Which Specialized Metabolites Does the Native Subantarctic Gastropod Notodiscus hookeri Extract from the Consumption of the Lichens Usnea taylorii and Pseudocyphellaria crocata?

The effect of lichen secondary metabolites on Aspergillus fungi

A systematic review of literature data on the antifungal potential of extracted lichen compounds and individual secondary metabolites against mold species of the genus Aspergillus is provided. Crude extracts from 49 epiphytic, 16 epigeic and 22 epilithic species of lichens and 44 secondary metabolites against 10 species, Aspergillus candidus, A. flavus, A. fumigatus, A. nidulans, A. niger, A. ochraceus, A. parasiticus, A. restrictus, A. stellatus and A. ustus, were analysed. Several measuring techniques were employed for such analyses. Lichen substances were extracted with alcoholic and other organic solvents mainly using the Soxhlet apparatus. Among the three most-studied mold species, the results showed that the crude extracts from the thalli of the lichens Cladonia foliacea, Hypotrachyna cirrhata, Leucodermia leucomelos, Platismatia glauca and Pseudevernia furfuracea against Aspergillus flavus, from C. foliacea, Nephroma arcticum and Parmelia sulcata against A. fumigatus and from Evernia prunastri, Hypogymnia physodes, Umbilicaria cylindrica and Variospora dolomiticola against A. niger have the greatest antifungal potential. The lichen secondary metabolites showed a higher inhibitory potential, e.g. protolichesterinic acid against A. flavus, lecanoric acid against A. fumigatus and orsellinic acid against A. niger; the other seven species of Aspergillus have been poorly studied and require further investigation. A comparison of the inhibitory potential of the tested mixtures of lichen substances and their secondary metabolites shows that they can compete with commonly used antifungal substances, such as ketoconazole and clotrimazole against A. flavus, A. nidulans, A. niger and A. parasiticus and fluconazole in the case of A. fumigatus.

Usnea sp.: Antimicrobial potential, bioactive compounds, ethnopharmacological uses and other pharmacological properties; a review article

ETHNOPHARMACOLOGICAL RELEVANCE

Usnea sp. is a fruticose thalli lichen with interesting medicinal properties. Since ancient times, Usnea sp. has been used in traditional medicine worldwide to treat various diseases. The broad scientific studies on this lichen have proved its multidirectional biological effect, such as antimicrobial activity, which is attributed to its usnic acid content.

PURPOSE

The main aim of this review is to provide an up-to-date overview of the antimicrobial activities of Usnea sp., including the traditional and medicinal uses, and a critical evaluation of the presented data. Also, the mechanism of this type of action will be explained.

METHODS

To prepare this manuscript, the information was extracted from scientific databases (Pubmed, ScienceDirect, Wiley, Springer, and Google Scholar), books, and theses. The available scientific information was critically analysed.

RESULTS

Analysis of the scientific literature regarding traditional uses and bioactivity research showed that Usnea sp. extracts exhibit high antibacterial activity. The Gram-positive bacteria (Staphylococcus aureus, Bacillus subtilis, Bacillus cereus, and Mycobacterium tuberculosis) and aquatic oomycetous fungi were the most sensitive Usnea sp. extracts. Moderate activity against Malassezia furfur and dermatophytes was observed, as well. Gram-negative bacteria, yeast, and fungi were more frequently resistant to Usnea sp. extracts (included Escherichia coli, Candida sp., Saccharomyces cerevisiae, and Aspergillus sp.). The antiviral activity of Usnea sp. was limited.

CONCLUSION

The results show that the use of Usnea sp. in traditional medicine can be scientifically documented. Studies show that usnic acid is the active compound present in Usnea sp. extracts. This compound, which has a high antibacterial and cytotoxic activity, exists in large quantities in low-polarity extracts, and low concentration in these of high-polarity. Usnea sp. extracts contain compounds other than usnic acid as well with biological effects. Usnea barbata is a species that has been employed in modern-day cosmetic and pharmaceutical preparations. The information presented in the review can be considered as a source of knowledge about the Usnea sp. It presents research on biological properties reported for different species of Usnea genus and thus can facilitate their use in medicine.

