Which feature of some species of blue green algae helps promoted them as bio-fertilizers?

Many cyanobacteria (blue‑green algae) possess the enzyme nitrogenase and, in filamentous heterocyst‑forming species, specialized cells (heterocysts) that protect nitrogenase from O2 and allow biological nitrogen fixation — the reduction of atmospheric N2 into ammonia (NH3), which is readily assimilable by plants. This capacity is the basis for using cyanobacteria (e.g., Anabaena in symbiosis with Azolla and free‑living nitrogen‑fixing strains) as biofertilizers in wetland/rice culture and other agricultural systems [1]. Thus the feature that promotes some blue‑green algae as biofertilizers is their intrinsic mechanism to convert atmospheric nitrogen into plant‑available forms (ammonium/ammonia).

Sources

1. [1] https://www.nature.com/scitable/knowledge/library/biological-nitrogen-fixation-23570419/