Terricolous lichens are abundant in semi-arid areas, where they are exposed to high
irradiation. Photoprotection is essential for the algae as the photobiont provides
the primer carbon source for both symbionts. The UV-protectant lichen metabolites
and different quenching procedures of the alga ensure adequate photoprotection. Since
the long-term effect of diminishing UV-protectant lichen metabolites is unknown, a
major part of lichen secondary metabolites was removed from Cladonia foliacea thalli
by acetone rinsing, and the lichens were then maintained under field conditions to
investigate the effect on both symbionts for 3 years. Our aim was to determine if
the decreased level of UV-protectant metabolites caused an elevated photoprotection
in the algae and to reveal the dynamics of production of the metabolites. Photosynthetic
activity and light protection were checked by chlorophyll a fluorescence kinetics
measurements every 6 months. The concentrations of fumarprotocetraric and usnic acids
were monitored by chromatographic methods. Our results proved that seasonality had
a more pronounced effect than that of acetone treatment on the function of lichens
over a long-term scale. Even after 3 years, the acetone-treated thalli contained half
as much usnic acid as the control thalli, and the level of photoprotection remained
unchanged in the algae. However, the amount of available humidity was a more critical
limiting environmental factor than the amount of incoming irradiation affecting usnic
acid production. The lichenicolous fungus Didymocyrtis cladoniicola became relatively
more abundant in the acetone-treated samples than in the control samples, indicating
a slight change caused by the treatment.
