Inhibition of Energy-Transfer to Photosystem II in Lichens by Dehydration: Different Properties of Reversibility with Green and Blue-green Phycobionts

Abstract

Dry lichens with green algal phycobionts are able to recover net photosynthesis through rehydration with water vapor, whereas lichens with blue-green algae lack this ability (Lange, Kilian and Ziegler, Oecologia 71, 104–110, 1986). By measurements of 77 K fluorescence emission and excitation spectra, it is investigated whether this basic difference in rehydration properties of green and blue-green lichens is due to the different organization of the antenna pigments. Emission spectra obtained from a lichen with a blue-green alga (Peltigera rufescens) and from a free-living blue-green alga (Nostoc cf. commune) were essentially identical after normalization at 710 nm and revealed the following major features: Equilibration of a dry organism with air of 99 % r.h. resulted in a pronounced increase of fluorescence emission at 650 nm when excited at 580 nm (i.e. at maximal absorbance for the phycobilin pigments). Additional spraying with liquid water led to a strong fluorescence increase around 690 nm. With excitation of chlorophyll at 430 nm no such changes could be observed with both procedures. Excitation spectra for emission at 695 nm revealed that the efficiency for fluorescence excitation at 565 nm compared to that at 435 nm was minimal after equilibration at 99 % r.h., whereas spraying with liquid water resulted in a strong enhancement. It is concluded that desiccation induces a functional detachment of the phycobilisomes (PBS) from photosystem II (PS II). Energy transfer from PBS to PS II is restored only when rehydration occurs with liquid water. Similar experiments with the lichen Ramalina maciformis, which contains a green phycobiont (Trebouxia spec.) yielded distinctly different responses. The desiccation induced lowering of fluorescence emission was almost totaly reversed by equilibration with air of 93 % r.h. Excitation spectra of 695 nm emission showed a large reduction of the emission ratio with 480 nm excitation over that with 435 nm induced by desiccation. Rehydration by increasing air humidity resulted in a gradual increase of this ratio until it was almost maximal at about 99 % r.h. It is concluded that with green algal symbionts desiccation induces a functional interruption of energy transfer between the light harvesting chl a/b pigment complex and PS II and that this can be largely restored by rehydration with humidified air, in contrast to the situation with blue-green algal symbionts.