Evolution of asymbiotic nitrogen fixation

Recent observations on the nature of the enzyme complex, nitrogenase, prepared from a variety of nitrogen-fixing micro-organisms, on its substrate specificity, energy requirements, source of reducing power and sensitivity to O 2 now permit speculation on the evolution of biological nitrogen fixation in asymbiotic micro-organisms. Ability to fix N 2 is restricted to procaryotic organisms and is particularly widespread among those having characteristics (e.g. hydrogenase, ferredoxin) regarded as primitive. If the primitive environment was devoid of O 2, the earliest N 2-fixing prokaryote would have been a strict anaerobe, not unlike Clostridium pasteurianum. Yet N 2-fixation seems unnecessary in a primitive ammonia-containing environment, and ammonia represses this function in contemporary species. This apparent paradox, the development of the ability to fix N 2 in circumstances in which it was apparently unnecessary suggests that a substance other than N 2 might have been primary substrate of the primeval enzyme. Substances such as acetylene, cyanide, cyanogen, nitriles or isonitriles are all substrates for nitrogenase and are all probable components of the primitive terrestrial environment. Biologically useful functions which a nitrogenase-like reductase system might have served involving substrates other than N 2 include: (a) a detoxification reaction to nullify the effects of cyanide or cyanogen; (b) a means of generating ATP anaerobically; (c) a hydrogen “escape valve”. Functions (b) and (c) are improbable because they would be physiologically uneconomic; function (a) is plausible. With the emergence of an oxidizing atmosphere, facultative and aerobic N 2-fixing micro-organisms could only retain the nitrogenase system if the O 2-sensitive component was protected from inactivation. In the Azotobacteraceae this is achieved by “conformational protection” together with a high respiration rate; in blue-green algae, a structural compartmentation occurs in the more highly evolved species.