DIRECT MEASUREMENT OF RELEASE AND ASSIMILATION OF AMMONIA IN THE GUNNERA-NOSTOC SYMBIOSIS

In Gunnera, Nostoc cells invade secretory tissue forming well defined symbiotic areas within the stems and are termed internal nodules (Silv ester, 1976). Excised, but intact, internal stem Nostoc nodules taken from Gunnera magellanica show light-stimulated nitrogenase activity an d release a small, but measurable, proportion of their current N-2 fix ation as NH3 into the external solution. When nodules are disrupted an d Nostoc extracted anaerobically, 90% or more of the estimated N-2 fix ation is released from the Nostoc cells as NH3 into the surrounding me dium. Use of N-15(2) confirmed that only 12% of N-2 fixed is retained within the cells of Nostoc. The remaining 88% was identified as NH3 re leased outside the cells. Within the intact nodule system, N-15(2) upt ake showed that 2-5% of recently fixed N-2 remains within the Nostoc c ells and up to 30% of extracellular N is in asparagine after 1 h. Evid ence is presented that stimulation of nitrogenase by light in the inta ct Gunnera/Nostoc system produces more NH3 than can be assimilated by the host cells, resulting in significant NH3 accumulation.