The effect of Paraquat, a herbicide known to stimulate oxygen-centred free-radical production in green plants, is compared with visible injury symptoms due to air pollution in Parmotrema reticulatum, an SO2-sensitive lichen species. This is in order to verify the hypothesized SO2 toxicity mechanism via the generation of free radicals. A new SEM-visible symptom is described: the production of heavy crystalline deposition on the thallus surface in SO2-stressed and Paraquat-treated specimens. The examination of the crystalline deposition by X-ray diffraction, correlated with SEM studies, showed that it is calcium oxalate (weddellite).

The decomposition of ground, dead (oven-dried) lichens has been studied in laboratory experiments that aimed to simulate decomposition in nature. Enzymes, common in soil animal guts and soil microbes, solubilized 29% of the lichen weight and 25% of lichen nitrogen. Approximately 9% of lichen nitrogen was mobilized to mineral forms during 97 days' incubation in soil or sand. Low moisture levels reduced the amount of nitrogen mobilized. In short-term (30 days) incubation studies, lichens at optimum moisture but low temperature lost 9% of their initial dry weight due to microbial metabolism. In contrast, lichens incubated for 30 and 135 days at similar moisture levels but higher temperatures lost 19% and 30% respectively of their initial dry weight due to microbial metabolism.

Using the response of epiphytic lichens in and around the city of Thessaloniki (Greece) to an air pollution source, the value of sampling lichens at breast height and the base of trees was compared. The data obtained were processed with detrended correspondence analysis and polythetic divisive two-way indicator species analysis, and IAP (index of atmospheric purity) values were estimated for each experimental site. The tree base IAP values were higher than those at breast height, reflecting the higher number of species present, the generally increased cover and frequency of occurrence. IAP values at the two sampling heights suggest a linear relationship and the same holds for the tree base IAP values and the first axis ordination scores. Sampling at the base resulted in better discrimination of sites, and may, therefore, offer more accurate monitoring of changes induced by air pollution.

The weathering phenomena resulting from the growth of six foliose and crustose lichens (Parmelia subrudecta, Xanthoria ectaneoides, Parmelia conspersa, Aspicilia radiosa, Caloplaca sp. and Ochrolechia parella) on three mafic rocks have been studied. The bioweathering results in more or less extensive fragmentation and corrosion of the mineral surfaces immediately beneath the lichen thalli and in the formation, in the thallus or at the rock-lichen interface, of secondary products. The significant amounts of whewellite found in all interfaces, and the bipiramids of weddellite detected at the serpentine rock-Ochrolechia parella interface, suggest that the oxalic acid secreted by the mycobiont is the chemical substance principally involved. The capacity of the lichens to alter their rock substrata does not appear to be related to their thallus morphology.

A laboratory experiment was conducted to determine the damage caused by snail grazing to 35 calcicolous lichen species and cyanobacteria from the Baltic island of Oland, Sweden. Individuals of four species of land snails (Balea perversa, Chondrina cliema, Clausilia bidentata and Helicigona lapicida) were allowed to graze on identified lichen species growing on pieces of limestone. Snail grazing on thalli and ascocarps was classified into four categories ranging from no damage to completely eaten. In general, some lichen species were heavily grazed, whereas others were only slightly damaged or not injured. Aspiciha calcarea, Tephromela atra and Verrucaria nigrescens were preferred by all snail species except Chondrina cliema. In general, lichen thalli that were not immersed in the calcareous rock and cyanobacteria were preferred, whereas ascocarps were avoided by one of the snail species (C. clienta). Immersed perithecia with a carbonized outer layer were avoided by all snail species except C. bidentata. Possibilities of chemical and mechanical defence properties in calcicolous lichens are discussed.

In contrast to green algal lichens, cyanobacterial species of different families, growth forms and habitats proved to be unable to attain positive net CO2 assimilation when the dry thalli were treated with air of high relative humidity; they needed liquid water for the reactivation of their photosynthetic apparatus. Identical behaviour is shown by all of the 47 lichen species with cyanobacterial photobionts, from six different genera, studied so far. This suggests a widely distributed, if not general, characteristic of cyanobacterial lichens. The difference in performance between both groups of photobionts was maintained when the lichen thallus was macerated. Furthermore, cultures of Chroococcidiopsis were unable to make use of water vapour hydration for positive net photosynthesis, and were similar in this respect to some free-living aerophilic cyanobacteria tested earlier. Possible physiological implications as well as ecological consequences for water-relation-dependent habitat selection of green-algal and cyanobacterial lichens are discussed.