A Cobalt Requirement for Symbiotic Growth of Azolla filiculoides in the Absence of Combined Nitrogen.

Cobalt was shown to be essential for the symbiotic growth of Azolla filiculoides and Anabacna azollac in the absence of fixed nitrogen. Addition of 0.01 mug/liter cobalt resulted in large increases in yield, chlorophyll content and nitrogen fixation as compared to control cultures without cobalt. Cobalt was not required for the growth of Azolla when nitrate nitrogen was supplied. The number of Anabaena azollae cells in the fronds of Azolla appeared to be decreased by ommission of cobalt from the culture medium containing nitrate nitrogen. It is concluded that cobalt is essential for the symbiotic growth of Azolla in the absence of combined nitrogen and it is suggested that the cobalt requirement is associated with the growth of Anabaena azollae.