Schematic representation of nitrogen cycle (Source: studyacs.com) Physiology of Nitrogen Fixation by the Cyanobacteria (BGA) Cyanobacteria (BGA) are the sole organisms that amalgamate the metabolic capabilities of photosynthetic O 2 production and nitrogen fixation. This is often prominent because the nitrogenase enzyme and reaction are highly unstable to O 2 . Cyanobacteria have advanced strategies to deal with this problem.