Lichens often grow in microhabitats where they experience severe abiotic stresses. Some species respond to high UV radiation by synthesizing dark brown melanic pigments in the upper cortex. However, unlike the melanized structures of non-lichenized fungi, the morphology of the melanic layer in lichens remains unstudied. Here, we analyzed the morphology, ultrastructure, and elemental composition of the melanized layer in UV-exposed thalli of the lichen Lobaria pulmonaria (L.) Hoffm. Using light microscopy, we detected a pigmented layer sensitive to staining with 3,4-L-dihydroxyphenylalanine, a precursor of eumelanin, in the upper cortex of melanized thalli. Analysis of cross-sections of melanized thalli using scanning electron microscopy revealed that melanin-like granules are deposited into the hyphal lumens. Melanized thalli also possessed thicker hyphal cell walls compared to pale thalli. Energy-dispersive X-ray spectroscopy analysis of the elemental composition of the hyphal walls and extracted melanin indicated that the type of melanin synthesized by L. pulmonaria is eumelanin. Transmission electron microscopy was used to show that during melanization melanosome-like dark vesicles are transported to the cell surface and secreted into the cell walls of the fungal hyphae. Results from this study provide new insights into the effects of melanin synthesis on the microstructure of lichen thalli.

Synthetic dyes are widely used in the industry; they are chemically stable, difficult to neutralize, and therefore they are a threat to the environment when released into wastewaters. The dyes have a significant impact on plant performance by impairing photosynthesis, inhibiting growth, and entering the food chain and may finally result in the toxicity, mutagenicity and carcinogenicity of food products. Implementation of the dark piment melanin for the adsorption of the synthetic dyes is a new ecologically friendly approach for bioremediation. The aim of the present work was to study the physico-chemical characteristics of melanins from the lichens Lobaria pulmonaria and Lobaria retigera, analyze their adsorption/desorption capacities towards synthetic dyes, and assess the capacity of melanins to mitigate toxicity of the dyes for a common soil bacterium Bacillus subtilis. Unique chelating properties of melanins determine the perspectives of the use of these high molecular weight polymers for detoxification of xenobiotics.

Melanins are universal biopolymers, the main biological function of which is to protect living organisms from adverse factors. Interest in these dark pigments arises due to the prospects for their use in medicine, cosmeceuticals, bioremediation, bioelectronics, and other fields. In this work, a complex scheme was developed to isolate melanins from the lichen Leptogium furfuraceum (Harm.), which is a symbiotic extremophilic organism. Screening of the conditions for the extraction of melanin, its subsequent precipitation and purification made it possible to isolate three samples of melanin with a total yield of 7.5% of the dry matter of the lichen. It was established that all isolated melanins have a polyaromatic structure, they contain carbohydrates and proteins, the content of which, depending on the conditions for melanin isolation, is 7.3–9.9% and 13.5–32.7%, respectively. Among the isolated melanins, melanin, which is a water-soluble fraction of acid-precipitated melanin, displays the highest antioxidant activity. This melanin, unlike other melanins, is almost fully soluble in distilled water, 95% ethanol, 99% dimethyl sulfoxide, and phosphate buffer (pH 7.4). The differences in solubility, antioxidant activity, and chelating capacity of isolated lichen melanins make it possible to determine potential areas for their practical use.

Lichens are unique extremophilic organisms due to their phenomenal resistance to adverse environmental factors, including ultraviolet (UV) irradiation. Melanization plays a special role in the protection of lichens from UV-B stress. In the present study, we analyzed the binding of melanins with the components of cell walls of the mycobiont of the upper cortex in the melanized lichen thalli Lobaria pulmonaria. Using scanning electron and atomic force microscopy, the morphological and nanomechanical characteristics of the melanized layer of mycobiont cells were visualized. Melanization of lichen thalli led to the smoothing of the surface relief and thickening of mycobiont cell walls, as well as the reduction in adhesion properties of the lichen thallus. Treatment of thalli with hydrolytic enzymes, especially chitinase and lichenase, enhanced the yield of melanin from melanized thalli and promoted the release of carbohydrates, while treatment with pectinase increased the release of carbohydrates and phenols. Our results suggest that melanin can firmly bind with hyphal cell wall carbohydrates, particularly chitin and 1,4-β-glucans, strengthening the melanized upper cortex of lichen thalli, and thereby it can contribute to lichen survival under UV stress.

The invasive plant Heracleum sosnowskyi Manden. is a valuable source of a number of bioactive metabolites that can be used in the pharmaceutical industry and medicine and may have some other applications as well. Today, there is a need to summarize data on these substances as well as analyze the toxicological profile of the metabolites of H. sosnowskyi. In this study, we collected a dataset of 225 metabolites of H. sosnowskyi from different literature sources and performed cluster analysis of their chemical structures; we revealed five main clusters of compounds: terpenoids, aromatic compounds, polyaromatic compounds, fatty acids, and furanocoumarins. In order to fill the gaps in the experimental data on the toxicity of the studied substances, we used machine learning (ML) algorithms previously designed for high-accuracy prediction of toxicity end-points. The ML-based approach allowed us to fill in up to 90% of the missing median lethal dose $L{D}_{50}$ (mouse) data for the studied molecules. The validity of each predicted value was confirmed by analyzing the applicability domain of the used ML models. For the calculations and ML modeling, we used the Syntelly chemoinformatics platform. For the most toxic compounds—hydroxycoumarins and furanocoumarins of H. sosnowskyi—the values for hepatotoxicity, drug-induced liver injury (DILI), cardiotoxicity, and carcinogenicity were predicted. Based on the analysis of $L{D}_{50}$ values for the mouse animal model, the greatest toxicity for furanocoumarins is expected with the intravenous route of administration (62–450 mg/kg), which can cause drug-induced liver injury. At the same time, the data do not show high cardiotoxicity risks for the studied furanocoumarins. Based on the presented results, we discuss prospects of using some of the compounds as pharmaceutical agents.

Lichens are unique extremophilic organisms due to their phenomenal resistance to adverse environmental factors, including ultraviolet (UV) irradiation. Melanization plays a special role in the protection of lichens from UV-B stress. In present study, we analyzed the binding of melanins with the components of cell walls of mycobiont of upper cortex in the melanized lichen thalli Lobaria pulmonaria. Using scanning electron and atomic force microscopy, the morphological and nanomechanical characteristics of melanized layer of mycobiont cells were visualized. Melanization of lichen thalli led to the smoothing the surface relief and thickening mycobiont cell walls and resulted in the increase of deformation values and reduction of adhesion properties of lichen thallus. Treatment of thalli with hydrolytic enzymes, especially chitinase and lichenase, enhanced the yield of melanin from melanized thalli and release of carbohydrates, while treatment with pectinase increased the release of carbohydrates and phenols. Our results suggest that melanin by firmly binding with hyphal cell wall carbohydrates, particularly chitin and 1,4-β-glucans, can form associations, strengthen the melanized upper cortex of lichen thalli, and thereby can contribute to lichen survival under UV stress.