Low-temperature limitation of primary photosynthetic processes in Antarctic lichens Umbilicaria antarctica and Xanthoria elegans

Temperature response curves of chlorophyll a fluorescence parameters were used to assess minimum sub-zero temperature assuring functioning of photosynthetic photochemical processes in photosystem II (PS II) of Antarctic lichens. Umbilicaria Antarctica and Xanthoria elegans were measured within the temperature range from ‑20 to +10°C by a fluorometric imaging system. For potential (F_V/F_M) and actual (Φ_II) quantum yields of photochemical processes the minimum temperature was found to be between ‑10 and ‑20°C. Non-photochemical quenching (NPQ) of absorbed excitation energy increased with temperature drop reaching maximum NPQ at ‑15°C. Image analysis revealed intrathalline heterogeneity of chlorophyll a fluorescence parameters with temperature drop. Temperature response of Φ_II exhibited an S-curve with pronounced intrathalline differences in X. elegans. The same relation was linear with only limited intrathalline difference in U. antarctica. The results showed that Antarctic lichen species were well adapted to sub-zero temperatures and capable of performing primary photosynthesis at ‑15°C.