Freezing temperature effects on photosystem II in Antarctic lichens evaluated by chlorophyll fluorescence

This study explores and compares the limits for photosynthesis in subzero temperatures of six Antarctic lichens: Sphaerophorus globosus, Caloplaca regalis, Umbilicaria antarctica, Pseudephebe minuscula, Parmelia saxatilis and Lecania brialmontii combining linear cooling and chlorophyll fluorescence methods. The results revealed triphasic S-curves in the temperature response of the maximum quantum yield (FV/FM) and effective quantum yield of photosystem II (OPSII) for all species. All investigated species showed a high level of cryoresistance with critical temperatures (Tc) below -20 degrees C. However, record low Tc temperatures have been discovered for L. brialmotii (-54 degrees C for FV/FM and -40 degrees C for OPSII) and C. regalis (-52 degrees C for FV/FM and -38 degrees C for OPSII). Additionally, the yield differentials (FV/FM - OPSII) in functions of temperature revealed one or two peaks, with the larger one occurring for temperatures below -20 degrees C for the above-mentioned species. Finally, Kautsky kinetics were measured and compared at different temperatures (20 degrees C, 10 degrees C, 0 degrees C and -10 degrees C and then -10 degrees C after 1 h of incubation). This research serves as a foundation for further developing investigations into the biophysical mechanisms by which photosynthesis is carried out at subzero temperatures.