Mass Spectrometry Imaging of Specialized Metabolites for Predicting Lichen Fitness and Snail Foraging

Lichens are slow-growing organisms supposed to synthetize specialized metabolites to protect themselves against diverse grazers. As predicted by the optimal defense theory (ODT), lichens are expected to invest specialized metabolites in higher levels in reproductive tissues compared to thallus. We investigated whether Laser Desorption Ionization coupled to Mass Spectrometry Imaging (LDI-MSI) could be a relevant tool for chemical ecology issues such as ODT. In the present study, this method was applied to cross-sections of thalli and reproductive tissues of the lichen Pseudocyphellaria crocata. Spatial mapping revealed phenolic families of metabolites. A quantification of these metabolites was carried out in addition to spatial imaging. By this method, accumulation of specialized metabolites was observed in both reproductive parts (apothecia and soralia) of P. crocata, but their nature depended on the lichen organs: apothecia concentrated norstictic acid, tenuiorin, and pulvinic acid derivatives, whereas soralia mainly contained tenuiorin and pulvinic acid. Stictic acid, tenuiorin and calycin, tested in no-choices feeding experiments, were deterrent for N. hookeri while entire thalli were consumed by the snail. To improve better knowledge in relationships between grazed and grazing organisms, LDI-MSI appears to be a complementary tool in ecological studies.

Overcoming deterrent metabolites by gaining essential nutrients: A lichen/snail case study

Specialised metabolites in lichens are generally considered repellent compounds by consumers. Nevertheless, if the only food available is lichens rich in specialised metabolites, lichenophages must implement strategies to overcome the toxicity of these metabolites. Thus, the balance between phagostimulant nutrients and deterrent metabolites could play a key role in feeding preferences. To further understand lichen-gastropod interactions, we studied the feeding behaviour and consumption in Notodiscus hookeri, the land snail native to sub-Antarctic islands. The lichen Usnea taylorii was used because of its simple chemistry, its richness in usnic acid (specialised metabolite) and arabitol (primary metabolite) and its presence in snail habitats. Choice tests in arenas with intact lichens versus acetone-rinsed lichens were carried out to study the influence of specialised metabolites on snail behaviour and feeding preference. Simultaneously, usnic acid and arabitol were quantified and located within the lichen thallus using HPLC-DAD-MS and in situ imaging by mass spectrometry to assess whether their spatial distribution explained preferential snail grazing. No-choice feeding experiments, with the pure metabolites embedded in an artificial diet, defined a gradual gustatory response, from strong repellence (usnic acid) to high appetence (D-arabitol). This case study demonstrates that the nutritional activity of N. hookeri is governed by the chemical quality of the food and primarily by nutrient availability (arabitol), despite the presence of deterrent metabolite (usnic acid).

Intrathalline Metabolite Profiles in the Lichen Argopsis friesiana Shape Gastropod Grazing Patterns

Lichen-gastropod interactions generally focus on the potential deterrent or toxic role of secondary metabolites. To better understand lichen-gastropod interactions, a controlled feeding experiment was designed to identify the parts of the lichen Argopsis friesiana consumed by the Subantarctic land snail Notodiscus hookeri. Besides profiling secondary metabolites in various lichen parts (apothecia, cephalodia, phyllocladia and fungal axis of the pseudopodetium), we investigated potentially beneficial resources that snails can utilize from the lichen (carbohydrates, amino acids, fatty acids, polysaccharides and total nitrogen). Notodiscus hookeri preferred cephalodia and algal layers, which had high contents of carbohydrates, nitrogen, or both. Apothecia were avoided, perhaps due to their low contents of sugars and polyols. Although pseudopodetia were characterized by high content of arabitol, they were also rich in medullary secondary compounds, which may explain why they were not consumed. Thus, the balance between nutrients (particularly nitrogen and polyols) and secondary metabolites appears to play a key role in the feeding preferences of this snail.