Sensitivity of photosynthetic processes to freezing temperature in extremophilic lichens evaluated by linear cooling and chlorophyll fluorescence.

Extremophilic lichens and their photosynthesizing photobionts from the cold regions of Earth are adapted to perform photosynthesis at subzero temperatures. To evaluate interspecific differences in the critical temperature for primary photochemical processes of photosynthesis, we exposed lichen thalli of Usnea antarctica, Usnea aurantiaco-atra, and Umbilicaria cylindrica to linear cooling from +20 to -50 °C at a constant rate of 2 °C min-1. Simultaneously, two chlorophyll fluorescence parameters (FV/FM - potential yield of photosynthetic processes in photosystem II, ΦPSII - effective quantum yield of PS II) evaluating a gradual subzero temperature-induced decline in photosynthetic processes were measured by a modulated fluorometer. For the studied species, the response of FV/FM and ΦPSII to declining temperature showed an S-curve shape. The decline in FV/FM and ΦPSII at low temperatures started at -5 and +5 °C, respectively in the majority of cases. The decline was, however, species-specific. U. aurantiaco-atra showed a constant-rate decline of ΦPSII from the physiological temperature 20 °C. U. antarctica exhibited the first sign of FV/FM decline at -12 °C. The critical temperature related to full inhibition of the photosynthetic processes in PSII (FV/FM), was found at -20 °C. However, this occurred at -30 °C for U. cylindrica. In an individual sample, the critical temperature for FV/FM was typically lower than for ΦPSII. The method of linear cooling combined with simultaneous measurements of chlorophyll fluorescence parameters proved to be an efficient tool in the estimation of extremophilic species sensitivity/resistance to freezing.