Photosynthetic traits of freshwater lichens are consistent with the submersion conditions of their habitat

Abstract

In this study, we compared the photosynthetic performance of epilithic freshwater lichens on siliceous stream rock submerged for: more than 9 (hyper-), 6-9 (meso-) or 3-6 months (sub-hydrophilic lichens). In the dry state, neither variable fluorescence nor respiration activity could be detected. In the wet state, rates of dark respiration (O2 uptake and CO2 production for immerged and in-air samples) were in the lower range of that reported for non-aquatic lichens. With 200 (under water) or 500 μmol.m-2.s-1 photosyntheticallly active photon flux density (PPFD) (aerial), photosynthesis was positive but rates were lower than that published for non-aquatic species. Under intense PPFD (2000 μmol.m-2.s-1, aerial), photosynthesis increased in sub- but became negative in hyper-hydrophilic species. After hydration, dry samples increased photosystem II (PSII) efficiency, which reached near steady state in <6-7 min. Hyper-hydrophilic lichen took longer than sub-hydrophilic species. A long period of desiccation (4 months) had a negative effect on subsequent PSII photochemistry of hyper- but not of sub-hydrophilic hydrated lichens. When thalli were allowed to dehydrate, all types of lichens lost PSII activity after about 15-20 min. Deactivation was faster in the hyper- than in the sub-hydrophilic species. The metabolic traits presented here are thus consistent with the ecological amplitude of the freshwater lichens studied.