Predicting CO2 gain and photosynthetic light acclimation from fluorescence yield and quenching in cyano-lichens

Modulated chlorophylla fluorescence is useful for eco-physiological studies of lichens as it is sensitive, non-invasive and specific to the photobiont. We assessed the validity of using fluorescence yield to predict CO₂ gain in cyano-lichens, by simultaneous measurements of CO₂ gas exchange and chlorophylla fluorescence in five species withNostoc-photobionts. For comparison, O₂ evolution and fluorescence were measured in isolated cells ofNostoc, derived fromPeltigera canina (Nostoc PC). At irradiances up to the growth light level, predictions from fluorescence yield underestimated true photosynthesis, to various extents depending on species. This reflected the combined effect of a state transition in darkness, which was not fully relaxed until the growth light level was reached, and a phycobilin contribution to the minimum fluorescence yield (F_o). Above the growth light level, the model progressively overestimated assimilation, reflecting increased electron flow to oxygen under excess irradiance. In cyanobacteria, this flow maintains photosystem II centres open even up to photoinhibitory light levels without contributing to CO₂ fixation. Despite this we show that gross CO₂ gain may be predicted from fluorescence yield also in cyanolichens when the analysis is made near the acclimated growth light level. This level can be obtained even when measurements are performed in the field, since it coincides with a minimum in non-photochemical fluorescence quenching (NPQ). However, the absolute relation between fluorescence yield and gross CO₂ gain varies between species. It may therefore be necessary to standardise the fluorescence prediction for each species with CO₂ gas exchange.Abbreviations CCM CO₂-Concentrating mechanism - Chl chlorophyll - C_i inorganic carbon - 0 convexity (curvature of the light response curve) - ETR electron transport rate - F_o minimum fluorescence yield - F_m maximal fluorescence yield - F_s fluorescence yield at steady-state photosynthesis - F_v variable fluorescence yield - F_v/F_m dark ratio of variable to maximal fluorescence yield after dark adaptation - F_vF_mmax ratio of variable to maximal fluorescence yield in the absence of quenching - _CO2 maximum quantum yield of CO₂ assimilation - _PS quantum yield of photosystem II photochemistry - GP gross photosynthesis - I irradiance (mol quanta·m^–2·s^–1) - NPQ non photochemical fluorescence quenching - qp photochemical fluorescence quenching