Desiccation and low temperature attenuate the effect of UVC254 nm in the photobiont of the astrobiologically relevant lichens Circinaria gyrosa and Buellia frigida

Several investigations on lichen photobionts (PBs) after exposure to simulated or real-space parameters consistently reported high viability and recovery of photosynthetic activity. These studies focused on PBs within lichen thalli, mostly exposed in a metabolically inactive state. In contrast, a recent study exposed isolated and metabolically active PBs to the non-terrestrial stressor UVC254nm and found strong impairment of photosynthetic activity and photo-protective mechanisms (Mee ss en et al. in 2014b). Under space and Mars conditions, UVC is accompanied by other stressors as extreme desiccation and low temperatures. The present study exposed the PBs of Buellia frigida and Circinaria gyrosa, to UVC in combination with desiccation and subzero temperatures to gain better insight into the combined stressors' effect and the PBs' inherent potential of resistance. These effects were examined by chlorophyll a fluorescence which is a good indicator of photosynthetic activity (Luttge & Budel in 2010) and widely used to test the viability of PBs after (simulated) space exposure. The present results reveal fast recovery of photosynthetic activity after desiccation and subzero temperatures. Moreover, they demonstrate that desiccation and cold confer an additional protective effect on the investigated PBs and attenuate the PBs' reaction to another stressor - even if it is a non-terrestrial one such as UVC. Besides other protective mechanisms (anhydrobiosis, morphological-anatomical traits and secondary lichen compounds), these findings may help to explain the high resistance of lichens observed in astrobiological studies.