The Cephalodia and their Nitrogenase Activity in the Lichen Stereocaulon paschale

Abstract

In the lichen Stereocaulon paschale (L.) FR. the cephalodia composed on average 6 % of the total thallus biomass. The general morphology of the cephalodia was examined in the scanning electron microscope. In the cephalodia the blue-green alga ( Stigonema sp.) had a heterocyst frequency of c. 16 %. There was always a reduction in nitrogenase activity when the cephalodia were quantitatively excised from the thallus and on average 59 % of the activity was retained in the excised cephalodia. There was also a reduction in activity when the thallus was cut in small pieces without rupturing the connection between a cephalodium and the rest of the thallus. Except for the lowered nitrogenase activity the cephalodia behaved as intact thalli. Nitrogenase activity in excised cephalodia showed a direct response to alternating light/dark periods. Free cephalodia kept in the light had a constant nitrogenase activity during at least two days.