NITROGEN-FIXATION BY NATURALLY-OCCURRING DUCKWEED CYANOBACTERIAL ASSOCIATIONS

N fixation as measured by acetylene conversion to ethylene was found to be common for duckweed blooms on ponds, lakes, and streams in Michigan [USA]. Twenty-six of 29 sites with duckweed sampled over a 2-yr period showed acetylene reduction activity (ARA). These activities corresponded to N inputs of 3.7-7.5 kg N .cntdot. ha-1 per annual cycle for typical blooms but dense Lemna trisula blooms could be 10 times greater. The ARA was stimulated 5- to 6-fold by light, was not removed when plants were shaken in water, and was usually associated with the leaves and not the roots. Colonies of heterocyst-bearing cyanobacteria of teh genera Nostoc, Gloeotrichia, Anabaena, Calothrix and Cylindrospermum were observed attached to the lower epidermis or in the reproductive pockets of leaves of Spirodela and Lemna plants but not on Wolffia plants. The ARA correlated reasonally well with the density of cyanobacterial colonies observed. The duckweed appeared to provide a more favorable environment harboring duckweed. The N2 fixation was not sufficient, however, to meet all the N needs of the duckweed bloom.