Hydrogen sulphide inhibits PSII of lichen photobionts

The effects of hydrogen sulphide (H2S) on five lichens with different photobiont, ecology, and tolerance to the pollutant were studied by means of samples exposed in closed chambers containing two known H2S solutions. The H2S concentration in the void volume at equilibrium with the liquid phase was measured through gas chromatography-mass spectrometry, combined with the use of solid phase micro extraction (GC/MS SPME). It was determined in 8 and 28 ppm H2S in absence of lichen material, and in c. 2 and 10 ppm H2S respectively with living lichen material inserted for 8 hours in the exposure chambers. Significant differences in the species-specific emission of chlorophyll a fluorescence (ChlaF) were observed, with a pronounced depression of Fv/Fm already detectable after 2 h exposure at 28 ppm H2S in all the species. The decrease intensity was positively correlated to sample surface and, to a lesser extent, to the species-specific pre-exposure Fv/Fm value. Dark-exposed samples were less affected than light-exposed ones. All four chlorolichens could recover the pre-exposure ChlaF emission after two days in absence of H2S, both in the light and in the dark, whereas the cyanolichen did not recover when kept in the dark. The results are thoroughly discussed on the basis of the known action mechanisms of H2S on the photosynthetic apparatus of vascular plants and cyanobacteria.