Which features of some species of blue – green algae helps promote them as bio-fertilizers? [UPSC 2010 (CS-P)]

They convert atmospheric methane into ammonia which the crop plants can absorb readily.
They induce the crop plants to produce the enzymes which help convert atmospheric nitrogen to nitrates.
They have the mechanism to convert atmospheric nitrogen into a form that the crop plants can absorb readily.
They induce the roots of the crop plants to absorb the soil nitrates in larger quantities.