Morphology and physicochemical properties of melanin particles from the lichen Lobaria pulmonaria.

Abstract

Melanins are the pigments with diverse protective functions in a wide range of organisms. The ability of lichens to synthesize melanins is an important adaptation in response to adverse light conditions, including ultraviolet (UV) irradiation and high light. Previously, we demonstrated that melanization of the lichen Lobaria pulmonaria is accompanied by the accumulation of melanin granules in the upper cortex. However, very little information is available on the morphology and physicochemical properties of these particles. Here, for the first time, we used polyclonal antibodies against eumelanin to visualize melanin in the upper cortex of L. pulmonaria and confirm its identity. Using scanning electron microscopy, we showed that extracted melanins are large, spherical aggregates, with an average size of 230 nm. The aggregates had an irregular and unstable structure and displayed high adhesive properties, according to atomic force microscopy. Purification using size exclusion chromatography showed that melanins coeluted with polysaccharides. Purified particles were visualized as discrete structures (subaggregates) with an average size of 42 nm and displayed higher stability and lower adhesive properties than non-purified aggregates. In colloidal aqueous systems, melanin particles formed aggregates with particle sizes of 279 and 212 nm for the non-purified and purified samples, respectively, suggesting that the removal of the polysaccharide component during purification causes the condensation of the polyaromatic structure of melanin. Taken together, our results suggest that melanin particles extracted from UV-melanized thalli of the lichen L. pulmonaria comprise a supramolecular assembly of melanin and polysaccharides. Analysis of the size, ζ-potential, and polydispersity index suggests that melanin particles are capable of structural rearrangements that lead to changes in their nanomechanical properties. The ability of the melanin to bind to polysaccharides may enhance the structural robustness of mycobiont cell walls and increase the tolerance of lichen thalli to stressful environments.