Efficient fungal UV-screening provides a remarkably high UV-B tolerance of photosystem II in lichen photobionts.

Lichen photobionts in situ have an extremely UV-B tolerant photosystem II efficiency (Fv/Fm). We have quantified the UV-B-screening offered by the mycobiont and the photobiont separately. The foliose lichens Nephroma arcticum and Umbilicaria spodochroa with 1: intact or 2: removed cortices were exposed to 0.7 Wm-2 UV-BBE for 4 h. Intact thalli experienced no reduction in Fv/Fm, whereas cortex removal lowered Fv/Fm in exposed photobiont layers by 22% for U. spodochroa and by 14% for N. arcticum. We also gave this UV-B dose to algal cultures of Coccomyxa and Trebouxia, the photobiont genera of N. arcticum and U. spodochroa, respectively. UV-B caused a 56% reduction in Fv/Fm for Coccomyxa, and as much as 98% in Trebouxia. The fluorescence excitation ratio (FER) technique comparing the fluorescence from UV-B or UV-A-excitation light with blue green excitation light using a Xe-PAM fluorometer showed that these photobiont genera did not screen any UV-B or UV-A The FER technique with a Multiplex fluorometer estimated the UV-A screening of isolated algae to be 13-16%, whereas intact lichens screened 92-95% of the UV-A. In conclusion, the cortex of N. arcticum and U. spodochroa transmitted no UV-B and little UV-A to the photobiont layer beneath. Thereby, the upper lichen cortex forms an efficient fungal solar radiation screen providing a high UV-B tolerance for studied photobionts in situ. By contrast, isolated photobionts have no UV-B screening and thus depend on their fungal partners in nature.