IN-SITU PHOTOSYNTHETIC DIFFERENTIATION OF THE GREEN ALGAL AND THE CYANOBACTERIAL PHOTOBIONT IN THE CRUSTOSE LICHEN PLACOPSIS-CONTORTUPLICATA

In situ photosynthetic activity in the green algal and the cyanobacter ial photobionts of Placopsis contortuplicata was monitored within the same thallus using chlorophyll a fluorescence methods. It proved possi ble to show that the response to hydration of the green algal and the cyanobacterial photobionts is different within the same thallus. Measu rements of the photochemical efficiency of PS II, Fv/Fm, reveal that i n the dry lichen thallus photosynthetic activity could be induced in t he green algal photobiont by water vapour uptake, in the cyanobacteria l photobiont only if it was hydrated with liquid water. However, rates of apparent electron flow through PS II as well as rates of CO2 gas e xchange were suboptimal after hydration with water vapour alone and ma ximum rates could only be observed when the thallus was saturated with liquid water. The differences in the water-related photosynthetic per formance and different light response curves of apparent electron tran sport rate through PS II indicate that the two photobionts act highly independently of each other. It was shown that the cyanobacteria from the cephalodia in P. contortuplicata act as photobiont. The rate of el ectron flow through PS II was found to be saturated at 1500 mumol phot on m-2 s-1, despite a considerable increase of non-photochemical quenc hing in the green algal photobiont which is lacking in the cyanobacter ial photobiont. No evidence of photoinhibition could be found in eithe r photobiont. Pronounced competition between the green algal and the c yanobacterial thallus can be observed in the natural habitat, indicati ng that the symbiosis in P contortuplicata should be regarded as a ver y variable adaptation to the extreme environmental conditions in the m aritime Antarctic.