Mycobiont‐cyanobiont interactions during dark nitrogen fixation by the lichen Peltigera aphthosa

Abstract

Acetylene reduction (nitrogenase activity) by excised cephalodia of Peltigera aphthosa Willd. slowly declined on transfer of the cephalodia from light to darkness. The decline was more rapid in the absence of CO2 or when phosphoenolpyruvate carboxylase activity was inhibited by adding maleic acid or malonic acid. When glutamine synthetase (GS) activity was totally inhibited by adding l‐methionine‐dl‐sulphoximine (MSX) the decline in nitrogenase activity in the absence of CO2 still occurred. However, this loss of activity did not occur when the mycobiont was disrupted using digitonin (0.01 % w/v) and the fixed NH4+ was released into the medium. The data suggest that dark CO2 fixation by the fungus supplies carbon skeletons which remove newly fixed NH4+ produced by the cyanobacterium. When such carbon skeletons are not available MH4+ accumulates and inhibits nitrogenase activity even in the absence of GS activity. It is probable that NH4+ and a product of GS exert independent inhibitory effects on nitrogenase activity.