Response of 19 Azolla strains to a high concentration of ammonium ions

The response of 19 Azolla strains from five species to 20 mM ammonium ions was examined. The response varied even among strains of the same species. The tolerance indexes of both relative growth rate and nitrogen fixation [(values for plants grown in the presence of ammonium ions)/(values for plants grown in the absence of ammonium ions)] showed that ammonium ions did not always simultaneously inhibit growth and nitrogen fixation of individual strains. The tolerance of each Azolla–Anabaena association to ammonium ions is assumed to be determined separately by both the host fern and the symbiotic Anabaena. The inhibitory effects of ammonium appeared predominantly in the mature region of Azolla–Anabaena associations. In the sensitive strains, both chlorophyll content and the number of cyanobionts were reduced only in the mature region when ammonium ions were present. Possible strains for treatment of wastewater, which contains a high concentration of ammonium ions, are discussed.