells (IC₅₀ 9.2047 × 10⁻⁵ g/mL), also showing an increase in protein levels of AMP-activated kinases-α1 (AMPK-α1) and ERK1/2 ^[37][124]. The compound protolichesterinic acid, isolated from Cetraria islandica, decreased cell viability and caused morphological changes at a concentration of 20 µg/mL on breast carcinomas T-47D and ZR-75-1 and erythro-leukaemia K-562 cells. Moreover, this secondary metabolite inhibited DNA synthesis at the concentrations of 1.1 µg/mL on ZR-75-1 cells, 3.8 µg/mL on T-47D cells and 11.2 µg/mL on K-562 cells. This activity is related to the ability of protolichesterinic acid to inhibit 5-lipoxygenase ^[38][125]. Thorsteinsdottir et al. conducted a study in which it was observed that protolichesterinic acid, in addition to reducing human lung cancer cells’ (A549) viability, also induced a decrease in Leucine Rich Repeat Containing 8 VRAC Subunit A (LRRC8A) protein expression, as well as volume-sensitive taurine release under hypotonic conditions ^[39][126]. On the other hand, the fumarprotocetraric acid, isolated from Cetraria islandica, did not inhibit the cell growth of the human cells T-47D (breast) and Panc-1 (pancreas) ^[40][127]. Moreover, the lichenan from Cetraria islandica was also not active against the human myeloid leukemia U937 cells ^[41][128]. In addition to Cetraria islandica, the cytotoxic activity of Cetraria aculeata has been studied. The acetone extract of this lichen species was active against HeLa (human uterus carcinoma) (IC₅₀ value of 200 μg/mL), A549 (human small lung carcinoma) (IC₅₀ value of 500 μg/mL) and 5RP7 (c-H-ras transformed-rat embryonic fibroblasts) (IC₅₀ value of 280 μg/mL) ^[42][129]. Regarding the studies performed to evaluate the genotoxic/antigenotoxic activities of Cetraria species, it has been highlighted that the species Cetraria aculeata showed a significant antigenotoxic effect on TA98 and TA100 strains of Salmonella typhimurium but had no effect in human lymphocytes ^[42][129]. On the other hand, the methanol extract showed antigenotoxic activity against Salmonella typhimurium strains TA1535 and TA1537, and a slight decrease in sister chromatid exchange (SCE) formation ^[43][14]. In another work, it has been shown that the methanol extract of Cetraria islandica (from 50 to 200 μg/mL) had genotoxic potential in cultured peripheral venous blood from healthy donors by increasing both the number of BN cells containing micronuclei (MNi) and the number of MNi in BN cells ^[18][15].

2.5. Cell Differentiation and Depigmentation Activities

The effect of β-1,3/1,4-Glucan (Lichenan), isolated from Cetraria islandica, on cell differentiation has recently been evaluated in primary normal human epidermal keratinocytes (NHEK) and HaCaT keratinocytes using immunofluorescence. A decrease in cell proliferation and an increase in protein expression of specific differentiation-related markers such as cytokeratin 10 (CK10) and involucrin were observed in NHEK cells. In addition, the gene expressions of CK, involucrin, transglutaminase, loricrin and filaggrin genes, which are involved in cell differentiation, were also increased ^[44][97]. The depigmenting activity was evaluated in chloroform, chloroform–methanol, methanol, and water extracts of Cetraria islandica species. The chloroform–methanol extract showed the highest inhibition capacity on tyrosinase (IC₅₀ 86 µg/mL). A cell viability assay was performed on human melanoma cells (MeWo) with IC₅₀ values of 264 µg/mL. Melanin assays demonstrated a significant reduction in melanin levels. On the other hand, zebrafish embryo models were used to determine the melanogenesis effects of the extract showing an inhibition of melanogenesis, and therefore a reduction in pigmentation ^[45][130].