Disappearance of symbiotic algae in theAzolla-Anabaenaassociation subjected to transitory-exposure to ammonium

When Azolla japonica was grown on media containing ammonium at high concentrations, the ammonium levels within the Azolla tissue increased rapidly and reached the same levels as those in the media. When these Azolla fronds were transferred to a N-free medium, yellow spots were observed. In the crushed yellow spots, hardly any filaments of symbiotic algae could be detected under a microscope. Furthermore, the yellow spots cut from whole plants were able to regenerate to normal fronds only on a medium containing combined nitrogen at low concentrations. These regenerated fronds were free from Anabaena, did not exhibit acetylene reduction activity and failed to survive on a N-free medium.