| Summary: | <i>Cladonia arbuscula</i> 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 CO<sub>2</sub> 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<sup>−2</sup> s<sup>−1</sup>. As PAR intensity increased to 2000 µmol m<sup>−2</sup> s<sup>−1</sup>, the rate of Myc-Glu(OH) synthesis slowed down. In turn, under dark/PAR irradiation with simultaneous deprivation of CO<sub>2</sub> in the atmosphere surrounding <i>C. arbuscula</i> and during darkness with continuous access to atmospheric CO<sub>2</sub>, its production was insignificant. Obtained data confirmed that Myc-Glu(OH) plays an important role in protecting <i>C. arbuscula</i> 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.
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