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

¹ Université Paul Sabatier, Département Biologie et Géosciences – Centre Etude Spatiale Biosphère – UMR 5126, 5 18 av. Edouard Belin, bpi 2801, 31401 Toulouse cedex 9, France
² Université de Toulouse, UPS, INPT, EcoLab, 118 route de Narbonne, 31062 Toulouse cedex 9, France
³ CNRS, EcoLab, 31062 Toulouse cedex 9, France
⁴ Université de Toulouse, INP-ENSAT, UMR 1248 AGIR (INPT-INRA), 31326 Castanet Tolosan, France

^* Corresponding author: cloter@wanadoo.fr

Received: 9 September 2015
Accepted: 17 February 2016

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.