Nitrogen fixation in the lichen Stereocaulon paschale
Abstract: The thesis is a summary and discussion of six papers.The purpose of the investigation was to study the influence of (i) environmental factors and (ii) the physiological condition of the thallus on nitrogen fixation in the lichen Stereocaulon paschale (L.) Fr. The nitrogen input to the site, a sparsely stocked pine forest in northern Sweden, was also studied. Nitrogen fixation (nitrogenase activity) was measured as acetylene reduction both in the field and in the laboratory. Usually intact lichen thalli were used, but also excised cephalodia were studied.All nitrogenase activity was located in the external cephalodia containing the blue-green alga Stigonema sp. There was always a reduction in nitrogenase activity when the cephalodia were quantitatively excised from the thallus. Moisture was found to be the most important environmental factor in the field during the snow free part of the year. At the site, with 14 % of the ground covered by S. paschale3 the yearly nitrogen fixation was estimated to c. 0.1 g nitrogen per m^. The lichen thalli could withstand several months in a very dry condition and at a low temperature without significant decrease in nitrogenase activity. A higher capacity for nitrogenase activity was found in lichen thalli collected from bare ground than in thalli collected under the snow. The light conditions before as well as during the nitrogenase activity measurements affected the nitrogenase activity. Thalli incubated with acetylene in the dark had only c. two thirds of their activities in the light. Lichen thalli pretreated in the light showed increased nitrogenase activities, probably due to raised content of carbohydrates available for nitrogenase activity. The necessary energy for nitrogenase activity is supplied by either oxidative phosphorylation or photophosphorylation.
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