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 (Silvester, 1976). Excised, but intact, internal stem Nostoc nodules taken from Gunnera magellanica show light-stimulated nitrogenase activity and release a small, but measurable, proportion of their current N fixation as NH into the external solution. When nodules are disrupted and Nostoc extracted anaerobically, 90% or more of the estimated N fixation is released from the Nostoc cells as NH into the surrounding medium. Use of N confirmed that only 12% of N fixed is retained within the cells of Nostoc. The remaining 88% was identified as NH released outside the cells. Within the intact nodule system, N uptake showed that 2-5% of recently fixed N remains within the Nostoc cells and up to 30% of extracellular N is in asparagine after 1 h. Evidence is presented that stimulation of nitrogenase by light in the intact Gunnera/Nostoc system produces more NH than can be assimilated by the host cells, resulting in significant NH accumulation.