The presence of the symbiont in the Azolla-Anabaena symbiosis is essential for the ability of the fern to utilize combined nitrogen

In the Azolla-Anabaena azollae association, the fern provides the photo-assimilates while the N2-fixing cyanobacterium supplies the entire N requirement (Peters and Meeks, 1989). Endophyte-free Azolla fern (EF), cured of its association, was dependent on combined N yet grew much slower than the association, displayed higher carbon (C) content, but lower N and protein contents (Kaplan and Peters, 1998), and developed a massive root system. We hypothesized that EF suffered from chronic N deficiency in the absence of the endophyte. Indeed, an A. pinnata association grown under a low-N₂ atmosphere develops symptoms of N deficiency, which are overcome by providing combined N to the medium (Table 1). Growth of EF was accelerated by replacing inorganic combined N with an organic N source (chasein hydrolysate + glutamine).