Metabolism of Mycosporine-Glutamicol in the Lichen Cladonia arbuscula subsp. squarrosa under Seasonal Changes and Elevated Exposure to UV-B or PAR Irradiation

Cladonia arbuscula in its environmental niches is regularly affected by daily and annual variations in solar radiation. Mycosporine-glutamicol, Myc-Glu(OH), which it synthesizes, may act as a significant cellular UV-protector. Therefore, we studied this compound concentration in lichen thalli concerning seasonal changes and increased exposure to UV-B and photosynthetically active radiation (PAR) with/without simultaneous CO2 deprivation. Myc-Glu(OH) occurred year-round and exhibited a strong seasonality. The most crucial role in the control of its synthesis played UV-B radiation, although its high concentration was also found after PAR irradiation at 1000 µmol m−2 s−1. As PAR intensity increased to 2000 µmol m−2 s−1, the rate of Myc-Glu(OH) synthesis slowed down. In turn, under dark/PAR irradiation with simultaneous deprivation of CO2 in the atmosphere surrounding C. arbuscula and during darkness with continuous access to atmospheric CO2, its production was insignificant. Obtained data confirmed that Myc-Glu(OH) plays an important role in protecting C. arbuscula from UV damage and favours its adaptation to environmental stress in its natural habitat. They also suggest that its synthesis is a synergism of multiple factors. Consequently, further studies should focus on their evaluation and the identification of a lichen partner actively involved in Myc-Glu(OH) biogenesis.