Assimilation of ammonia by the azolla‐anabaena symbiosis

Abstract 15N‐labeled ammonia was rapidly assimilated by Azolla caroliniana and incorporated into plant material even though sustained growth of the fern‐algae symbiosis cannot be maintained with ammonia as nitrogen source. During ammonia uptake, the nitrogenous pool of the fern rapidly increases and contains large amounts of free ammonia and glutamine. N2 fixation activity of the algal symbiont declines during assimilation of ammonia, but it is restored to a high level upon transfer of plants to nitrogen‐free media, as the pool ammonia content decreases. During growth of the fern on N2, the algal symbiont supplies ammonia in a manner permitting sustained growth of the plant. Exogenous ammonia, therefore, appears to interrupt regulation of inorganic nitrogen metabolism of the plant‐algal symbiosis.