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

Abstract

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 (F_V/F_M) and effective quantum yield of photosystem II (Φ_PSII) for all species. All investigated species showed a high level of cryoresistance with critical temperatures (T_c) below -20 °C. However, record low T_c temperatures have been discovered for L. brialmotii (-54 °C for F_V/F_M and -40 °C for Φ_PSII) and C. regalis (-52 °C for F_V/F_M and -38 °C for Φ_PSII). Additionally, the yield differentials (F_V/F_M - Φ_PSII) in functions of temperature revealed one or two peaks, with the larger one occurring for temperatures below -20 °C for the above-mentioned species. Finally, Kautsky kinetics were measured and compared at different temperatures (20 °C, 10 °C, 0 °C and -10 °C and then -10 °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.

Keywords: Antarctic lichens; Chlorophyll fluorescence; Freezing tolerance; Kautsky kinetic; Linear cooling; Photosynthesis.