Fungal melanins as a sun screen for symbiotic green algae in the lichen Lobaria pulmonaria

The mycobiont of the high-light-susceptible forest lichen Lobaria pulmonari a was shown to deposit brown, melanic compounds in the outer layer of the u pper cortex, depending on the long-term level of solar radiation in its nat ural habitat. Furthermore, pale thalli from a shady habitat produced melani c compounds after transplantation to a sunny habitat. This browning of the cortex appeared to be a physiologically active process, taking place only d uring periods with frequent hydration. Melanin production was slow. After t ransplantation. more than 1 year was needed for a shade-adapted thallus to reduce the cortical transmittance (230-1000 nm) to a similar level to that of naturally sun-exposed specimens. Melanic compounds acted as a sun screen , especially reducing UVB and UVA wavelengths, but also visible wavelengths , at the photobiont Level. In the near infrared range, there was only a sma ll difference in transmittance between shade- and sun-adapted cortices. A n egative correlation was found between the natural light level and the corti cal transmittance of wavelengths below 700 nm. However, previous studies ha ve shown that even photobionts of melanic L. pulmonaria thalli are relative ly susceptible to high-light exposure. Since melanins also increase the abs orbance of solar energy for the whole thallus, it appears that what is gain ed in terms of UV- and light protection in melanic L. pulmonaria specimens may be offset by increased exposure to excess temperatures for this highly heat-susceptible lichen.