Melanisation of lichens : the composition of melanin and the role of ultraviolet light (uv) in peltigeralean and non-peltigeralean lichens.

Abstract

Lichens are unique organisms widely known for their ability to tolerate extreme environmental conditions due to the symbiotic relationship between a fungus and algae or cyanobacteria. This includes the ability to synthesise melanins to protect themselves from ultraviolet radiation and high light. Melanins are found in different forms, the eumelanins (DOPA), often synthesised by Peltigeralean lichens while the non-Peltigeralean produce dark pigments that appear not to be DOPA melanins. Increased levels of UV and high light affect the physiology of many organisms, as a result this study investigates the effects of the photoprotective pigment, melanin on the photosynthetic apparatus of both chlorophycean and cyanobacterial bionts. The first aim of this thesis was to study the effect of using different light regimes to induce melanins. The second aim was to compare the properties of melanin between different lichens with those from free-living fungi. Lastly, the effect of melanisation on the photosynthesis was investigated. Results presented here suggest that melanins are insoluble in organic solvents, except DMSO and strongly absorb in the UVB and UVA wavelengths. The induction of melanin was slow and was better induced beneath the screens that transmitted UV. Transplanting nonmelanised Lobaria pulmonaria, to an open site for four weeks induced melanic pigments. Melanised thalli had normal chlorophyll contents and normal maximum rates of photosynthesis. Chlorophyll fluorescence analysis showed that the maximum quantum yield and relative electron transfer rates were similar to those of nonmelanised thalli. However, at light levels lower than 100 μmol photons m-2 s-1 melanisation of the upper cortex of the lichen reduced rates of CO2 fixation by more than 40%. Melanic thalli also had a higher chlorophyll a/b ratio and more xanthophyll cycle pigments. In Lobaria retigera, the +UV screen decreased the photosynthetic rate more than other light treatments. Photoinhibition of wet thalli was rapid compared to dry, though complete recovery was reached after a day. From these results, it can be confirmed that melanisation has a protective action against high light as melanised thalli were more tolerant to excess light. While protecting photobionts from high light, melanisation reduced photosynthetic efficiency and protects lichens from photoinhibition in both the wet and dry states.