$N_2$-fixing blue-green alga Anabaena which is extensively used in rice cultivation, forms symbiotic association with

Answer: BSolution: Symbiotic assiciation has been found between Anabaena (blue green alga) and Azolla (a water fern). Azolla pinnata has anabeana azollae in its fronds. It is often inoculated to rice fields for nitrogen fixationl. It is an efficient fertiliser of paddy fierlds. Estimates of the annual quantities of $N_2$ fixed by Azolla range between 120 and 312 kg nitrogen per hactare.