Ecophysiology of calcicolous endolithic lichens: progress and problems

Abstract

The biology of calcicolous endolithic lichens is far from being well-known. In this paper the results of a multidisciplinar research are presented. The study is based on laboratory and field data, and is divided in four parts: (1) anatomical investigation of the species; (2) characterization of gas exchange rates of selected epi- and endolithic lichens; response curves were obtained in dependence to the main abiotic factors (thallus water content, light, CO₂ concentration, temperature); (3) characterization of chemical-physical parameters of the substratum (porosity, capillarity, etc.); (4) field measurements of microclimatic parameters in the habitat of each species, and quantification of thallus hydration in situ during selected days at different weather conditions. Six species, five of them endolithic (Acrocordia conoidea, Petractis clausa, Rinodina immersa, Verrucaria marmorea and Verrucaria sp.) and one epilithic (Aspicilia radiosa) were investigated in the VG 46 abyss near Gropada (Trieste Karst, NE Italy). The results show that: (1) the maximum photosynthetic rates of endolithic lichens are small, ranging between 0.2 and 1.5 mmol CO₂ m^-2 h^-1 at optimal conditions, although chlorophyll content is relatively high, ranging between 66 and 183 mg Chl _(a+b) m^-2; (2) the reduction in photosynthesis at high thallus water content is larger than in foliose and fruticose lichens; (3) the algae are light-saturated at low values; there is a good relationship among compensation point, light saturation, and light regimes of the microhabitat of each species, as shown by the multivariate analysis of laboratory and field data; (4) CO₂ saturation is reached only at values higher than 1,300 ppm; (5) the substratum probably plays a protective fuction against several factors, such as excessive light, prolonged dehydration, and herbivory; CO₂ produced by the dissolution of the substratum is not apparently used in photosynthesis, as suggested by d¹³C data; (6) rainfall is the main water source for both epi- and endolithic species; on the contrary, dew can be efficiently used only by endolithic lichens, because their biomass is considerably smaller and can be saturated with a smaller amount of water; dew is an important water source for species growing in exposed habitat, such as R. immersa and V. marmorea; (7) Verrucaria sp., which grows on vertical faces of the most humid and shaded parts of the abyss, is apparently able to use air water vapour to reach high hydration values, and is therefore less dependent on rainfall than the other species; this species also has the largest biomass, and is particularly rich in lipids, stored in a thick layer of “oil-hyphac”. On the basis of these data some hypotheses on the significance of the adaptive value of endolithism are made.

Key Words:

• CO₂ exchange
• Ecophysiology
• Endolithic
• Lichen
• Limestone
• Photosynthesis
• Respiration