Cryoresistance of Antarctic endemic lichen <i>Himantormia lugubris</i>: Analysis of photosystem II functionality using a constant-rate cooling approach

It is well established that lichens from polar regions of the Earth are capable to perform photosynthesis at sub-zero temperatures. Majority of them show a high degree of cryoresistance, however, species-specific differences exist. Therefore, the aim of our study was to evaluate behaviour of primary photochemical processes of photosynthesis in Antarctic endemic species Himantormia lugubris at sub-zero temperature. For the purpose, the method of constant rate (2°C min-1) cooling (from +20 to -40°C) with simultaneous measurements of chlorophyll fluorescence parameters related to photosystem II (PSII) was used. During the cooling, potential yield of photosynthetic processes in PSII (FV/FM), and effective quantum yield of PSII (ΦPSII) were measured in 30 s interval. From the FV/FM and ΦPSII data sets, S-curves reflecting temperature dependence of the two chlorophyll fluorescence parameters were constructed and analyzed. The S-curves were found tri-phasic in response to sample temperature decline: (1) slight or no decline phase, (2) rapid decline phase, followed by (3) slow change reaching critical temperature at which the primary photosynthetic processes were fully inhibited. Critical temperature was found -30 and -20°C for FV/FM, and ΦPSII, respectively. The latter critical temperature was accompanied by an increase in background chlorophyll fluorescence (F0) indicating inhibition of energy transfer from light-harvesting complexes to core of PSII. A newly-designed chlorophyll fluorescence parameter (a differential, i.e. the difference between the maximum value-normalized FV/FM, and ΦPSII) was used in order to evaluate the temperature at which the processes related to photosynthetic electron flow through thylakoid membrane carriers (ΦPSII) and the energy flow through PSII (FV/FM) differed to a largest extent. This parameters proved to be temperature-dependent and useful in the evaluation of cryoresistance. Based on our study, H. lugubris, its primary photosynthetic processes in particular, might be considered as higly resistant to sub-zero temperature.