The Azolla-Anabaena azollae relationship

Abstract

The heterocystous blue-green alga, Anabaena azollae, was isolated from the leaf cavities of the water fern, Azolla caroliniana, where it occurs as an endophyte. The isolated alga was capable of light dependent CO2 fixation and acetylene reduction. Aerobic dark acetylene reduction occurred and was dependent upon endogenous substrates. Vegetative cells of the alga reduced nitro-blue tetrazolium chloride (NBT) to blue formazan. Heterocysts did not. Heterocysts reduced triphenyl tetrazolium chloride (TTC) to red formazan faster than vegetative cells. Reduction of TTC by both heterocysts and vegetative cells was much more rapid than has been reported for free-living heterocystous blue-green algae. Both NBT and TTC inhibited acetylene reduction and CO2 fixation. The inhibition by TTC was more closely correlated to the time of exposure of the cells to the reagent and to the amount of deposition per cell than to the number of cells containing red formazan. No differential inhibition of acetylene reduction versus CO2 fixation was observed. Autoradiography showed that CO2 fixation occurred only in vegetative cells. Heterocysts caused a darkening of nuclear emulsions (chemography). This observation has been employed by others as an index of reducing activity in these cells. DCMU inhibited the acetylene reducing capacity of alga isolated from dark pretreated fronds more rapidly and to a greater extent than that in alga isolated from light pretreated fronds. Ammonia in excess of 5 mM was required before any inhibition of acetylene reduction was observed under either aerobic or anaerobic conditions in the light.