The responses of lichens to fluoride pollution from an aluminium smelter
1) Fluoride concentrations were investigated in 6 lichens and a moss growing on stone walls around the aluminium smelter at Invergordon. 2) These plants were able to accumulate elevated amounts of fluoride from the atmosphere. The highest level of fluoride occurred near the smelter and in the direction of prevailing winds. 3) Lichen surveys on three different substrates (fence-posts, trees and stone walls) revealed that lichens colonizing all these habitats were badly damaged in polluted areas. 4) A lichen desert was observed on the fence posts closer to the smelter. a5) Some species of lichens were absent from these substrates in the vicinity of smelter and did not appear until some distance away from the emission source. 6) These observations suggested that crustose lichens, due to their presence closer to the smelter, were more resistant to fluoride pollution than either foliose or fruticose lichens. 7) Transplant studies showed that lichens absorbed large amounts of fluoride from the polluted atmosphere around the aluminium smelter. a8) Fluoride concentration in the transplanted lichens increased with increasing time of exposure. 9) Four lichens transplanted in the same locality accumulated varying amounts of fluoride in their thalli. 10) The transplanted lichens were visually damaged and discoloured in zones of high pollution. 11) This discolouration was concurrent with decline in net photosynthesis and chlorophyll content of the thalli. 12) The reduction of net photosynthesis was also related to an increasing amount of fluoride in the lichen thalli. 13) The declining rate of net photosynthesis in a 'sensitive' species (i.e. H. physodes) was greater than in the 'resistant' species (P. saxatilis). 14) Laboratory studies revealed that lichens were able to accumulate large amounts of fluoride from hydrogen fluoride gas and sodium fluoride solutions. 15) Various lichens absorbed different quantities of fluoride from the same pollution levels. 16) The net photosynthetic rates of lichens decreased with increasing concentrations of hydrogen fluoride gas in fumigation chambers or fluoride ions in sodium fluoride solutions. 17) The effects of fluoride applied as hydrogen fluoride gas were more acute and rapid than those from sodium fluoride solutions. 18) Various lichens showed different physiological responses to the same external concentrations of fluoride (whether gaseous or in solutions). 19) The internal concentrations at which damage occurred were similar for each species, whether fluoride was applied as hydrogen fluoride gas or sodium fluoride solution. 20) The order of species sensitivity remains the same in whatever the form of fluoride applied and whether it is applied in the field or under laboratory conditions. 21) Field transplantation suggested that H. physodes exposed to the rain water was less affected by fluoride pollution than samples which were protected from rain. 22) The "unexposed" samples accumulated more fluoride in their thalli than the "exposed" lichens and consequently showed greater morphological and physiological abnormalities. 23) The chlorophyll content and net photosynthesis of both "exposed" and "unexposed" lichen samples decreased after 12 weeks transplantation. The lichens which were "exposed" showed a 50% reduction in net photosynthesis, whereas "unexposed" samples protected from rain water showed no positive net photosynthesis and contained considerably less chlorophyll. 24) Ultrastructural studies of H. physodes samples, fumigated with graded series of hydrogen fluoride gas, revealed disruption of chloroplasts in algal cells (i.e. Trobouxia). 25) The disruption of chloroplast i.e. formation of vesicles and their size increased with increasing the fluoride accumulation in the lichen thalli. No major change in the fungal cells was observed.